Time evolution of entanglement and bistability of two coupled quantum dots in a driven cavity

Abstract

The time evolution of entanglement between two quantum dots (QDs) trapped inside a cavity driven by a coherent quantized field is studied. In the presence of dissipation, entanglement shows many interesting features such as sudden death and revival, and finite steady state value after sudden death. We also investigate dependence of entanglement on dot variables and its relation to bistability. It is found that entanglement vanishes when the cavity field intensity approaches the upper branch of the bistability curve. When the cavity is driven by a modulated field in the presence of dissipation, it can periodically generate entanglement, which is much larger than the maximum value attained in the steady-state for this system but the dots are never fully entangled.     

The evolution and stability of quantum correlations in dissipative cavity

We investigate the dynamics of quantum correlations such as entanglement and quantum discord between two atoms in a lossy cavity. It is found that a stable quantum discord could be induced even when the atoms remain separable at all times. Also, we show that it is possible to amplify and protect the quantum discord under cavity decay for certain types of initial states. Moreover, entanglement decoherence-free subspaces are obtained which may be useful in quantum information and quantum computation.     

The dynamics of entanglement and quantum discord of two atoms in coupled cavities

We investigate the dynamics of quantum correlations such as entanglement and quantum discord between two noninteracting atoms, each of which is trapped inside one of two coupled cavities. We find that the cavity decay can induce both entanglement and quantum discord between the two atoms when they are initially prepared in doubly excited state. The result shows the sudden death and sudden birth of entanglement and robustness of the quantum discord to sudden death. It is also found that the doubly excited state is responsible for the sudden death of entanglement. Moreover, the sudden death of entanglement can be controlled by the intercavity hopping rate.     

Frozen geometric discord in dissipative cavity quantum electrodynamics system

We investigate the dynamics of geometric quantum correlations for certain decoherence channels and discuss the necessary conditions for the existence of frozen geometric discord. As illustrative examples, we study the phenomenon of double sudden transitions in geometric discord for a system consisting of two noninteracting atoms inserted in two independent dissipative cavities and how the initial state parameters and decay rate of dissipative cavities affect the frozen time during which the geometric discord remains constant. We also explore the dynamics of geometric discord between two noninteracting atoms trapped in a common dissipative cavity and find that the geometric discord exhibits sudden transition between classical and quantum decoherence. Moreover, a nonzero stationary geometric discord can arise in both the independent cavity case and common cavity case.     

Continuous-Variable Quantum Network Coding Based on Quantum Discord

Establishing entanglement is an essential task of quantum communication technology. Beyond entanglement, quantum discord, as a measure of quantum correlation, is a necessary prerequisite to the success of entanglement distribution. To realize efficient quantum communication based on quantum discord, in this paper, we consider the practical advantages of continuous variables and propose a feasible continuous-variable quantum network coding scheme based on quantum discord. By means of entanglement distribution by separable states, it can achieve quantum entanglement distribution from sources to targets in a butterfly network. Compared with the representative discrete-variable quantum network coding schemes, the proposed continuous-variable quantum network coding scheme has a higher probability of entanglement distribution and defends against eavesdropping and forgery attacks. Particularly, the deduced relationship indicates that the increase in entanglement is less than or equal to quantum discord.     

Researching the Link Between the Geometric and Rènyi Discord for Special Canonical Initial States Based on Neural Network Method

Quantum correlation which is different to the entanglement and classical correlation plays important role in quantum information field. In our setup, neural network method is adopted to simulate the link between the Rènyi discord (α = 2) and the geometric discord (Bures distance) for special canonical initial states in order to show the consistency of physical results for different quantification methods. Our results are useful for studying the differences and commonalities of different quantizing methods of quantum correlation.     

Relation between quantum probe and entanglement in n-qubit systems within Markovian and non-Markovian environments

We address in detail the process of parameter estimation for an n-qubit system dissipating into a cavity in which the qubits are coupled to the single-mode cavity field via coupling constant g which should be estimated. In addition, the cavity field interacts with an external field considered as a set of continuum harmonic oscillators. We analyse the behaviour of the quantum Fisher information (QFI) for both weak and strong coupling regimes. In particular, we show that in strong coupling regime, the memory effects are dominant, leading to an oscillatory variation in the dynamics of the QFI and consequently information flowing from the environment to the quantum system. We show that when the number of the qubits or the coupling strength rises, the oscillations, signs of non-Markovian evolution of the QFI, increase. This indicates that in the strong-coupling regime, increasing the size of the system or the coupling strength remarkably enhances the reversed flow of information. Moreover, we find that it is possible to retard the QFI loss during the time evolution and therefore enhance the estimation of the parameter using a cavity with a larger decay rate factor. Furthermore, analysing the dynamics of the QFI and negativity of the probe state, we reveal a close relationship between the entanglement of probes and their capability for estimating the parameter. It is shown that in order to perform a better estimation of the parameter, we should avoid measuring when the entanglement between the probes is maximized.     

