Transparency, nonclassicality, and nonreciprocity in chiral waveguide quantum electrodynamics

We examine quantum statistical properties of transmission and reflection from a chiral waveguide coupled to qubits for arbitrary input powers. We report on several remarkable features of output fields such as transparency, quantum nonreciprocity, and the second-order correlation function <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msup><a:mi>g</a:mi><a:mrow><a:mo>(</a:mo><a:mn>2</a:mn><a:mo>)</a:mo></a:mrow></a:msup><a:mrow><a:mo>(</a:mo><a:mn>0</a:mn><a:mo>)</a:mo></a:mrow></a:mrow></a:math> values less than unity. In particular, for two qubits detuned antisymmetrically with respect to the central waveguide frequency, we find transparency in forward transmission and in photon numbers for arbitrary values of the input powers provided the phase separation between qubits is an integer multiple of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>π</b:mi></b:math>. Values of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msup><c:mi>g</c:mi><c:mrow><c:mo>(</c:mo><c:mn>2</c:mn><c:mo>)</c:mo></c:mrow></c:msup><c:mrow><c:mo>(</c:mo><c:mn>0</c:mn><c:mo>)</c:mo></c:mrow></c:mrow></c:math> less than unity can be reached even for nonzero value of the intrinsic damping by using phase separation different from integer multiple of <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mi>π</d:mi></d:math>, marking the transition from classical to quantum light. We also uncover a different type of critical coupling regime for qubits that enables complete suppression of forward-propagating amplitude transmission at specific driving powers, giving rise to enhanced nonreciprocal effects in both transmission and quantum fluctuations in amplitudes. Forward propagation amplifies the quantum fluctuations in amplitudes, while backward propagation significantly suppresses them. These findings open pathways for controlling light-matter interactions in chiral quantum electrodynamics, with potential applications in quantum information and nonreciprocal quantum devices.