Quantum light swimmer breaks Newton's third

A swimmer made of quantum light can move upstream without expending its own energy by breaking Newton's third law of motion. Researchers led by Yi Hu at Nankai University in China demonstrated this effect using a fluid and a swimmer both constructed from interacting photons.

For most objects, every action has an equal and opposite reaction. In this quantum experiment, however, the interaction between the swimmer and the fluid was non-reciprocal. The swimmer experienced an attractive force towards the fluid, while the fluid felt a repulsive force from the swimmer. This imbalance created a net force that pushed the swimmer against the current.

Creating a quantum light river

The team created their quantum light fluid by firing lasers into a special crystal under an applied electric voltage. This setup made the photons interact, forming a river-like system. The swimmer was a separate beam of quantum light shaped into a single, solitary wave. Past experiments with quantum light provided the precise control needed to construct both elements.

Yi Hu says the key finding is that active behaviour does not require an intrinsically active particle. "For me, the biggest takeaway is that active behaviour does not necessarily require an intrinsically active particle or swimmer," he said.

A different mechanism for motion

Mathias Albert at Côte d’Azur University in France notes that upstream motion has been seen before in other quantum light fluids. Previous demonstrations relied on creating tiny vortices behind the swimmer, with the recoil from vortex creation providing the push. The new experiment shows a fundamentally different mechanism driven by a broken action-reaction principle.

Non-reciprocal interactions are known to shape motion in "active matter" systems, where objects like bacteria or robot swarms consume energy to move. This experiment focused on a passive object that does not consume energy yet still benefits from such interactions. The work could lead to a new type of active matter within a quantum system.

Potential for quantum devices

Albert says combining active matter concepts with quantum fluids and photonic systems is a current focus for several research groups. Controlling chunks of light to move in a preferred direction could be useful in devices that use quantum light, such as those for communications or quantum information processing.

The team now aims to build more complex experiments. They want to control the swimmer's motion more precisely or add multiple swimmers to the fluid at once.