Quantum Physics (quant-ph)

Abstract: Quantum Key Distribution (QKD) is a physical layer encryption technique that enables two distant parties to exchange secure keys with information-theoretic security. In the last two decades, QKD has transitioned from laboratory research to real-world applications, including multi-user quantum access networks (QANs). This network structure allows users to share single-photon detectors at a network node through time-division multiplexing, thereby significantly reducing the network cost. However, current QAN implementations require additional hardware for auxiliary tasks such as time synchronization. To address this issue, we propose a cost-efficient QAN that uses qubit-based synchronization. In this approach, the transmitted qubits facilitate time synchronization, eliminating the need for synchronization hardware. We tested our scheme by implementing a network for two users and successfully achieved average secure key rates of $53.84$ kbps and $71.90$ kbps for each user over a 50-km commercial fiber spool. In addition, we investigated the capacity of the access network under cross-talk and loss conditions. The simulation results demonstrate that this scheme can support a QAN with 64 users with key rates up to 1070~bps. Our work provides a feasible and cost-effective way to implement a multi-user QKD network, further promoting the widespread application of QKD.

Abstract: Quantum key distribution (QKD) theoretically offers unconditional security. Unfortunately, the gap between theory and practice threatens side-channel attacks on practical QKD systems. Many well-known QKD protocols use weak coherent laser pulses to encode the quantum information. These sources differ from ideal single photon sources and follow Poisson statistics. Many protocols, such as decoy state and coincidence detection protocols, rely on monitoring the photon statistics to detect any information leakage. The accurate measurement and characterization of photon statistics enable the detection of adversarial attacks and the estimation of secure key rates, strengthening the overall security of the QKD system. We have rigorously characterized our source to estimate the mean photon number employing multiple detectors for comparison against measurements made with a single detector. Furthermore, we have also studied intensity fluctuations to help identify and mitigate any potential information leakage due to state preparation flaws. We aim to bridge the gap between theory and practice to achieve information-theoretic security.

Abstract: The definition of a consistent evolution equation for statistical hybrid quantum-classical systems is still an open problem. In this paper we analyze the case of Ehrenfest dynamics on systems defined by a probability density and identify the relations of the non-linearity of the dynamics with the obstructions to define a consistent dynamics for the first quantum moment of the distribution. This first quantum moment represents the physical states as a family of classically-parametrized density matrices $\hat \rho(\xi)$, for $\xi$ a classical point; and it is the most common representation of hybrid systems in the literature. Due to this obstruction, we consider higher order quantum moments, and argue that only a finite number of them are physically measurable. Because of this, we propose an effective solution for the hybrid dynamics problem based on approximating the distribution by those moments and representing the states by them.

Abstract: The quantum annealing machine manufactured by D-Wave Systems is expected to find the optimal solution for QUBO (Quadratic Unconstrained Binary Optimization) accurately and quickly. This would be useful in future applications where real-time calculation is needed. One such application is traffic signal optimization. Some studies use quantum annealing for this. However, they are formulated in unrealistic settings, such as only crossroads on the map. Therefore, we suggest a QUBO, which can deal with T-junctions and multi-forked roads. To validate the efficiency of our approach, SUMO (Simulation of Urban MObility) is used. This enables us to experiment with geographic information data very close to the real world. We compared results with those using the Gurobi Optimizer in the experiment to confirm that quantum annealing can find a ground state. The results show that the quantum annealing cannot find the ground state, but our model can reduce the time that vehicles wait at a red light. It is also inferior to the Gurobi Optimizer in calculation time. This seems to be due to the D-Wave machine's hardware limitations and noise effects, such as ambient temperature. If these problems are solved, and the number of qubits is increased, the use of quantum annealing is likely to be superior in terms of the speed of calculating an optimal solution.

Abstract: In the quest for quantum gravity, we have lacked experimental verification, hampered by the weakness of gravity and decoherence. Recently, various experiments have been proposed to verify quantum entanglement induced by Newtonian gravitational interactions. However, they are not yet certainly feasible with existing techniques. To search for a new setup, we compute the logarithmic negativity of two oscillators with arbitrary quadratic potential coupled by gravity. We find that unstable inverted oscillators generate gravity-induced entanglement most quickly and are most resistant to decoherence from environmental fluctuations. As an experimental realization, we propose a setup of the optical levitation of mirrors with the anti-spring effect. To avoid decoherence due to photon shot noise, a sandwich configuration that geometrically creates the anti-spring is promising.

