Quantum Optics Beyond the Visible: A New Era for X-Ray Science

Quantum optics has reshaped imaging and spectroscopy by exploiting nonclassical correlations, yet most demonstrations and applications remain in the visible and near-infrared. Bringing quantum-optical concepts to the hard x-ray regime is especially compelling because x-rays offer intrinsic access to sub-nanoscale structures and sub-attosecond dynamics, while also posing severe constraints on dose, background, and detector performance. Quantum resources such as entanglement and time-energy correlations can, in principle, enable lower-dose measurements, improved signal-to-noise in noisy environments, and new routes to spatial and temporal resolution beyond what is practical with classical x-ray approaches.

Recent experiments have begun to realize this vision. Time-energy correlations from x-ray spontaneous parametric down-conversion have been used to enhance visibility and suppress background, enabling meaningful measurements even at very low photon flux. In parallel, progress toward x-ray analogs of canonical quantum-optics tools, such as beam splitters and single-photon interferometric protocols that rely on quantum correlations, has opened the door to quantum interferometry and phase-sensitive sensing at x-ray energies.

In this colloquium, I will survey these emerging capabilities in x-ray quantum sensing, highlight what they can uniquely offer relative to classical x-ray methods, and discuss how quantum technologies may transform future x-ray science and applications.