QUANTUM-NONDEMOLITION DEMONSTRATION VIA REPEATED BACKACTION EVADING MEASUREMENTS

Repeated backaction evading measurements have been performed for the f irst time. The first backaction evading device prepares the field stat e 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 coe fficient is 1.34. The conditional variance of the final signal given t he first device measurement is also 1.8 dB below the shot noise and th e normalized correlation between the two meter outputs is 0.3. These f igures are clearly in the quantum regime, and they constitute a full d emonstration of the quantum nondemolition principle.