New articles on Quantum Physics

[1] 2312.00832

Optimized General Uniform Quantum State Preparation

Quantum algorithms for unstructured search problems rely on the preparation of a uniform superposition, traditionally achieved through Hadamard gates. However, this incidentally creates an auxiliary search space consisting of nonsensical answers that do not belong in the search space and reduce the efficiency of the algorithm due to the need to neglect, un-compute, or destructively interfere with them. Previous approaches to removing this auxiliary search space yielded large circuit depth and required the use of ancillary qubits. We have developed an optimized general solver for a circuit that prepares a uniform superposition of any N states while minimizing depth and without the use of ancillary qubits. We show that this algorithm is efficient, especially in its use of two wire gates, and that it has been verified on an IonQ quantum computer and through application to a quantum unstructured search algorithm.

[2] 2312.00864

Quantum Acceleration Limit

The speed limit provides an upper bound of the dynamical evolution time of a quantum system. Here, we introduce the notion of quantum acceleration limit for unitary time evolution of quantum systems under time-dependent Hamiltonian. We prove that the quantum acceleration is upper bounded by the fluctuation in the derivative of the Hamiltonian. We illustrate the quantum acceleration limit for a two-level quantum system. This notion can have important applications in quantum computing, quantum control and quantum thermodynamics.

[3] 2312.00875

A perspective on protein structure prediction using quantum computers

Despite the recent advancements by deep learning methods such as AlphaFold2, \textit{in silico} protein structure prediction remains a challenging problem in biomedical research. With the rapid evolution of quantum computing, it is natural to ask whether quantum computers can offer some meaningful benefits for approaching this problem. Yet, identifying specific problem instances amenable to quantum advantage, and estimating quantum resources required are equally challenging tasks. Here, we share our perspective on how to create a framework for systematically selecting protein structure prediction problems that are amenable for quantum advantage, and estimate quantum resources for such problems on a utility-scale quantum computer. As a proof-of-concept, we validate our problem selection framework by accurately predicting the structure of a catalytic loop of the Zika Virus NS3 Helicase, on quantum hardware.

[4] 2312.00892

Black-Litterman Portfolio Optimization with Noisy Intermediate-Scale Quantum Computers

In this work, we demonstrate a practical application of noisy intermediate-scale quantum (NISQ) algorithms to enhance subroutines in the Black-Litterman (BL) portfolio optimization model. As a proof of concept, we implement a 12-qubit example for selecting 6 assets out of a 12-asset pool. Our approach involves predicting investor views with quantum machine learning (QML) and addressing the subsequent optimization problem using the variational quantum eigensolver (VQE). The solutions obtained from VQE exhibit a high approximation ratio behavior, and consistently outperform several common portfolio models in backtesting over a long period of time. A unique aspect of our VQE scheme is that after the quantum circuit is optimized, only a minimal number of samplings is required to give a high approximation ratio result since the probability distribution should be concentrated on high-quality solutions. We further emphasize the importance of employing only a small number of final samplings in our scheme by comparing the cost with those obtained from an exhaustive search and random sampling. The power of quantum computing can be anticipated when dealing with a larger-size problem due to the linear growth of the required qubit resources with the problem size. This is in contrast to classical computing where the search space grows exponentially with the problem size and would quickly reach the limit of classical computers.

[5] 2312.00898

On the Lorentz Boosted Parallel Plate Casimir Cavity

Two perfectly conducting, infinite parallel plates will restrict the electromagnetic vacuum, producing an attractive force. This phenomenon is known as the Casimir effect. Here we use electromagnetic field correlators to define the local interaction between the plates and the vacuum, which gives rise to a renormalized stress-energy tensor. We then show that a Lorentz boost of the underlying electric and magnetic fields that comprise the correlators will produce the correct stress-energy tensor in the boosted frame. The infinite surface divergences of the field correlators will transform appropriately, such that they cancel out in the boosted frame and produce the desired finite result.

[6] 2312.00997

Scaling Whole-Chip QAOA for Higher-Order Ising Spin Glass Models on Heavy-Hex Graphs

We show through numerical simulation that the Quantum Alternating Operator Ansatz (QAOA) for higher-order, random-coefficient, heavy-hex compatible spin glass Ising models has strong parameter concentration across problem sizes from $16$ up to $127$ qubits for $p=1$ up to $p=5$, which allows for straight-forward transfer learning of QAOA angles on instance sizes where exhaustive grid-search is prohibitive even for $p>1$. We use Matrix Product State (MPS) simulation at different bond dimensions to obtain confidence in these results, and we obtain the optimal solutions to these combinatorial optimization problems using CPLEX. In order to assess the ability of current noisy quantum hardware to exploit such parameter concentration, we execute short-depth QAOA circuits (with a CNOT depth of 6 per $p$, resulting in circuits which contain $1420$ two qubit gates for $127$ qubit $p=5$ QAOA) on $100$ higher-order (cubic term) Ising models on IBM quantum superconducting processors with $16, 27, 127$ qubits using QAOA angles learned from a single $16$-qubit instance. We show that (i) the best quantum processors generally find lower energy solutions up to $p=3$ for 27 qubit systems and up to $p=2$ for 127 qubit systems and are overcome by noise at higher values of $p$, (ii) the best quantum processors find mean energies that are about a factor of two off from the noise-free numerical simulation results. Additional insights from our experiments are that large performance differences exist among different quantum processors even of the same generation and that dynamical decoupling significantly improve performance for some, but decrease performance for other quantum processors. Lastly we show $p=1$ QAOA angle mean energy landscapes computed using up to a $414$ qubit quantum computer, showing that the mean QAOA energy landscapes remain very similar as the problem size changes.

[7] 2312.01023

A More Efficient Postprocessing Procedure for Evaluating Hamiltonian Expectation Values in VQE

Variational quantum eigensolvers (VQEs) are considered one of the main applications of quantum computers in the noisy intermediate-scale quantum (NISQ) era. Here, we propose a simple strategy to improve VQEs by reducing the classical overhead of evaluating Hamiltonian expectation values. Observing the fact that $\left< b \middle| G \middle| b \right>$ is fixed for a measurement outcome bit string b in the corresponding basis of a mutually commuting observable group G in a given Hamiltonian, we create a measurement memory (MM) dictionary for every commuting operator group G in a Hamiltonian and store $b$ and $\left< b \middle| G \middle| b \right>$ as key and value. The first time a measurement outcome bit string b appears, $\left< b \middle| G \middle| b \right>$ is calculated and stored. The next time the same bit string appears, we can retrieve $\left< b \middle| G \middle| b \right>$ from the memory, rather than evaluating it once again. We further analyze the complexity of MM and compare it with commonly employed post-processing procedure, finding that MM is always more efficient in terms of time complexity. We implement this procedure on the task of minimizing a fully connected Ising Hamiltonians up to 20 qubits, and $H_{2}$, $H_{4}$, LiH, and $H_{2}O$ molecular Hamiltonians with different grouping methods. For Ising Hamiltonian, where all $O(N^2)$ terms commute, our method offers an $O(N^2)$ speedup in terms of the percentage of time saved. In the case of molecular Hamiltonians, we achieved over $O(N)$ percentage time saved, depending on the grouping method.

[8] 2312.01031

Non-Markovian dynamics of a superconducting qubit in a phononic bandgap

The overhead to construct a logical qubit from physical qubits rapidly increases with the decoherence rate. Current superconducting qubits reduce dissipation due to two-level systems (TLSs) by using large device footprints. However, this approach provides partial protection, and results in a trade-off between qubit footprint and dissipation. This work introduces a new platform using phononics to engineer superconducting qubit-TLS interactions. We realize a superconducting qubit on a phononic bandgap metamaterial that suppresses TLS-mediated phonon emission. We use the qubit to probe its thermalization dynamics with the phonon-engineered TLS bath. Inside the phononic bandgap, we observe the emergence of non-Markovian qubit dynamics due to the Purcell-engineered TLS lifetime of 34 $\mu s$. We discuss the implications of these observations for extending qubit relaxation times through simultaneous phonon protection and miniaturization.

