QUROPE Quantum Information Processing and Communication in Europe

We propose a feasible experimental scheme to improve the few-photon optomechanical effects, including photon blockade and mechanical-Schrodinger cat-state generation, as well as photon-phonon entanglement in a tripartite microwave optomechanical circuit. The system under consideration is formed by a single-Cooper-pair transistor, a microwave LC resonator, and a micromechanical resonator. Our scheme is based on an additional higher-order (generalized) nonlinear cross-Kerr type of coupling, linearly dependent on photon number while quadratically dependent on mechanical phonon one, which can be realized via adjusting the gate charge of the Cooper-pair transistor. We show, both analytically and numerically, that the presence of both cross-Kerr and generalized cross-Kerr nonlinearities not only may give rise to the enhancement of one- and two-photon blockades as well as photon induced tunneling but can also provide more controllability over them. Furthermore, it is shown that in the regime of zero optomechanical coupling, with the aid of generalized cross-Kerr nonlinearity, one can generate multi-components mechanical superposition states which exhibit robustness against system dissipations. We also study the steady-state entanglement between the microwave and mechanical modes, the results of which signify the role of generalized cross-Kerr nonlinearity in enhancing the entanglement in the regime of large-red detuning. The proposed generalized cross-Kerr optomechanical system can be found potential applications in microwave quantum sensing, quantum telecommunication, and quantum information protocols.

Geometrically, quantum mechanics is defined by a complex line bundle $L_\hbar$ over the classical particle phase space $T^*{R}^3\cong{R}^6$ with coordinates $x^a$ and momenta $p_a$, $a,...=1,2,3$. This quantum bundle $L_\hbar$ is endowed with a connection $A_\hbar$, and its sections are standard wave functions $\psi$ obeying the Schr\"odinger equation. The components of covariant derivatives $\nabla_{A_\hbar}^{}$ in $L_\hbar$ are equivalent to operators ${\hat x}^a$ and ${\hat p}_a$. The bundle $L_\hbar=: L_{C}^+$ is associated with symmetry group U(1)$_\hbar$ and describes particles with quantum charge $q=1$ which is eigenvalue of the generator of the group U(1)$_\hbar$. The complex conjugate bundle $L^-_{C}:={\overline{L_{C}^+}}$ describes antiparticles with quantum charge $q=-1$. We will lift the bundles $L_{C}^\pm$ and connection $A_\hbar$ on them to the relativistic phase space $T^*{R}^{3,1}$ and couple them to the Dirac spinor bundle describing both particles and antiparticles. Free relativistic quarks and leptons are described by the Dirac equation on Minkowski space ${R}^{3,1}$. This equation does not contain interaction with the quantum connection $A_\hbar$ on bundles $L^\pm_{C}\to T^*{R}^{3,1}$ because $A_\hbar$ has non-vanishing components only along $p_a$-directions in $T^*{R}^{3,1}$. To enable the interaction of elementary fermions $\Psi$ with quantum connection $A_\hbar$ on $L_{C}^\pm$, we will extend the Dirac equation to the phase space while maintaining the condition that $\Psi$ depends only on $t$ and $x^a$. The extended equation has an infinite number of oscillator-type solutions with discrete energy values as well as wave packets of coherent states. We argue that all these normalized solutions describe virtual particles and antiparticles living outside the mass shell hyperboloid. The transition to free particles is possible through squeezed coherent states.

Quantum error mitigation is a technique used to post-process errors occurring in the quantum system, which reduces the expected errors and achieves higher accuracy. One method of quantum error mitigation is zero-noise extrapolation, which involves amplifying the noise and then extrapolating the observable expectation of interest back to a noise-free point. This method usually relies on the error model of the noise, as error rates for different levels of noise are assumed during the noise amplification process. In this paper, we propose that the purity of output states in noisy circuits can assist in the extrapolation process, eliminating the need for assumptions about error rates. We also introduce the quasi-polynomial model from the linearity of quantum channel for extrapolation of experimental data, which can be reduced to other proposed models. Furthermore, we verify our purity-assisted zero-noise extrapolation by performing numerical simulations and experiments on the online public quantum computation platform, Quafu, to compare it with the routine zero-noise extrapolation and virtual distillation methods. Our results demonstrate that this modified method can suppress the random fluctuation of operator expectation measurement, and effectively reduces the bias in extrapolation to a level lower than both the zero-noise extrapolation and virtual distillation methods, especially when the error rate is moderate.

