Quantum Nonreciprocity with Nonlinearity and Weyl semimetals

The emerging field of quantum computing has been rapidly growing and has shown interesting opportunities to overcome the limitations of classical computers for many currently unfeasible problems. A key technology that will be required for quantum computation devices is the unidirectional signal propagation and routing, whereby electromagnetic radiation propagates asymmetrically between two points. Most modern nonreciprocal components are realized based on the magneto-optical effect in ferrite materials. These devices are expensive, barely tunable, bulky, and incompatible with planar technologies, including transmission-line quantum circuits. In this work, we present our recent results on isolators suitable for quantum systems. We first discuss the isolation effect obtained by a suitable combination of quantum nonlinearities and symmetry breaking. By an example of a two-qubit system, we show that the presence of the dark state and its properties are crucial to establish large nonreciprocity in this class of systems. Then we discuss a novel approach to tunable isolation based on twisted bilayered Weyl semimetals. The approach enables highly efficient tuning of both direction and value of isolation with the relative rotation of Weyl semimetals.