[9] 2312.01036

Optimal Clifford Initial States for Ising Hamiltonians

Evaluating quantum circuits is currently very noisy. Therefore, developing classical bootstraps that help minimize the number of times quantum circuits have to be executed on noisy quantum devices is a powerful technique for improving the practicality of Variational Quantum Algorithms. CAFQA is a previously proposed classical bootstrap for VQAs that uses an initial ansatz that reduces to Clifford operators. CAFQA has been shown to produce fairly accurate initialization for VQA applied to molecular chemistry Hamiltonians. Motivated by this result, in this paper we seek to analyze the Clifford states that optimize the cost function for a new type of Hamiltonian, namely Transverse Field Ising Hamiltonians. Our primary result connects the problem of finding the optimal CAFQA initialization to a submodular minimization problem which in turn can be solved in polynomial time.

[10] 2312.01055

Coherence Distillation Unveils Einstein-Podolsky-Rosen Steering

Quantum coherence is a fundamental property in quantum information science. Recent developments have provided valuable insights into its distillability and its relationship with nonlocal quantum correlations, such as quantum discord and entanglement. In this work, we focus on quantum steering and the local distillable coherence for a steered subsystem. We propose a steering inequality based on collaborative coherence distillation. Notably, we prove that the proposed steering witness can detect one-way steerable and all pure entangled states. Through linear optical experiments, we corroborate our theoretical efficacy in detecting pure entangled states. Furthermore, we demonstrate that the violation of the steering inequality can be employed as a quantifier of measurement incompatibility. Our work provides a clear quantitative and operational connection between coherence and entanglement, two landmark manifestations of quantum theory and both key enablers for quantum technologies.

[11] 2312.01070

Multi-photon Rabi oscillations in the presence of the classical noise in a quantum nonlinear oscillator

We consider the model of a single-mode quantum nonlinear oscillator with the fourth (Kerr) and sixth (over-Kerr) orders of nonlinearity in the presence of fluctuations of the driving field. We demonstrate that the presence of the amplitude noise does not significantly affect the multi-photon Rabi transitions for the Kerr oscillator, and, in contrast, suppresses these oscillations for the over- Kerr oscillator. We explain the suppression of multi-photon transitions in the over-Kerr oscillator by quasienergy fluctuations caused by noise in field amplitude. In contrast, for the Kerr oscillator, these fluctuations cancel each other for two resonant levels due to the symmetry in the oscillator quasienergy spectrum.

[12] 2312.01079

Spin interaction of non-relativistic neutrons with an ultrashort laser pulse

The non-relativistic Pauli equation is used to study the interaction of slow neutrons with a short magnetic pulse. In the extreme limit, the pulse is acting on the magnetic moment of the neutron only at one instant of time. We obtain the scattering amplitude by deriving the junction conditions for the Pauli wave function across the pulse. Explicit expressions are given for a beam of polarized plane wave neutrons subjected to a pulse of spatially constant magnetic field strength. Assuming that the magnetic field is generated by an ultrashort laser pulse, we provide crude numerical estimates.

[13] 2312.01084

General noise-resilient quantum amplitude estimation

Quantum advantage requires overcoming noise-induced degradation of quantum systems. Conventional methods for reducing noise such as error mitigation face scalability issues in deep circuits. Specifically, noise hampers the extraction of amplitude and observable information from quantum systems. In this work, we present a novel algorithm that enhances the estimation of amplitude and observable under noise. Remarkably, our algorithm exhibits robustness against noise that varies across different depths of the quantum circuits. We assess the accuracy of amplitude and observable using numerical analysis and theoretically analyze the impact of gate-dependent noise on the results. This algorithm is a potential candidate for noise-resilient approaches that have high computational accuracy.

[14] 2312.01086

Non-Abelian quantum geometric tensor in degenerate topological semimetals

The quantum geometric tensor (QGT) characterizes the complete geometric properties of quantum states, with the symmetric part being the quantum metric, and the antisymmetric part being the Berry curvature. We propose a generic Hamiltonian with global degenerate ground states, and give a general relation between the corresponding non-Abelian quantum metric and unit Bloch vector. This enables us to construct the relation between the non-Abelian quantum metric and Berry or Euler curvature. To be concrete, we present and study two topological semimetal models with global degenerate bands under CP and $C_2T$ symmetries, respectively. The topological invariants of these two degenerate topological semimetals are the Chern number and Euler class, respectively, which are calculated from the non-Abelian quantum metric with our constructed relations. Based on the adiabatic perturbation theory, we further obtain the relation between the non-Abelian quantum metric and the energy fluctuation. Such a non-adiabatic effect can be used to extract the non-Abelian quantum metric, which is numerically demonstrated for the two models of degenerate topological semimetals. Finally, we discuss the quantum simulation of the model Hamiltonians with cold atoms.

[15] 2312.01094

On reconstruction of states from evolution induced by quantum dynamical semigroups perturbed by covariant measures

In this work, we show the ability to restore states of quantum systems from evolution induced by quantum dynamical semigroups perturbed by covariant measures. Our procedure describes reconstruction of quantum states transmitted via quantum channels and as a particular example can be applied to reconstruction of photonic states transmitted via optical fibers. For this, the concept of perturbation by covariant operator-valued measure in a Banach space is introduced and integral representation of the perturbed semigroup is explicitly constructed. Various physically meaningful examples are provided. In particular, a model of the perturbed dynamics in the symmetric (boson) Fock space is developed as covariant measure for a semiflow of shifts and its perturbation in the symmetric Fock space, and its properties are investigated. Another example may correspond to the Koopman-von Neumann description of a classical oscillator with bounded phase space.

[16] 2312.01115

Simple and general unitarity conserving numerical real time propagators of time dependent Schrödinger equation based on Magnus expansion

Magnus expansion provides a general way to expand the real time propagator of a time dependent Hamiltonian within the exponential such that the unitarity is satisfied at any order. We use this property and explicit integration of Lagrange interpolation formulas for the time dependent Hamiltonian within each time interval and derive approximations that preserve unitarity for the differential time evolution operators of general time dependent Hamiltonians. The resulting second order approximation is the same as using the average of Hamiltonians for two end points of time. We identify three fourth order approximations involving commutators of Hamiltonians at different times, and also derive a sixth order expression. Test of these approximations along with other available expressions for a two state time dependent Hamiltonian with sinusoidal time dependences provide information on relative performance of these approximations, and suggest that the derived expressions can serve as useful numerical tools for time evolution for time resolved spectroscopy, quantum control, quantum sensing, and open system quantum dynamics.

[17] 2312.01156

Efficient Light Source Placement using Quantum Computing

NP-hard problems regularly come up in video games, with interesting connections to real-world problems. In the game Minecraft, players place torches on the ground to light up dark areas. Placing them in a way that minimizes the total number of torches to save resources is far from trivial. In this paper, we use Quantum Computing to approach this problem. To this end, we derive a QUBO formulation of the torch placement problem, which we uncover to be very similar to another NP-hard problem. We employ a solution strategy that involves learning Lagrangian weights in an iterative process, adding to the ever growing toolbox of QUBO formulations. Finally, we perform experiments on real quantum hardware using real game data to demonstrate that our approach yields good torch placements.

[18] 2312.01157

Quantum dissipative adaptation with cascaded photons

Classical dissipative adaptation is a hypothetical non-equilibrium thermodynamic principle of self-organization in driven matter, relating transition probabilities with the non-equilibrium work performed by an external drive on dissipative matter. Recently, the dissipative adaptation hypothesis was extended to a quantum regime, with a theoretical model where only one single-photon pulse drives each atom of an ensemble. Here, we further generalize that quantum model by analytically showing that N cascaded single-photon pulses driving each atom still fulfills a quantum dissipative adaptation. Interestingly, we find that the level of self-organization achieved with two pulses can be matched with a single effective pulse only up to a threshold, above which the presence of more photons provide unparalleled degrees of self-organization.

[19] 2312.01205

Understanding central spin decoherence due to interacting dissipative spin baths

We propose a new approach to simulate the decoherence of a central spin coupled to an interacting dissipative spin bath with cluster-correlation expansion techniques. We benchmark the approach on generic 1D and 2D spin baths and find excellent agreement with numerically exact simulations. Our calculations show a complex interplay between dissipation and coherent spin exchange, leading to increased central spin coherence in the presence of fast dissipation. Finally, we model near-surface NV centers in diamond and show that accounting for bath dissipation is crucial to understanding their decoherence. Our method can be applied to a variety of systems and provides a powerful tool to investigate spin dynamics in dissipative environments.

