Interferometry reveals 'social behavior' of Photons

The photon is one of the particle types, called bosons, that is able to form a Bose-Einstein condensate. The ability of photons to condense in the state known as a Bose-Einstein condensate is how liquid light is derived.

To understand the physical mechanisms that control the formation of a Bose-Einstein condensate composed of light, scientists at the University of Twente (UT) investigated the Bose-Einstein condensation of photons in a controlled environment, using a Mach-Zehnder (MZ) interferometer.

The scientists specifically studied the switching behavior of the interferometer and collected the resulting measurements. By partially or completely closing the outputs of the interferometer, the scientists systematically varied the degree of dissipation and feedback in the system, which allowed them to identify the underlying physical principles that determined the formation of Bose-Einstein condensates under nonequilibrium conditions.

The researchers used a microsize mirror structure (an MZ interferometer) with channels through which photons could flow. The structure contained a channel that split in two and then rejoined as one. At the reunification point, the photons could either take a channel with a closed end or a channel with an open end.

A depiction of mirror structures with channels. Researchers at the University of Twente used interferometry to help gain understanding of the physical mechanisms that determine condensate matter’s state under controlled dissipation. Courtesy of University of Twente.

Professor Jan Klärs and his team found that the liquid light could “decide” for itself which path to take by adjusting its frequency of oscillation.

The team observed that the photons tried to stay together by choosing the path that led to the lowest losses — the channel with the closed end. When their frequency was adjusted, the photons naturally sought to minimize particle loss and destructive interference of their environment. This capability became visible when the condensation occurred under nonequilibrium conditions.

“You could call it ‘social behavior,’” Klärs said. Other types of bosons, such as fermions, “preferred” to remain separated.

The interferometer reflected light between two mirrors, similar to a laser. However, unlike a laser, the mirrors reflected a percentage of the light that made it almost impossible for photons to escape. The photons reached room temperature through the process of thermalization. When the photons traveled through the channels, they behaved like a super fluid and moved in a preferred direction, even at room temperature.

Thermalization, the researchers said, is the crucial difference between a normal laser and a Bose-Einstein condensate of photons. They said that technically speaking, the photon gas resembles a black body, where radiation is in equilibrium with matter.

The experiments showed the physical mechanisms involved in the formation of a Bose-Einstein condensate, which typically remain hidden when the system is close to thermal equilibrium. Beyond a deeper understanding of Bose-Einstein condensation, the results could be useful for the experimental realization of unconventional computing schemes for solving optimization problems based on coherent networks of condensates and lasers. Understanding the physical mechanisms that determine the state of a condensate under controlled dissipation and feedback, as demonstrated in the work of the UT team, is essential to the design of these systems.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-26087-0).