Usage of cutting-edge artificial intelligence will be the baseline at future high energy colliders such as the High Luminosity Large Hadron Collider, to cope with the enormously increasing demand of the computing resources. The rapid development of quantum machine learning could bring in further paradigm-shifting improvement to this challenge. One of the two highest CPU-consuming components, the charged particle reconstruction, the so-called track reconstruction, can be considered as a quadratic unconstrained binary optimization (QUBO) problem. The Quantum Approximate Optimization Algorithm (QAOA) is one of the most promising algorithms to solve such combinatorial problems and to seek for a quantum advantage in the era of the Noisy Intermediate-Scale Quantum computers. It is found that the QAOA shows promising performance and demonstrated itself as one of the candidates for the track reconstruction using quantum computers.

We introduce a generalized classical model of Brownian motion for describing thermal relaxation processes which is thermodynamically consistent. Applying the canonical quantization to this model, a quantum equation for the density operator is obtained. This equation has a thermal equilibrium state as its stationary solution, but the time evolution is not necessarily a Completely Positive and Trace-Preserving (CPTP) map. In the application to the harmonic oscillator potential, however, the requirement of the CPTP map is shown to be satisfied by choosing parameters appropriately and then our equation reproduces a Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) equation satisfying the detailed balance condition. This result suggests a quantum-classical correspondence in thermal relaxation processes and will provide a new insight to the study of decoherence.

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.

This study presents an investigation into the thermodynamic properties of a dirty black hole immersed in a uniform electric field within the framework of the Einstein-Nonlinear Electrodynamics (ENE)-dilaton theory. The analysis delves into various thermodynamic aspects, including heat capacity, Helmholtz free energy, and internal energy, providing insights into the behavior of the black hole under the influence of the electric field. Furthermore, the article explores the intricate interplay between quantum effects and thermodynamic behavior through the examination of quantum-corrected entropy. The study aims to shed light on the non-perturbative corrections that arise in this complex system, offering a comprehensive understanding of the modified thermodynamics of dirty black holes within the specified theoretical framework.

Ising machines have emerged as a promising solution for rapidly solving NP-complete combinatorial optimization problems, surpassing the capabilities of traditional computing methods. By efficiently determining the ground state of the Hamiltonian during the annealing process, Ising machines can effectively complement CPUs in tackling optimization challenges. To realize these Ising machines, a bi-stable oscillator is essential to emulate the atomic spins and interactions of the Ising model. This study introduces a Josephson parametric oscillator (JPO)-based tile structure, serving as a fundamental unit for scalable superconductor-based Ising machines. Leveraging the bi-stable nature of JPOs, which are superconductor-based oscillators, the proposed machine can operate at frequencies of 7.5GHz while consuming significantly less power (by three orders of magnitude) than CMOS-based systems. Furthermore, the compatibility of the proposed tile structure with the Lechner-Hauke-Zoller (LHZ) architecture ensures its viability for large-scale integration. We conducted simulations of the tile in a noisy environment to validate its functionality. We verified its operational characteristics by comparing the results with the analytical solution of its Hamiltonian model. This verification demonstrates the feasibility and effectiveness of the JPO-based tile in implementing Ising machines, opening new avenues for efficient and scalable combinatorial optimization in quantum computing.

Cavity magnonics, which studies the interaction of light with magnetic systems in a cavity, is a promising platform for quantum transducers, quantum memories, and devices with non-reciprocal behaviour. At microwave frequencies, the coupling between a cavity photon and a magnon, the quasi-particle of a spin wave excitation, is a consequence of the Zeeman interaction between the cavity's magnetic field and the magnet's macroscopic spin. For each photon/magnon interaction, a coupling phase factor exists, but is often neglected in simple systems. However, in "loop-coupled" systems, where there are at least as many couplings as modes, the coupling phases become relevant for the physics and lead to synthetic gauge fields. We present experimental evidence of the existence of such coupling phases by considering two spheres made of Yttrium-Iron-Garnet and two different re-entrant cavities. We predict numerically the values of the coupling phases, and we find good agreement between theory and the experimental data. Theses results show that in cavity magnonics, one can engineer synthetic gauge fields, which can be useful for building nonreciprocal devices.

