Swimming against a superfluid flow: Self-propulsion via vortex-antivortex shedding in a quantum fluid of light

TL;DR

This paper demonstrates how a finite-mass impurity in a flowing quantum fluid of light can self-propel against the flow by emitting vortex-antivortex pairs, linking quantum fluid dissipation to active-matter propulsion.

Contribution

It introduces a novel mechanism of impurity self-propulsion in a quantum fluid of light via vortex shedding, supported by a quantitative point-vortex model.

Findings

Impurity can swim against superfluid flow by vortex-antivortex emission.

The propulsion mechanism depends on impurity shape and flow velocity.

Vortex backreaction enables passive locomotion in quantum fluids.

Abstract

A superfluid flows without friction below a critical velocity, exhibiting zero drag force on impurities. Above this threshold, superfluidity breaks down, and the internal energy is redistributed into incoherent excitations such as vortices. We demonstrate that a finite-mass, mobile impurity immersed in a flowing two-dimensional paraxial superfluid of light can \textit{swim} against the superfluid current when this critical velocity is exceeded. This self-propulsion is achieved by the periodic emission of quantized vortex-antivortex pairs downstream, which impart an upstream recoil momentum that results in a net propulsive force. Analogous to biological systems that minimize effort by exploiting wake turbulence, the impurity harnesses this vortex backreaction as a passive mechanism of locomotion. Reducing the impurity dynamics to the motion of its center of mass and using a point-vortex model, we quantitatively describe how this mechanism depends on the impurity geometry and the surrounding flow velocity. Our findings establish a fundamental link between internal-energy dissipation in quantum fluids and concepts of self-propulsion in active-matter systems, and opens new possibilities for exploiting vortices for controlled quantum transport at the microscale.