Continuous Variable Quantum Entanglement Research Articles

Continuous variable quantum entanglement in optomechanical systems: A short review

Cavity optomechanics deals with the radiation pressure induced interaction between photons and mechanical motion in a cavity. It has promising applications in quantum information science. In this review, the authors discuss quantum entanglement in this emerging area of research. After giving a brief historical background on the topic of entanglement and cavity optomechanics, measures of continuous variable entanglement are discussed somewhat in great details. This is followed by a short discussion on cavity quantum optomechanics, relevant to the topic on entanglement. Then the authors discuss most of the prominent ideas and proposals pertaining to entanglement research in cavity optomechanics up until now. The authors have emphasized the key theoretical concepts without too much rigor and provided relevant experimental details whenever deemed appropriate. Finally, the authors conclude by giving a perspective on other quantum correlations such as quantum discord and quantum synchronization.

Generation of 8.3 dB continuous variable quantum entanglement at a telecommunication wavelength of 1550 nm

Continuous variable quantum entanglement at a telecommunication wavelength of 1550 nm is experimentally generated using a single nondegenerate optical parametric amplifier based on a type-II periodically poled KTiOPO4 crystal. The triply resonant of the nondegenerate optical parametric amplifier is adjusted by tuning the crystal temperature and tilting the orientation of the crystal in the optical cavity. Einstein–Podolsky–Rosen-entangled beams with quantum correlations of 8.3 dB for both the amplitude and phase quadratures are experimentally generated. This system can be used for continuous variable fibre-based quantum communication.

Distribution of continuous variable quantum entanglement at a telecommunication wavelength over 20 km of optical fiber

The distribution of continuous variable (CV) Einstein-Podolsky-Rosen (EPR)-entangled beams at a telecommunication wavelength of 1550 nm over single-mode fibers is investigated. EPR-entangled beams with quantum entanglement of 8.3 dB are generated using a single nondegenerate optical parametric amplifier based on a type-II periodically poled KTiOPO4 crystal. When one beam of the generated EPR-entangled beams is distributed over 20 km of single-mode fiber, 1.02 dB quantum entanglement can still be measured. The degradation of CV quantum entanglement in a noisy fiber channel is theoretically analyzed considering the effect of depolarized guided acoustic wave Brillouin scattering in optical fibers. The theoretical prediction is in good agreement with the experimental results.

Distillation of Atmospherically Disturbed Continuous Variable Quantum Entanglement with Photon Subtraction**Supported by the National Natural Science Foundation of China under Grant Nos U1304613, 11574400, 11204197, 11204379 and 11074244, the National Basic Research Program of China under Grant No 2011CBA00200, and the Doctor Foundation of Education Ministry of China under Grant No 20113402110059.

Entanglement distillation is a probabilistic quantum operation which tries to repair entanglement after environment-induced decoherence. All known distillations of continuous variable entanglement always assume that the entanglement is distributed via a standard attenuating channel with fixed photon loss rate, which is fairly idealized without many realistic effects taken into account. Here we investigate the problem of distillation of entanglement transmitted via atmospheric channel, with transmittance being highly probabilistic and unfixed. Two typical distillation schemes, i.e., single-sided photon subtraction and bi-sided photon subtraction, are investigated.

Quantum correlations of two-mode Gaussian systems in a thermal environment

In the framework of the theory of open systems based on completely positive quantum dynamical semigroups, we give a description of continuous variable quantum entanglement and quantum discord for a system consisting of two non-interacting non-resonant bosonic modes embedded in a thermal environment. We study the time evolution of logarithmic negativity, which characterizes the degree of entanglement, and show that in the case of an entangled initial squeezed thermal state, entanglement suppression takes place for all the temperatures of the environment, including zero temperature. We analyze the time evolution of the Gaussian quantum discord, which is a measure of all the quantum correlations in the bipartite state, including entanglement, and show that discord decays asymptotically in time under the effect of the thermal bath. We describe also the time evolution of classical correlations and quantum mutual information, which measures the total correlations of the quantum system.

Generation of two-color continuous variable quantum entanglement at 0.8 and 1.5 μm

Two-color continuous variable quantum entanglement at 0.8 and 1.5 μm was experimentally demonstrated by using an above-threshold optical parametric oscillator based on a periodically poled KTiOPO4 crystal. The system presented here has potential applications in future quantum information networks, e.g., it can be used to made a connection between a quantum memory device based on alkaline atoms and a quantum communication device based on telecommunication optical fibers.

Quantum correlations among superradiant Bose-Einstein condensate atoms

Quantum correlations among atoms in superradiant Bose-Einstein condensates are discussed. It is shown that atoms in the superradiant atomic condensate can exhibit continuous variable quantum entanglement analogous to Einstein-Podolsky-Rosen (EPR)-type quantum correlations. Comparison to quantum entanglement in the Dicke model in thermal equilibrium is provided.

The quantum state of a laser field

Optical implementations of quantum communication protocols typically involve laser fields. However, the standard description of the quantum state of a laser field is surprisingly insufficient to understand the quantum nature of such implementations. In this paper, we give a quantum information-theoretic description of a propagating continuous-wave laser field and reinterpret various quantum-optical experiments in light of this. A timely example is found in a recent controversy about the quantum teleportation of continuous variables. We show that contrary to the claims of T. Rudolph and B.C. Sanders [Phys. Rev. Lett. {\bf 87}, 077903 (2001)], a conventional laser can be used for quantum teleportation with continuous variables and for generating continuous-variable quantum entanglement. Furthermore, we show that optical coherent states do play a privileged role in the description of propagating laser fields even though they cannot be ascribed such a role for the intracavity field.