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Physical Review A

Macroscopic quantum superposition states in Bose-Einstein condensates: Decoherence and many modes

P. Louis, P. M. R. Brydon, Craig M. Savage

Published Oct 12, 2001• 36 citations
Publisher

Abstract

We investigate the feasibility of a particular scheme for generating macroscopic quantum superposition states in two-species dilute gas Bose-Einstein condensates. The scheme utilizes two-body interactions and Josephson coupling between the species. We report numerical studies that extend a previous two-mode model to include dissipation and extra modes.

DOI
10.1103/physreva.64.053613

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We investigate the feasibility of a particular scheme for generating macroscopic quantum superposition states in two-species dilute gas Bose-Einstein condensates. The scheme utilizes two-body interactions and Josephson coupling between the species. We report numerical studies that extend a previous two-mode model to include dissipation and extra modes.
We use a simple two-mode model to investigate the quantum state dynamics of a two-species Bose-Einsten condensate, such as that produced in recent experiments, undergoing weak Josephson coupling. We find that in certain parameter regimes the quantum state of the system evolves into a macroscopic superposition of two states which differ in the atom number difference between the two species. The ``size'' of the macroscopic superposition created by such a method can be varied by adjusting the Josephson coupling coefficient, and is found to be near maximal for a certain critical value of this coefficient.
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.
Due to their coherence properties, dilute atomic gas Bose-Einstein condensates seem a versatile platform for controlled creation of mesoscopically entangled states with a large number of particles and allow controlled studies of their decoherence. However, the creation of such states intrinsically involves many-body quantum dynamics that cannot be captured by mean-field theory and, thus, invalidates the most common methods for the description of condensates. We follow up on a proposal in which a condensate cloud as a whole is brought into a superposition of two different spatial locations by mapping entanglement from a strongly interacting Rydberg atomic system onto the condensate using off-resonant laser dressing [Phys. Rev. Lett. 115 040401 (2015)]. A variational many-body ansatz akin to recently developed multiconfigurational methods allows us to model this entanglement mapping step explicitly, while still preserving the simplicity of mean-field physics for the description of each branch of the superposition. In the second part of the article, we model the decoherence process due to atom losses in detail. Altogether we confirm earlier estimates that tightly localized clouds of 400 atoms can be brought into a quantum superposition of two locations about $3\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ apart and remain coherent for about 1 ms.
We present a scheme to prepare generalized coherent states in a system with two species of Bose-Einstein condensates. First, within the two-mode approximation, we demonstrate that a Schrodinger cat-like can be dynamically generated and, by controlling the Josephson-like coupling strength, the number of coherent states in the superposition can be varied. Later, we analyze numerically the dynamics of the whole system when interspecies collisions are inhibited. Variables such as fractional population, Mandel parameter and variances of annihilation and number operators are used to show that the evolved state is entangled and exhibits sub-Poisson statistics.
Macroscopic superposition states enable fundamental tests of quantum mechanics and hold a huge potential in metrology, sensing, and other quantum technologies. We propose to generate macroscopic superposition states of a large number of atoms in the ground state of a spin-1 Bose-Einstein condensate. Measuring the number of particles in one mode prepares with large probability highly entangled macroscopic superposition states in the two remaining modes. The macroscopic superposition states are heralded by the measurement outcome. Our protocol is robust under realistic conditions in current experiments, including finite adiabaticity, particle loss, and measurement uncertainty.