Unconditional Quantum-Noise Suppression via Measurement-Based Quantum Feedback

We demonstrate unconditional quantum-noise suppression in a collective spin system via feedback control based on quantum nondemolition measurement. We perform shot-noise limited collective spin measurements on an ensemble of 3.7 X 10(5) laser-cooled Yb-171 atoms in their spin-1/2 ground states. Correlation between two sequential quantum nondemolition measurements indicates -0.80(-0.12)(+0.11) dB quantum-noise suppression in a conditional manner. Our feedback control successfully converts the conditional quantum-noise suppression into the unconditional one without significant loss of the noise reduction level. DOI: 10.1103/PhysRevLett.110.163602