Magnetic field-free non-reciprocal Raman amplification of fiber-guided light enabled by chirally coupled spin-polarized atoms

Non-reciprocal elements are key components for fiber-optical networks and integrated optical chips. For example, they allow one to protect lasers from harmful optical feedback and to implement optical add-drop multiplexers or cascaded quantum systems. A novel class of such non-reciprocal elements utilizes the internal spin of quantum emitters in order to break Lorentz reciprocity.

While respective isolators and circulators have been demonstrated recently, a corresponding non-reciprocal amplifier is hitherto missing. Here, we experimentally show non-reciprocal Raman amplification of light pulses using spin-polarized atoms that are chirally coupled to an optical nanofiber.

We control the direction in which amplification occurs via the Zeeman state in which the atoms are prepared. We observe an exponential increase of the optical output power with the number of atoms and obtain over 40 % single-pass gain for about 2000 atoms. In addition, we show that non-reciprocal amplification prevails in the absence of an offset magnetic field. Our results can be readily transferred to other types of nanophotonic waveguides and quantum emitters which feature circularly polarized optical transitions.