Mary M. Cola

Remote state preparation and teleportation in phase space

Continuous variable remote state preparation and teleportation are analysed using Wigner functions in phase space. We suggest a remote squeezed state preparation scheme between two parties sharing an entangled twin beam, where homodyne detection on one beam is used as a conditional source of squeezing for the other beam. The scheme also works with noisy measurements, and provides squeezing if the homodyne quantum efficiency is larger than 50%. The phase space approach is shown to provide a convenient framework to describe teleportation as a generalized conditional measurement, and to evaluate relevant degrading effects, such the finite amount of entanglement, the losses along the line and the nonunit quantum efficiency at the sender location.

Teleportation of bipartite states using a single entangled pair

A class of quantum protocols to teleport bipartite (entangled) states of two qubits is suggested. Our schemes require a single entangled pair shared by the two parties and the transmission of three bits of classical information, as well as a two-qubit gate with an additional qubit at the receivers location. Noisy quantum channels are considered and the effects on both the teleportation fidelity and the entanglement of the replica are evaluated.

A quantum model for collective recoil lasing

Free Electron Laser (FEL) and Collective Atomic Recoil Laser (CARL) are described by the same model of classical equations for properly defined scaled variables. These equations are extended to the quantum domain describing the particles motion by a Schrodinger equation coupled to a self-consistent radiation field. The model depends on a single collective parameter which represents the maximum number of photons emitted per particle. We demonstrate that the classical model is recovered in the limit >> 1, in which the Wigner function associated to the Schrodinger equation obeys to the classical Vlasov equation. On the contrary, for ≤ 1, a new quantum regime is obtained in which both FELs and CARLs behave as a two-state system coupled to the self-consistent radiation field and described by Maxwell-Bloch equations.

Quantum-state engineering assisted by entanglement

We suggest a general scheme for continuous variable quantum-state engineering based on conditional measurements carried out on entangled twin beam of radiation. Realistic detection schemes such as on/off photodetection, homodyne detection, and joint measurement of two-mode quadratures are analyzed in detail. Imperfections of the apparatuses, such as nonunit quantum efficiency and finite resolution, are taken into account. We show that conditional on/off photodetection provides a reliable scheme to verify nonclassicality, whereas conditional homodyning represents a tunable and robust source of squeezed light. We also describe optical continuous variable teleportation as a conditional measurement, and evaluate the degrading effects of finite amount of entanglement, decoherence due to losses, and nonunit quantum efficiency.

Robust generation of entanglement in Bose-Einstein condensates by collective atomic recoil

We address the dynamics induced by collective atomic recoil in a Bose-Einstein condensate in the presence of radiation losses and atomic decoherence. In particular, we focus on the linear regime of the lasing mechanism, and analyze the effects of losses and decoherence on the generation of entanglement. The dynamics is that of three bosons, two atomic modes interacting with a single-mode radiation field, coupled with a bath of oscillators. The resulting three-mode dissipative Master equation is solved analytically in terms of the Wigner function. We examine in details the two complementary limits of high-Q cavity and bad cavity, the latter corresponding to the so-called superradiant regime, both in the quasiclassical and quantum regimes. We found that three-mode entanglement as well as two-mode atom-atom and atom-radiation entanglement is generally robust against losses and decoherence, thus making the present system a good candidate for the experimental observation of entanglement in condensate systems. In particular, steady-state entanglement may be obtained both between atoms with opposite momenta and between atoms and photons.

Recoil-induced subradiance in an ultracold atomic gas

Subradiance, i.e., the cooperative inhibition of spontaneous emission by destructive interatomic interference, can be realized in a cold atomic sample confined in a ring cavity and lightened by a two-frequency laser. The atoms, scattering the photons of the two laser fields into the cavity mode, recoil and change their momentum. Under proper conditions the atomic initial momentum state and the first two momentum recoil states form a three-level degenerate cascade. A stationary subradiant state is obtained after the scattered photons have left the cavity, leaving the atoms in a coherent superposition of the three collective momentum states. Both a semiclassical description of the process and the quantum subradiant state with its Wigner function are given. Antibunching, quantum correlations, and entanglement between the atomic modes of the subradiant state are demonstrated.

Superradiant light scattering from a moving Bose–Einstein condensate

We investigate the interaction of a moving BEC with a far detuned laser beam. Superradiant Rayleigh scattering arises from the spontaneous formation of a matter-wave grating due to the interference of two wavepackets with different momenta. The system is described by the CARL-BEC model which is a generalization of the Gross–Pitaevskii model to include the self-consistent evolution of the scattered field. The experiment gives evidence of a damping of the matter-wave grating which depends on the initial velocity of the condensate. We describe this damping in terms of a phase-diffusion decoherence process, in good agreement with the experimental results. � 2004 Elsevier B.V. All rights reserved. PACS: 42-50.Fx; 42.50.Vk; 03.75.)b

Radiation to atom quantum mapping by collective recoil in a Bose-Einstein condensate

We propose a scheme to realize radiation to atom continuous variable quantum mapping, i.e., to teleport the quantum state of a single mode radiation field onto the collective state of atoms with a given momentum out of a Bose– Einstein condensate. The atoms-radiation entanglement needed for the teleportation protocol is established through the interaction of a single mode with the condensate in the presence of a strong far off-resonant pump laser, whereas the coherent atomic displacement is obtained by the same interaction with the radiation in a classical coherent field. 2003 Elsevier B.V. All rights reserved. PACS: 03.67.Mn; 03.75.Gg

Transverse Effects in Collective Atomic Recoil Lasing

We investigate transverse effects in collective atomic recoil lasing (CARL), where a cold atomic sample is lightened by a far detuned laser beam resonant with the internal atomic transition. The gradient force of the scattered radiation field produces a collective self-focusing on the atoms, which could be observed in a Bose-Einstein condensate stored in a bidirectional optical ring cavity or in the superradiant CARL-BEC regime.

Entanglement in a Bose–Einstein condensate by collective atomic recoil

We address the interaction between a Bose–Einstein condensate and a single-mode quantized radiation field in the presence of a strong far off-resonant pump laser. The generation of atom–atom and atom–field entanglement is demonstrated in the linear regime. The effects of cavity losses are taken into account and an analytic solution of the corresponding master equation is given in terms of the Wigner function of the system.