Controllable Slow-to-Fast-Light System in Waveguide Quantum Electrodynamics

Slowing down the speed of light is crucial for quantum memory and synchronization in quantum processors. We demonstrate a controllable slow-to-fast-light system in the microwave regime using waveguide quantum electrodynamics. Our setup involves a high-coherence superconducting artificial atom coupled to the end of a transmission line with a low relaxation rate (around 2 MHz). This results in a π phase shift within a narrow linewidth, causing light pulse to slow down by 273 ns while maintaining 86% energy efficiency.

Furthermore, through the utilization of the artificial atom's higher-level Autler-Townes splitting, we can manipulate the group delay, allowing us to transition from negative to positive delays by adjusting the control tone power of the higher-level transition. This effect also enables us to switch the slow/fast-light phenomenon on and off as desired.