Fast, low-loss all-optical phase modulation in warm rubidium vapour

We demonstrated a new way to modulate the phase of light in free space.

Phase modulators are a core building block of photonic circuits: they route, multiplex, and control weak optical signals, including single photons. On-chip modulators are fast and mature, but doing the same job for free-space beams is typically much slower, which limits high-clock-speed applications.

Our approach uses all-optical control in a warm rubidium vapour to imprint a controllable phase shift on a signal beam. Because it’s driven by light rather than electronics, the modulation is naturally fast. We also outline a path to integrate the same mechanism into hollow-core fibre, making it a promising component for large-scale quantum networks.