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.     

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.     

Enhancement of discord and entanglement with detuning for two coupled quantum dots in a driven cavity

We study the quantum discord for a system of two identical coupled quantum dots interacting with quantized cavity field in the presence of cavity as well as dot decay and detuning. The cavity is externally driven by a coherent light. These results are compared with the entanglement of the quantum dots in various parameter regimes in which system may or may not show bistability. We show that the discord in the steady state is nonzero for any nonzero cavity field amplitude. The system has higher discord in the upper branch of the bistability curve where the entanglement is zero. We also find many other interesting results including high discord and entanglement in the presence of detuning, a phenomenon which we further examine by approximating the density matrix in the appropriate limit.     

Generating EPR-type entanglement of degenerate optomechanical parametric oscillators

Einstein–Podolski–Rosen (EPR) entanglement states are achievable by combining two single-mode position and momentum squeezed states at a 50:50 beam splitter (BS). To generate the EPR mechanical entanglement, we consider the system consisted of two parametric optomechanical resonators, where two mechanical oscillators are linearly coupled. The linear coupling forms the symmetric and antisymmetric combinations of two mechanical modes, parallel to a 50:50 BS mixing. In the weak optomechanical coupling regime and via applying the opposite phases of parametric interactions, the symmetric and antisymmetric mechanical modes can be position and momentum squeezed, respectively. Therefore, two original mechanical modes are EPR entangled. Moreover, the mechanical thermal noise can decrease the entanglement. But with the parametric interaction enhanced optomechanical cooling, the influence of thermal noise on entanglement can be significantly suppressed, and the mechanical entanglement can be generated under a relatively high temperature. We also discuss the critical thermal occupation where the entanglement disappears, which is proportional to the optomechanical cooperativity parameter.     

Sub-Poissonian photon squeezing and entanglement in optical chain second harmonic generation

Nonclassical properties exhibited by a chain of cavity modes second harmonic generation in coupled oscillators system, designed by using multichannel optical waveguides, is explored. The solution for the Hamiltonian of the coupled-modes driven by coherent excitation is obtained via an exact formulation of the normal-ordered Fokker-Planck equation. Nonclassical effects, namely the sub-Poissonian photons, squeezing and entanglement are noticed. Multichannel coupling of the coupled oscillators induces new possibilities for correlation between the modes in different channels, henceforth, provides an effective way towards manipulation of quantum light.     

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.     

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.     

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.     

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.     

Entanglement generated in a nanomechanical oscillator system

We consider a Fabry–Perot cavity with one fixed mirror and a movable perfectly reflecting mirror. Applying a linearised fluctuation analysis, we derive the quantum fluctuations and correlation matrix between the cavity and nanomechanical oscillator. We investigate the continuous-variable (CV) entanglement and squeezing between the cavity and nanomechanical oscillator. It is found that high intensity of entanglement and squeezing between the cavity and nanomechanical oscillator can be achieved.     

Creating quantum entanglement between multi-atom Dicke states and two cavity modes

We present a scheme to create quantum entanglement between multi-atom Dicke states and two cavity modes by passing N three-level atoms in Λ configuration through a resonant two-mode cavity one by one. We further show that such a scheme can be used to generate arbitrary two-mode N-photon entangled states, arbitrary superposition of Dicke states, and a maximal entangled state of Dicke states. These states may find applications in the demonstration of quantum non-locality, high-precision spectroscopy and quantum information processing.     

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.     

Entangling a single NV centre with a superconducting qubit via parametric couplings between photons and phonons in a hybrid system

We propose an efficient scheme for generating entangled states between a single nitrogen-vacancy (NV) centre in diamond and a superconducting qubit in a hybrid set-up. In this device, the NV centre and the superconducting qubit couple to a nanomechanical resonator and a superconducting coplanar waveguide cavity, respectively, while the microwave cavity and the mechanical resonator are parametrically coupled with a tunable coupling strength. We show that, highly entangled states between the NV centre and the superconducting qubit can be achieved, by means of the Jaynes–Cummings interactions in the NV-resonator and qubit-cavity subsystems which transfer the entanglement between the vibration phonons and the cavity photons to the NV centre and the superconducting qubit. This work may provide interesting applications in quantum computation and communication with single NV spins and superconducting qubits.     

Entangling atoms and ions in dissipative environments

Abstract

Quantum information processing rests on our ability to manipulate quantum superpositions through coherent unitary transformations, and to establish entanglement between constituent quantum components of the processor. The quantum information processor (a linear ion trap, or a cavity confining the radiation field for example) exists in a dissipative environment. We discuss ways in which entanglement can be established within such dissipative environments. We can even make use of a strong interaction of the system with its environment to produce entanglement in a controlled way.     

