Quantum Noise and QND Measurements

Michael B. Mensky, Jürgen Audretsch

Mark S. Hillery, Marlan O. Scully

We examine the effects of quantum noise in the readout of a proposed quantum non-demolition (QND) system. The equations of motion of the system are solved approximately both with and without noise. The effect of the noise upon the measurement accuracy of the system is determined and it is found that, in principle, the presence of quantum noise in the readout should not prevent one from measuring the amplitude of the oscillator to better than the standard quantum limit.

Philippe Grangier

Summary form only given. The present techniques allow one to perform continuous or discrete quantum measurements at or even beyond the standard "quantum noise" level, even on single quantum objects. In this context, Kip Thorne and Vladimir Braginsky introduced in the seventies the concept of "Quantum Non Demolition (QND) measurement", which was initially meant to be applied to mechanical harmonic oscillators designed for detecting gravitational waves, the so-called "Weber's bars". But the QND ideas encountered a real success in quantum optics, where quantum fluctuations are readily accessible, and can also be circumvented. We review the state of the art of QND measurements, in this broad sense of sub-shot-noise optical information processing. Most of the initial objectives of QND measurements have now been successfully reached through proof-of-principle experiments. The domain is now opening on future perspectives related to quantum information processing (e.g. quantum cryptography), either with "discrete" or "continuous" quantum variables. In this context the close relationship between QND measurements and controlled-NOT (CNOT) gates are discussed and illustrated.

Crispin W. Gardiner, P. Zoller

Crispin W. Gardiner

Stephen R. Friberg, Takaaki Mukai, Susumu Machida

Quantum mechanics allows quantum nondemolition (QND) variables to be measured without being changed. This requires QND variables to be initially in an eigenstate and measurement backaction noise to be directed into conjugate variables. Experimental demonstrations thus require two measurements: the first to collapse variables toward an eigenstate and the second to show results identical to the first. Here, we report results from two successive soliton-collision QND measurements that optical correlation measurements show to be nearly identical.