  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 measurement scheme for the spin states of laser-trapped
atoms. On $^{171}$Yb atoms held in an optical dipole trap, a transition that is
simultaneously cycling, spin-state selective, and spin-state preserving is
created by introducing a circularly polarized beam of control laser to
optically dress the spin states in the excited level, while leaving the spin
states in the ground level unperturbed. We measure the phase of spin precession
of $5\times10^{4}$ atoms in a bias magnetic field of 20 mG. This QND approach
reduces the optical absorption detection noise by $\sim$19 dB, to a level of
2.3 dB below the atomic quantum projection noise. In addition to providing a
general approach for efficient spin-state readout, this all-optical technique
allows quick switching and real-time programming for quantum sensing and
quantum information processing.
