Enhanced quantum spin fluctuations in a binary Bose-Einstein condensate

TL;DR

This paper proposes a novel regime for enhancing quantum spin fluctuations in a dilute two-component Bose-Einstein condensate by engineering effective attractive interactions, enabling experimental observation without collapse risks.

Contribution

It introduces a new method to significantly amplify quantum spin fluctuations in BECs through engineered interactions, avoiding collapse and enabling testing of beyond-mean-field theories.

Findings

Quantum spin fluctuations are significantly enhanced in the proposed regime.

The regime is experimentally accessible via Bragg spectroscopy.

Theoretical predictions match numerical simulations for the immiscibility transition.

Abstract

For quantum fluids, the role of quantum fluctuations may be significant in several regimes such as when the dimensionality is low, the density is high, the interactions are strong, or for low particle numbers. In this paper we propose a fundamentally different regime for enhanced quantum fluctuations without being restricted by any of the above conditions. Instead, our scheme relies on the engineering of an effective attractive interaction in a dilute, two-component Bose-Einstein condensate (BEC) consisting of thousands of atoms. In such a regime, the quantum spin fluctuations are significantly enhanced (atom bunching with respect to the noninteracting limit) since they act to reduce the interaction energy - a remarkable property given that spin fluctuations are normally suppressed (anti-bunching) at zero temperature. In contrast to the case of true attractive interactions, our approach is not vulnerable to BEC collapse. We numerically demonstrate that these quantum fluctuations are experimentally accessible by either spin or single-component Bragg spectroscopy, offering a useful platform on which to test beyond-mean-field theories. We also develop a variational model and use it to analytically predict the shift of the immiscibility critical point, finding good agreement with our numerics.