Environment-induced Entanglement Research Articles

Effects of dipolar coupling on an entanglement storage device

Quantum computation requires efficient long-term storage devices to preserve quantum states. An attractive candidate for such storage devices is qubits connected to a common dissipative environment. The common environment gives rise to persistent entanglements in these qubit systems. Hence these systems act efficiently as a storage device of entanglement. However, the existence of a common environment often requires the physical proximity of the qubits and hence results in direct dipolar coupling between the qubits. In this work, we investigate effects of the secular and the nonsecular part of the dipolar coupling on the environment-induced entanglement using a recently-proposed fluctuation-regulated quantum master equation (Chakrabarti and Bhattacharyya 2018 Phys. Rev. A 97 063837). We show that nonsecular part of the dipolar coupling results in reduced entanglement and hence less efficiency of the storage devices. We also discuss the properties of efficient storage that mitigates the detrimental effects of the dipolar coupling on the stored entanglement.

Divergence of entanglement entropy in quantum systems: Zero-modes

To this day, von Neumann definition of entropy remains the most popular measure of quantum entanglement. Much of the literature on entanglement entropy, particularly in the context of field theory, has focused on isolating the UV divergences. Zero-mode divergences of the entanglement entropy are less studied in this context, and apart from being easier to isolate, they offer an interesting insight into the physics of the system. To gain a better understanding of the system in this limit, we develop the free particle approximation of Harmonic oscillator, with which we investigate the properties of entropy divergence in continuous bi-partite quantum systems such as the coupled Harmonic oscillators and the Hydrogen atom. We also show zero-mode divergence of the entropy of environment-induced entanglement in a tri-partite oscillator system. We discuss the implications of our result for field theory and IR structure of gravity.

Experimental investigation of environment-induced entanglement using an all-optical setup

We investigate the generation of entanglement between two non interacting qubits coupled to a common reservoir. An experimental setup was conceived to encode one qubit on the polarization of an optical beam and another qubit on its transverse mode. The action of the reservoir is implemented as conditional operations on these two qubits, controlled by the longitudinal path as an ancillary degree of freedom. An entanglement witness and the two-qubit concurrence are easily evaluated from direct intensity measurements showing an excellent agreement with the theoretical prediction.

Gaussian entanglement induced by an extended thermal environment

We study stationary entanglement among three harmonic oscillators which are dipole coupled to a one-dimensional or a three-dimensional bosonic environment. The analysis of the open-system dynamics is performed with generalized quantum Langevin equations which we solve exactly in Fourier representation. The focus lies on Gaussian bipartite and tripartite entanglement induced by the highly non-Markovian interaction mediated by the environment. This environment-induced interaction represents an effective many-parties interaction with a spatial long-range feature: a main finding is that the presence of a passive oscillator is detrimental for the stationary two-mode entanglement. Furthermore, our results strongly indicate that the environment-induced entanglement mechanism corresponds to uncontrolled feedback which is predominantly coherent at low temperatures and for moderate oscillator-environment coupling as compared to the oscillator frequency.

Environment-Induced Entanglement for a Tripartite System in Non-Markovian Dissipation

We show that entanglement between three two-level atoms can be generated if the three atoms interact with a common structured reservoir, but do not interact directly with each other. Comparing with the results in Markovian case, we find that much higher values of bipartite and tripartite entanglement can be obtained for an initially factorized state of the tripartite system. The maximal value of entanglement depends on the detuning and the strength of relative coupling between the atoms and the reservoir.

Environment-induced entanglement with a single photon

We propose an all-optical setup, which couples different degrees of freedom of a single photon, to investigate entanglement generation by a common environment. The two qubits are represented by the photon polarization and Hermite-Gauss transverse modes, while the environment corresponds to the photon path. For an initially two-qubit separable state, the increase of entanglement is analyzed as the probability of an environment-induced transition ranges from 0 to 1. An entanglement witness that is invariant throughout the evolution of the system yields a direct measurement of the concurrence of the two-qubit state.

