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Physical Review A

Sub-shot-noise measurement of modulated absorption using parametric down-conversion

P. R. Tapster, S F Seward, John Rarity

Published Sep 1, 1991• 82 citations
Publisher

Abstract

We use a lithium iodate nonlinear crystal as a source of two quantum correlated beams at about 830 nm. A liquid-crystal cell is placed in one of the beams and is electrically driven to produce a very small modulation of its transmission coefficient. Low-noise high-efficiency photodiodes detect the light in each beam and the spectrum of the photocurrents, and the spectrum of the difference between the photocurrents, are measured. The difference spectrum shows a reduction of the noise below the shot-noise level of 4 dB compared with the classical difference technique, where the shot noise in the two detectors simply adds. However, the classical noise in one beam is low enough to make a single-beam shot-noise-limited measurement. Using an optimized difference technique, we show a reduction of the noise level compared to the single-beam shot noise of 2 dB.

DOI
10.1103/physreva.44.3266

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Nondegenerate parametric downconversion is used to produce two identical photon trains. Shot-noise fluctuations in one photon train are fed back to reduce the noise in the other to below the shot-noise limit. A watt of 413-nm light illuminates a deuterated potassium dihydrogen phosphate (KD*P) crystal. The intensity of this light is classically stabilized by a feedback loop using an electrooptic modulator (EOM). Apertures are used to select two downconverted beams from the cone of downconverted light produced in the crystal. The aperture positions are adjusted to satisfy the phase matching conditions for 826 ± 50-nm photon pairs. The beams are detected using silicon PIN diodes with low-noise (<1-fA/Hz1/2) preamplifiers. When the apertures are correctly aligned a high degree of shot-noise correlation is observed between detectors. One of the detector outputs is fed back to the EOM, and the noise in both detectors falls below the shot noise limit.

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Journal of Modern Optics•Oct 1, 1994•23 citations
We use a lithium iodate nonlinear crystal as a source of two quantum correlated beams at about 830 nm. A liquid-crystal cell is placed in one of the beams and is electrically driven to produce a very small modulation of its transmission coefficient. Low-noise high-efficiency photodiodes detect the light in each beam and the spectrum of the photocurrents, and the spectrum of the difference between the photocurrents, are measured. The difference spectrum shows a reduction of the noise below the shot-noise level of 4 dB compared with the classical difference technique, where the shot noise in the two detectors simply adds. However, the classical noise in one beam is low enough to make a single-beam shot-noise-limited measurement. Using an optimized difference technique, we show a reduction of the noise level compared to the single-beam shot noise of 2 dB.
Nondegenerate parametric downconversion is used to produce two identical photon trains. Shot-noise fluctuations in one photon train are fed back to reduce the noise in the other to below the shot-noise limit. A watt of 413-nm light illuminates a deuterated potassium dihydrogen phosphate (KD*P) crystal. The intensity of this light is classically stabilized by a feedback loop using an electrooptic modulator (EOM). Apertures are used to select two downconverted beams from the cone of downconverted light produced in the crystal. The aperture positions are adjusted to satisfy the phase matching conditions for 826 ± 50-nm photon pairs. The beams are detected using silicon PIN diodes with low-noise (<1-fA/Hz1/2) preamplifiers. When the apertures are correctly aligned a high degree of shot-noise correlation is observed between detectors. One of the detector outputs is fed back to the EOM, and the noise in both detectors falls below the shot noise limit.
Recently, we have observed as much as 7 dB of quantum-noise reduction in the direct difference detection of twin-beam signal and idler outputs from a traveling-wave optical parametric amplifier (OPA).1 The OPA, which consists of a type-II phase-matched KTiOPO4 crystal, is pumped by the second harmonic of a mode-locked and Q-switched Nd:YAG laser. Here we describe an experiment in which we used these twin-beam pulses to improve on the optimal (shot-noise-limited) direct-detection signal-to-noise ratio (SNR) that can be achieved with coherent-state pulses of light of the same peak power.
Low-current, high-impedance microelectronic devices can be affected by electric current shot noise more than they are affected by Nyquist noise, even at room temperature. An approach to implementing a sub-shot noise current source for powering such devices is based on direct conversion of amplitude-squeezed light to photocurrent. The phenomenon of optical squeezing allows for the optical measurements below the fundamental shot noise limit, which would be impossible in the domain of classical optics. This becomes possible by affecting the statistical properties of photons in an optical mode, which can be considered as a case of information encoding. Once encoded, the information describing the photon (or any other elementary excitations) statistics can be also transmitted. In fact, it is such information transduction from optics to an electronics circuit, via photoelectric effect, that has allowed the observation of the optical squeezing. It is very difficult, if not technically impossible, to directly measure the statistical distribution of optical photons except at extremely low light level. The photoelectric current, on the other hand, can be easily analyzed using RF spectrum analyzers. Once it was observed that the photocurrent noise generated by a tested light source in question is below the shot noise limit (e.g. produced by a coherent light beam), it was concluded that the light source in question possess the property of amplitude squeezing. The main novelty of this technology is to turn this well-known information transduction approach around. Instead of studying the statistical property of an optical mode by measuring the photoelectron statistics, an amplitude-squeezed light source and a high-efficiency linear photodiode are used to generate photocurrent with sub-Poissonian electron statistics. By powering microelectronic devices with this current source, their performance can be improved, especially their noise parameters. Therefore, a room-temperature sub-shot noise current source can be built that will be beneficial for a very broad range of low-power, low-noise electronic instruments and applications, both cryogenic and room-temperature. Taking advantage of recent demonstrations of the squeezed light sources based on optical micro-disks, this sub-shot noise current source can be made compatible with the size/power requirements specific of the electronic devices it will support.
Quantum-intensity-correlated twin beams of light can be used to measure absorption with precision beyond the classical shot-noise limit. The degree to which this can be achieved with a given estimator is defined by the quality of the twin-beam intensity correlations, which is quantified by the noise reduction factor. We derive an analytical model of twin-beam experiments, incorporating experimental parameters such as the relative detection efficiency of the beams, uncorrelated optical noise, and uncorrelated detector noise. We show that for twin beams without excessive noise, measured correlations can be improved by increasing the detection efficiency of each beam; notwithstanding, this may unbalance detection efficiency. However, for beams with excess intensity or other experimental noise, one should balance detection efficiency, even at the cost of reducing detection efficiency—we specifically define these noise conditions and verify our results with statistical simulation. This has application in design and optimization of absorption spectroscopy and imaging experiments.
Photocurrent shot-noise suppression is observed in closed optoelectronic loop below the limit (1 − η) of standard shot-noise level (SNL) imposed by conventional theory of photodetector with quantum efficiency η. Experimental results on photocurrent noise are presented for different values of feedback strength and quantum efficiency η of in-loop photodiode. It is shown that at fixed feedback strength the suppression factor does not depend on η. Minimum noise spectral density is observed below the SNL by a factor 17·8 (− 12·5 dB). Such large suppression cannot be explained in terms of an ordinary sub-Poissonian light state in the loop. We suggest the concept of an anticorrelation light state, where the light in the loop is described as a non-stationary state with Poissonian quantum fluctuations, and superimposed anticorrelated classical fluctuations. While the degree of noise reduction in the photocurrent can be arbitrary large, the degree of anticorrelation between the classical and quantum fluctuations in the light beam remains limited by the quantum efficiency of the feedback photodiode. Calculations based on this concept are in a good agreement with experimental data on super-shot-noise power, obtained for the beam extracted from the loop by a beam splitter.