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Optics Letters

Detection of amplitude modulation with squeezed light for sensitivity beyond the shot-noise limit

Min Xiao, Ling-An Wu, Harry J. Kimble

Published Jun 1, 1988• 63 citations
Publisher

Abstract

An improvement in precision beyond the limit set by the vacuum-state or zero-point fluctuations of the electromagnetic field is reported for the detection of amplitude modulation encoded on a weak signal beam. The improvement is achieved by employing the squeezed light from an optical parametric oscillator to reduce the level of fluctuations below the shot-noise limit. An increase in signal-to-noise ratio of 2.5 dB relative to the shot-noise limit is demonstrated.

DOI
10.1364/ol.13.000476

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An improvement in precision beyond the limit set by the vacuum-state or zero-point fluctuations of the electromagnetic field is reported for the detection of amplitude modulation encoded on a weak signal beam. The improvement is achieved by employing the squeezed light from an optical parametric oscillator to reduce the level of fluctuations below the shot-noise limit. An increase in signal-to-noise ratio of 2.5 dB relative to the shot-noise limit is demonstrated.
Improvements in precision beyond the limit set by the vacuum state or zero-point fluctuations of the field are reported for two optical experiments. Both experiments employ the squeezed light from an optical parametric oscillator to reduce the level of fluctuations in one of two quadrature phase amplitudes below the shot-noise level (SNL). The first experiment follows the work of Caves1 on precision interferometry with squeezed states. An increase in signal-to-noise of 2.5 dB relative to the SNL is demonstrated for the detection of phase modulation in a Mach-Zehnder interferometer. In the second experiment amplitude modulation is encoded on a weak signal beam with an acoustooptic modulator. For the case of a squeezed signal beam compared with a beam in a coherent state, an improvement in sensitivity of 2.5 dB is achieved for the detection of amplitude changes of the field. The observed increases in sensitivity in both experiments are currently limited by simple linear losses in propagation and detection and not by the degree of available squeezing.
An improvement in precision beyond the limit set by the vacuum-state or zero-point fluctuations of the electromagnetic field is reported for the measurement of phase modulation in an optical interferometer. The experiment makes use of squeezed light to reduce the level of fluctuations below the shot-noise limit. An increase in the signal-to-noise ratio of 3.0 dB relative to the shot-noise limit is demonstrated, with the improvement currently limited by losses in propagation and detection and not by the degree of available squeezing.
Squeezed states of the electromagnetic field are characterized by a reduction of fluctuations for one of two quadrature phase amplitudes below the level of fluctuations for the vacuum state. This reduction is a manifestly quantum or nonclassical feature which is of great interest in its own right but also potentially important for precision measurement with sensitivity beyond the shot-noise or vacuum-state limit. We describe three different experiments in which squeezed light has been generated in our laboratory involving parametric downconversion, intracavity frequency doubling, and optical bistability with two-state atoms. By employing the squeezed light produced by a subthreshold optical parametric oscillator, we have achieved improvements in sensitivity beyond the vacuum-state limit for the detection of both phase and amplitude changes of the electromagnetic field. An alternate route to sensitivity beyond the vacuum-state limit is through quantum nondemolition measurement, where we have suggested and are implementing a scheme involving parametric conversion and polarization mixing.
Enhancement of the sensitivity of a polarization interferometer beyond the limit set by vacuum fluctuations of the electromagnetic field has been obtained with use of squeezed light generated by a KTiOPO4{\mathrm{KTiOPO}}_{4}KTiOPO4​ optical parametric amplifier. The increase in signal-to-noise ratio relative to the shot-noise limit is 2 dB. The corresponding improvement in the response time of the interferometer is measured to be a factor of 1.6.
We experimentally demonstrate a novel sub-shot-noise-limited heterodyne detection scheme to measure a weak optical signal field by employing amplitude-squeezed light as the local oscillator (LO) field. The amplitude-squeezed LO field at 428.8 nm is generated from a high efficiency single-pass second-harmonic generation in a KNbO3{\mathrm{KNbO}}_{3}KNbO3​ crystal pumped by femtosecond (130 fs) pulses at 857.6 nm, and the signal field is combined with the generated squeezed LO field through the crystal. An enhancement of 0.7 dB (1.4 dB inferred) in signal-to-noise ratio beyond the shot-noise limit is directly observed.
Squeezed light has lower quantum noise in amplitude or phase than the quantum noise limit (QNL) of classical light. This enables enhancing sensitivity -- quantified by the signal-to-noise ratio (SNR) -- to beyond the QNL in optical techniques such as spectroscopy, gravitational wave detection, magnetometry and imaging. Precision -- the variance of repeated estimates -- has also been enhanced beyond the QNL using squeezed vacuum to estimate optical phase and transmission. However, demonstrations have been limited to femtowatts of probe power. Here we demonstrate simultaneous precision and sensitivity beyond the QNL for estimating modulated transmission using a squeezed amplitude probe of 0.2 mW average (25 W peak) power. This corresponds to 8 orders of magnitude above the power limitations of previous sub-QNL precision measurements. Our theory and experiment show precision enhancement scales with the amount of squeezing and increases with the resolution bandwidth of detection. We conclude that quantum enhanced precision in estimating a modulated transmission increases when observing dynamics at ∼\sim∼ kHz and above. This opens the way to performing measurements that compete with the optical powers of current classical techniques, but have superior precision and sensitivity beyond the classical limit.