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Physical Review Letters

Noiseless Signal Amplification using Positive Electro-Optic Feedforward

Ping Koy Lam, Timothy C. Ralph, Elanor H. Huntington, Hans‐A. Bachor

Published Aug 25, 1997• 64 citations
Publisher

Abstract

We propose an electro-optic feedforward scheme which can in principle produce perfect noiseless signal amplification (signal transfer coefficient of ${T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1$). We demonstrate the scheme experimentally and report, for a signal gain of 13.4 dB, a signal transfer coefficient of ${T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.88$ which is limited mainly by detector efficiencies (92%). The result clearly exceeds the standard quantum limit, ${T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5$, set by the high gain limit of a phase insensitive linear amplifier. We use the scheme to amplify a small signal carried by 35% amplitude squeezed light and demonstrate that, unlike the fragile squeezed input, the signal amplified output is robust to propagation losses.

DOI
10.1103/physrevlett.79.1471

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Kluwer Academic Publishers eBooks•Dec 19, 2005•1 citation
We propose an electro-optic feedforward scheme which can in principle produce perfect noiseless signal amplification (signal transfer coefficient of Ts=1{T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1Ts​=1). We demonstrate the scheme experimentally and report, for a signal gain of 13.4 dB, a signal transfer coefficient of Ts=0.88{T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.88Ts​=0.88 which is limited mainly by detector efficiencies (92%). The result clearly exceeds the standard quantum limit, Ts=0.5{T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.5Ts​=0.5, set by the high gain limit of a phase insensitive linear amplifier. We use the scheme to amplify a small signal carried by 35% amplitude squeezed light and demonstrate that, unlike the fragile squeezed input, the signal amplified output is robust to propagation losses.
We demonstrate an elegant way of handling optical signals which are generated using squeezed states of light without losing their improved signal to noise ratio. We do this by amplifing, without significant noise penalty, both signal and noise away from the quantum noise limit into the classical domain. This makes the information robust to losses. Our system achieves a signal transfer coefficient, T(s), close to unity. As a demonstration we amplify a small signal carried by 35% amplitude squeezed light and show that unlike the fragile squeezed input, the signal amplified output is robust to propagation losses. A signal transfer coefficient of T(s) = 0.75 is achieved even in the presence of large introduced (86%) downstream losses.
Theoretical results are presented that show that noiseless phase quadrature amplification is possible, and limited experimentally only by the efficiency of the phase-detection system. Experimental results obtained using a neodymium-doped yttrium aluminum garnet laser show a signal gain of 10 dB and a signal transfer ratio of Ts=0.9{T}_{s}=0.9Ts​=0.9. This result easily exceeds the standard quantum limit for signal transfer. The results also explicitly demonstrate the phase-sensitive nature of the amplification process.
We used a spatially broadband optical parametric amplifier for image amplification. In the phase-sensitive configuration of the amplifier, we observed noiseless amplification of input images. For a gain of $\ensuremath{\simeq}2.5$, we measured noise-figure values of ( $0.2\ifmmode\pm\else\textpm\fi{}0.6$) and ( $0.4\ifmmode\pm\else\textpm\fi{}0.5$) dB, respectively, for two amplifiers of different lengths. These experimental values agree with theory (for an ideal noiseless amplifier the noise figure is 0 dB) and are almost 2 dB lower than the quantum limit of an ideal phase-insensitive amplifier.
We analyze an optical-phase feed-forward scheme to produce a squeezed-light beam from intensity-correlated twin beams generated by nondegenerate parametric amplification. The quadrature measurement of the idler beam at the output of the parametric amplifier is linearly fed forward onto the output signal beam with an electro-optic phase modulator. The minimum available squeezing factor is shown to be 1/(2g - 1), where g is the parametric amplifier gain, and is equal to the nonclassical intensity-noise reduction factor of the original twin beams at the output of the parametric amplifier. By changing the frequency of the input signal beam, one can generate tunable squeezed light with this scheme for application in precision spectroscopy.