Spin–orbit-coupling-induced broadband single-photon nonreciprocity on a solid-state platform

Nonreciprocal optical devices (NRODs) working in the quantum regime are the key building blocks for nonreciprocal quantum information processing. Conventional NRODs require a bias field, such as a strong control field or an external magnetic field, increasing the complexity in fabrication and operation. Here, we propose a bias-free nonmagnetic scheme for single-photon isolators and circulators on a solid-state platform. Our single-photon isolator is made of a chiral optical waveguide embedded with a negatively charged carbon nanotube quantum dot (CNQD), while the circulator with four ports consists of two optical waveguides and a whispering-gallery-mode optical microresonator coupling to a CNQD in a chiral way. The time-reversal symmetry in these two systems breaks because the inherent spin–orbit coupling of electrons and holes in the CNQD strongly lifts the energy degeneracies of two circularly polarized transitions. Under realistic experimental parameters, we obtain a low-loss and high-isolation-contrast single-photon isolator over a 10 gigahertz bandwidth and a circulator over a one gigahertz bandwidth, respectively, without requiring any bias (control) fields. Our bias-free design can greatly simplify the implementation and operation of NRODs and provide a compact solid-state platform for nonreciprocal quantum information processing.