[20] 2312.01246

Co-Designed Superconducting Architecture for Lattice Surgery of Surface Codes with Quantum Interface Routing Card

Facilitating the ability to achieve logical qubit error rates below physical qubit error rates, error correction is anticipated to play an important role in scaling quantum computers. While many algorithms require millions of physical qubits to be executed with error correction, current superconducting qubit systems contain only hundreds of physical qubits. One of the most promising codes on the superconducting qubit platform is the surface code, requiring a realistically attainable error threshold and the ability to perform universal fault-tolerant quantum computing with local operations via lattice surgery and magic state injection. Surface code architectures easily generalize to single-chip planar layouts, however space and control hardware constraints point to limits on the number of qubits that can fit on one chip. Additionally, the planar routing on single-chip architectures leads to serialization of commuting gates and strain on classical decoding caused by large ancilla patches. A distributed multi-chip architecture utilizing the surface code can potentially solve these problems if one can optimize inter-chip gates, manage collisions in networking between chips, and minimize routing hardware costs. We propose QuIRC, a superconducting Quantum Interface Routing Card for Lattice Surgery between surface code modules inside of a single dilution refrigerator. QuIRC improves scaling by allowing connection of many modules, increases ancilla connectivity of surface code lattices, and offers improved transpilation of Pauli-based surface code circuits. QuIRC employs in-situ Entangled Pair (EP) generation protocols for communication. We explore potential topological layouts of QuIRC based on superconducting hardware fabrication constraints, and demonstrate reductions in ancilla patch size by up to 77.8%, and in layer transpilation size by 51.9% when compared to the single-chip case.

[21] 2312.01257

Directing entanglement spreading by means of a quantum East/West heterojunction structure

We extend the translationally invariant quantum East model to an inhomogeneous chain with East/West heterojunction structure. In analogy to the quantum diffusion of substantial particles, we observe a cyclic entanglement entropy spreading in the heterojunction during time evolution, which can be regarded as continuous cycles in a quantum heat engine. In order to figure out the possibility of manipulating the entanglement entropy as a quantum resource, the entropy growth is shown to be determined by the initial occupation and the site-dependent chemical potential, and the former is equivalent to an effective temperature. Through fine adjustment of these parameters, we discover the entanglement flow is simply superposed with those from two sources of the chain. An intriguing relation between our model and the traditional heat engines is subsequently established.

[22] 2312.01298

Noiselessly amplified thermal states and after multi-photon addition or subtraction

In this paper, we introduce a noiselessly amplified thermal state (ATS), by operating the noiseless amplification operator ($g^{\hat{n}}$) on the thermal state (TS) with corresponding mean photon number (MPN) $\bar{n}$. Actually, the ATS is an new TS with MPN $\bar{N}=g^{2}\bar{n}/[1-\bar{n}\left(g^{2}-1\right)]$. Furthermore, we introduce photon-added-ATS (PAATS) and photon-subtracted-ATS (PSATS) by operating $m$-photon addition ($\hat{a}^{\dag m}$) and $m$-photon subtraction ($\hat{a}^{m}$) on the ATS, respectively. We study photon number distributions (PNDs), purities, and Wigner functions (WFs) for all these states.

[23] 2312.01311

Using non-convex optimization in quantum process tomography: Factored gradient descent is tough to beat

We propose a non-convex optimization algorithm, based on the Burer-Monteiro (BM) factorization, for the quantum process tomography problem, in order to estimate a low-rank process matrix $\chi$ for near-unitary quantum gates. In this work, we compare our approach against state of the art convex optimization approaches based on gradient descent. We use a reduced set of initial states and measurement operators that require $2 \cdot 8^n$ circuit settings, as well as $\mathcal{O}(4^n)$ measurements for an underdetermined setting. We find our algorithm converges faster and achieves higher fidelities than state of the art, both in terms of measurement settings, as well as in terms of noise tolerance, in the cases of depolarizing and Gaussian noise models.

[24] 2312.01316

Separation of wave and particle attributes of two entangled photons

We put forward a feasible scheme to spatially separate the wave and particle properties of two entangled photons by properly choosing the pre- and post-selection of path states. Our scheme, which implements the quantum Cheshire cat phenomenon for two-photon states, also guarantees that the observation of wave and particle properties of the two entangled photons always obey the Bohr's complementarity principle.

[25] 2312.01396

Quantum dense coding with gravitational cat states

A protocol of quantum dense coding with gravitational cat states is proposed. We explore the effects of temperature and system parameters on the dense coding capacity and provide an efficient strategy to preserve the quantum advantage of dense coding for these states. Our results might open new opportunities for secure communication and possibly insights into the fundamental nature of gravity in the context of quantum information processing.

[26] 2312.01401

Quantum Simulation of Dissipative Energy Transfer via Noisy Quantum Computer

In recent years, due to its formidable potential in computational theory, quantum computing has become a very popular research topic. However, the implementation of practical quantum algorithms, which hold the potential to solve real-world problems, is often hindered by the significant error rates associated with quantum gates and the limited availability of qubits. In this study, we propose a practical approach to simulate the dynamics of an open quantum system on a noisy computer, which encompasses general and valuable characteristics. Notably, our method leverages gate noises on the IBM-Q real device, enabling us to perform calculations using only two qubits. The results generated by our method performed on IBM-Q Jakarta aligned with the those calculated by hierarchical equations of motion (HEOM), which is a classical numerically-exact method, while our simulation method runs with a much better computing complexity. In the last, to deal with the increasing depth of quantum circuits when doing Trotter expansion, we introduced the transfer tensor method(TTM) to extend our short-term dynamics simulation. Based on quantum simulator, we show the extending ability of TTM, which allows us to get a longer simulation using a relatively short quantum circuits.

[27] 2312.01402

More Quantum Speedups for Multiproposal MCMC

Multiproposal Markov chain Monte Carlo (MCMC) algorithms choose from multiple proposals at each iteration in order to sample from challenging target distributions more efficiently. Recent work demonstrates the possibility of quadratic quantum speedups for one such multiproposal MCMC algorithm. Using $P$ proposals, this quantum parallel MCMC (\QP) algorithm requires only $\mathcal{O}(\sqrt{P})$ target evaluations at each step. Here, we present a fast new quantum multiproposal MCMC strategy, \QPP, that only requires $\mathcal{O}(1)$ target evaluations and $\mathcal{O}(\log P)$ qubits. Unlike its slower predecessor, the \QPP\ Markov kernel (\textcolor{red}{1}) maintains detailed balance exactly and (\textcolor{red}{2}) is fully explicit for a large class of graphical models. We demonstrate this flexibility by applying \QPP\ to novel Ising-type models built on bacterial evolutionary networks and obtain significant speedups for Bayesian ancestral trait reconstruction for 248 observed salmonella bacteria.

[28] 2312.01448

Driven transparent quantum graphs

In this paper, we discuss the concept of quantum graphs with transparent vertices by considering the case where the graph interacts with an external time-independent field. In particular, we address the problem of transparent boundary conditions for quantum graphs, building on previous work on transparent boundary conditions for the stationary Schrodinger equation on a line. Physically relevant constraints making the vertex transparent under boundary conditions in the form of (weight) continuity and Kirchhoff rules are derived using two methods, the scattering approach and transparent boundary conditions for the time-independent Schrodinger equation. The latter is derived by extending the transparent boundary condition concept to the time-independent Schrodinger equation on driven quantum graphs. We also discuss how the eigenvalues and eigenfunctions of a quantum graph are influenced not only by its topology, but also by the shape(type) of a potential when an external field is involved.

[29] 2312.01503

Impact of temporal correlations, coherence, and postselection on two-photon interference

Two-photon interference is an indispensable resource of quantum photonics, nevertheless, not straightforward to achieve. The cascaded generation of photon pairs intrinsically contain temporal correlations, which negatively affect the ability of such sources to perform two-photon interference, hence hindering applications. We report on how such correlation interplays with decoherence and temporal postselection, and under which conditions the temporal postselection could improve the two-photon interference visibility. Our study identifies crucial parameters of the performance and indicates the path towards achieving a source with optimal performance.

[30] 2312.01506

Universal Control of Symmetric States Using Spin Squeezing

The manipulation of quantum many-body systems is a frontier challenge in quantum science. Entangled quantum states that are symmetric to permutation between qubits are of growing interest. Yet, the creation and control of symmetric states has remained a challenge. Here, we find a way to universally control symmetric states, proposing a scheme that relies solely on coherent rotations and spin squeezing. We present protocols for the creation of different symmetric states including Schrodinger cat and Gottesman-Kitaev-Preskill states. The obtained symmetric states can be transferred to traveling photonic states via spontaneous emission, providing a powerful mechanism for the creation of desired quantum light states.

