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.     

Effects of cavity-field statistics on atomic entanglement in a two-mode Jaynes–Cummings model with intensity-dependent coupling

We study the entanglement properties of a pair of two-level Rydberg atoms passing one after another into a lossless cavity with two modes. The atoms interact with the cavity field via an intensity-dependent, non-degenerate two-photon transition. The initial joint state of two successive atoms that enter the cavity is unentangled. Interactions mediated by the two-mode cavity photon field result in the final two-atom mixed entangled type state. The entanglement of formation of the joint two-atom state as a function of the Rabi angle, gt, is calculated for the two-mode Fock state field, coherent field, and thermal field, respectively, inside the cavity. The change in the magnitude of atomic entanglement with cavity photon number in two modes has been studied.     

Quantum decoherence with the Unruh single-particle state having right and left components

We investigate the effects of quantum decoherence generated by the Unruh effect in non-inertial frames under an amplitude damping channel and a phase damping channel, respectively, when the Unruh single particle has right and left components. We not only consider the influence of acceleration on entanglement, but also consider the influence of different rates between right and left components of the Unruh single-particle state on entanglement. We find that when the Unruh single-particle state has right and left components, i.e. |1?_U?=?q _L|0?_I|1?_II?+?q _R|1?_I|0?_II, |q _R|²?+?|q _L|²?=?1, with q _L?≠?0, there appears the sudden death of entanglement, which occurs earlier under both the amplitude damping channel and the phase damping channel with the increase of acceleration than that when the Unruh single-particle state only has a right component (q _R?=?1, q _L?=?0). We also find that the initial entanglement decreases with decrease of q _R from 1 to 1/2^1/2.     

Entanglement in two-site Bose–Hubbard model with non-linear dissipation

In this paper, we study the decoherence and entanglement properties for the two-site Bose–Hubbard model in the presence of a non-linear damping. We apply the techniques of thermo field dynamics and then use Hartree-Fock approximation to solve the corresponding master equation. The expectation values of the approximated field operators appearing in the solution of master equation are computed self-consistently. We solve this master equation for a small time t so that we get the analytical solution, thereby we compute the decoherence and entanglement properties of the solution of the two-mode bosonic system. We have found that for a small initial time t, the entanglement of the system increases but at the same time the system decoheres exponentially.     

Local entanglement in a bimodal high- Q cavity: Production and utilization

A new conditional scheme for entangling the two quantized modes of a bimodal high-Q cavity field is presented. We show that, injecting one at time k atoms inside the cavity, it is possible to guide the field toward k-dependent linear combinations of k + 1 bimodal Fock states, each one possessing the same total number of photons. The two simple cases corresponding to the passage of one or two atoms only through the resonator are considered. Their practical feasibility against cavity losses, spontaneous emission and other sources of imprecision of the experimental set-up is discussed. Two examples illustrating the usefulness of and the interest toward the creation of such a local entanglement, are reported. The first one relates the specific features of these bimodal entangled states to the occurrence of a non-classical correlation effect in the dynamics of a two-level atom interacting with the entangled cavity. The second one demonstrates that the peculiar entanglement initially stored in the cavity in accordance with our method, provides an effectively exploitable resource to entangle two spatially separated cavities.     

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.     

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.     

Nanostructures,Entanglement and the Physics of Quantum Control

Abstract

Nanostructures might be viewed as solid-state approximations to SU(N) networks, the nodes of which would, in simplest form, be analogous to elementary spins. Owing to interactions with an external light field and coupling between the local nodes these networks allow for single- and multiple-node coherence (entanglement), despite damping. By means of stochastic simulations we demonstrate how such a ‘quantum machinery’ embedded in a qualified environment would look like in terms of measurement protocols. These protocols give evidence for the underlying complex behaviour of the network, a complexity which is based on the non-local information contained in the entanglement and which would not be present in the ‘classical limit’ of using local information only.     

Cooling neutral particles in multimode cavities without spontaneous emission

Abstract

We discuss a scheme to cool, trap and manipulate an ensemble of polarizable particles moving in the field of a multimode optical cavity via the correlated dynamics of the field and the particle motion. Using a large detuning between the atoms and the field, spontaneous emission plays a negligible role in the dynamics and the cooling scheme only requires a sufficiently large optical dipole moment. Increasing the particle number slows down the cooling process but it can be accelerated using an increasing number of field modes with higher pump amplitudes. For the special case of a two mode ring cavity and assuming small deviations of the particle positions from the potential minima, the frequencies and damping rates of the vibrational excitation modes can be explicitly calculated. We find a rapid damping of the centre-of-mass motion and relatively slow damping rates for the relative particle oscillations. These predictions agree quite well with a quantum treatment of the atomic motion as used for the excitations of a non-interacting Bose gas (at T = 0) inside the cavity field. Due to the position-dependent mode coupling, the cooling process in a multimode configuration in general happens much faster than for a standing wave geometry. These analytical results are confirmed by N-particle simulations of the semiclassical equations and show even enhanced damping due to the anharmonicity of the full potential.     

