Classical Light Can Make Single Photons Settle Into Thermal Equilibrium, Theory Shows
Physicists in the U.S. and Germany show that intense classical waves in a nonlinear optical system can mediate effective interactions between photons that normally pass through each other.
Photons are the ultimate information carriers in modern quantum physics. They are coherent and travel at the speed limit of the universe. But those advantages come with a basic limitation: photons do not interact with one another directly. A new theoretical study shows a way around that, using the chaotic dynamics of intense classical light to push individual photons toward thermal equilibrium.
The research, published in the journal Science Advances, was carried out by scientists from Saint Louis University in the United States, the Max Born Institute in Berlin and Humboldt University of Berlin. The paper is titled "Thermalization of quantum light induced by classical nonlinear wave dynamics." The first author is Fouad Chahrour.
The problem they address is well known. When intense classical light travels through nonlinear materials, the waves affect themselves and each other, changing phase and amplitude through effects such as self-phase and cross-phase modulation. Reproducing that behavior at the level of single photons is extremely difficult, because ordinary materials have vanishingly small nonlinear responses at ultralow intensity. Individual photons usually pass right through one another, which has kept researchers from exploring many-body quantum dynamics in optical systems.
The team's idea is to avoid the weak nonlinearity of materials by sending quantum light alongside relatively intense classical light in a multimode nonlinear optical system. In numerical experiments, the classical field evolves chaotically and produces fluctuations in the effective refractive index that the quantum light experiences. The researchers found that single-photon and two-photon states in this setup do not simply keep their initial state or fall apart into noise. They thermalize.
The evidence is in the statistics of the output. The photon occupation of the system's modes matches Boltzmann and Rayleigh-Jeans distributions, the characteristic signatures of thermal equilibrium, for single-photon and two-photon light respectively. In effect, the classical waves mediate effective interactions between photons.
The authors say this offers a way to simulate complex quantum behavior in standard optical hardware, without ultrastrong single-photon nonlinear materials, which are hard to make and scale. The work reaches into statistical mechanics and condensed matter physics. It could provide a controllable testbed for open quantum systems and many-body thermodynamics, and could be used to explore richer phenomena such as aspects of quantum phase transitions in ordinary laboratory settings.
The study is theoretical and based on numerical simulation. The team has not reported a laboratory demonstration, so the next test is whether the predicted thermalization can be seen in a real multimode optical experiment.
