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The Review of Laser Engineering

Generation of Quantum-Correlated Twin Beams and Its Application

Katsuyuki Kasai

Published Jan 1, 2003• 0 citations
Publisher

Abstract

The signal and idler beams from a non-degenerate, above threshold, optical parametric oscillator have a strong quantum correlation, which are called twin beams. This correlation arises from the simultaneous generation of the signal and idler photons in the form of twin pairs of photons through the parametric down conversion, and the generated twin beams have exactly the same photon statistics. As a result, the fluctuations on the difference between the intensities of the twin beams are reduced with respect to the shot noise limit, that is, intensity-difference squeezing. We give a comprehensive overview of the generation of twin beams using the optical parametric oscillator, and the applications exploiting the quantum correlation of twin beams are presented.

DOI
10.2184/lsj.31.592

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The signal and idler beams from a non-degenerate, above threshold, optical parametric oscillator have a strong quantum correlation, which are called twin beams. This correlation arises from the simultaneous generation of the signal and idler photons in the form of twin pairs of photons through the parametric down conversion, and the generated twin beams have exactly the same photon statistics. As a result, the fluctuations on the difference between the intensities of the twin beams are reduced with respect to the shot noise limit, that is, intensity-difference squeezing. We give a comprehensive overview of the generation of twin beams using the optical parametric oscillator, and the applications exploiting the quantum correlation of twin beams are presented.
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.
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).
We have generated bright squeezed light and quantum-correlated twin photon beams by using a cw semimonolithic KTP Optical Parametric Oscillator (OPO), which exhibits a low oscillation threshold and is capable of oscillating even in the free-running (no external-feedback) condition. The pump beam reflected by the OPO, after a cascading nonlinearity process in the crystal, appears as bright squeezed light. The squeezing observed on the reflected pump beam from the semimonolithic OPO was 30% (inferred squeezing: 48%). The signal and idler beams generated by the optical parametric oscillator are highly quantum-correlated twin photon beams. Since the squeezing property of the intensity difference between the twin photonbeams is robust in the condition where the two fields are balanced, the squeezing can be observed even in the freerunning condition of the OPO. This intensity-difference squeezing observed between the twin photon beams from the free-running semimonolithic OPO was 70% (inferred squeezing: 87%).
This chapter provides a review of both theoretical and experimental results that are obtained on the Non-Degenerate Optical Parametric Oscillator above threshold operation. In particular it is shown that most of oscillator's characteristics can be understood from the fact that the non-linear crystal emits highly correlated twin photons. The chapter studies the correlation properties between the intensities of the corresponding twin fields. It provides a simple treatment in terms of “corpuscular” photons and gives a proof of the corpuscular model in a fully quantum manner. The chapter provides a classical representation of the fields but in which vacuum fluctuations are properly accounted. This “classical” approach yields the noise spectra of the various measurable quantities in the general case. The chapter presents observation of quantum noise reduction on the difference between the intensities of the two twin beams. It deals with new possible applications of the twin photons to spectroscopy and to the generation of amplitude squeezed laser beams.
Measurement slight amounts of absorption of light with accuracy beyond the standard quantum limit has been experimentally demonstrated. The quantum-correlated twin beams used in the measurement were generated from a nondegenerate optical parametric oscillator including an alpha-cut KTiOPO(4) crystal pumped by an intracavity frequency-doubled Nd:YAG laser. The noise in the intensity difference between the twin beams was reduced by 88% below the standard quantum limit (SQL). The signal-to-noise ratio was improved by 7 dB with respect to the SQL of the total light employed in the experiment and by 4 dB with respect to that of the signal light.