arXiv subjects

Yusui Chen

Publications and source records attributed to Yusui Chen.

11 recordsLinked to original sources

arXiv2025

Non-Markovian two-time correlation functions for optomechanical systems

In this paper, we focus on the two-time correlation function (TTCF) of the cavity optomechanical system, which serves as the most popular tool in precision detection technologies. We utilize the stochastic Schrodinger equation approach to study TTCF for the cavity optomechanical system in the long-time steady state TTCF and time-dependent case. Our numerical simulations support two major conclusions: (1) long-time steady states in Markovian and non-Markovian regimes are different, resulting in the distinct TTCF, and (2) the time-dependent TTCF can reveal more information about the environment, rather than the traditional spectral function method.

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arXiv2022

Positivity Preserving non-Markovian Master Equation for Open Quantum System Dynamics: Stochastic Schr\"{o}dinger Equation Approach

Positivity preservation is naturally guaranteed in exact non-Markovian master equations for open quantum system dynamics. However, in many approximated non-Markovian master equations, the positivity of the reduced density matrix is not guaranteed. In this paper, we provide a general class of time-local, perturbative and positivity-preserving non-Markovian master equations generated from stochastic Schrodinger equations, particularly quantum-state-diffusion equations. Our method has an expanded range of applicability for accommodating a variety of non-Markovian environments. We show the positivity-preserving master equation for a three-level system coupled to a dissipative bosonic environment as a particular example to exemplify our general approach. We illustrate the numerical simulations with an analysis explaining why the previous approximated non-Markovian master equations cannot guarantee positivity. Our work provides a consistent master equation for studying the non-Markovian dynamics in ultrafast quantum processes and strong-coupling systems.

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arXiv2021

Feasible Architecture for Quantum Fully Convolutional Networks

Fully convolutional networks are robust in performing semantic segmentation, with many applications from signal processing to computer vision. From the fundamental principles of variational quantum algorithms, we propose a feasible pure quantum architecture that can be operated on noisy intermediate-scale quantum devices. In this work, a parameterized quantum circuit consisting of three layers, convolutional, pooling, and upsampling, is characterized by generative one-qubit and two-qubit gates and driven by a classical optimizer. This architecture supplies a solution for realizing the dynamical programming on a one-way quantum computer and maximally taking advantage of quantum computing throughout the calculation. Moreover, our algorithm works on many physical platforms, and particularly the upsampling layer can use either conventional qubits or multiple-level systems. Through numerical simulations, our study represents the successful training of a pure quantum fully convolutional network and discusses advantages by comparing it with the hybrid solution.

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arXiv2020

Impact of the Central Frequency of Environment on Non-Markovian Dynamics in Piezoelectric Optomechanical Devices

The piezoelectric optomechanical devices supply a promising experimental platform to realize the coherent and effective control and measurement of optical circuits working in Terahertz (THz) frequencies via superconducting electron devices typically working in Radio (MHz) frequencies. However, quantum fluctuations are unavoidable when the size of mechanical oscillators enters into the nanoscale. The consequences of the noisy environment are still challenging due to the lack of analytical tools. In this paper, a semi-classical and full-quantum model of piezoelectric optomechanical systems coupled to a noisy bosonic quantum environment are introduced and solved in terms of quantum-state diffusion (QSD) trajectories in the non-Markovian regime. We show that the noisy environment, particularly the central frequency of the environment, can enhance the entanglement generation between optical cavities and LC circuits in some parameter regimes. Moreover, we observe the critical points in the coefficient functions, which can lead the different behaviors in the system. Besides, we also witness the entanglement transfers between macroscopic objects due to the memory effect of the environment. Our work can be applied in the fields of electric/ optical switches, and long-distance distribution in a large-scale quantum network.

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arXiv2019

Quantumness protection for open systems in a double-layer environment

We study the dynamics of the two-level atomic systems (qubits) under a double-layer environment that is consisted of a network of single-mode cavities coupled to a common reservoir. A general exact master equation for the dynamics can be obtained by the quantum-state-diffusion (QSD) equation. The quantumness of the atoms including coherence and entanglement is investigated within various configurations of the external environment. It is shown that the preservation and generation of the quantumness can be controlled by regulating the parameters of the cavity network. Moreover the underlying physics of the results can be profoundly revealed by an effective model via a unitary transformation. Our work provides an interesting proposal in protecting the quantumness of open systems in the framework of a double-layer environment containing bosonic modes.

