One-sided photon interactions pushed a light beam upstream through a quantum fluid of light

The study, "Active upstream motion in a quantum fluid of light arising from nonreciprocal interactions" by Siyu Li et al, was published in Physical Review A (2026), DOI 10.1103/lwyj-7m5f.

Yi Hu at Nankai University in China and his colleagues carried out the work.

The team sent two timed laser beams through a nonlinear crystal, one representing the fluid and the other the swimmer; tilting the fluid beam let them control its transverse flow direction and speed.

The study authors write: "The swimmer tends to reshape the fluid, passing through it asymmetrically, which in turn exerts on the swimmer a force opposite to the fluid flow direction. Such a counterintuitive process stems from the nonreciprocity of their mutual interactions, which results in an inverted recoil compared to that in equilibrium systems."

The upstream motion is due to the asymmetric distribution of the fluid caused by the swimmer: the intensity of the fluid on one side of the swimmer is higher than on the other.

Upstream motion had been demonstrated in previous experiments, but the mechanism involved vortex shedding, which imparts a net upstream recoil momentum and still obeys Newton's third law; the new study is the first to use non-reciprocal interactions, violating action-reaction symmetry.

Mathias Albert at Cote d'Azur University in France says upstream motion has previously been achieved in other quantum fluids of light, but it always relied on creating tiny vortices behind the swimmer in the fluid, with the recoil from their creation pushing the swimmer, so the new experiment demonstrates a different mechanism for motion.

Action-reaction symmetry is not broken just because something is out of equilibrium, but it can be broken when particles exhibit non-reciprocal interactions, in which internal interactions can be converted into net momentum.

Non-reciprocal interactions are known to be crucial for shaping the motion of active matter systems where objects consume energy to move, such as bacterial mixtures, flocks of birds or swarms of tiny robots.

In the experiments, the narrow laser beam acting as the swimmer moved opposite to the flow of a broader optical fluid.

In the usual reciprocal setup, the beam instead moved downstream, providing a direct comparison.

By adjusting fluid speeds and densities, the team found the upstream motion was strongest at intermediate fluid speeds and densities, rather than at the highest or lowest values.

The team combined theoretical scattering analysis, computer simulations and laboratory experiments.

Albert says: "This question is very much in the air at the moment, with several groups exploring how the concepts of active matter can be combined with quantum fluids and photonic systems."

The experiments involved an optical analog of a quantum fluid, not a demonstration using material quantum fluids such as ultracold atoms or liquid helium.

Hu says: "For me, the biggest takeaway is that active behaviour does not necessarily require an intrinsically active particle or swimmer."

The experiments by Hu and his colleagues focused on an object that does not consume energy yet still benefited from non-reciprocal interactions, and their work could usher in the creation of a new type of active matter in a quantum system.

Neither published account reports a momentum budget for the crystal or the pump light; the non-reciprocity described is between the swimmer beam and the fluid beam.

The reported contrast between the two configurations is a reversal of travel direction: upstream when the interaction is non-reciprocal, downstream when it is reciprocal.

Hu says the team now wants to construct more complex experiments where they could control the swimmer's motion more precisely or add several swimmers to the fluid at once.

Future research could test whether related effects occur in other quantum platforms, such as atomic gases or engineered quantum materials.

Albert says that, eventually, the ability to control chunks of light so they move in some preferred direction could be useful within devices that use quantum light, such as those used in communications or quantum information processing.