[31] 2312.01519

Error Correlations in Photonic Qudit-Mediated Entanglement Generation

Generating entanglement between distributed network nodes is a prerequisite for the quantum internet. Entanglement distribution protocols based on high-dimensional photonic qudits enable the simultaneous generation of multiple entangled pairs, which can significantly reduce the required coherence time of the qubit registers. However, current schemes require fast optical switching, which is experimentally challenging. In addition, the higher degree of error correlation between the generated entangled pairs in qudit protocols compared to qubit protocols has not been studied in detail. We propose a qudit-mediated entangling protocol that completely circumvents the need for optical switches, making it more accessible for current experimental systems. Furthermore, we quantify the amount of error correlation between the simultaneously generated entangled pairs and analyze the effect on entanglement purification algorithms and teleportation-based quantum error correction. We find that optimized purification schemes can efficiently correct the correlated errors, while the quantum error correction codes studied here perform worse than for uncorrelated error models.

[32] 2312.01545

Diverse Entanglement Mechanisms in Multimode Nonlinear Continuous Variables

Non-Gaussian entangled states play a crucial role in harnessing quantum advantage in continuous-variable quantum information. However, how to fully characterize N-partite (N > 3) non-Gaussian entanglement without quantum state tomography remains elusive, leading to a very limited understanding of the underlying entanglement mechanism. Here, we propose several necessary and sufficient conditions for the positive-partial-transposition separability of multimode nonlinear quantum states resulting from high-order Hamiltonians and successive beam splitting operations. When applied to the initial state, the beam-splitter operations induce the emergence of different types of entanglement mechanisms, including pairwise high-order entanglement, collective high-order entanglement and the crossover between the two. We show numerically that for the four-mode scenario, the threshold for the existence of entanglement for any bipartition does not exceed the entanglement of the original state at fixed high-order moments. These results provide a new perspective for understanding multipartite nonlinear entanglement and will promote their application in quantum information processing.

[33] 2312.01557

External Potentials and Ehrenfest Relations in Lagrangian Field Theories

This paper develops a general method to construct Ehrenfest-like relations for Lagrangian field theories when an external, coordinate-dependent scalar potential is applied. To do so, we derive continuity equations in which the spatial and temporal derivatives of the potential can be interpreted as a source of field momentum and field energy, respectively. For a non-relativistic Schr\"odinger field theory, these continuity equations yield Ehrenfest's theorem for energy, linear momentum, and angular momentum. We then derive a relativistic counterpart for these relations using complex Klein-Gordon fields coupled with an electric potential.

[34] 2312.01562

Kernel Alignment for Quantum Support Vector Machines Using Genetic Algorithms

The data encoding circuits used in quantum support vector machine (QSVM) kernels play a crucial role in their classification accuracy. However, manually designing these circuits poses significant challenges in terms of time and performance. To address this, we leverage the GASP (Genetic Algorithm for State Preparation) framework for gate sequence selection in QSVM kernel circuits. We explore supervised and unsupervised kernel loss functions' impact on encoding circuit optimisation and evaluate them on diverse datasets for binary and multiple-class scenarios. Benchmarking against classical and quantum kernels reveals GA-generated circuits matching or surpassing standard techniques. We analyse the relationship between test accuracy and quantum kernel entropy, with results indicating a positive correlation. Our automated framework reduces trial and error, and enables improved QSVM based machine learning performance for finance, healthcare, and materials science applications.

[35] 2312.01570

Parallelizing quantum simulation with decision diagrams

Recent technological advancements show promise in leveraging quantum mechanical phenomena for computation. This brings substantial speed-ups to problems that are once considered to be intractable in the classical world. However, the physical realization of quantum computers is still far away from us, and a majority of research work is done using quantum simulators running on classical computers. Classical computers face a critical obstacle in simulating quantum algorithms. Quantum states reside in a Hilbert space whose size grows exponentially to the number of subsystems, i.e., qubits. As a result, the straightforward statevector approach does not scale due to the exponential growth of the memory requirement. Decision diagrams have gained attention in recent years for representing quantum states and operations in quantum simulations. The main advantage of this approach is its ability to exploit redundancy. However, mainstream quantum simulators still rely on statevectors or tensor networks. We consider the absence of decision diagrams due to the lack of parallelization strategies. This work explores several strategies for parallelizing decision diagram operations, specifically for quantum simulations. We propose optimal parallelization strategies. Based on the experiment results, our parallelization strategy achieves a 2-3 times faster simulation of Grover's algorithm and random circuits than the state-of-the-art single-thread DD-based simulator DDSIM.

[36] 2312.01590

Criteria for Grover Search on Weighted Databases

The Grover algorithm stands as a pivotal solution for unstructured search problems and has become a fundamental quantum subroutine in numerous complex algorithms. This study delves into Grover's search methodology within non-uniformly distributed databases, a scenario more commonly encountered in real-world problems. We uncover that in such cases, the Grover evolution displays distinct behavior compared to uniform or 'unstructured databases'. The search enabled by this evolution doesn't consistently yield a speed-up, and we establish criteria for such occurrences. Additionally, we apply this theory to databases whose distributions relate to coherent states, substantiating the speed-up via Grover evolution through numerical verification. Overall, our findings offer an effective extension of the original Grover algorithm, enriching implementation strategies and widening its application scope.

[37] 2312.01602

Quantum Time Series Similarity Measures and Quantum Temporal Kernels

This article presents a quantum computing approach to the design of similarity measures and kernels for classification of stochastic symbol time series. The similarity is estimated through a quantum generative model of the time series. We consider classification tasks where the class of each sequence depends on its future evolution. In this case a stochastic generative model provides natural notions of equivalence and distance between the sequences. The kernel functions are derived from the generative model, exploiting its information about the sequences evolution.We assume that the stochastic process generating the sequences is Markovian and model it by a Quantum Hidden Markov Model (QHMM). The model defines the generation of each sequence through a path of mixed quantum states in its Hilbert space. The observed symbols are emitted by application of measurement operators at each state. The generative model defines the feature space for the kernel. The kernel maps each sequence to the final state of its generation path. The Markovian assumption about the process and the fact that the quantum operations are contractive, guarantee that the similarity of the states implies (probabilistic) similarity of the distributions defined by the states and the processes originating from these states. This is the heuristic we use in order to propose this class of kernels for classification of sequences, based on their future behavior. The proposed approach is applied for classification of high frequency symbolic time series in the financial industry.

[38] 2312.01651

Experimental Optimal Quantum State Estimation with Genuine Three-copy Collective Measurements

Nonclassical phenomena tied to entangled states are focuses of foundational studies and powerful resources in many applications. By contrast, the counterparts on quantum measurements are still poorly understood. Notably, genuine multipartite nonclassicality is barely discussed, not to say experimental realization. Here we experimentally demonstrate the power of genuine tripartite nonclassicality in quantum measurements based on a simple estimation problem. To this end we realize an optimal genuine three-copy collective measurement via a nine-step two-dimensional photonic quantum walk with 30 elaborately designed coin operators. Then we realize an optimal estimation protocol and achieve an unprecedented high estimation fidelity, which can beat all strategies based on restricted collective measurements by more than 11 standard deviations. These results clearly demonstrate that genuine collective measurements can extract more information than local measurements and restricted collective measurements. Our work opens the door for exploring genuine multipartite nonclassical measurements and their power in quantum information processing.

[39] 2312.01655

Quantum Polar Metric Learning: Efficient Classically Learned Quantum Embeddings

Deep metric learning has recently shown extremely promising results in the classical data domain, creating well-separated feature spaces. This idea was also adapted to quantum computers via Quantum Metric Learning(QMeL). QMeL consists of a 2 step process with a classical model to compress the data to fit into the limited number of qubits, then train a Parameterized Quantum Circuit(PQC) to create better separation in Hilbert Space. However, on Noisy Intermediate Scale Quantum (NISQ) devices. QMeL solutions result in high circuit width and depth, both of which limit scalability. We propose Quantum Polar Metric Learning (QPMeL) that uses a classical model to learn the parameters of the polar form of a qubit. We then utilize a shallow PQC with $R_y$ and $R_z$ gates to create the state and a trainable layer of $ZZ(\theta)$-gates to learn entanglement. The circuit also computes fidelity via a SWAP Test for our proposed Fidelity Triplet Loss function, used to train both classical and quantum components. When compared to QMeL approaches, QPMeL achieves 3X better multi-class separation, while using only 1/2 the number of gates and depth. We also demonstrate that QPMeL outperforms classical networks with similar configurations, presenting a promising avenue for future research on fully classical models with quantum loss functions.

