Dynamic Stimulation of Quantum Coherence in Systems of Lattice Bosons

Thermal fluctuations tend to destroy long-range phase correlations. Consequently, bosons in a lattice will undergo a transition from a phase-coherent superfluid as the temperature rises. Contrary to common intuition, however, we show that nonequilibrium driving can be used to reverse this thermal decoherence. This is possible because the energy distribution at equilibrium is rarely optimal for the manifestation of a given quantum property. We demonstrate this in the Bose-Hubbard model by calculating the nonequilibrium spatial correlation function with periodic driving. We show that the nonequilibrium phase boundary between coherent and incoherent states at finite bath temperatures can be made qualitatively identical to the familiar zero-temperature phase diagram, and we discuss the experimental manifestation of this phenomenon in cold atoms.

Figure 1

Numerical density plots of $\mathrm{Re}{\mathcal{G}}_{00}^K$ for bath temperature $\beta U=25$, coupling width $\Lambda /U=3$, and strength $\eta =0.01$. (a) Equilibrium: $V=0$. (b) Superfluid enhancement: $V/U=0.1$, $\hslash \Omega /U=0.05$, $\mathbf{k}=\frac{\pi }{a}\mathbf{x}$, $n_p=5$. (c) Superfluid enhancement: $V/U=0.1$, $\hslash \Omega /U=0.005$, $\mathbf{k}=\frac{\pi }{a}\mathbf{x}$, $n_p=5$. Note the similarity to the $T=0$ equilibrium diagram.

Figure 2

The $T=0.04U$ equilibrium phase boundary (dotted line) plotted with examples from different regions of driving parameter space.