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arXiv:cond-mat/0006349 · Acta Physica Polonica B

Decoherence and long-lived Schroedinger cats in BEC

Diego A. R. Dalvit, Jacek Dziarmaga, Wojciech Hubert Zurek

Published Jun 22, 2000• 0 citations
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Abstract

We consider quantum superposition states in Bose-Einstein condensates. A decoherence rate for the Schrödinger cat is calculated and shown to be a significant threat to this macroscopic quantum superposition of BEC's. An experimental scenario is outlined where the decoherence rate due to the thermal cloud is dramatically reduced thanks to trap engineering and "symmetrization" of the environment. We show that under the proposed scenario the Schroedinger cat belongs to an approximate decoherence-free pointer subspace. This paper is a simplified and more pedagogical version of our previous work, Phys.Rev. A62, 13607 (2000).

DOI
10.48550/arxiv.cond-mat/0006349

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We consider quantum superposition states in Bose-Einstein condensates. A decoherence rate for the Schrödinger cat is calculated and shown to be a significant threat to this macroscopic quantum superposition of BEC's. An experimental scenario is outlined where the decoherence rate due to the thermal cloud is dramatically reduced thanks to trap engineering and "symmetrization" of the environment. We show that under the proposed scenario the Schroedinger cat belongs to an approximate decoherence-free pointer subspace. This paper is a simplified and more pedagogical version of our previous work, Phys.Rev. A62, 13607 (2000).
We consider a quantum superposition of Bose-Einstein condensates (BEC's) in two immiscible internal states. A decoherence rate for the resulting Schr\"odinger cat is calculated and shown to be a significant threat to this macroscopic quantum superposition of BEC's. An experimental scenario is outlined where the decoherence rate due to the thermal cloud is dramatically reduced thanks to trap engineering and ``symmetrization'' of the environment, which allow the Schr\"odinger cat to be an approximate pointer state.
We study the possibility to create many-particle Schrödinger cat-like states by using a Feshbach resonance to reverse the sign of the scattering length of a Bose-Einstein condensate trapped in a double-well potential. To address the issue of experimental verification of coherence in the cat-like state, we study the revival of the initial condensate state in the presence of environmentally-induced decoherence. As a source of decoherence, we consider the interaction between the atoms and the electromagnetic vacuum, due to the polarization induced by an incident laser field. We find that the resulting decoherence is directly related to the rate at which spontaneously scattered photons carry away sufficient information to distinguish between the two atom-distributions which make-up the cat state. We show that for a 'perfect' cat-state, a single scattered photon will bring about a collapse of the superposition, while a less-than-perfect cat-like state can survive multiple scatterings before collapse occurs. In addition, we study the dephasing effect of atom-atom collisions on the cat-like states.
Quantum superpositions of macroscopically distinguishable states having distinct phases can be created with a Bose-Einstein condensate trapped in a periodic potential. The experimental signature is contained in the phase distribution of the interference patterns obtained after releasing the traps. Moreover, in the double-well case, this distribution exhibits a dramatic dependence on the parity of the total number of atoms. We finally show that, for single-well occupations up to a few hundred atoms, the macroscopic quantum superposition can be robust enough against decoherence to be experimentally revealable within current technology.
"As a rule, the coupling of a quantum system to an uncontrollable thermal reservoir (a “bath”) gives rise to the system’s decoherence, i.e., the destruction of its unitary coherent evolution [1]. Not less common is the rule that the more complex the quantum system, the more detrimental are the bath effects [2]. Here we point out that the coupling of a complex quantum system to a bath may actually induce advantageous coherent dynamics. Namely, an exact solution for a quantum many-body system characterized by large angular momentum (or spin) that is coupled to a thermal bath reveals a hitherto unexplored general effect: bath-induced effectively nonlinear evolution. This evolution can drive the large-spin system into a macroscopic quantum-superposition (“Schroedinger-cat”) state [3, 4]. Such counter-intuitive bath-induced effects should be observable in various setups on long (Markovian) time scales. They change our perspective of non-classicality in open many-body quantum systems. Namely, the bath may cause rather than impede the formation of distinctly non-classical Schroedinger-cat states, despite their fragility in the presence of a bath [2, 5]."