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Physical Review Letters

Experimental Realization of a Quantum Measurement for Intensity Difference Fluctuation Using a Beam Splitter

Hai Wang, Yun Zhang, Qing Pan, Hong Te Su, A. Porzio, Changde Xie, Kunchi Peng

Published Feb 15, 1999• 51 citations
Publisher

Abstract

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, ${T}_{s}{+T}_{m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.31$ and ${V}_{s/m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}2.1\mathrm{dB}$.

DOI
10.1103/physrevlett.82.1414

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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 Vs/m=\ensuremath−2.1dB{V}_{s/m}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{-}2.1\mathrm{dB}Vs/m​=\ensuremath−2.1dB.
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.
We give the intensity fluctuation joint probability of the twin-beam quantum state, which was generated with an optical parametric oscillator operating above threshold. Then we present what to our knowledge is the first measurement of the intensity fluctuation conditional probability distributions of twin beams. The measured inference variance of twin beams 0.62+/-0.02, which is less than the standard quantum limit of unity, indicates inference with a precision better than that of separable states. The measured photocurrent variance exhibits a quantum correlation of as much as -4.9+/-0.2 dB between the signal and the idler.
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.