Quantifying quantumness of correlations using Gaussian Rényi-2 entropy in optomechanical interfaces

Using the Gaussian Rényi-2 entropy, we analyse the behaviour of two different aspects of quantum correlations (entanglement and quantum discord) in two optomechanical subsystems (optical and mechanical). We work in the resolved sideband and weak coupling regimes. In experimentally accessible parameters, we show that it is possible to create entanglement and quantum discord in the considered subsystems by quantum fluctuations transfer from either light to light or light to matter. We find that both mechanical and optical entanglement are strongly sensitive to thermal noises. In particular, we find that the mechanical one is more affected by thermal effects than that optical. Finally, we reveal that under thermal noises, the discord associated with the entangled state decays aggressively, whereas the discord of the separable state (quantumness of correlations) exhibits a freezing behaviour, seeming to be captured over a wide range of temperature.     

A simple model for a minimal environment: the two-atom Tavis–Cummings model revisited

Individual quantum systems may be interacting with surrounding environments having a small number of degrees of freedom. Here we discuss a simple toy model: a system constituted by a two-level atom (atom 1) interacting with a single mode cavity field which is (weakly) coupled to a small environment (atom 2). We investigate the influence of the minimal environment on the dynamics of the linear entropy and the atomic dipole squeezing of atom 1, as well as the entanglement between atom 1 and the field. We also obtain the full analytical solution of the two-atom Tavis–Cummings model for both arbitrary coupling strengths and frequency detunings, necessary to analyse the influence of the field-environment detuning on the evolution of the system’s quantum properties. For complementarity, we discuss the role of the degree of mixedness of the environment by analysing the time-averaged linear entropy of atom 1.     

Entanglement-induced population trapping by Schrödinger's cat

Abstract

We propose an experiment that is a variation of the Schrödinger's cat ′paradox' wherein the entanglement between a microscopic system and a macroscopic system is of primary interest. The experiment involves tunable entanglement and serves as a model for controllable decoherence in the context of cavity quantum electrodynamics where atoms interact dispersively with a cavity field initially in a coherent state. The interaction produces an entanglement between the atom and the field, and the degree of entanglement can be probed by subjecting the atom to resonant classical radiation after it leaves the cavity. The amplitude of the resulting Rabi oscillations reflects the degree of the entanglement, there being no Rabi oscillations when the entanglement is maximum. We show that the cavity damping does not affect the experiment.     

Measure of Entanglement States of Two Interacting Electrons in Vertically Coupled Quantum Dots Induced by a Time-Dependent Electric Field

We study the dynamics of two electrons located in two vertically tunnel-coupled quantum dots in the presence of an oscillatory electric field. By solving the time-dependent Schrödinger equation, we predict the dynamical generation of entangled electron states, such as the EPR (Einstein, Podolsky, and Rosen) pairs or Bell states. The Schmidt rank and the von Neumann entropy are evaluated to characterize the degree of entanglement of the two electron states.     

Magnetic field control of exchange and noise immunity in double quantum dots

We employ density functional calculated eigenstates as a basis for exact diagonalization studies of semiconductor double quantum dots, with two electrons, through the transition from the symmetric bias regime to the regime where both electrons occupy the same dot. We calculate the singlet-triplet splitting J(epsilon) as a function of bias detuning epsilon and explain its functional shape with a simple, double anticrossing model. A voltage noise suppression "sweet spot," where d J(epsilon)/d(epsilon) = 0 with nonzero J(epsilon), is predicted and shown to be tunable with a magnetic field B.     

Quantum Entanglement and Correlations in Superconducting Flux Qubits

We report on the quantum correlations dissipative dynamics followed by coupled superconducting flux qubits. The coupling between the superconducting quantum register and the reservoir is described by two different mechanisms: collective and independent decoherence. By means of the Bloch?CRedfield formalism, we solve the quantum master equation and show that coupling under collective quantum noise is more robust to decoherence. This result is demonstrated for different flux qubit initial preparations, taking into account the influence due to external fields and temperature. Furthermore, we compute the entanglement and the quantum discord dissipative dynamics as controlled by external parameters. We show that the discord is more robust against decoherence effects. This fact could be harnessed in the realization of quantum computing tasks that do not need to invoke entanglement in their implementation.     