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arXiv2019

Exact coherence dynamics mediated by a single cavity mode in the presence of leakage

We present a quantum-state-diffusion equation to characterize the dynamics of a generic atomic system coupled to a leaky cavity mode. As quantum resources, the population, the coherence and even the entanglement of the system would gradually leak out of the cavity. The effect from the leakage of the cavity-mode to the uncontrollable degree of freedom, e.g., environment, is however not always negative to particular targets. A well-established scenario is that a photon counter attached to the cavity would absorb and then record the leaking photon. As a medium between the system and the photon counter, a strong coupling between system and cavity is necessary to enhance the measurement efficiency. While we find it also reduce the reading efficiency of the photon counter at the same time as fewer photons leak out of the cavity. We investigate the competition between these two mechanisms in the framework of non-Markovian open-quantum-system dynamics, where the photon counter serves as a general bosonic environment. Our results provide an optimized parameter space for system entanglement preservation and generation.

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arXiv2018

Open system approach to non-equilibrium dynamical theory of quantum dot systems

We theoretically investigate the non-equilibrium quantum dynamical theory of a quantum dot system coupled to fermionic reservoirs using the recently developed stochastic fermionic quantum state diffusion (FQSD) equation. The exact or approximate dynamical equations associated with the FQSD equation can describe the non-equilibrium quantum transport processes beyond the long-time limit leading to a steady state. We study in details the electron transport of a quantum-dot system coupled to two fermionic environments with different chemical potentials. We report the onset of Coulomb blockade in quantum dots in two distinctive cases: one involving a spin degeneracy one-quantum dot model, and the other a specific spin non-degeneracy two-quantum dot model. While the spin degeneracy case shows that the current in the quantum dot may be blockaded non-monotonically with respect to Coulomb energy, the non-degeneracy case exhibits significant non-Markovian effects, and it enables us to study the relations between initial conditions of the dots and the steady state currents.

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arXiv2018

Measurement Back Action on Qubit Systems by a Cavity Probe

We study the back action on a coupled multiple-qubit system induced by a quantum cavity probe in a non-demolition quantum measurement scheme. The exact quantum state stochastic Schrödinger equation is derived to systematically investigate the dynamics of quantum entanglement of the multiple-qubit system inside the cavity probe. Although quantum entanglement cannot be directly measured through experiments, the back action on quantum entanglement in the multi-qubit system is witnessed by applying the exact quantum state stochastic equations. Our results demonstrate that the analysis on the sensitivity of the quantum measurement should include not only the standard quantum limit of the canonical operators, but also the back action on the quantum entanglement. Our new method proposed a theoretical approach to investigate the effects of the measurement back action on the multi-qubit systems induced by the cavity probe.

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arXiv2017

Non-Markovian quantum interference in multilevel quantum systems: Exact master equation approach

We study the non-Markovian quantum interference phenomenon of a multi-state atomic system coupled to a bosonic dissipative environment by using the exact master equations derived in this paper. Two examples involving four-level systems with or without external control fields are investigated. Our results show that non-Markovian master equations are capable of exhibiting quantum interference phenomena in a temporal domain that has not fully explored before. In particular, we show that the environmental memory is instrumental in the onset of non-Markovian quantum interference pattern in different time scales.

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arXiv2014

Crossover Between Non-Markovian and Markovian Dynamics Induced by a Hierarchical Environment

Non-Markovian evolution of an open quantum system can be induced by the memory effects of a reservoir. Although a reservoir with stronger memory effects may seem like it should cause stronger non-Markovian effects on the system of interest, this seemingly intuitive thinking may not always be correct. We illustrate this by investigating a qubit (a two-level atom) that is coupled to a hierarchical environment, which contains a single-mode cavity and a reservoir consisting of infinite numbers of modes. We show how the non-Markovian character of the system is influenced by the coupling strength between the qubit and cavity and the correlation time of the reservoir. In particular, we found a new phenomenon whereby the qubit Markovian and non-Markovian transition exhibits a anomalous pattern in a parameter space depicted by the coupling strength and the correlation time of the reservoir.

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arXiv2014

Exact non-Markovian master equations for multiple qubit systems: quantum trajectory approach

A wide class of exact master equations for a multiple qubit system can be explicitly constructed by using the corresponding exact non-Markovian quantum state diffusion equations. These exact master equations arise naturally from the quantum decoherence dynamics of qubit system as a quantum memory coupled to a collective colored noisy source. The exact master equations are also important in optimal quantum control, quantum dissipation and quantum thermodynamics. In this paper, we show that the exact non-Markovian master equation for a dissipative N-qubit system can be derived explicitly from the statistical average of the corresponding non-Markovian quantum trajectories. We illustrated our general formulation by an explicit construction of a three-qubit system coupled to a non-Markovian bosonic environment. This multiple qubit master equation offers an accurate time evolution of quantum systems in various domains, and paves a way to investigate the memory effect of an open system in a non-Markovian regime without any approximation.

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