Researchers from the Skolkovo Institute of Science and Technology and the National Academy of Sciences of Belarus achieved a remarkable breakthrough in photonics. They observed a classical counterpart of the quintessential Hong-Ou-Mandel effect in quantum mechanics for the first time using a condensate of polaritons – hybrid particles made of light and matter – caught in an optical trap.
This breakthrough, reported in Physical Review B, brings researchers one step closer to creating compact quantum devices that can use light instead of electrons for computation and simulations. The study was supported by a grant (No. 24-72-10118) from the Russian Science Foundation. The research was conducted at the Skoltech Hybrid Photonics Laboratory in collaboration with the National Academy of Sciences of Belarus.
A classical quantum experiment in a new light
The Hong-Ou-Mandel (HOM) effect, a salient feature of quantum physics, was first demonstrated in 1987. In the original experiment, two identical photons reached the inputs of a beam splitter simultaneously, bunched together and exited through one output, never through different ones. The intensity correlation function measured between the outputs suggested that the photons were indistinguishable. The correlation function displayed the anticorrelation Hong-Ou-Mandel dip, indicating the degree to which the photons were identical.
The Skoltech team recreated this effect using exciton-polaritons – hybrid quasiparticles formed by strong interactions between light and electronic excitations (excitons) in a semiconductor. Like ultracold atoms forming a Bose-Einstein condensate, polaritons can condense into a single quantum state and therefore behave like a macroscopic quantum wave.
The researchers “caught” polaritons in a microscopic optical trap, splitting their light into two beams, and then directed single photons from each beam into a Hong-Ou-Mandel interferometer to check how similar the photons were. The resulting interference patterns clearly showed how the anticorrelation effect varied based on the statistical properties of the polariton condensate and the optical delay between the split beams.
When exposed to circularly polarized excitation, the condensate behaved like a stable laser, generating light with characteristic photon statistics. The Hong-Ou-Mandel dip, which is a measure of two-particle interference, matched the shape of the condensate’s coherence curve and approached the classical limit as the optical delay increased.
In the case of linearly polarized excitation, the researchers observed the “photon bunching” effect. Rather than flowing smoothly over time, the photons were often emitted in succession, as if glued together into small beams. As a result, the flow was redistributed into random intervals of higher and lower density, enhancing the system’s interference response and making the HOM dip twice as deep compared to the coherent regime.
Excitation with an elliptical polarization made the condensate’s internal spin rotate at gigahertz frequencies, causing it to enter a self-induced Larmor precession regime. Consequently, the HOM dip periodically disappeared and reappeared as the optical delay was adjusted. In other words, the condensate’s spin dynamics directly determined the moment of interference recovery. This vividly illustrates how light and matter can function as a finely controlled, single quantum system.
“We were amazed to witness the revival of two-photon interference at the Larmor precession frequency,” says Stepan Baryshev, the study author and a senior research scientist at the Skoltech Hybrid Photonics Laboratory. “This demonstrates our ability to control two-particle quantum effects by merely adjusting the polarization of the light that creates the polariton condensate.”
Bridging quantum optics and solid-state physics
Unlike ultracold atomic systems, which require complex vacuum and laser cooling systems, polariton condensates can form within inorganic semiconductor microcavities at temperatures just a few degrees above absolute zero. In the case of advanced organic materials, they can even form at room temperature. This makes polariton condensates a viable, scalable platform for studying classical and quantum effects in integrated devices.
The theoretical model that the team developed to describe their observations can be used to explore the transition between quantum and classical radiation statistics, as well as the mechanisms of coherence loss and collective quantum phenomena in controlled optical systems.
The ability to control and observe quantum interference in polariton condensates opens up exciting technological possibilities. Such systems could form the core of:
quantum simulators that use controlled light-matter states to simulate complex materials and chemical reactions;
all-optical logic gates and quantum transistors, where information is processed through interference rather than electric current;
sources of non-classical light, such as entangled photon pairs or squeezed light, useful for quantum communication and sensing;
neuromorphic and analog computing architectures, in which interacting polariton condensates mimic the behavior of neural networks or complex physical systems.
“Polaritons combine the best of both worlds,” said Pavlos Lagoudakis, a Skoltech Distinguished Professor who led the study. “They behave like particles of light, which are fast and easy to control, while interacting like particles of matter. This makes them ideal candidates for the next generation of hybrid quantum technologies.”