Abstract: Electrically controllable quantum-dot molecules (QDMs) are a promising platform for deterministic entanglement generation and, as such, a resource for quantum-repeater networks. We develop a microscopic open-quantum-systems approach based on a time-dependent Bloch-Redfield equation to model the generation of entangled spin states with high fidelity. The state preparation is a crucial step in a protocol for deterministic entangled-photon-pair generation that we propose for quantum repeater applications. Our theory takes into account the quantum-dot molecules' electronic properties that are controlled by time-dependent electric fields as well as dissipation due to electron-phonon interaction. We quantify the transition between adiabatic and non-adiabatic regimes, which provides insights into the dynamics of adiabatic control of QDM charge states in the presence of dissipative processes. From this, we infer the maximum speed of entangled-state preparation under different experimental conditions, which serves as a first step towards simulation of attainable entangled photon-pair generation rates. The developed formalism opens the possibility for device-realistic descriptions of repeater protocol implementations.

Abstract: Quantum Brownian motion serves as a fundamental paradigm for investigations in open quantum systems, where a harmonic oscillator interacts with a bosonic thermal bath. A generalized coupling of the environment to the harmonic oscillator system via both its position and momentum was developed in recent times. To this end, we take up this generalized model of quantum Brownian motion and study it from the perspective of quantum thermodynamics. The system of interest is envisaged as a quantum battery interacting with the bath acting as a charger (dissipation) mechanism. We probe into the problem of maximum work that can be extracted through such a system using ergotropy and its (in)-coherent parts along with the instantaneous and average powers of the battery. We examine the effect of bath temperature and momentum coupling on the charging-discharging behavior of the battery. A connection between the memory effects of the system with charging-discharging behavior is further explored.

Abstract: By means of the Dirac bispinor formalism, we show that the state of a (massive) oscillating neutrino produced by a weak interaction process, is an hyperentangled state of flavor, chirality, and spin. Since chirality is not a conserved quantity, chiral oscillations take place and affects the flavor transition probability. By means of the complete complementarity relations, we analyze how correlations and coherence are redistributed in time between the different degrees-of-freedoms of the system. In a similar way, we consider a spin entangled lepton-antineutrino pair and describe the redistribution of the spin-spin entanglement into correlations between the other degrees-of-freedom. In both cases the effects of chiral oscillations are relevant in the non-relativistic regime. Our analysis provides a complete characterization of the quantum correlations involved in lepton-antineutrino pairs and in single particle neutrino evolution, and provides a further insight on possible routes to interpret and measure chiral oscillations.

Abstract: The integration of Quantum Computers (QC) within High-Performance Computing (HPC) environments holds significant promise for solving real-world problems by leveraging the strengths of both computational paradigms. However, the integration of a complex QC platform in an HPC infrastructure poses several challenges, such as operation stability in non-laboratory like environments, and scarce access for maintenance. Currently, only a few fully-assembled QCs currently exist worldwide, employing highly heterogeneous and cutting-edge technologies. These platforms are mostly used for research purposes, and often bear closer resemblance to laboratory assemblies rather than production-ready, stable, and consistently-performing turnkey machines. Moreover, public cloud services with access to such quantum computers are scarce and their availability is generally limited to few days per week. In November 2022, we introduced the first cloud-accessible general-purpose quantum computer based on single photons. One of the key objectives was to maintain the platform's availability as high as possible while anticipating seamless compatibility with HPC hosting environment. In this article, we describe the design and implementation of our cloud-accessible quantum computing platform, and demonstrate one nine availability (92 %) for external users during a six-month period, higher than most online services. This work lay the foundation for advancing quantum computing accessibility and usability in hybrid HPC-QC infrastructures.

Abstract: The set of two-body reduced states of translation invariant, infinite quantum spin chains can be approximated from inside and outside using matrix product states and marginals of finite systems, respectively. These lead to hierarchies of algebraic approximations that become tight only in the limit of infinitely many auxiliary variables. We show that this is necessarily so for any algebraic ansatz by proving that the set of reduced states is not semialgebraic. We also provide evidence that additional elementary transcendental functions cannot lead to a finitary description.

