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arXiv:quant-ph/0609119 · Physical Review Letters

Ultracold Bosons in a Tilted Multilevel Double-Well Potential

Dimitri R. Dounas-Frazer, Ann M. Hermundstad, Lincoln D. Carr

Published Nov 14, 2007• 94 citations
arXivHTMLPDF

Abstract

The N-body problem in a tilted double well requires new features for macroscopic quantum superposition in ultracold atoms. In particular, one needs to go beyond the single-particle ground state in each well. We provide explicit criteria for when two energy levels are needed to describe the state space. For typical experimental parameters, two levels are indeed required for the creation of macroscopic superposition states. Furthermore, we show that a small tilt causes the collapse of such states. However, partial macroscopic superposition states reappear when the tilt can be compensated by atom-atom interactions.

DOI
10.1103/physrevlett.99.200402

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The N-body problem in a tilted double well requires new features for macroscopic quantum superposition in ultracold atoms. In particular, one needs to go beyond the single-particle ground state in each well. We provide explicit criteria for when two energy levels are needed to describe the state space. For typical experimental parameters, two levels are indeed required for the creation of macroscopic superposition states. Furthermore, we show that a small tilt causes the collapse of such states. However, partial macroscopic superposition states reappear when the tilt can be compensated by atom-atom interactions.
We study macroscopic superpositions in the orbital rather than the spatial degrees of freedom, in a three-dimensional, double-well system. We show that the ensuing dynamics of NNN interacting excited ultracold bosons, which, in general, requires at least eight single-particle modes and (N+7N)(\genfrac{}{}{0pt}{}{N+7}{N})(NN+7​) Fock vectors, is described by a surprisingly small set of many-body states. An initial state with half the atoms in each well, and purposely excited in one of them, gives rise to the tunneling of axisymmetric and transverse vortex structures. We show that transverse vortices tunnel orders of magnitude faster than axisymmetric ones and are therefore more experimentally accessible. The tunneling process generates macroscopic superpositions only distinguishable by their orbital properties and within experimentally realistic times.
We present a thorough description of the physical regimes for ultracold bosons in double wells, with special attention paid to macroscopic superpositions (MSs). We use a generalization of the Lipkin-Meshkov-Glick Hamiltonian of up to eight single particle modes to study these MSs, solving the Hamiltonian with a combination of numerical exact diagonalization and high-order perturbation theory. The MS is between left and right potential wells; the extreme case with all atoms simultaneously located in both wells and in only two modes is the famous NOON state, but our approach encompasses much more general MSs. Use of more single particle modes brings dimensionality into the problem, allows us to set hard limits on the use of the original two-mode LMG model commonly treated in the literature, and also introduces a mixed Josephson-Fock regime. Higher modes introduce angular degrees of freedom and MS states with different angular properties.
We present exact expressions for the quantum sloshing of Bose-Einstein condensates in a tilted two-well potential. Tunneling is suppressed by a small potential difference between wells, or tilt. However, tunneling resonances occur for critical values of the tilt when the barrier is high. At resonance, tunneling times on the order of 10–100 ms are possible. Furthermore, such tilted resonances lead to a dynamical scheme for creating few-body NOON-like macroscopic superposition states which are protected by the many-body wave function against potential fluctuations.
A variationally optimised basis allows an accurate description of the quantum \nbehaviour of ultra-cold atoms, even in the strongly correlated regime. A rescaling \nscheme corrects discrepancies caused by using a reduced Hilbert space. This \napproach also allows the modelling of experimentally realizable double-well potentials, \nwhich still reveals the maximally-entangled states seen in fixed basis models. \nTime dynamics of these double-well systems show macroscopic tunnelling between \nwells for bosons with a sufficient interaction strength. \nThe many-body problem of interacting bosons in the highly-correlated regime \nis difficult. The number of basis states needed to describe this quantum system \naccurately quickly grows beyond computational reach. Rescaling the interaction \nstrength proves a simple and effective method of calculating exact eigenvalues in a \nreduced Hilbert space. \nBosonic systems in the double-well potential are investigated next. First, how \ndifferent eigen-states depend on the interaction strength is examined. The variationally \noptimised method has advantages over a standard fixed basis method with \nthe ability to model experimentally viable systems and explore more stronglycorrelated \nregimes. Secondly, tunnelling dynamics in the double well are studied, \nspecifically for a system where all particles initially occupy a single well. Oscillations \ncorresponding to collective tunnelling between wells are found in regimes \nwhere there are zero interactions or bosons lie in a maximally-entangled state. \nWhat governs the dynamics outside these two regimes is also considered.
Abstract Ultracold interacting atoms are an excellent tool to study correlation functions of many-body systems that are generally eluding detection and manipulation. Herein, we investigate the ground state of bosons in a tilted triple-well potential and characterize the many-body state by the eigenvalues of its reduced one-body density matrix and Glauber correlation functions. We unveil how the interplay between the interaction strength and the tilt can be used to control the number of correlated wells as well as the fragmentation, i.e. the number of macroscopic eigenvalues of the reduced one-body density matrix.