Single-photon absorber based on strongly interacting Rydberg atoms
  Removing exactly one photon from an arbitrary input pulse is an elementary operation in quantum optics and enables applications in quantum information processing and quantum simulation. Here we demonstrate a deterministic single-photon absorber based on the saturation of an optically thick free-space medium by a single photon due to Rydberg blockade. Single-photon subtraction adds a new component to the Rydberg quantum optics toolbox, which already contains photonic logic building-blocks such as single-photon sources, switches, transistors, and conditional $\pi$-phase shifts. Our approach is scalable to multiple cascaded absorbers, essential for preparation of non-classical light states for quantum information and metrology applications, and, in combination with the single-photon transistor, high-fidelity number-resolved photon detection. 
Christoph Tresp, Christian Zimmer, Ivan Mirgorodskiy, Hannes   Gorniaczyk, Asaf Paris-Mandoki, Sebastian Hofferberth
quant-ph physics.atom-ph