Comment on "Coherent Ratchets in Driven Bose-Einstein Condensates"

TL;DR

This paper critiques a previous claim that noninteracting quantum particles can exhibit directed ratchet currents under time-reversal symmetry, clarifying that such symmetry implies zero current.

Contribution

It provides a correction to prior work by demonstrating that time-reversal symmetry necessarily leads to zero ratchet current in the studied system.

Findings

Time-reversal symmetry implies zero ratchet current.

Previous claim of finite current under symmetry is incorrect.

Clarifies the theoretical understanding of quantum ratchets.

Abstract

C. E. Creffield and F. Sols (Phys. Rev. Lett. 103, 200601 (2009)) recently reported finite, directed time-averaged ratchet current, for a noninteracting quantum particle in a periodic potential even when time-reversal symmetry holds. As we explain in this Comment, this result is incorrect, that is, time-reversal symmetry implies a vanishing current.