Phys. Rev. Lett. 107, 173902 (2011)
Unwanted feedback caused by reflections between optical components can have a deleterious effect on quantum optical devices. Avoiding feedback requires the use of optical diodes, also known as isolators, which support the propagation of photons in one direction while prohibiting propagation in the opposite direction. Traditional solid-state optical isolators include Faraday rotators, which provide isolation by rotating the polarization of a light beam. Now, Jung-Tsung Shen and colleagues at Washington University in St Louis, USA, have demonstrated that near-complete optical isolation can be achieved at the single-photon level by coupling a quantum impurity to a passive, linear waveguide that has a locally planar, circular polarization. Their single-photon optical diode operates on individual photons, thus enabling unidirectional propagation. Furthermore, this configuration does not rely on the use of bulk nonlinear materials or quasi-phase-matching and can be implemented in various types of waveguide. The researchers have also demonstrated that the performance of their diode is not sensitive to the intrinsic dissipation of the quantum impurity.
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