Surface plasmon-assisted optical bistability in the quantum dot-metal nanoparticle hybrid system

We theoretically investigated optical bistability (OB) of a coupled excition–plasmon hybrid system in a unidirectional ring cavity. It is found that the threshold and the region of OB can be tuned by adjusting the center–center distance between the quantum dot and metal nanoparticle (MNP), the Rabi frequency of the control field and the radius of the MNP. Due to the significantly enhanced optical nonlinearity by the surface plasmon effect, the threshold of OB can be decreased greatly when the probe field is parallel to the major axis of the hybrid system. The enhanced OB may have promising applications in optical switching and optical storage.     

Entanglement dynamics and quasi-periodicity in discrete quantum walks

We study the entanglement dynamics of discrete time quantum walks acting on bounded finite sized graphs. We demonstrate that, depending on system parameters, the dynamics may be monotonic, oscillatory but highly regular, or quasi-periodic. While the dynamics of the system are not chaotic since the system comprises linear evolution, the dynamics often exhibit some features similar to chaos such as high sensitivity to the system's parameters, irregularity and infinite periodicity. Our observations are of interest for entanglement generation, which is one primary use for the quantum walk formalism. Furthermore, we show that the systems we model can easily be mapped to optical beamsplitter networks, rendering experimental observation of quasi-periodic dynamics within reach.     

Electrical and optical control of entanglement entropy in a coupled triple quantum dot system

We investigated theoretically the entanglement creation through tunneling rate and fields in a four-level triple quantum dot molecule based on InAs/GaAs/AlGaAs heterostructure in both steady state and transient state. We demonstrate that the entanglement entropy among the QDM and its spontaneous emission fields can be controlled by coherent and incoherent pumping field and tunnel-coupled electronics levels. The results may provide some new possibilities for technological applications in solid-state quantum information science, quantum computing, teleportation, encryption, compression codec, and optoelectronics.     

On the quantumness of correlations in nuclear magnetic resonance

Nuclear magnetic resonance (NMR) was successfully employed to test several protocols and ideas in quantum information science. In most of these implementations, the existence of entanglement was ruled out. This fact introduced concerns and questions about the quantum nature of such bench tests. In this paper, we address some issues related to the non-classical aspects of NMR systems. We discuss some experiments where the quantum aspects of this system are supported by quantum correlations of separable states. Such quantumness, beyond the entanglement-separability paradigm, is revealed via a departure between the quantum and the classical versions of information theory. In this scenario, the concept of quantum discord seems to play an important role. We also present an experimental implementation of an analogue of the single-photon Mach-Zehnder interferometer employing two nuclear spins to encode the interferometric paths. This experiment illustrates how non-classical correlations of separable states may be used to simulate quantum dynamics. The results obtained are completely equivalent to the optical scenario, where entanglement (between two field modes) may be present.     

Effects of Magnetic Quantum Dots on Magnetoconductance in Quantum Wires

We present a theoretical study of electronic transport in quantum wires (narrow two-dimensional electron gas) with array of magnetic quantum dots. Each magnetic quantum dot is defined by a small circular region where the strength of perpendicular magnetic field is modulated. By making use of a newly developed calculation method based on the gauge transformations, we calculated the conductance as a function of the external perpendicular magnetic field. Our numerical calculations show that the magnetoconductance is very sensitive to the number of magnetic quantum dots in the field region where the direction of the net magnetic field in dot regions is antiparallel to the external magnetic field.     

Quantum properties and dynamics of X states

X states are a broad class of two-qubit density matrices that generalize many states of interest in the literature. In this work, we give a comprehensive account of various quantum properties of these states, such as entanglement, negativity, quantum discord and other related quantities. Moreover, we discuss the transformations that preserve their structure both in terms of continuous time evolution and discrete quantum processes.     

Dynamics of quantum steerability for two atoms in coupled cavities

We study the dynamics of quantum steerability between two non-interacting atoms, each of which is trapped inside one of two coupled cavities. Compared with entanglement, quantum steerability manifests sudden birth and sudden death phenomenon during the time evolution. We find that the cavity decay plays a destruction role for both steerability and entanglement. It is also shown that the survival time as well as the maximal value of steerability are sensitive to the asymmetry of the cavities. Moreover, it is found the sudden death of steerability can be controlled by the hopping rate of the coupled cavities.