Superluminal Spacetime Boundary, Time Reflection and Quantum Light Generation From Relativistic Plasma Mirrors

A plasma mirror is an optical device for high‐power, ultrashort‐wavelength electromagnetic fields, utilizing a sheet of relativistic oscillating electrons to generate and manipulate light. This work proposes that the spatiotemporally varying plasma oscillation, induced by an ultra‐high‐intensity laser beam, functions as a “spacetime mirror” with significant potential for exploring quantum light. This study finds that the spacetime mirror exhibits several exotic features: i) a superluminal spacetime boundary, ii) time reflection and refraction, and iii) quantum light sources with pair generation via vacuum squeezing. These theoretical and simulation results are in excellent agreement, and experimental verification is underway. This work demonstrates the interplay with emerging fields such as time‐varying media, suggesting the plasma mirror as an ideal platform to study strong‐field quantum optics at extremes.