Transparency, Nonreciprocity and Nonclassicality in Chiral Waveguide Quantum Electrodynamics

We examine transmission and reflection characteristics of qubits coupled to a chiral waveguide for arbitrary input powers. In particular, we focus on the existence of transparency, nonreciprocity, and quantum fluctuations of the output field. The quantumness is imparted by the qubits. When two qubits are separated by half-integer multiples of the drive wavelength and assigned antisymmetric detunings, the system exhibits perfect transparency, remarkably independent of input power. Additionally, we uncover strong nonreciprocal effects in both transmission and quantum fluctuations. Forward propagation amplifies the quantum fluctuations, while backward propagation significantly suppresses them. When the qubits are symmetrically detuned and when the intrinsic damping is nonzero, the two-qubit system reaches a critical regime, where transmission completely vanishes at specific driving powers, giving rise to enhanced nonreciprocity. In the critical regime, we find that the second-order correlation function g⁽²⁾(0) of the reflected light drops below 1, marking the transition from classical to quantum light. These findings open new pathways for controlling light-matter interactions in chiral quantum electrodynamics, with potential applications in quantum information and nonreciprocal quantum devices.