Nonlinear Optics Mediated by Chiral Waveguide QED: Generation of Momentum-anticorrelated Photon Pairs

Research Overview

This paper investigates the nonlinear optical properties of superconducting transmons chirally coupled to a one-dimensional waveguide. The authors examine how these artificial atoms manipulate the coherence and statistics of transmitted light. By analyzing the system through both semi-classical master equations and full-wavefunction quantum calculations, the study identifies a mechanism for generating and controlling incoherent photon states.

Key Findings

The researchers demonstrate that a single chiral transmon acts as a nonlinear element, converting coherent incident light—characterized by Poissonian statistics and long-range coherence—into an incoherent field with spatially localized coherence and photon bunching. This effect is attributed to a two-photon process where the transmon converts two independent incoming photons into a pair with opposite momentum shifts.

Crucially, the authors show that adding a second chiral transmon can reverse this effect, effectively acting as a filter that restores the coherence of the transmitted field. This "revival of coherence" occurs because the two transmons form a collective dark state that decouples from the waveguide. Furthermore, by introducing local driving with independently tunable amplitude and phase, the authors demonstrate that the quantum statistics of the transmitted field can be engineered to span anti-bunched, coherent, and strongly bunched regimes.

This work provides a framework for using chiral waveguide QED to engineer quantum light states. The ability to toggle between coherent and incoherent states, and to tune photon statistics, is highly relevant for quantum information processing, particularly for applications requiring deterministic quantum state transfer or the generation of specific correlated photon states. The findings highlight the potential of superconducting circuits as a versatile platform for nonlinear quantum optics.