Cambridge researchers demonstrate light beam moving upstream in quantum fluid

Researchers at the University of Cambridge have demonstrated that a narrow beam of light can move upstream through a flowing quantum fluid of polaritons, a result announced in a paper published online on Monday. The experiment, carried out in a semiconductor microcavity at cryogenic temperatures, showed the light pulse travelling against a polariton stream at speeds up to 0.3 mm s⁻¹, effectively “swimming” upstream while the surrounding fluid continued downstream. The finding overturns the usual expectation that objects are dragged along by a moving medium unless an external force is applied, and it appears to violate Newton’s third law in the conventional sense.

The work builds on a decade of research into quantum fluids of light, where photons acquire mass and interact strongly enough to behave like a superfluid. In such a state, particles can flow without friction, and disturbances normally generate drag forces that push them downstream. Earlier studies showed that light can flow without resistance, but they never observed a negative drag, where the moving object pushes the fluid forward while moving backward. Understanding this counter‑intuitive behaviour is important for both fundamental physics and emerging photonic technologies, because it reveals new ways to control light‑matter interactions at the quantum level.

In the Cambridge setup, a continuous‑wave laser created a polariton condensate that was set into motion by a gradient in the cavity’s potential. A second, weaker laser injected a narrow probe beam into the flow. High‑speed cameras recorded the probe’s trajectory, showing it bending upstream and maintaining its shape over several hundred micrometres. The team measured a reversal of the drag coefficient, confirming the negative‑drag regime predicted by recent theoretical models. Lead author Dr Lydia Chen said the result opens a path toward low‑energy optical circuits where signals can be routed against a background flow without extra power. The researchers plan to explore larger‑scale devices and to test whether similar upstream motion can be achieved in room‑temperature materials, a step that could bring quantum‑fluid concepts into practical photonic chips.