Abstract: Duan-Lukin-Cirac-Zoller quantum repeater protocol provides a feasible scheme to implement long-distance quantum communication and large-scale quantum networks. The elementary link, namely the entanglement between two atomic ensembles, is a fundamental component of quantum repeater. For practical quantum repeater, it is required that the elementary link can be prepared with high yield and the spin waves stored in atoms can be efficiently converted into photons on demand. However, so far, such quantum repeater link has not been demonstrated in experiments. Here, we demonstrate a proof-of-principle multiplexed quantum repeater link by entangling two temporally multiplexed quantum memory. Compared with a single-mode link, the successful preparation rate of the multiplexed link is increased by one order of magnitude. By using the cavity-enhanced scheme, the on-demand retrieval efficiency of atomic spin waves is improved to 70%, which is beneficial for the subsequent entanglement swapping between adjacent links. The realization of temporally multiplexed quantum repeater link with high retrieval efficiency lays a foundation for the development of practical quantum networks.

Abstract: Despite its fully unitary dynamics, the bosonic Kitaev chain (BKC) displays key hallmarks of non-Hermitian physics including non-reciprocal transport and the non-Hermitian skin effect. Here we demonstrate another remarkable phenomena: the existence of an entanglement phase transition (EPT) in a variant of the BKC that occurs as a function of a Hamiltonian parameter g, and which coincides with a transition from a reciprocal to a non-reciprocal phase. As g is reduced below a critical value, the post-quench entanglement entropy of a subsystem of size l goes from a volume-law phase where it scales as l to a super-volume law phase where it scales like lN with N the total system size. This EPT occurs for a system undergoing purely unitary evolution and does not involve measurements, post-selection, disorder or dissipation. We derive analytically the entanglement entropy out of and at the critical point for the $l=1$ and $l/N \ll 1$ case.

Abstract: Building large-scale superconducting quantum circuits will require miniaturisation and integration of supporting devices including microwave circulators, which are currently bulky, stand-alone components. Here we report the realisation of a passive on-chip circulator which is made from a loop consisting of three tunnel-coupled superconducting islands, with DC-only control fields. We observe the effect of quasiparticle tunnelling, and we dynamically classify the system into different quasiparticle sectors. When tuned for circulation, the device exhibits strongly non-reciprocal 3-port scattering, with average on-resonance insertion loss of 2 dB, isolation of 14 dB, power reflectance of -11 dB, and a bandwidth of 200 MHz.

Abstract: Quantum computing stands at the vanguard of science, focused on exploiting quantum mechanical phenomena like superposition and entanglement. Its goal is to create innovative computational models that address intricate problems beyond classical computers' capabilities. In the Noisy Intermediate-Scale Quantum (NISQ) era, developing algorithms for nonlinear function calculations on density matrices is of paramount importance. This project endeavors to design scalable algorithms for calculating power functions of mixed quantum states. This study introduces two algorithms based on the Hadamard Test and Gate Set Tomography. Additionally, a comparison of their computational outcomes is offered, accompanied by a meticulous assessment of errors inherent in the Gate Set Tomography-based approach.

Abstract: Schr\"odinger cat states, quantum superpositions of macroscopically distinct classical states, are an important resource for quantum communication, quantum metrology and quantum computation. Especially, cat states in a phase space protected against phase-flip errors can be used as a logical qubit. However, cat states, normally generated in three-dimensional cavities, are facing the challenges of scalability and controllability. Here, we present a novel strategy to generate and store cat states in a coplanar superconducting circuit by the fast modulation of Kerr nonlinearity. At the Kerr-free work point, our cat states are passively preserved due to the vanishing Kerr effect. We are able to prepare a 2-component cat state in our chip-based device with a fidelity reaching 89.1% under a 96 ns gate time. Our scheme shows an excellent route to constructing a chip-based bosonic quantum processor.

Abstract: We explore the physics of quantum systems that suffer from partial decoherence, in the intermediate range between coherent quantum evolution and incoherent classical physics. It has been predicted that new physics and technology are enabled in this intermediate regime. In particular we explore the asymmetric transmission through an Aharonov-Bohm (AB) ring that supports a 3:1 asymmetry in transmission times, augmented with de-phasing features that act preferentially on the longer-lingering quantum waves. Such a device is realized as a microwave analogue quantum graph utilizing a gyrator to create the 3:1 transmission time delay asymmetry, along with both homogeneous and localized losses to mimic the effects of de-phasing in the analogous mesoscopic electron system. Measurements and simulations of this device demonstrate the required non-reciprocal transmission time delay, as well as an asymmetry in transmission probability. The measurements and simulations are performed in both the frequency domain, and in the time domain using wave packets. We demonstrate asymmetric transmission through the AB-ring graph as a function of loss/de-phasing in both simulation and experiment, in both the frequency- and time-domains, and compare to expectations for the corresponding quantum system. The results are consistent with the hypothesis that the transmission asymmetry and loss of detailed balance is an equilibrium property of the analogous mesoscopic quantum graph.

