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arXiv:cond-mat/9801196 · Physical Review Letters

Macroscopic Quantum Tunneling of a Bose-Einstein Condensate with Attractive Interaction

Masahito Ueda, Anthony J. Leggett

Published Feb 23, 1998• 148 citations
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Abstract

A Bose-Einstein condensate with attractive interaction can be metastable if it is spatially confined and if the number of condensate bosons ${N}_{0}$ is below a certain critical value ${N}_{\mathrm{c}}$. By applying a variational method and the instanton technique to the Gross-Pitaevskii energy functional, we find analytically the frequency of the collective excitation and the rate of macroscopic quantum tunneling. We show that near the critical point the tunneling exponent vanishes according to $({1\ensuremath{-}N}_{0}/{N}_{\mathrm{c}}{)}^{5/4}$ and that macroscopic quantum tunneling can be a dominant decay mechanism of the condensate for ${N}_{0}$ very close to ${N}_{\mathrm{c}}$.

DOI
10.1103/physrevlett.80.1576

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A Bose-Einstein condensate with attractive interaction can be metastable if it is spatially confined and if the number of condensate bosons N0{N}_{0}N0​ is below a certain critical value Nc{N}_{\mathrm{c}}Nc​. By applying a variational method and the instanton technique to the Gross-Pitaevskii energy functional, we find analytically the frequency of the collective excitation and the rate of macroscopic quantum tunneling. We show that near the critical point the tunneling exponent vanishes according to (1\ensuremath−N0/Nc)5/4({1\ensuremath{-}N}_{0}/{N}_{\mathrm{c}}{)}^{5/4}(1\ensuremath−N0​/Nc​)5/4 and that macroscopic quantum tunneling can be a dominant decay mechanism of the condensate for N0{N}_{0}N0​ very close to Nc{N}_{\mathrm{c}}Nc​.
The ground state of Bose-Einstein condensates with attractive particle interactions is metastable. One of the decay mechanisms of the condensate is a collapse by macroscopic quantum tunneling, which can be described by the bounce trajectory as a solution of the time-dependent Gross-Pitaevskii equation in imaginary time. For condensates with an electromagnetically induced gravity-like interaction, the bounce trajectory is computed with an extended variational approach using coupled Gaussian functions and is simulated numerically exact within the mean-field approach on a spacetime lattice. It is shown that the variational computations converge very rapidly to the numerically exact result with increasing number of Gaussians. The tunneling rate of the condensate is obtained from the classical action and additional parameters of the bounce trajectory. The converged variational and numerically exact results drastically improve by several orders of magnitude the decay rates obtained previously with a simple variational approach using a single Gaussian-type orbital for the condensate wave function.
We show theoretically the existence of a metastable state and the possibility of decay to the ground state through macroscopic quantum tunneling in two-component Bose-Einstein condensates with repulsive interactions. Numerical analysis of the coupled Gross-Pitaevskii equations clarifies the metastable states whose configuration preserves or breaks the symmetry of the trapping potential, depending on the interspecies interaction and the particle number. We calculate the tunneling decay rate of the metastable state by using the collective coordinate method under the WKB approximation. Then the height of the energy barrier is estimated by the saddle point solution. It is found that macroscopic quantum tunneling is observable in a wide range of particle numbers. Macroscopic quantum coherence between two distinct states is discussed; this might give an additional coherent property of two-component Bose condensed systems. Thermal effects on the decay rate are estimated.
The decay of Bose-Einstein condensates from a metastable ground state into collapse due to macroscopic quantum tunneling is investigated using a semiclassical approximation to Feynman’s path integral formalism. Applying a variational ansatz of a single Gaussian to the wave function determined by the Gross-Pitaevskii equation, a special choice of the Gaussian width parameters yields a mean-field energy functional in Hamiltonian form. The temporal spatial extension of the condensate then is described in the picture of a particle moving in an external potential. In this picture the decay of the condensate wave function is investigated using the bounce trajectory method, accounting for the action of the tunneling orbit in imaginary time and quadratic fluctuations around it, the latter described by the Gelfand-Yaglom differential equation. Additionally, the tunneling formalism is used for describing the condensate wave function by a trial function of a superposition of several Gaussians. Using this ansatz a Hamiltonian form of the mean-field energy does not exist and the formalism of describing the tunneling process has to be extended. The bounce trajectory is described in the parameter space of the Gaussian width parameters and the equations of motion for the imaginary time evolution are obtained by applying a time-dependent variational principle. The action of the bounce trajectory is investigated as a function of the number of Gaussians taken into account and the contribution of the fluctuations is evaluated using the monodromy matrix.
We study the Bose-Einstein condensation for a cloud of 7Li{}^{7}\mathrm{Li}7Li atoms with attractive nonlocal (finite-range) interaction in a harmonic trap. In addition to the low-density metastable branch, which is present also in the case of local interaction, a new stable branch appears at higher densities. For a large number of atoms, the size of the cloud in the stable high-density branch is independent of the trap size and the atoms are in a macroscopic quantum self-trapped configuration. We analyze the macroscopic quantum tunneling between the low-density metastable branch and the high-density one by using the instanton technique. Moreover, we consider the decay rate of the Bose condensate due to inelastic two- and three-body collisions.
We investigate the macroscopic quantum tunneling of the attractive Bose-Einstein condensate. Within the effective Lagrangian framework, we find bounce solutions and explicitly calculate the decay rate of the condensate trapped in a cylindrically symmetric potential. In particular, in the case where the number of condensed bosons is slightly below a certain critical number, we present a detailed analysis of the bounce solutions and discuss the approximations employed in our calculations. The effects of finite temperatures and the shape of the trapping potential are evaluated.