A quantum optics laboratory

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For most objects, for every action, there is an equal and opposite reaction. But if you are swimming upstream in a river of quantum light, the normal rules don’t apply. Physicists have demonstrated that breaking Isaac Newton’s third law of motion in such a setting can be useful, leading to new ways of controlling quantum light in experiments and devices.

Moving upstream against the current is possible, as some animals and boats do, but at the cost of expending energy. Yi Hu at Nankai University in China and his colleagues wondered whether a passive swimmer that doesn’t do so could still move upstream.

The researchers studied upstream swimming in a quantum system. Because past experiments established precise ways of controlling quantum light, they used it to make both their river and swimmer. They created this quantum light fluid by shooting lasers into a special crystal that was exposed to an electric voltage, which made the photons interact and form a river-like system. The swimmer was another beam of quantum light, but this one was shaped to form a single, solitary wave. 

The action-reaction principle of Newton’s third law would have the swimmer and the river experience the same type of force – either repulsive or attractive – similar to how gravity leads to mutual attraction between our bodies and Earth. Earth attracts us downwards, and experiences an upwards attraction towards us. In contrast, in the experiment, the swimmer experienced an attractive interaction with the river, but the river experienced a repulsive force on the swimmer. This was a “non-reciprocal” interaction that breaks the action-reaction principle. Its net effect was an upstream force on the swimmer, says Hu.

“For me, the biggest takeaway is that active behaviour does not necessarily require an intrinsically active particle or swimmer,” he says.

Mathias Albert at Côte d’Azur University in France says that 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 fundamentally different mechanism for motion. 

We already know that non-reciprocal interactions can 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. But the experiments by Hu and his colleagues focused on an object that doesn’t consume energy, yet still benefited from non-reciprocal interactions. Their work could therefore usher in the creation of a new type of active matter in a quantum system.

“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,” says Albert. 

He 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 devices are prevalent in, for example, communications or quantum information processing.

Hu says that 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.