Light-mediated strong coupling between a mechanical oscillator and atomic spins 1 meter apart.

Engineering strong interactions between quantum systems is essential for many phenomena of quantum physics and technology. Typically, strong coupling relies on short-range forces or on placing the systems in high-quality electromagnetic resonators, which restricts the range of the coupling to small distances. We used a free-space laser beam to strongly couple a collective atomic spin and a micromechanical membrane over a distance of 1 meter in a room-temperature environment.

Second-order photon correlation measurement with picosecond resolution using frequency upconversion.

The second-order correlation function of light g(τ) constitutes a pivotal tool to quantify the quantum behavior of an emitter and in turn its potential for quantum information applications. The experimentally accessible time resolution of g(τ) is usually limited by the jitter of available single-photon detectors. Here, we present a versatile technique allowing g(τ) to be measured from a large variety of light signals with a time resolution given by the pulse length of a mode-locked laser.