Dicke model and environment-induced entanglement in ion-cavity QED

The Dicke model describes the dynamics of N identical two-level atoms interacting with a quantized threedimensional electromagnetic (EM) field [1]. Under certain conditions the model predicts that the atoms interact with the quantized EM field collectively, giving rise to the widely studied phenomena of superradiance and subradiance [2, 3]. In free space ideal superradiance and subradiance take place in the so called small sample limit, i.e., when the atoms are so close to each other that one can ignore any effect resulting from their different spatial positions. In this case the atoms are indistinguishable with respect to their emission and absorption properties; hence, the presence of equivalent paths through which the emission process may occur gives rise to fully constructive (superradiance) or destructive (subradiance) interference. Ideal superradiance or subradiance in free space is very difficult to observe in the experiments since it requires that the atoms are placed in a regular pattern within a sample smaller than the wavelength of the EM field they interact with (small sample case). The requirement of a regular pattern is due to the presence of the dipoledipole forces that would otherwise break the symmetry under permutation of any two atoms necessary to observe superradiant-subradiant behavior. Such a regularity can be achieved, e.g., with trapped-ion crystals [4] or atoms in optical lattices [5]. In these systems, however, the separation between the particles is typically larger or on the same order of magnitude than the resonant wavelength (large sample case). In the large sample case, cooperative effects still occur but the subradiant state is not completely decoupled from the dynamics. Indeed, partial subradiance and superradiance have been observed with trapped ions [6].

Origination and survival of qudit-qudit entanglement in open systems

The exact reduced dynamics for two independent qudits dephasingly coupled to a common zero-temperature, super-Ohmic, bosonic environment is studied. It is shown that the qudit-qudit subsystem initially prepared in a separable state evolves into an entangled state. Details of the entanglement origination depend on both the coupling strength and the spectral properties of the environment as well as on the dimension of the qudit; the environment-induced entanglement can survive in the long-time limit. The entanglement dynamics of initially maximally entangled qudit-qudit states is analyzed for two examples: the qubit-qubit and qutrit-qutrit systems. The theory is applied to negative partial transposition entangled states of a two-qutrit system, which are shown to be distillable (for the assumed initial state) for arbitrary environment parameters.

Environment-induced two-mode entanglement in quantum Brownian motion

The time evolution of quantum correlations of entangled two-mode continuous variable states is examined in single-reservoir as well as two-reservoir models, representing noisy correlated or uncorrelated non-Markovian quantum channels. For this purpose the model of quantum Brownian motion is extended. Various separability criteria for Gaussian continuous variable systems are applied. In both types of reservoir models moderate non-Markovian effects prolong the separability time scales. However, in these models the properties of the stationary state may differ. In the two-reservoir model the initial entanglement is completely lost and both modes are finally uncorrelated. In a common reservoir both modes interact indirectly via the coupling to the same bath variables. Below a critical bath temperature entanglement between the two modes is preserved even in the steady state. A separability criterion is derived, which depends on the bath temperature and the response function of the open quantum system. Thus, the extended quantum Brownian motion model of a two-mode continuous variable system in a common reservoir provides an example of environment-induced entanglement.

Environment-induced disentanglement of the Einstein–Podolsky–Rosen system

We study the entanglement dynamics of the Einstein, Podolsky and Rosen (EPR) system in which two spatially separated particles are coupled with their own cavity field. The environment-induced entanglement sudden death depends on the parameters of an initial entangled state and a mean photon number as well. We moreover investigate the time-evolution behaviour for both concurrence and energy transfer between the EPR particles and the environment, from which it is found that the maximal concurrence indeed corresponds to the minimal energy of the EPR system. However, their time variations are neither in one-to-one correspondence nor in step. In particular, when the state of two particles becomes disentangled, the time variation of energy transfer still exists.

Entanglement within the quantum trajectory description of open quantum systems.

The degree of entanglement in an open quantum system varies according to how information in the environment is read. A measure of this contextual entanglement is introduced based on quantum trajectory unravelings of the open system dynamics. It is used to characterize the entanglement in a driven quantum system of dimension 2 x infinity where the entanglement is induced by the environmental interaction. A detailed mechanism for the environment-induced entanglement is given.