Entanglement generation in atom-cavity networks with complete connectivity

In this paper, we consider the situation that four identical two-level atoms are separately trapped in separated tetrahedral structure single-mode optical cavities, which are placed at the vertices of a tetrahedron and are coupled by four fibres. Each atom resonantly interacts with cavity via a one-photon hopping. The evolution of the state vector of the system is given by solving the Schrödinger equation when the total excitation number of the system equals one. Negativity is adopted to quantify the degree of entanglement between two subsystems. The entanglement dynamics between atoms and between cavities is studied. The influences of atom-cavity coupling coefficient on the entanglement between atoms and that between cavities are discussed. The results obtained using the numerical method show that the atom–atom entanglement and the cavity–cavity entanglement are all strengthened with increase of atom-cavity coupling coefficient.     

The synchronization and entanglement of optomechanical systems

We investigate synchronization and entanglement in two coupled cavity optomechanical systems. The classical synchronization, quantum synchronization and entanglement of the two cavity fields and the two mechanical oscillators are analysed, respectively. Our results show that the two cavity resonators are synchronization without entanglement, while the two mechanical oscillators are entangled with quantum-phase synchronization. We conclude that the quantum synchronization and entanglement have no affirmatory relationship although they are both signature of correlation.     

Effect of the Stark shift on entanglement in a double two-photon JC model

Considering a double two-photon JC model, we investigate the entanglement between the two two-level atoms and that between the two cavity fields, and study the effect of the Stark shift on entanglement. The results show that, on the one hand the atom–atom and cavity–cavity concurrences evolve periodically with time and the periods are affected by the Stark shift; on the other hand, the two atoms are not disentangled at any time when the Stark shift is considered, and for large values of the Stark shift parameter, the two atoms can remain in a stationary entangled state. In addition, we find that the so-called entanglement sudden death can occur under appropriate conditions on the dynamic Stark shift for a certain initial state of the system.     

Spontaneously induced atom–radiation entanglement in an ensemble of trapped two-level atoms

Analysis of the effects of the spontaneously induced correlation on atom–radiation entanglement in an ensemble of two-level atoms initially prepared in the upper energy level and then trapped in a cavity containing a source of a squeezed radiation employing the method of evaluating the coherent-state propagator is presented. It is found that the cavity radiation exhibits squeezing which is directly attributed to the squeezed radiation initially present in the cavity. The intensity of the cavity radiation increases with the squeeze parameter and interaction time. It is also shown that a substantial degree of entanglement between the atomic state and radiation mode exits at a particular time which depends on the coupling constant and squeeze parameter. It is understood that although the squeezed radiation is directly accountable for the cavity squeezing, it significantly destroys the atom–radiation entanglement induced by the correlation between spontaneously emitted radiation and the atoms.     

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.     

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.     

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.     

Enhanced entanglement in a non-degenerate four-level atom with a parametric oscillator

We discuss the generation and evolution of a macroscopic entanglement light with a subthreshold non-degenerate parametric oscillator, coupled to a vacuum reservoir. The four-level atoms driven by two classical fields interact with the parametric oscillator. The master equation for the cavity modes of the scheme is derived and analyzed, the entanglement properties of the two-mode light generated by this scheme inside and outside the cavity is studied. We show that the light produced by this system is strongly entangled with time evolution inside and outside a cavity.     

Fock states generation in a kicked cavity with a nonlinear medium

Abstract

We discuss a model of a cavity filled with a passive nonlinear ?Kerr‘ medium and periodically kicked by a series of ultra-short laser pulses. The nonlinear medium is described by the (2q ? 1)th nonlinearity X ^(2q?1). We find analytical formulas describing the field states inside the cavity. We show that such a system can produce, depending on the order of the nonlinearity, superpositions of several Fock states with the small photon numbers (0,1; 0,1,2; etc). In particular, the one-photon state can be approached during the evolution of the system with X ⁽³⁾ nonlinearity provided the cavity losses are negligible. The purity of states generated in this process, however, can be seriously degraded by the cavity damping. We perform numerical calculations to validate our analytical results.     

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.     

Preparation of W,GHZ, and two-qutrit states using bimodal cavities

Abstract

We show how one can prepare three-qubit entangled states like W-states, Greenberger-Horne-Zeilinger states as well as two-qutrit entangled states using the multi-atom two-mode entanglement. We propose a technique of preparing such a multi-particle entanglement using stimulated Raman adiabatic passage. We consider a collection of three-level atoms in Λ configuration simultaneously interacting with a resonant two-mode cavity for this purpose. Our approach permits a variety of multi-particle extensions.