QUANTUM-NONDEMOLITION DEMONSTRATION VIA REPEATED BACKACTION EVADING MEASUREMENTS

Repeated backaction evading measurements have been performed for the first time. The first backaction evading device prepares the field state while the second one measures it again. The conditional variance of the signal at the output of the first device given the measurement is 1.8 dB below the shot noise and the signal to noise ratio transfer coefficient is 1.34. The conditional variance of the final signal given the first device measurement is also 1.8 dB below the shot noise and the normalized correlation between the two meter outputs is 0.3. These figures are clearly in the quantum regime, and they constitute a full demonstration of the quantum nondemolition principle.