[40] 2312.01660

Feedback Cooling of an Insulating High-Q Diamagnetically Levitated Plate

Levitated systems in vacuum have many potential applications ranging from new types of inertial and magnetic sensors through to fundamental issues in quantum science, the generation of massive Schrodinger cats, and the connections between gravity and quantum physics. In this work, we demonstrate the passive, diamagnetic levitation of a centimeter-sized massive oscillator which is fabricated using a novel method that ensures that the material, though highly diamagnetic, is an electrical insulator. By chemically coating a powder of microscopic graphite beads with silica and embedding the coated powder in high-vacuum compatible wax, we form a centimeter-sized thin square plate which magnetically levitates over a checkerboard magnet array. The insulating coating reduces eddy damping by almost an order of magnitude compared to uncoated graphite with the same particle size. These plates exhibit a different equilibrium orientation to pyrolytic graphite due to their isotropic magnetic susceptibility. We measure the motional quality factor to be Q~1.58*10^5 for an approximately centimeter-sized composite resonator with a mean particle size of 12 microns. Further, we apply delayed feedback to cool the vertical motion of frequency ~19 Hz from room temperature to 320 millikelvin.

[41] 2312.01664

Quantum Algorithm for Radiative Transfer Equation

The radiation transfer equation is widely used for simulating such as heat transfer in engineering, diffuse optical tomography in healthcare, and radiation hydrodynamics in astrophysics. By combining the lattice Boltzmann method, we propose a quantum algorithm for radiative transfer. This algorithm encompasses all the essential physical processes of radiative transfer: absorption, scattering, and emission. Our quantum algorithm exponentially accelerates radiative transfer calculations compared to classical algorithms. In order to verify the quantum algorithm, we perform quantum circuit simulation using IBM Qiskit Aer and find good agreement between our numerical result and the exact solution. The algorithm opens new application of fault-tolerant quantum computers for plasma engineering, telecommunications, nuclear fusion technology, healthcare and astrophysics.

[42] 2312.01684

Orbital angular momentum-enhanced phase estimation using non-Gaussian state with photon loss

This study investigates the use of orbital angular momentum (OAM) to enhance phase estimation in Mach-Zehnder interferometers (MZIs) by employing non-Gaussian states as input resources in the presence of noise. Our research demonstrates that non-Gaussian states, particularly the photonsubtraction-then-addition (PSA) state, exhibit the best sensitivity in the presence of symmetric noise. Additionally, higher-order of Bose operator of non-Gaussian states provide better sensitivity for symmetric noise. OAM can mitigate the deterioration of noise, making it possible to estimate small phase shifts theta close to 0. OAM enhances the resolution and sensitivity of all input states and mitigating the deterioration caused by photon loss. Additionally, OAM enhances the resolution and sensitivity of all input states, enabling the sensitivity to approach the 1/N limit even under significant photon loss (e.g.,50% symmetric photon loss). These results hold promise for enhancing the sensitivity and robustness of quantum metrology, particularly in the presence of significant photon loss.

[43] 2312.01710

Efficiency at maximum $\dotΩ$ figure of merit of a spin half quantum heat engine in the presence of external magnetic field

We consider a finite time quantum heat engine analogous to finite time classical Carnot heat engine with a working substance of spin half particles. We study the efficiency at maximum $\dot{\Omega}$ figure of merit of the quantum heat engine of spin half particles as a working substance in the presence of external magnetic field. The efficiency of this engine at maximum $\dot{\Omega}$ figure of merit shows anomalous behavior in certain region of particles population levels. Further, we find that the efficiency at maximum $\dot{\Omega}$ figure exceeds all the known bounds and even approaches the Carnot efficiency at finite time. Our study indicates that the population of spin half particles plays a crucial role in quantum heat engine whose collective effect in the quantum regime can provide superior engine performance with higher efficiency.

[44] 2312.01802

Comment on "the Spin Dependence of Detection Times and the Nonmeasurability of Arrival Times"

We respond to the recent article by S. Goldstein, R. Tumulka, and N. Zangh\`i [arXiv:2309.11835] concerning the spin-dependent arrival-time distributions reported in [S. Das and D. D\"urr, Sci. Rep. 9: 2242 (2019)].

[45] 2312.01926

The variational quantum eigensolver self-consistent field method within a polarizable embedded framework

We formulate and implement the Variational Quantum Eigensolver Self Consistent Field (VQE-SCF) algorithm in combination with polarizable embedding (PE), thereby extending PE to the regime of quantum computing. We test the resulting algorithm, PE-VQE-SCF, on quantum simulators and demonstrate that the computational stress on the quantum device is only slightly increased in terms of gate counts compared to regular VQE-SCF. On the other hand, no increase in shot noise was observed. We illustrate how PE-VQE-SCF may lead to the modeling of real chemical systems using a simulation of the reaction barrier of the Diels-Alder reaction between furan and ethene as an example.

[46] 2312.01947

Maximising Quantum-Computing Expressive Power through Randomised Circuits

In the noisy intermediate-scale quantum era, variational quantum algorithms (VQAs) have emerged as a promising avenue to obtain quantum advantage. However, the success of VQAs depends on the expressive power of parameterised quantum circuits, which is constrained by the limited gate number and the presence of barren plateaus. In this work, we propose and numerically demonstrate a novel approach for VQAs, utilizing randomised quantum circuits to generate the variational wavefunction. We parameterize the distribution function of these random circuits using artificial neural networks and optimize it to find the solution. This random-circuit approach presents a trade-off between the expressive power of the variational wavefunction and time cost, in terms of the sampling cost of quantum circuits. Given a fixed gate number, we can systematically increase the expressive power by extending the quantum-computing time. With a sufficiently large permissible time cost, the variational wavefunction can approximate any quantum state with arbitrary accuracy. Furthermore, we establish explicit relationships between expressive power, time cost, and gate number for variational quantum eigensolvers. These results highlight the promising potential of the random-circuit approach in achieving a high expressive power in quantum computing.

[47] 2312.01965

Optimal Phase Estimation in Finite-dimensional Fock Space

Phase estimation is a major mission in quantum metrology. In the finite-dimensional Fock space the NOON state ceases to be optimal when the particle number is fixed yet not equal to the space dimension minus one, and what is the true optimal state in this case is still undiscovered. Hereby we present three theorems to answer this question and provide a complete optimal scheme to realize the ultimate precision limit in practice. These optimal states reveal an important fact that the space dimension could be treated as a metrological resource, and the given scheme is particularly useful in scenarios where weak light or limited particle number is demanded.

[48] 2312.01974

Polarization-insensitive microwave electrometry using Rydberg atoms

We investigate the Autler-Townes splitting for Rydberg atoms dressed with linearly polarized microwave radiation, resonant with generic $S_{1/2}\leftrightarrow{P}_{1/2}$ and $S_{1/2}\leftrightarrow{P}_{3/2}$ transitions. The splitting is probed using laser light via electromagnetically-induced transparency measurements, where transmission of probe laser light reveals a two-peak pattern. In particular, this pattern is invariant under rotation of the microwave field polarization. In consequence, we establish $S \leftrightarrow P$ Rydberg transitions as ideally suited for polarization-insensitive electrometry, contrary to recent findings [A. Chopinaud and J.D. Pritchard, Phys. Rev. Appl. $\mathbf{16}$, 024008 (2021)].

[49] 2312.01992

Whence Nonlocality? Removing spooky action at a distance from the de Broglie Bohm pilot-wave theory using a time-symmetric version of de Broglie double solution

In this work, we review and extend a version of the old attempt made by Louis de broglie for interpreting quantum mechanics in realistic terms, namely the double solution. In this theory quantum particles are localized waves, i.e, solitons, that are solutions of relativistic nonlinear field equations. The theory that we present here is the natural extension of this old work and relies on a strong time-symmetry requiring the presence of advanced and retarded waves converging on particles. Using this method, we are able to justify wave-particle duality and to explain the violations of Bell's inequalities. Moreover, the theory recovers the predictions of the pilot-wave theory of de Borglie and Bohm, often known as Bohmian mechanics. As a direct consequence, we reinterpret the nonlocal action at a distance presents in the pilot-wave theory. In the double solution developed here there is fundamentally no action at a distance but the theory requires a form of superdeterminism driven by time-symmetry.

