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Europhysics Letters (EPL)

Generation of frequency-tunable twin beams and its application in sub–shot-noise FM spectroscopy

Haibo Wang, Zehui Zhai, Shaokai Wang, Jiangrui Gao

Published Oct 1, 2003• 8 citations
Publisher

Abstract

Widely frequency-tunable quantum-correlated twin beams have been experimentally demonstrated. The noise of the intensity difference between the signal and idler beams of the twin beams is reduced to more than 3 dB below the shot noise limit (SNL) throughout a wavelength-tuning range of 5.6 nm. A continuously frequency-tuning range of 2.8 GHz appears around the center wavelength of 1041.506 nm, where the maximum noise reduction of about 5.3 dB is observed. The tunable twin beams are used in frequency modulation (FM) spectroscopy of Cs 2 molecules and a spectrum with a sensitivity of about 4 dB beyond that of the SNL is obtained.

DOI
10.1209/epl/i2003-00110-7

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Widely frequency-tunable quantum-correlated twin beams have been experimentally demonstrated. The noise of the intensity difference between the signal and idler beams of the twin beams is reduced to more than 3 dB below the shot noise limit (SNL) throughout a wavelength-tuning range of 5.6 nm. A continuously frequency-tuning range of 2.8 GHz appears around the center wavelength of 1041.506 nm, where the maximum noise reduction of about 5.3 dB is observed. The tunable twin beams are used in frequency modulation (FM) spectroscopy of Cs 2 molecules and a spectrum with a sensitivity of about 4 dB beyond that of the SNL is obtained.
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
We have developed a frequency-modulated, tunable, amplitude-squeezed, diode-laser-based source and used it to perform FM spectroscopy on rubidium. The setup consists of a free-running diode laser injection locked by a frequency-stabilized, current-modulated diode laser. The injection-locked slave laser beam adopted the frequency spectrum of the master laser beam while rejecting residual AM in the master laser beam by more than 50 dB. Injection locking also enhanced amplitude squeezing in the slave laser beam by suppressing uncorrelated longitudinal sidemodes. The noise floor of the measurement was 0.8 dB below the shot-noise level.
A frequency tunable source of squeezed light has been developed for application to a variety of problems in atomic physics. The basic experimental arrangement consists of a subthreshold optical parametric oscillator (OPO) that is pumped by the frequency-doubled output of a titanium-sapphire laser. Squeezed light with continuous tunability of 2 GHz has been generated with a directly observed nonclassical noise reduction of 5.0 dB relative to the vacuumstate limit in a balanced homodyne detector (5.2 dB when corrected for detector noise). Although our source is potentially broadly tunable over the noncritical phase-matching bandwidth of potassium niobate, current research has concentrated on operation at 852 nm for excitation of the D2 line of atomic cesium. Relative to the usual quantum limit associated with vacuumstate fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in FM saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB after correction for detector noise.