Quantum metrology with atoms and photons

We describe our recent work in quantum metrology using atomic and photonic systems for measurements with sensitivity beyond the standard quantum limit, e.g., beyond the shot-noise and projection-noise limits. We demonstrate two optical magnetometry systems using rubidium atoms as field sensors, with optical read-out by paramagnetic Faraday rotation. Our hot-atom magnetometer operates beyond the shot-noise limit of sensitivity using a polarization-squeezed optical probe. Our next-generation cold-atom magnetometer operates on similar principles but with much greater atomic control. We demonstrate quantum-non-demolition measurement of the collective spin of the atom cloud at both the shot-noise and projection-noise limits of sensitivity.