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arXiv:quant-ph/0103106

Quantum nondemolition measurement of a light field component by a feedback compensated beam splitter

Holger Friedrich Hofmann, Takayoshi Kobayashi, Akira Furusawa

Published Mar 19, 2001• 0 citations
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Abstract

In conventional quantum nondemolition measurements, the interaction between signal and probe preserves the measured variable. Alternatively, it is possible to restore the original value of the variable by feedback. In this paper, we describe a quantum nondemolition measurement of a quadrature component of the light field using a feedback compensated beam splitter. The noise induced by the vacuum port of the beam splitter is compensated by a linear feedback resulting in an effective amplification of the observed variable. This amplification is then be reversed by optical parametric amplification to restore the original value of the field component.

DOI
10.48550/arxiv.quant-ph/0103106

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In conventional quantum nondemolition measurements, the interaction between signal and probe preserves the measured variable. Alternatively, it is possible to restore the original value of the variable by feedback. In this paper, we describe a quantum nondemolition measurement of a quadrature component of the light field using a feedback compensated beam splitter. The noise induced by the vacuum port of the beam splitter is compensated by a linear feedback resulting in an effective amplification of the observed variable. This amplification is then be reversed by optical parametric amplification to restore the original value of the field component.
Abstract Quantum nondemolition (QND) measurement plays an essential role in quantum optics and quantum information. Here, a feedback-assisted atom-light-correlated interferometer (FALCI) is proposed and applied to the QND measurement of photon number, in which the phase of the probe acquires information about the photon number of the signal without perturbing it due to the AC-Stark effect. A beam splitter with adjustable reflectivity is employed as a feedback controller to enhance the performance of QND measurement. The FALCI harnesses interference between quantum-correlated atoms and light to perform the measurement. It is found that the quantum correlation can be enhanced by manipulating the feedback ratio. Furthermore, the QND measurement sensitivity is theoretically studied via homodyne detection. It is demonstrated that the FALCI can achieve the sensitivity enhancement with suitable feedback ratio. For the lossy case, feedback control can mitigate the effect of losses on the sensitivity.
Methods for the enhancement of optical quantum nondemolition (QND) measurements are discussed. We review the use of meter squeezing as a QND enhancement tool and present a method of QND enhancement using an electro-optic feed-forward amplifier. By applying a linearized theory it is shown that these techniques work very well together. The combined effect of these enhancement methods is modeled for two QND systems, a squeezed light beam splitter and an optical parametric amplifier. We also discuss the conflict between the normal QND criteria and QND systems that involve noiseless amplification. We use an additional parameter to quantify the problem. A method for correcting the effects of noiseless amplification is discussed and modeled. We also discuss a special case of QND that eliminates the optical interaction between the meter and signal input beams. This system is shown to be a very effective QND device.
A beam splitter is the simplest and clearest example of a coupling device with which quantum limited measurements can be performed. The limitations due to the vacuum fluctuations entering the usually unused port are circumvented by injection of a 3.7 dB squeezed wave into this port. Thereby phase-sensitive measurements that satisfy the quantum nondemolition criteria are realized (signal transfer Ts+Tm=1.29{T}_{s}{+T}_{m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.29Ts​+Tm​=1.29, 32% correlation). The dependences of the signal transfer and correlation on beam splitter transmissivity are measured and found to be in quantitative agreement with a calculation that takes into account various inefficiencies. Continuous stable operation for 36 hours is achieved.
A new quantum measurement scheme of intensity difference fluctuation between two light beams of equal mean intensity is presented. In this system a beam splitter is used as the coupling device and the twin beams with high quantum correlation are injected into its dark port as the input meter wave instead of the usual vacuum field. A measurement satisfying all the quantum nondemolition criteria is experimentally achieved. The measured sum of the transfer coefficients and the conditional variance are, respectively, Ts+Tm=1.31{T}_{s}{+T}_{m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.31Ts​+Tm​=1.31 and ${V}_{s/m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}2.1\mathrm{dB}$.
Finite resolution quantum nondemolition (QND) measurements allow a determination of light field properties while preserving some of the original quantum coherence of the input state. It is thus possible to measure correlations between the photon number and a quadrature component of the same light field mode. Nonclassical features emerge as photon number quantization is resolved. In particular, a strong anti-correlation of quantization and coherence is observable in QND measurements of photon number, and a correlation between measurement induced quantum jumps and quadrature component measurement results is obtained in QND measurements of quadrature fluctuations in the photon vacuum. Such nonclassical correlations represent fundamental quantum properties of the light field and may provide new insights into the nature of quantization itself.