Experimental results of upstream and
downstream motion. Credit: Physical Review A (2026). DOI: 10.1103/lwyj-7m5f
Just as a leaf drifts along with a
stream, objects in other moving fluids normally drift along with the flow. That
is, unless they exert energy to move against it. Although it may be less
intuitive, light waves or photons work similarly. To move against a stream of
light, an object or particle, like a photon, must either have an external force
acting on it or actively use energy to move upstream.
In a new study, published in Physical Review A, a team of physicists
demonstrates how a beam of light can "swim" upstream in a quantum
fluid of light by breaking action-reaction symmetry and reshaping how the
surrounding forces affect the flow.
Non-reciprocal interactions and upstream motion
Matter is not in equilibrium when
it is actively creating a force to oppose motion, like a motorized boat moving
up a river. Newton's third law of motion states that for every action, there is
an equal and opposite reaction. This is also known as action-reaction symmetry.
Action-reaction symmetry is not
broken just because something is out of equilibrium, but this symmetry can be
broken when particles exhibit non-reciprocal interactions. In these
interactions, one object influences another differently than the other influences
it. In non-reciprocal interactions, internal interactions can be converted into
net momentum.
Scientists have proposed that these
unusual interactions could create active behavior in quantum-like systems.
Upstream motion has 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.
A swimming light beam driven by nonreciprocal interactions
The team involved in the new study
combined theoretical scattering analysis, computer simulations and laboratory
experiments. They sent two timed laser beams through a nonlinear crystal, with
one representing the fluid and the other representing the swimmer. By tilting
the fluid beam, they were able to control its transverse flow direction and
speed. In their 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.
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 that on the other side. By
adjusting fluid speeds and densities, they also found that the upstream motion
was strongest at intermediate fluid speeds and densities, rather than at the
highest or lowest values.
The experiments involved an optical
analog of a quantum fluid, not a demonstration using material quantum fluids
such as ultracold atoms or liquid helium. However, the study offers a test bed
for studying active-like behavior under non-reciprocal interactions. Future
research could test whether related effects occur in other quantum platforms,
such as atomic gases or engineered quantum materials.
The study authors write, "This
study advances the fundamental understanding of nonreciprocal interactions in
the quantum regime, providing key insights for designing novel quantum devices
with active functionalities."
Source: Light beam 'swims' upstream through a quantum fluid by violating Newton's third law
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