Counterflow superfluidity in a two-component Mott insulator

Abstract

Counterflow superfluidity is an anomalous quantum phase that was predicted two decades ago in the context of a two-component Bose–Hubbard model. In this phase, although both components exhibit fluidity, their correlated counterflow currents cancel each other out, resulting in the system behaving as an incompressible Mott insulator. However, realizing and identifying this phase experimentally has proven challenging due to the stringent requirements for a single set-up, including defect-free state preparation, minimal heating during coherent manipulations, and spin- and site-resolved detection of the phases. Here, we report on the observation of counterflow superfluidity in a binary Bose mixture in optical lattices. After preparing a low-entropy spin-Mott state by conveying two spin-1/2 bosonic atoms at every single lattice site to form a doublon, we adiabatically drove the system to the counterflow superfluid phase at approximately 1 nK. We observed features of antipair correlations through site- and spin-resolved quantum-gas microscopy in both real and momentum spaces. Finally, we measured long-range off-diagonal spin correlations in the rotated basis, revealing a correlation length approaching the system size. These techniques and observations demonstrated here provide accessibility to Borromean counterfluids.