Abstract
Photons are nonchiral particles: their handedness can be both left and right. However, when light is transversely confined, it can locally exhibit a transverse spin whose orientation is fixed by the propagation direction of the photons. Confined photons thus have chiral character. Here, we employ this to demonstrate nonreciprocal transmission of light at the single-photon level through a silica nanofibre in two experimental schemes. We either use an ensemble of spin-polarised atoms that is weakly coupled to the nanofibre-guided mode or a single spin-polarised atom strongly coupled to the nanofibre via a whispering-gallery-mode resonator. We simultaneously achieve high optical isolation and high forward transmission. Both are controlled by the internal atomic state. The resulting optical diode is the first example of a new class of nonreciprocal nanophotonic devices which exploit the chirality of confined photons and which are, in principle, suitable for quantum information processing and future quantum optical networks.
Highlights
Miniaturized components that control the flow of light are key to information processing in integrated optical circuits
We demonstrate that the chiral nature of photons can be exploited for the realization of an optical diode when the photons interact with spin-polarized atoms
We perform proof-of-concept experiments demonstrating nanophotonic optical isolators that operate at the single-photon level and with low loss
Summary
Miniaturized components that control the flow of light are key to information processing in integrated optical circuits. The interaction of quantum emitters with light fields that exhibit spin-orbit interaction has been observed in the strongly confined optical modes of whisperinggallery-mode (WGM) resonators [20,21] and nanoscale waveguides [22,23,24]. This opens the route towards a new class of nonreciprocal devices in which the quantum state of the emitter controls the light propagation in nanophotonic waveguides [25,26,27]. Both experiments are carried out in an effective single-photon regime, i.e., a regime where every quantum emitter interacts with at most one photon at a time
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