Abstract: Pointlike systems coupled to quantum fields are often employed as toy models for measurements in quantum field theory. In this paper, we identify the field observables recorded by such models. We show that in models that work in the strong coupling regime, the apparatus is correlated with smeared field amplitudes, while in models that work in weak coupling the apparatus records particle aspects of the field, such as the existence of a particle-like time of arrival and resonant absorption. Then, we develop an improved field-detector interaction model, adapting the formalism of Quantum Brownian motion, that is exactly solvable. This model confirms the association of field and particle properties in the strong and weak coupling regimes, respectively. Further, it can also describe the intermediate regime, in which the field-particle characteristics `merge'. In contrast to standard perturbation techniques, this model also recovers the relativistic Breit-Wigner resonant behavior in the weak coupling regime. The modulation of field-particle-duality by a single tunable parameter is a novel feature that is, in principle, experimentally accessible.

Abstract: We study chaotic dynamics and anomalous transport in a Bose-Hubbard chain in the semiclassical regime (the limit when the number of particles goes to infinity). We find that the system has mixed phase space with both regular and chaotic dynamics, even for long chains with up to hundred wells. The consequence of the mixed phase space is strongly anomalous diffusion in the space of occupation numbers, with a discrete set of transport exponents. After very long times the system crosses over to the hydrodynamic regime with normal diffusion. Anomalous transport is quite universal, almost completely independent of the parameters of the model (Coulomb interaction, chemical potential): it is mainly determined by the initial distribution of particles along the chain. We corroborate our findings by analytical arguments: scaling analysis for the anomalous regime and the Langevin equation for the normal diffusion regime.

Abstract: The ability to engineer cavity-mediated interactions has emerged as a powerful tool for the generation of non-local correlations and the investigation of non-equilibrium phenomena in many-body systems. Levitated optomechanical systems have recently entered the multi-particle regime, with promise for using arrays of massive strongly coupled oscillators for exploring complex interacting systems and sensing. Here, by combining advances in multi-particle optical levitation and cavity-based quantum control, we demonstrate, for the first time, programmable cavity-mediated interactions between nanoparticles in vacuum. The interaction is mediated by photons scattered by spatially separated particles in a cavity, resulting in strong coupling ($G_\text{zz}/\Omega_\text{z} = 0.238\pm0.005$) that does not decay with distance within the cavity mode volume. We investigate the scaling of the interaction strength with cavity detuning and inter-particle separation, and demonstrate the tunability of interactions between different mechanical modes. Our work paves the way towards exploring many-body effects in nanoparticle arrays with programmable cavity-mediated interactions, generating entanglement of motion, and using interacting particle arrays for optomechanical sensing.

Abstract: Variational quantum algorithms exploit the features of superposition and entanglement to optimize a cost function efficiently by manipulating the quantum states. They are suitable for noisy intermediate-scale quantum (NISQ) computers that recently became accessible to the worldwide research community. Here, we implement and demonstrate the numerical processes on the 5-qubit and 7-qubit quantum processors on the IBM Qiskit Runtime platform. We combine the commercial finite-element-method (FEM) software ABAQUS with the implementation of Variational Quantum Eigensolver (VQE) to establish an integrated pipeline. Three examples are used to investigate the performance: a hexagonal truss, a Timoshenko beam, and a plane-strain continuum. We conduct parametric studies on the convergence of fundamental natural frequency estimation using this hybrid quantum-classical approach. Our findings can be extended to problems with many more degrees of freedom when quantum computers with hundreds of qubits become available in the near future.

Abstract: Quantum transport across discrete structures is a relevant topic of solid state physics and quantum information science, which can be suitably studied in the context of continuous-time quantum walks. The addition of phases degrees of freedom, leading to chiral quantum walks, can also account for directional transport on graphs with loops. We discuss criteria for quantum transport and study the enhancement that can be achieved with chiral quantum walks on chain-like graphs, exploring different topologies for the chain units and optimizing over the phases. We select three candidate structures with optimal performance and investigate their transport behaviour with Krylov reduction. While one of them can be reduced to a weighted line with minor couplings modulation, the other two are truly chiral quantum walks, with enhanced transport probability over long chain structures.