We propose a scheme to enhance the sensitivity of Non-Hermitian optomechanical mass-sensors. The benchmark system consists of two coupled optomechanical systems where the mechanical resonators are mechanically coupled. The optical cavities are driven either by a blue or red detuned laser to produce gain and loss, respectively. Moreover, the mechanical resonators are parametrically driven through the modulation of their spring constant. For a specific strength of the optical driving field and without parametric driving, the system features an Exceptional Point (EP). Any perturbation to the mechanical frequency (dissipation) induces a splitting (shifting) of the EP, which scales as the square root of the perturbation strength, resulting in a sensitivity-factor enhancement compared with conventional optomechanical sensors. The sensitivity enhancement induced by the shifting scenario is weak as compared to the one based on the splitting phenomenon. By switching on parametric driving, the sensitivity of both sensing schemes is greatly improved, yielding to a better performance of the sensor. We have also confirmed these results through an analysis of the output spectra and the transmissions of the optical cavities. In addition to enhancing EP sensitivity, our scheme also reveals nonlinear effects on sensing under splitting and shifting scenarios. This work sheds light on new mechanisms of enhancing the sensitivity of Non-Hermitian mass sensors, paving a way to improve sensors performance for better nanoparticles or pollutants detection, and for water treatment.

In this study, we carry out a non-perturbative approach to a quantum Otto engine, employing an Unruh-DeWitt particle detector to extract work from a quantum Klein-Gordon field in an arbitrary globally hyperbolic curved spacetime. We broaden the scope by considering the field in any quasi-free state, which includes vacuum, thermal, and squeezed states. A key aspect of our method is the instantaneous interaction between the detector and the field, which enables a thorough non-perturbative analysis. We demonstrate that the detector can successfully extract positive work from the quantum Otto cycle, even when two isochoric processes occur instantaneously, provided the detector in the second isochoric process receives a signal from the first interaction. This signaling allows the detector to release heat into the field, thereby the thermodynamic cycle is completed. As a demonstration, we consider a detector at rest in flat spacetime and compute the work extracted from the Minkowski vacuum state.

A new perspective in terms of inverter-chain link (ICL) diagrams of quantum entanglement faithfully captures the fundamental concept of quantum teleportation and superdense coding. The ICL may be considered a series of {\sigma}_{x} Pauli-matrix operations, where a physical/geometric representation provides the mysterious link raised by EPR. Here, we employ discrete phase space and ICL analyses of quantum entanglement as a resource for quantum teleportation and superdense coding. We underscore the quantum superposition principle and Hadamard transformation under a local single-qubit operation. On the fundamental question posed by EPR, our result seems to lend support to the geometric nature of quantum entanglement. In concluding remarks, we discuss very briefly a bold conjecture in physics aiming to unify general relativity with quantum mechanics, namely, ER=EPR.

In this paper, we address the nature of spacetime in quantum gravity in light of a new version of the holographic principle that has established a relationship between string theory and polymer holonomy structures similar to Loop Quantum Cosmology spin networks. In front of the results found out, it is possible to argue that, for such a relationship to work, spacetime must be seen as emergent from a fundamental structure whose degrees of freedom correspond to quantum correlations only.

In this paper, we show that a possible relationship between the Hubble-Lema\^{i}tre constant and the universe holographic complexity can be established in the context of a new proposal for the emergence of spacetime, according to which spacetime must emerge from quantum information encoded in quantum correlations without correlate. Such a bridge between the Hubble-Lema\^{i}tre constant and the universe holographic complexity can shed some light on the issue of the Hubble tension.

Heralded photons from a silicon source are temporally multiplexed utilizing thin film lithium niobate photonics. The time-multiplexed source, operating at a rate of R = 62.2 MHz, enhances single photon probability by 3.25 $\pm$ 0.05.