[50] 2312.02002

End-to-End Demonstration for CubeSatellite Quantum Key Distribution

Quantum key distribution (QKD) provides a method of ensuring security using the laws of physics, avoiding the risks inherent in cryptosystems protected by computational complexity. Here we investigate the feasibility of satellite-based quantum key exchange using low-cost compact nano-satellites. This paper demonstrates the first prototype of system level quantum key distribution aimed at the Cube satellite scenario. It consists of a transmitter payload, a ground receiver and simulated free space channel to verify the timing and synchronisation (T&S) scheme designed for QKD and the required high loss tolerance of both QKD and T&S channels. The transmitter is designed to be deployed on various up-coming nano-satellite missions in the UK and internationally. The effects of channel loss, background noise, gate width and mean photon number on the secure key rate (SKR) and quantum bit error rate (QBER) are discussed. We also analyse the source of QBER and establish the relationship between effective signal noise ratio (ESNR) and noise level, signal strength, gating window and other parameters as a reference for SKR optimization. The experiment shows that it can tolerate the 40 dB loss expected in space to ground QKD and with small adjustment decoy states can be achieved. The discussion offers valuable insight not only for the design and optimization of miniature low-cost satellite-based QKD systems but also any other short or long range free space QKD on the ground or in the air.

[51] 2312.02004

Geometric aspects of mixed quantum states inside the Bloch sphere

When studying the geometry of quantum states, it is acknowledged that mixed states can be distinguished by infinitely many metrics. Unfortunately, this freedom causes metric-dependent interpretations of physically significant geometric quantities such as complexity and volume of quantum states. In this paper, we present an insightful discussion on the differences between the Bures and the Sj\"oqvist metrics inside a Bloch sphere. First, we begin with a formal comparative analysis between the two metrics by critically discussing three alternative interpretations for each metric. Second, we illustrate explicitly the distinct behaviors of the geodesic paths on each one of the two metric manifolds. Third, we compare the finite distances between an initial and final mixed state when calculated with the two metrics. Interestingly, in analogy to what happens when studying topological aspects of real Euclidean spaces equipped with distinct metric functions (for instance, the usual Euclidean metric and the taxicab metric), we observe that the relative ranking based on the concept of finite distance among mixed quantum states is not preserved when comparing distances determined with the Bures and the Sj\"oqvist metrics. Finally, we conclude with a brief discussion on the consequences of this violation of a metric-based relative ranking on the concept of complexity and volume of mixed quantum states.

[52] 2312.02009

Chiral excitation flows of multinode network based on synthetic gauge fields

Chiral excitation flows have drawn a lot of attention for their unique unidirectionality. Such flows have been studied in three-node networks with synthetic gauge fields (SGFs), while they are barely realized as the number of nodes increases. In this work, we propose a scheme to achieve chiral flows in $n$-node networks, where an auxiliary node is introduced to govern the system. This auxiliary node is coupled to all the network nodes, forming sub-triangle structures with interference paths in these networks. We find the implicit chiral symmetry behind the perfect chiral flow and propose the universal criteria that incorporate previous models, facilitating the implementation of chiral transmission in various networks. By investigating the symmetries within these models, we present different features of the chiral flow in bosonic and spin networks. Furthermore, we extend the four-node model into a ladder network, which is promising for remote state transfer in practical systems with less complexity. Our scheme can be realized in state-of-the-art experimental systems, such as superconducting circuits and magnetic photonic lattices, thereby opening up new possibilities for future quantum networks.

[53] 2312.02020

Hückel Molecular Orbital Theory on a Quantum Computer: A Scalable System-Agnostic Variational Implementation with Compact Encoding

H\"uckel molecular orbital (HMO) theory provides a semi-empirical treatment of the electronic structure in conjugated {\pi}-electronic systems. A scalable system-agnostic execution of HMO theory on a quantum computer is reported here based on a variational quantum deflation (VQD) algorithm for excited state quantum simulation. A compact encoding scheme is proposed here that provides an exponential advantage over direct mapping and allows quantum simulation of the HMO model for systems with up to 2^N conjugated centers in N qubits. The transformation of the H\"uckel Hamiltonian to qubit space is achieved by two different strategies: a machine-learning-assisted transformation and the Frobenius-inner-product-based transformation. These methods are tested on a series of linear, cyclic, and hetero-nuclear conjugated {\pi}-electronic systems. The molecular orbital energy levels and wavefunctions from the quantum simulation are in excellent agreement with the exact classical results. The higher excited states of large systems, however, are found to suffer from error accumulation in the VQD simulation. This is mitigated by formulating a variant of VQD that exploits the symmetry of the Hamiltonian. This strategy has been successfully demonstrated for the quantum simulation of C_{60} fullerene containing 680 Pauli strings encoded on six qubits. The methods developed in this work are system-agnostic and hence are easily adaptable to similar problems of different complexity in other fields of research.

[54] 2312.02031

Virtual Quantum Markov Chains

Quantum Markov chains generalize classical Markov chains for random variables to the quantum realm and exhibit unique inherent properties, making them an important feature in quantum information theory. In this work, we propose the concept of virtual quantum Markov chains (VQMCs), focusing on scenarios where subsystems retain classical information about global systems from measurement statistics. As a generalization of quantum Markov chains, VQMCs characterize states where arbitrary global shadow information can be recovered from subsystems through local quantum operations and measurements. We present an algebraic characterization for virtual quantum Markov chains and show that the virtual quantum recovery is fully determined by the block matrices of a quantum state on its subsystems. Notably, we find a distinction between two classes of tripartite entanglement by showing that the W state is a VQMC while the GHZ state is not. Furthermore, we establish semidefinite programs to determine the optimal sampling overhead and the robustness of virtual quantum Markov chains. We demonstrate the optimal sampling overhead is additive, indicating no free lunch to further reduce the sampling cost of recovery from parallel calls of the VQMC states. Our findings elucidate distinctions between quantum Markov chains and virtual quantum Markov chains, extending our understanding of quantum recovery to scenarios prioritizing classical information from measurement statistics.

[55] 2312.02035

Fisher information susceptibility for multiparameter quantum estimation

We extend the notion of the Fisher information measurement noise susceptibility to the multiparameter quantum estimation scenario. After giving its mathematical definition, we derive an upper and a lower bound to the susceptibility. We then apply these techniques to two paradigmatic examples of multiparameter estimation: the joint estimation of phase and phase-diffusion and the estimation of the different parameters describing the incoherent mixture of optical point sources. Our figure provides clear indications on conditions allowing or hampering robustness of multiparameter measurements.

[56] 2312.02039

Entanglement-magic separation in hybrid quantum circuits

Magic describes the distance of a quantum state to its closest stabilizer state. It is -- like entanglement -- a necessary resource for a potential quantum advantage over classical computing. We study magic, quantified by stabilizer entropy, in a hybrid quantum circuit with projective measurements and a controlled injection of non-Clifford resources. We discover a phase transition between a (sub)-extensive and area law scaling of magic controlled by the rate of measurements. The same circuit also exhibits a phase transition in entanglement that appears, however, at a different critical measurement rate. This mechanism shows how, from the viewpoint of a potential quantum advantage, hybrid circuits can host multiple distinct transitions where not only entanglement, but also other non-linear properties of the density matrix come into play.

