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Annual Meeting Optical Society of America

Sub-shot-noise light in parametric downconversion

P. R. Tapster, John Rarity, J.S. Satchell, Eric Jakeman

Published Jan 1, 1987• 0 citations
Publisher

Abstract

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.

DOI
10.1364/oam.1987.me5

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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.
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.
It has been recently shown theoretically1 and experimentally2 that nondegenerate optical parametric oscillators generate above threshold intense and highly correlated twin beams. More precisely, the noise on the difference between the intensities of the two beams is well below the shot noise for noise frequencies inside the cavity bandwidth. An application of this quantum correlation is to measure the intensity of one beam and use this information to correct the fluctuations of the other beam by optoelectronic means (photodiode, amplifier, and electrooptic modulator). The correction may be applied either on the pumping beam (feedback correction) or directly on the twin beam itself (feed-forward correction).
When a laser beam interacts with a nonlinear crystal, some of the incident photons fission into two lower frequency signal and idler photons through the process of spontaneous parametric downconversion. It is shown that, as a result of the strong positive correlation between signal and idler photons, the normalized intensity cross-correlation function λ12(τ) should be inversely proportional to the downconverted light intensity and ideally should be given by the inverse of the mean number of photons counted in the resolving time. This unusual relationship, which is a reflection of the quantum mechanical character of the downconverted optical field, has been confirmed in photoelectric counting measurements. The experiment makes use of the light from an argon-ion laser oscillating at 351.1 nm interacting with an 8-cm long crystal of KDP. The downconverted light is extracted at 633.5 and 788.0 nm in two directions that satisfy the phase-matching condition and is allowed to fall on two counting photomultipliers. The output pulses are fed to a time-to-digital converter that yields the cross-correlation function.
Nondegenerate optical parametric oscillators generate above-threshold signal and idler beams that have intensity fluctuations correlated at the quantum level (twin beams). We describe what is to our knowledge the first high-sensitivity spectroscopy experiment using twin beams emitted by a cw optical parametric oscillator: a very weak two-photon absorption signal, in the 10(-7) range, is recorded on the 4S(1/2)-5S(1/2) transition of atomic potassium with a noise background that is reduced by 1.9 dB with respect to the shot-noise limit of the light used in the experiment.
We demonstrate sub-shot-noise photon-number correlations in a (temporal) multimode mesoscopic ($\ensuremath{\sim}{10}^{3}$ detected photons) twin beam produced by picosecond-pulsed spontaneous nondegenerate parametric down-conversion. We have separately detected the signal and idler distributions of photons collected in twin coherence areas and found that the variance of the photon-count difference goes below the shot-noise limit by 3.25dB3.25\phantom{\rule{0.3em}{0ex}}\mathrm{dB}3.25dB. The number of temporal modes contained in the twin beam, as well as the size of the twin coherence areas, depends on the pump intensity. Our scheme is based on spontaneous down-conversion and thus does not suffer from limitations due to the finite gain of the parametric process. Twin beams are also used to demonstrate the conditional preparation of a nonclassical (sub-Poissonian) state.