We consider the fermionic (logarithmic) negativity between two fermionic modes in the Schwinger model. Recent results pointed out that fermionic systems can exhibit stronger entanglement than bosonic systems, exhibiting a negativity that decays only algebraically. The Schwinger model is described by fermionic excitations at short distances, while its asymptotic spectrum is the one of a bosonic theory. We show that the two-mode negativity detects this confining, fermion-to-boson transition, shifting from an algebraic decay to an exponential decay at distances of the order of the de Broglie wavelength of the first excited state. We derive analytical expressions in the massless Schwinger model and confront them with tensor network simulations. We also perform tensor network simulations in the massive model, which is not solvable analytically, and close to the Ising quantum critical point of the Schwinger model, where we show that the negativity behaves as its bosonic counterpart.

The bulk-boundary correspondence (BBC) relates in-gap boundary modes to bulk topological invariants. In certain non-Hermitian topological systems, conventional BBC becomes invalid in the presence of the non-Hermitian skin effect (NHSE), which manifests as distinct energy spectra under the periodic and open boundary conditions and massive eigenstate localization at boundaries. In this work, we introduce a scheme to induce NHSE without breaking conventional BBC, dubbed as the topologically compatible NHSE (TC-NHSE). In a general one dimensional two-band model, we unveil two types of TC-NHSE that do not alter topological phase transition points under any circumstance or only in a certain parameter regime, respectively. Extending our model into two dimension, we find that TC-NHSE can be selectively compatible to different sets of Weyl points between different bands of the resultant semimetallic system, turning some of them into bulk Fermi arcs while keeping the rest unchanged. Our work hence helps clarify the intricate interplay between topology and NHSE in non-Hermitian systems, and provides a versatile approach for designing non-Hermitian topological systems where topological properties and NHSE do not interfere each other.

Investigating the time evolution of complexity in quantum systems entails evaluating the spreading of the system's state across a defined basis in its corresponding Hilbert space. Recently, the Krylov basis has been identified as the one that minimizes this spreading. In this study, we develop a numerical exploration of the Krylov complexity in quantum states following a quench in the Lipkin-Meshkov-Glick model. Our results reveal that the long-term averaged Krylov complexity acts as an order parameter when the quench originates from a zero magnetic field. It effectively discriminates between the two dynamic phases induced by the quench, sharing a critical point with the conventional order parameter. Additionally, we examine the inverse participation ratio and Shannon entropy in both the Krylov basis and the energy basis. A matching dynamic behavior is observed in both bases when the initial state possesses a specific symmetry. This behavior is analytically elucidated by establishing the equivalence between the Krylov basis and the pre-quench energy eigenbasis.

During the last 30 years, stimulated by the quest to build superconducting quantum processors, a theory of quantum electrical circuits has emerged and this theory goes under the name of circuit quantum electrodynamics or circuit-QED. The goal of the theory is to provide a quantum description of the most relevant degrees of freedom. The central objects to be derived and studied are the Lagrangian and the Hamiltonian governing these degrees of freedom. Central concepts in classical network theory such as impedance and scattering matrices can be used to obtain the Hamiltonian and Lagrangian description for the lossless (linear) part of the circuits. Methods of analysis, both classical and quantum, can also be developed for nonreciprocal circuits. These lecture notes aim at giving a pedagogical overview of this subject for theoretically-oriented Master or PhD students in physics and electrical engineering, as well as Master and PhD students who work on experimental superconducting quantum devices and wish to learn more theory.

The coherence characteristics of a tin-vacancy color center in diamond are investigated through optical means including coherent population trapping between the ground state orbital levels and linewidth broadening effects. Due to the large spin-orbit splitting of the orbital ground states, thermalization between the ground states occurs at rates that are impractical to measure directly. Here, spectral information is transformed into its conjugate variable time, providing picosecond resolution and revealing an orbital depolarization timescale of ${\sim30{\rm~ps}}$. Consequences of the investigated dynamics are then used to estimate spin dephasing times limited by thermal effects.