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arXiv:quant-ph/9910073 · International Journal of Modern Physics B

ENTANGLEMENT BETWEEN BOSE–EINSTEIN CONDENSATES

Yu Shi

Published Sep 10, 2001• 19 citations

Abstract

For a Bose condensate in a double-well potential or with two Josephson-coupled internal states, the condensate wavefunction is a superposition. Here we consider coupling two such Bose condensates, and suggest the existence of a joint condensate wavefunction, which is in general a superposition of all products of the bases condensate wavefunctions of the two condensates. The corresponding many-body state is a product of such superposed wavefunctions, with appropriate symmetrization. These states may be potentially useful for quantum computation. There may be robustness and stability due to macroscopic occupation of a same single particle state. The nonlinearity of the condensate wavefunction due to particle–particle interaction may be utilized to realize nonlinear quantum computation, which was suggested to be capable of solving NP-complete problems.

DOI: 10.1142/s0217979201007154

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For a Bose condensate in a double-well potential or with two Josephson-coupled internal states, the condensate wavefunction is a superposition. Here we consider coupling two such Bose condensates, and suggest the existence of a joint condensate wavefunction, which is in general a superposition of all products of the bases condensate wavefunctions of the two condensates. The corresponding many-body state is a product of such superposed wavefunctions, with appropriate symmetrization. These states may be potentially useful for quantum computation. There may be robustness and stability due to macroscopic occupation of a same single particle state. The nonlinearity of the condensate wavefunction due to particle–particle interaction may be utilized to realize nonlinear quantum computation, which was suggested to be capable of solving NP-complete problems.

It is proposed that quantum computation can be implemented on the basis of macroscopic quantum coherence of a many-body system, especially the Bose-Einstein condensation. Since a Bose-Einstein condensate is described by a non-linear Schrödinger equation, and the non-linearity is tunable, in principle one may build a quantum computer composed of both linear and non-linear gates. Consequently NP-complete and #P problems can be solved. This idea is illustrated by representing the qubit as the atomic Bose-Einstein condensate trapped in a double-well potential.

In this paper, we investigated theoretically the dynamic effect induced by nonlinear tunneling of two weakly linked Bose–Einstein condensates in an extended boson Josephson-junction model, in which the two-body interaction is extended to neighboring lattice sites in strong interaction regime. As a consequence the dynamic properties compared with the usual Josephson-junction model are crucially modified by the atom-pair tunneling, which leads to the macroscopic quantum-states of coherent relative-phase between two condensates seen to be effective Schrödinger cat states. The relative-phase is controllable by system parameters and thus can be used to realize the qubit in quantum computation.

Dynamical evolution of quantum mechanical squeezing and entanglement in a two-mode Bose-Einstein condensate (TBEC) with an adiabatic, time-varying Raman coupling is studied by finding analytical expressions for these quantities. In particular, we study the entanglement between the atoms in the condensate as well as that between the two modes. The nature of the enhanced quantum correlations in TBEC is clarified by considering squeezing and entanglement both with and without the nonlinear interaction turned on; it is found that entanglement approaching maximal value can be achieved with the nonlinear interactions present. Somewhat counterintuitively, greater squeezing is found in the absence of nonlinear interactions. This is due to the collapses and revivals of the TBEC quantum state induced by the nonlinear interactions. In addition, results involving the self-trapping phase state of TBEC indicate potential for creating a dynamically stable, macroscopic entangled quantum state which is relatively robust with respect to atom number fluctuations.