Observation of a Bilayer Superfluid with Interlayer Coherence

TL;DR

This paper reports the experimental realization of a bilayer superfluid with tunable interlayer coherence in ultracold Bose gases, demonstrating a phase transition to a superfluid state mediated by interlayer coupling.

Contribution

It introduces a controllable bilayer system of 2D ultracold Bose gases and provides direct evidence of interlayer-coupled superfluidity through correlation measurements and phase diagram mapping.

Findings

Observation of a crossover from short-range to quasi-long-range order.

Direct evidence of bilayer superfluidity mediated by interlayer coupling.

Suppressed vortex excitations in the antisymmetric mode.

Abstract

Controlling the coupling between different degrees of freedom in many-body systems is a powerful technique for engineering novel phases of matter. We create a bilayer system of two-dimensional (2D) ultracold Bose gases and demonstrate the controlled generation of bulk coherence through tunable interlayer Josephson coupling. We probe the resulting correlation properties of both phase modes of the bilayer system: the symmetric phase mode is studied via a noise-correlation method, while the antisymmetric phase fluctuations are directly captured by matter-wave interferometry. The measured correlation functions for both of these modes exhibit a crossover from short-range to quasi-long-range order above a coupling-dependent critical point, thus providing direct evidence of bilayer superfluidity mediated by interlayer coupling. We map out the phase diagram and interpret it with renormalization-group theory and Monte Carlo simulations. Additionally, we elucidate the underlying mechanism through the observation of suppressed vortex excitations in the antisymmetric mode.