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Register a new userDissipation and fluctuations in elongated bosonic Josephson junctions
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We investigate the dynamics of bosonic atoms in elongated Josephson junctions. We find that these systems are characterized by an intrinsic coupling between the Josephson mode of macroscopic quantum tunneling and the sound modes. This coupling of Josephson and sound modes gives rise to a damped and stochastic Langevin dynamics for the Josephson degree of freedom. From a microscopic Lagrangian, we deduce and investigate the damping coefficient and the stochastic noise, which includes thermal and quantum fluctuations. Finally, we study the time evolution of relative-phase and population-imbalance fluctuations of the Josephson mode and their oscillating thermalization to equilibrium.
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We use the Bose-Hubbard Hamiltonian to study quantum fluctuations in canonical equilibrium ensembles of bosonic Josephson junctions at relatively high temperatures, comparing the results for finite particle numbers to the classical limit that is attained as $N$ approaches infinity. We consider both attractive and repulsive atom-atom interactions, with especial focus on the behavior near the T=0 quantum phase transition that occurs, for large enough $N$, when attractive interactions surpass a critical level. Differences between Bose-Hubbard results for small $N$ and those of the classical limit are quite small even when $N sim 100$, with deviations from the limit diminishing as 1/N.
We report on the experimental characterization of a spatially extended Josephson junction realized with a coherently-coupled two-spin-component Bose-Einstein condensate. The cloud is trapped in an elongated potential such that that transverse spin excitations are frozen. We extract the non-linear parameter with three different manipulation protocols. The outcomes are all consistent with a simple local density approximation of the spin hydrodynamics, i.e., of the so-called Bose-Josephson junction equations. We also identify a method to produce states with a well defined uniform magnetization.
We extend a recent method to shortcut the adiabatic following to internal bosonic Josephson junctions in which the control parameter is the linear coupling between the modes. The approach is based on the mapping between the two-site Bose-Hubbard Hamiltonian and a 1D effective Schrodinger-like equation, valid in the large $N$ (number of particles) limit. Our method can be readily implemented in current internal bosonic Josephson junctions and it improves substantially the production of spin-squeezing with respect to usually employed linear rampings.
We calculate the first two moments and full probability distribution of the work performed on a system of bosonic particles in a two-mode Bose-Hubbard Hamiltonian when the self-interaction term is varied instantaneously or with a finite-time ramp. In the instantaneous case, we show how the irreversible work scales differently depending on whether the system is driven to the Josephson or Fock regime of the bosonic Josephson junction. In the finite-time case, we use optimal control techniques to substantially decrease the irreversible work to negligible values. Our analysis can be implemented in present-day experiments with ultracold atoms and we show how to relate the work statistics to that of the population imbalance of the two modes.
We analyze the effects of the temperature on a bosonic Josephson junction realized with ultracold and dilute atoms in a double-well potential. Starting from the eigenstates of the two-site Bose-Hubbard Hamiltonian, we calculate the coherence visibility and the fluctuation of the on-site occupation number and study them as functions of the temperature. We show that, contrary to naive expectations, when the boson-boson interaction is suitably chosen thermal effects can increase the coherence visibility and reduce the on-site number fluctuation.
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