Magnetic-free unidirectional polarization rotation and free-space optical isolators and circulators

Optical nonreciprocity is an essential concept in optical signal processing and communication. The traditional way to achieve nonreciprocity is using magneto-optical nonreciprocal polarization rotation, which is incompatible with existing on-chip integration due to the applied magnetic field. Here, we theoretically and experimentally realize unidirectional polarization rotation with a magnetic-free mechanism using atomic ensembles. Free-space isolators and circulators are further formed based on the nonreciprocity of polarization rotation. As only one direction of the signal changes its polarization, the forward transmission and backward isolation can be adjusted separately. Compared with the existing magnetic-free circulators that rely on high-Q factor cavities or Mach–Zehnder interferometers, we experimentally realized circulators in free space. This dramatically simplifies the magnetic-free circulator structure and provides better stability.