Quantum Non-Demolition Measurement on Spin Precession of Laser-Trapped ¹⁷¹Yb Atoms

Quantum non-demolition (QND) measurement enhances the detection efficiency and measurement fidelity, and is highly desired for its applications in precision measurements and quantum information processing. We propose and demonstrate a QND scheme for the spin state of laser-trapped atoms, allowing the atoms in each spin state to be probed repeatedly. On ¹⁷¹Yb atoms held in an optical dipole trap, a spin-selective cycling transition is produced by introducing an ancillary laser. We measure the phase of spin precession of the ¹⁷¹Yb atoms in a 20-mG bias magnetic field. The QND approach reduces the measurement noise by ∼19 dB, to a level of 2.4 dB below the atom shot noise.