Twisted Cavity Is a One-Way Light Path
At the microscopic scale, most systems would behave the same if we ran the cosmic clock in reverse. But this time-reversal symmetry is broken in certain cases, such as for electrons inside materials exhibiting quantum Hall effects. A new optical device breaks time-reversal symmetry for photons using a twisted optical cavity containing spin-polarized atoms. Unlike previous time-asymmetric optical devices, this design is compatible with quantum simulation experiments, in which trapped photons are made to behave like massive, interacting particles.
Breaking time reversal for photons is well known from the Faraday effect—a rotation in the polarization of light passing through certain magnetized materials. The effect is based on a difference in the speed of left- and right-circularly-polarized waves, which are time-reversed partners. The Faraday effect is used in optical isolators that let light pass in the forward direction but block it from going backwards. However, this approach can’t work in a quantum simulation experiment since the materials are too lossy to be placed inside an optical cavity.
It is possible to generate a low-loss Faraday effect using a spin-polarized atomic gas, but such an effect cancels out for photons passing back and forth through the gas inside a conventional two-mirror optical cavity. The solution devised by Ningyuan Jia and colleagues from the University of Chicago is to use a twisted four-mirror cavity that has a different resonant energy for left- and right-circular polarization. Along one stretch of the cavity, the researchers placed spin-polarized rubidium atoms, which induced a small Faraday rotation in photons each time they passed through. As a result of the twisting and the atoms, the cavity attenuated backward-going photons by 99%.
This research is published in Physical Review A.
–Michael Schirber
Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.