[57] 2312.02066

Majorization theoretical approach to entanglement enhancement via local filtration

From the perspective of majorization theory, we study how to enhance the entanglement of a two-mode squeezed vacuum (TMSV) state by using local filtration operations. We present several schemes achieving entanglement enhancement with photon addition and subtraction, and then consider filtration as a general probabilistic procedure consisting in acting with local (non-unitary) operators on each mode. From this, we identify a sufficient set of two conditions on filtration operators for successfully enhancing the entanglement of a TMSV state, namely the operators must be Fock-orthogonal (i.e., preserving the orthogonality of Fock states) and Fock-amplifying (i.e., giving larger amplitudes to larger Fock states). Our results notably prove that ideal photon addition, subtraction, and any concatenation thereof always enhance the entanglement of a TMSV state in the sense of majorization theory. We further investigate the case of realistic photon addition (subtraction) and are able to upper bound the distance between a realistic photon-added (-subtracted) TMSV state and a nearby state that is provably more entangled than the TMSV, thus extending entanglement enhancement to practical schemes via the use of a notion of approximate majorization. Finally, we consider the state resulting from $k$-photon addition (on each of the two modes) on a TMSV state. We prove analytically that the state corresponding to $k=1$ majorizes any state corresponding to $2\leq k \leq 8$ and we conjecture the validity of the statement for all $k\geq 9$.

[58] 2312.00862

From Spin States to Socially Integrated Ising Models: Proposed Applications of Graph States, Stabilizer States, Toric States to Opinion Dynamics

Recent research has developed the Ising model from physics, especially statistical mechanics, and it plays an important role in quantum computing, especially quantum annealing and quantum Monte Carlo methods. The model has also been used in opinion dynamics as a powerful tool for simulating social interactions and opinion formation processes. Individual opinions and preferences correspond to spin states, and social pressure and communication dynamics are modeled through interactions between spins. Quantum computing makes it possible to efficiently simulate these interactions and analyze more complex social networks.Recent research has incorporated concepts from quantum information theory such as Graph State, Stabilizer State, and Surface Code (or Toric Code) into models of opinion dynamics. The incorporation of these concepts allows for a more detailed analysis of the process of opinion formation and the dynamics of social networks. The concepts lie at the intersection of graph theory and quantum theory, and the use of Graph State in opinion dynamics can represent the interdependence of opinions and networks of influence among individuals. It helps to represent the local stability of opinions and the mechanisms for correcting misunderstandings within a social network. It allows us to understand how individual opinions are subject to social pressures and cultural influences and how they change over time.Incorporating these quantum theory concepts into opinion dynamics allows for a deeper understanding of social interactions and opinion formation processes. Moreover, these concepts can provide new insights not only in the social sciences, but also in fields as diverse as political science, economics, marketing, and urban planning.

[59] 2312.00873

Critical quantum dynamics of observables at eigenstate transitions

It is an outstanding goal to unveil the key features of quantum dynamics at eigenstate transitions. Focusing on quadratic fermionic Hamiltonians that exhibit localization transitions, we identify physical observables that exhibit scale-invariant critical dynamics at the transition when quenched from the initially localized states. The identification is based on two ingredients: (a) A relationship between the time evolution of observables in a many-body state and the transition probabilities of single-particle states, and (b) scale invariance of transition probabilities, which generalizes a corresponding recent result for survival probabilities [Phys. Rev. Lett. 131, 060404 (2023) and arXiv:2309.16005]. These properties suggest that there is also critical behavior in the quantum-quench dynamics of observables, which share the common eigenbasis with the Hamiltonian before the quench. Focusing on experimentally relevant observables such as site occupations and the particle imbalance we numerically demonstrate their critical behavior at the eigenstate transitions in the three-dimensional Anderson model and the one-dimensional Aubry-Andr\'e model.

[60] 2312.00903

Dual Operation of Gate-All-Around Silicon Nanowires at Cryogenic Temperatures: FET and Quantum Dot

As CMOS structures are envisioned to host silicon spin qubits, and for co-integrating quantum systems with their classical control blocks, the cryogenic behaviour of such structures need to be investigated. In this paper we characterize the electrical properties of Gate-All-Around (GAA) n-MOSFETs Si nanowires (NWs) from room temperature down to 1.7 K. We demonstrate that those devices can operate both as transistor and host quantum dots at cryogenic temperature. In the classical regime of the transistor we show improved performances of the devices and in the quantum regime we show systematic quantum dots formation in GAA devices.

[61] 2312.00954

Classical periodic orbits from coherent states in mesoscopic quantum elliptic billiards

An analytical construction of a wave function with localization in classical periodic orbits in an elliptic billiard has been achieved by appropriately superposing nearly coherent states expressed as products of Mathieu functions. We analyze and discuss the rotational and librational regimes of motion in the elliptic billiard. Simplified line equations corresponding to the classical trajectories can be extracted from the quantum coherent state as an integral equation involving angular Mathieu functions. The phase factors appearing in the integrals are connected to classical initial positions and velocity components. We analyze the probability current density, the phase maps, and the vortex distributions of the coherent states for both rotational and librational motions. The coherent state may represent traveling and standing trajectories inside the elliptic billiard.

[62] 2312.00976

Reversible phasonic control of a quantum phase transition in a quasicrystal

Periodic driving can tune the quasistatic properties of quantum matter. A well-known example is the dynamical modification of tunneling by an oscillating electric field. Here we show experimentally that driving the phasonic degree of freedom of a cold-atom quasicrystal can continuously tune the effective quasi-disorder strength, reversibly toggling a localization-delocalization quantum phase transition. Measurements agree with fit-parameter-free theoretical predictions, and illuminate a fundamental connection between Aubry-Andr\'e localization in one dimension and dynamic localization in the associated two-dimensional Harper-Hofstadter model. These results open up new experimental possibilities for dynamical coherent control of quantum phase transitions.

[63] 2312.01024

Hybrid Quantum Neural Network in High-dimensional Data Classification

The research explores the potential of quantum deep learning models to address challenging machine learning problems that classical deep learning models find difficult to tackle. We introduce a novel model architecture that combines classical convolutional layers with a quantum neural network, aiming to surpass state-of-the-art accuracy while maintaining a compact model size. The experiment is to classify high-dimensional audio data from the Bird-CLEF 2021 dataset. Our evaluation focuses on key metrics, including training duration, model accuracy, and total model size. This research demonstrates the promising potential of quantum machine learning in enhancing machine learning tasks and solving practical machine learning challenges available today.

[64] 2312.01041

Topological spin textures in electronic non-Hermitian systems

Non-Hermitian systems have been discussed mostly in the context of open systems and nonequilibrium. Recent experimental progress is much from optical, cold-atomic, and classical platforms due to the vast tunability and clear identification of observables. However, their counterpart in solid-state electronic systems in equilibrium remains unmasked although highly desired, where a variety of materials are available, calculations are solidly founded, and accurate spectroscopic techniques can be applied. We demonstrate that, in the surface state of a topological insulator with spin-dependent relaxation due to magnetic impurities, highly nontrivial topological soliton spin textures appear in momentum space. Such spin-channel phenomena are delicately related to the type of non-Hermiticity and correctly reveal the most robust non-Hermitian features detectable spectroscopically. Moreover, the distinct topological soliton objects can be deformed to each other, mediated by topological transitions driven by tuning across a critical direction of doped magnetism. These results not only open a solid-state avenue to exotic spin patterns via spin- and angle-resolved photoemission spectroscopy, but also inspire non-Hermitian dissipation engineering of spins in solids.

[65] 2312.01199

Potentials on the conformally compactified Minkowski spacetime and their application to quark deconfinement

We study a class of conformal metric deformations in the quasi-radial coordinate parameterizing the 3-sphere in the conformally compactified Minkowski spacetime $S^1\times S^3$. After reduction of the associated Laplace-Beltrami operators to a Schr\"odinger form, a corresponding class of exactly solvable potentials (each one containing a scalar and a gradient term) is found. In particular, the scalar piece of these potentials can be exactly or quasi-exactly solvable, and among them we find the finite range confining trigonometric potentials of P\"oschl-Teller, Scarf and Rosen-Morse. As an application of the results developed in the paper, the large compactification radius limit of the interaction described by some of these potentials is studied, and this regime is shown to be relevant to a quantum mechanical quark deconfinement mechanism.

[66] 2312.01259

Topological defect formation in a phase transition with tunable order

The Kibble-Zurek mechanism (KZM) describes the non-equilibrium dynamics and topological defect formation in systems undergoing second-order phase transitions. KZM has found applications in fields such as cosmology and condensed matter physics. However, it is generally not suitable for describing first-order phase transitions. It has been demonstrated that transitions in systems like superconductors or charged superfluids, typically classified as second-order, can exhibit weakly first-order characteristics when the influence of fluctuations is taken into account. Moreover, the order of the phase transition (i.e., the extent to which it becomes first rather than second order) can be tuned. We explore quench-induced formation of topological defects in such tunable phase transitions and propose that their density can be predicted by combining KZM with nucleation theory.