QUANTUM INFORMATION Quantum imaging,quantum lithography and the uncertainty principle

One of the most surprising consequences of quantum mechanics is the entanglement of two or more distance particles. The ‘ghost’ image experiment demonstrated the astonishing nonlocal behaviour of an entangled photon pair. Even though we still have questions with regard to fundamental issues of the entangled quantum systems, quantum entanglement has started to play an important role in practical applications. Quantum lithography is one of the hot topics. We have demonstrated a quantum lithography experiment recently, in which the experimental results have beaten the classical diffraction limit by a factor of two. This is a quantum mechanical two-photon phenomenon but not a violation of the uncertainty principle.     

Synchronization hypothesis in the Winfree model

We consider N oscillators coupled by a mean field as in the Winfree model. The model is governed by two parameters: the coupling strength κ and the spectrum width γ of the frequencies of each oscillator centred at 1. In the uncoupled regime, κ = 0, each oscillator possesses its own natural frequency, and the difference between the phases of any two oscillators grows linearly in time. In the zero-width regime for the spectrum, the oscillators are simultaneously in the death state if and only if κ is above some positive value κ_*. We say that N oscillators are synchronized if the difference between any two phases is uniformly bounded in time. We identify a new hypothesis for the existence of synchronization. The domain in (γ, κ) of synchronization contains {0} × [0, κ_*] in its closure. Moreover, the domain is independent of the number of oscillators and the distribution of the frequencies. We show numerically, on a specific family of Winfree models, that the above hypothesis seems to be a bifurcation criterion for the existence of synchronization domain. The transition is not, however, mathematically sharp.     

Optomechanics with silicon nanowires by harnessing confined electromagnetic modes

The optomechanical coupling that emerges in an optical cavity in which one of the mirrors is a mechanical resonator has allowed sub-Kelvin cooling with the prospect of observing quantum phenomena and self-sustained oscillators with very high spectral purity. Both applications clearly benefit from the use of the smallest possible mechanical resonator. Unfortunately, the optomechanical coupling largely decays when the size of the mechanical system is below the light wavelength. Here, we propose to exploit the optical resonances associated to the light confinement in subwavelength structures to circumvent this limitation, efficiently extending optomechanics to nanoscale objects. We demonstrate this mechanism with suspended silicon nanowires. We are able to optically cool the mechanical vibration of the nanowires from room temperature to 30-40 K or to obtain regenerative mechanical oscillation with a frequency stability of about one part per million. The reported optomechanical phenomena can be exploited for developing cost-optimized mass sensors with sensitivities in the zeptogram range.     

Preparation of an arbitrary density matrix of a harmonic oscillator

Abstract

We propose a deterministic method to generate an arbitrary (pure or mixed) density matrix of a harmonic oscillator. The general density matrices are achieved by manipulating quantum entanglement between the oscillator and an auxiliary oscillator. We discuss how our preparation scheme can be realized by cavity quantum electrodynamics interactions so that a general density matrix of a single-mode electromagnetic field can be created.     

Coherent coupling between an optomechanical membrane and an interacting photon Bose–Einstein condensate

In this paper, we study theoretically the optomechanical interaction of an interacting condensate of photons with an oscillating mechanical membrane in a microcavity. We show that in the Bogoliubov approximation, due to the large number of photons in the condensate, there is a linear strong effective coupling between the Bogoliubov mode of the photonic Bose–Einstein condensate (BEC) and the mechanical motion of the membrane which depends on the photon–photon scattering potential. This coupling leads to the cooling of the mechanical motion, the normal mode splitting (NMS), the squeezing of the output field and the entanglement between the excited mode of the cavity and the mechanical mode. Since the photon condensation occurs at room temperature, this hybrid system can be potentially considered as a room temperature source of squeezed light as well as a suited candidate for exploring the quantum effects. We show that, on one hand, the non-linearity of the photon gas increases the degree of the squeezing of the output field of the microcavity and the efficiency of the cooling process at high temperatures. On the other hand, it reduces the NMS in the displacement spectrum of the oscillating membrane and the degree of the optomechanical entanglement. In addition, the temperature of the photonic BEC can be used to control the above-mentioned phenomena.     

Conservation of Angular Momentum in a Flux Qubit

Oscillations of superconducting current between clockwise and counterclockwise directions in a flux qubit do not conserve the angular momentum of the qubit. To compensate for this effect the solid containing the qubit must oscillate in unison with the current. This requires entanglement of quantum states of the qubit with quantum states of a macroscopic body. The question then arises whether slow decoherence of quantum oscillations of the current is consistent with fast decoherence of quantum states of a macroscopic solid. This problem is analyzed within an exactly solvable quantum model of a qubit embedded in an absolutely rigid solid and for the elastic model that conserves the total angular momentum. We show that while the quantum state of a flux qubit is, in general, a mixture of a large number of rotational states, slow decoherence is permitted if the system is macroscopically large. Practical implications of entanglement of qubit states with mechanical rotations are discussed.