Quantum nondemolition measurements - Comment on recent developments. [detectability of extremely weak signals

James D. Macomber

Oldwig v. Roos

The limitations of the detectability of extremely weak signals (gravitational radiation for instance) imposed by Heisenberg's uncertainty principle on the sequential determination of those signals have been explored recently. A variety of schemes have been proposed to circumvent these limitations. Although all of the earlier attempts have been proven fruitless a recent proposal seems to be quite promising. The scheme, consisting of two harmonic oscillators interacting with each other in a peculiar way, allows for an exact analytical solution which is derived here. If it can be assumed that the expectation value of one of the canonical variables of the total system suffices to monitor the weak signal it can be shown that, in the absence of thermal noise, arbitrarily weak signals can in principle be measured without interference from the uncertainty principle.

Александр Семенович Холево

Stephen R. Friberg

In quantum mechanics, measurement is not external to an experiment but an integral part of it; for example, whether quanta act as waves or particles depends on what property we measure. However, certain quantum mechnical principles can be harnessed to perform extremely sensitive measurements without disturbing an extremely weak signal. Friberg discusses how quantum nondemolition measurements allow a signal and a probe to be entangled and then later read out without altering the original signal.

Gerard J Milburn, Daniel F. Walls

Guoxi Fang, Yancheng Zhao

In this paper, the method will be introduced, it's called Quantum Nondemolition measurement. The purpose is to realize a new measuring method to surpass quantum limit. It provides a new method of overcoming quantum noise. In principle, it can use any precision to detect a certain weak signal. Finally, the application of QND Measurement in optical communication will be introduced.