[67] 2312.01462

Categorical relations and bipartite entanglement in tensor cones for Toeplitz and Fejér-Riesz operator systems

The present paper aims to understand separability and entanglement in tensor cones, in the sense of Namioka and Phelps, that arise from the base cones of operator system tensor products. Of particular interest here are the Toeplitz and Fej\'er-Riesz operator systems, which are, respectively, operator systems of Toeplitz matrices and Laurent polynomials (that is, trigonometric polynomials), and which are related in the operator system category through duality. Some notable categorical relationships established in this paper are the C$^*$-nuclearity of Toeplitz and Fej\'er-Riesz operator systems, as well as their unique operator system structures when tensoring with injective operator systems. Among the results of this study are two of independent interest: (i) a matrix criterion, similar to the one involving the Choi matrix, for a linear map of the Fej\'er-Riesz operator system to be completely positive; (ii) a completely positive extension theorem for positive linear maps of $n\times n$ Toeplitz matrices into arbritary von Neumann algebras, thereby showing that a similar extension theorem of Haagerup for $2\times 2$ Toeplitz matrices holds for Toeplitz matrices of higher dimension.

[68] 2312.01567

Toward Automated Quantum Variational Machine Learning

In this work, we address the problem of automating quantum variational machine learning. We develop a multi-locality parallelizable search algorithm, called MUSE, to find the initial points and the sets of parameters that achieve the best performance for quantum variational circuit learning. Simulations with five real-world classification datasets indicate that on average, MUSE improves the detection accuracy of quantum variational classifiers 2.3 times with respect to the observed lowest scores. Moreover, when applied to two real-world regression datasets, MUSE improves the quality of the predictions from negative coefficients of determination to positive ones. Furthermore, the classification and regression scores of the quantum variational models trained with MUSE are on par with the classical counterparts.

[69] 2312.01579

Enhanced Persistent Orientation of Asymmetric-Top Molecules Induced by Cross-Polarized Terahertz Pulses

We investigate the persistent orientation of asymmetric-top molecules induced by time-delayed THz pulses that are either collinearly or cross polarized. Our theoretical and numerical results demonstrate that the orthogonal configuration outperforms the collinear one, and a significant degree of persistent orientation - approximately 10% at 5 K and nearly 3% at room temperature - may be achieved through parameter optimization. The dependence of the persistent orientation factor on temperature and field parameters is studied in detail. The proposed application of two orthogonally polarized THz pulses is both practical and efficient. Its applicability under standard laboratory conditions lays a solid foundation for future experimental realization of THz-induced persistent molecular orientation.

[70] 2312.01706

Experimental observation of the Yang-Lee quantum criticality in open systems

The Yang-Lee edge singularity was originally studied from the standpoint of mathematical foundations of phase transitions, and its physical demonstration has been of active interest both theoretically and experimentally. However, the presence of an imaginary magnetic field in the Yang-Lee edge singularity has made it challenging to develop a direct observation of the anomalous scaling with negative scaling dimension associated with this critical phenomenon. We experimentally implement an imaginary magnetic field and demonstrate the Yang-Lee edge singularity through a nonunitary evolution governed by a non-Hermitian Hamiltonian in an open quantum system, where a classical system is mapped to a quantum system via the equivalent canonical partition function. In particular, we directly observe the partition function in our experiment using heralded single photons. The nonunitary quantum criticality is identified with the singularity at an exceptional point. We also demonstrate unconventional scaling laws for the finite-temperature dynamics unique to quantum systems.

[71] 2312.01716

Density-wave-type supersolid of two-dimensional tilted dipolar bosons

We predict a stable density-waves-type supersolid phase of a dilute gas of tilted dipolar bosons in a two-dimensional (2D) geometry. This many-body phase is manifested by the formation of the stripe pattern and elasticity coexisting together with the Bose-Einstein condensation and superfluidity at zero temperature. With the increasing the tilting angle the type of the gas-supersolid transition changes from the first order to the second one despite the 2D character of the system, whereas the anisotropy and many-body stabilizing interactions play crucial role. Our approach is based on the numerical analysis of the phase diagram using the simulated annealing method for a free-energy functional. The predicted supersolid effect can be realized in a variety of experimental setups ranging from excitons in heterostructures to cold atoms and polar molecules in optical potentials.

[72] 2312.01736

Out-of-time-ordered correlators of mean-field bosons via Bogoliubov theory

Quantum many-body chaos concerns the scrambling of quantum information among large numbers of degrees of freedom. It rests on the prediction that out-of-time-ordered correlators (OTOCs) of the form $\langle [A(t),B]^2\rangle$ can be connected to classical symplectic dynamics. We rigorously prove a variant of this correspondence principle for mean-field bosons. We show that the $N\to\infty$ limit of the OTOC $\langle [A(t),B]^2\rangle$ is explicitly given by a suitable symplectic Bogoliubov dynamics. The proof uses Bogoliubov theory and extends to higher-order correlators of observables at different times. For these, it yields an out-of-time-ordered analog of the Wick rule. Our result spotlights a new problem in nonlinear dispersive PDE with implications for quantum many-body chaos.

[73] 2312.01891

A Christmas Story about Quantum Teleportation

Quantum teleportation is a concept that fascinates and confuses many people, in particular given that it combines quantum physics and the concept of teleportation. With quantum teleportation likely to play a key role in several communication technologies and the quantum internet in the future, it is imperative to create learning tools and approaches that can accurately and effectively communicate the concept. Recent research has indicated the importance of teachers enthusing students about the topic of quantum physics. Therefore, educators at both high school and early university level need to find engaging and perhaps unorthodox ways of teaching complex, yet interesting topics such as quantum teleportation. In this paper, we present a paradigm to teach about the concept of quantum teleportation using the Christmas gift-bringer Santa Claus. Using the example of Santa Claus, we use an unusual context to explore the key aspects of quantum teleportation, and all without being overly abstract. In addition, we outline a worksheet designed for use in the classroom setting which is based on common misconceptions from quantum physics.

[74] 2312.01913

Tunable exciton polaritons in band-gap engineered hexagonal boron nitride

We show that hexagonal boron nitride (hBN), a two-dimensional insulator, when subjected to an external superlattice potential forms a new paradigm for electrostatically tunable excitons in the near- and mid-ultraviolet (UV). The imposed potential has three consequences: (i) it renormalizes the effective mass tensor, leading to anisotropic effective masses; (ii) it renormalizes the band gap, eventually reducing it; (iii) it reduces the exciton binding energies. All these consequences depend on a single dimensionless parameter, which includes the product of strength of the external potential with its period. In addition to the excitonic energy levels, we compute the optical conductivity along two orthogonal directions, and from it the absorption spectrum. The results for the latter show that our system is able to mimic a grid polarizer. These characteristics make one-dimensional hBN superlattices a viable and unexplored platform for fine-tuned polaritonics in the UV to visible spectral range.

[75] 2312.01971

Directional spontaneous emission in photonic crystal slabs

Spontaneous emission is one of the most fundamental out-of-equilibrium processes in which an excited quantum emitter relaxes to the ground state due to quantum fluctuations. In this process, a photon is emitted that can interact with other nearby emitters and establish quantum correlations between them, e.g., via super and subradiance effects. One way to modify these photon-mediated interactions is to alter the dipole radiation patterns of the emitter, e.g., by placing photonic crystals near them. One recent example is the generation of strong directional emission patterns-key to enhancing super and subradiance effects-in two dimensions by employing photonic crystals with band structures characterized by linear isofrequency contours and saddle-points. However, these studies have predominantly used oversimplified toy models, overlooking the electromagnetic field's intricacies in actual materials, including aspects like geometrical dependencies, emitter positions, and polarization. Our study delves into the interaction between these directional emission patterns and the aforementioned variables, revealing the untapped potential to fine-tune collective quantum optical phenomena.

[76] 2312.02049

Gravitational-electromagnetic phase in the Kerr-Newman spacetime

We calculate the gravitational-electromagnetic phase for a charged particle in the Kerr-Newman spacetime. The result is applied to an interference experiment, in which the phase differences and the fringe shifts are derived. We find that both the charge of the particle and the charge of the black hole contribute to the gravitational phase difference, for which we give some qualitative explanations. Finally, we extend the results to the case of dyonic particles in the spacetime of a dyonic Kerr-Newman black hole.