Showing papers on "Quantum error correction published in 1982"

Quantum limits on noise in linear amplifiers

Abstract: How much noise does quantum mechanics require a linear amplifier to add to a signal it processes? An analysis of narrow-band amplifiers (single-mode input and output) yields a fundamental theorem for phase-insensitive linear amplifiers; it requires such an amplifier, in the limit of high gain, to add noise which, referred to the input, is at least as large as the half-quantum of zero-point fluctuations. For phase-sensitive linear amplifiers, which can respond differently to the two quadrature phases ("$cos\ensuremath{\omega}t$" and "$sin\ensuremath{\omega}t$"), the single-mode analysis yields an amplifier uncertainty principle---a lower limit on the product of the noises added to the two phases. A multimode treatment of linear amplifiers generalizes the single-mode analysis to amplifiers with nonzero bandwidth. The results for phase-insensitive amplifiers remain the same, but for phase-sensitive amplifiers there emerge bandwidth-dependent corrections to the single-mode results. Specifically, there is a bandwidth-dependent lower limit on the noise carried by one quadrature phase of a signal and a corresponding lower limit on the noise a high-gain linear amplifier must add to one quadrature phase. Particular attention is focused on developing a multimode description of signals with unequal noise in the two quadrature phases.

Quantum noise and quantum nondemolition measurements

Abstract: 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.

On the quantum theory of parametric frequency converter

Abstract: A quantum mechanical treatment is given of the acoustooptic parametric conversion processes in dielectric crystals when the signal acoustical and idle light waves are transformed into each other in the presence of intensive optical pumping. The approximate Heisenberg equations of motion are found and solved for the creation and annihilation operators of signal and idle modes with due regard to the interaction of these modes with other light and vibratory modes of the crystal (“the thermostat”). It is shown that the thermostat influence results in noise and attenuation effects. These persistent noises are also converted from one mode into another and vice versa. Threshold conditions and asymptotic levels of noise are discussed.