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arXiv:quant-ph/0406031 · Optics Letters

Sub-shot-noise heterodyne polarimetry

Sheng Feng, Olivier Pfister

Published Dec 1, 2004• 16 citations
arXivHTMLPDF

Abstract

We report an experimental demonstration of a heterodyne polarization rotation measurement with a noise floor 4.8 dB below the optical shot noise by use of classically phase-locked quantum twin beams emitted above threshold by an ultrastable type II Na:KTP cw optical parametric oscillator. We believe that this is the largest noise reduction achieved to date in optical phase-difference measurements.

DOI
10.1364/ol.29.002800

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We report an experimental demonstration of a heterodyne polarization rotation measurement with a noise floor 4.8 dB below the optical shot noise by use of classically phase-locked quantum twin beams emitted above threshold by an ultrastable type II Na:KTP cw optical parametric oscillator. We believe that this is the largest noise reduction achieved to date in optical phase-difference measurements.
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.
Quantum interference of twin optical beams, a typical example of correlated boson fields, is of great interest from the viewpoint of both fundamental and applied physics. Intensity correlated optical beams and their quantum interference bring the possibility of performing sub-shot-noise interferometry experiments which yield high-sensitivity in practical applications. A type-II doubly resonant optical parametric oscillator (OPO) has been built for research in quantum optics, to provide a source of ultrastable quantum correlated (twin) optical beams with continuously tunable frequency difference from zero up to several THz. Three servo loops have been built to control the frequency difference of the twin optical beams and sub-hertz stability has been achieved for frequency difference up to 100MHz. In particular, frequency degenerate twin beams are indistinguishable macroscopic quantum systems. A series of experimental results has been produced with these OPO-generated ultrastable twin beams: the observation of the generalized Hong-Ou-Mandel interference of untrastable twin optical beams, sub-shot-noise homodyne interferometry, and sub-shot-noise heterodyne polarimetry. Note: Abstract extracted from PDF text
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.
We investigate experimentally the number and phase quantum noises of the light emitted by an ultrastable nondegenerate optical parametric oscillator (OPO) above threshold. Strong intensity correlations at the photon level -- number-difference squeezing or twin beams -- have already been observed by several groups. We have recently observed the twin-beam interference of a classically phase-locked OPO and have shown that the phase-difference is antisqueezed, as expected from the number-phase Heisenberg (in)equality. In this paper we describe the realization of a stable classical measurement of the joint quadratures of an OPO, as part of our ongoing effort to generate ultrastable EPR light beams simultaneously squeezed in number difference and phase sum.