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Physical Review Letters

Spectroscopy with squeezed light

Eugene Simon Polzik, J. Carri, Harry J. Kimble

Published May 18, 1992• 443 citations
Publisher

Abstract

A frequency-tunable source of squeezed light is employed for spectroscopic measurements of atomic cesium. Relative to the usual quantum limit associated with vacuum-state fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB inferred after correcting for detector thermal noise.

DOI
10.1103/physrevlett.68.3020

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A frequency-tunable source of squeezed light is employed for spectroscopic measurements of atomic cesium. Relative to the usual quantum limit associated with vacuum-state fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB inferred after correcting for detector thermal noise.
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.
Squeezed and other nondassical fields can be employed for enhanced spectroscopic sensitivity surpassing the standard quantum limit.1 However, beyond improvements in quantitative analysis where the nondassical field acts only as a passive probe are a wealth of possibilities associated with the active alteration of fundamental atomic radiative processes.2 In this regard, we are currently pursuing two experiments which make use of our frequency tunable source of squeezed light. The first studies two- photon excitation by correlated photon pairs,3 while the second investigates line- width alterations in cavity QED with squeezed light.4
We report the experimental generation of a squeezed vacuum at frequencies ranging from 2.5 kHz to 200 kHz that is resonant on the cesium D2 line by using a below-threshold optical parametric oscillator (OPO). The OPO is based on a periodically-poled KTiOPO 4 (PPKTP) crystal that is pumped using a bow-tie four-mirror ring frequency doubler. The phase of the squeezed light is controlled using a quantum noise locking technique. At a pump power of 115 mW, maximum quadrature phase squeezing of 3.5 dB and anti-squeezing of 7.5 dB are detected using a home-made balanced homodyne detector. This squeezed vacuum at an atomic transition in the kilohertz range is an ideal quantum source for quantum metrology of enhancing measurement precision, especially for ultra-sensitive measurement of weak magnetic fields when using a Cs atomic magnetometer in the audio frequency range.
A frequency-tunable source of squeezed light would allow investigations of the atom-field interaction in a regime where fundamental new phenomena have been predicted.1 There are also opportunities for spectroscopy with sensitivity beyond the shot-noise limit. Toward these objectives, in our experiment we employ a single-frequency Ti-sapphire laser with tuning optimized around 852 nm (cesium resonance transition). The infrared output of the laser is frequency doubled in an external buildup cavity with a KNbO3 crystal as the nonlinear element. Efficient doubling provides 0.65 W of tunable blue output for 1.35 W of infrared input. This blue output is then used to pump an optical parametric oscillator (OPO) containing a second KNbO3 crystal for downconversion back to the infrared. The high power of the blue pump that we have achieved and the large nonlinearity of KNbO3 allows us to employ an OPO that is resonant only for the subharmonic infrared light. This design facilitates tuning of the OPO compared with the case in which the blue pump and the infrared are both resonant. The doubling cavity, OPO, and laser itself are locked relative to a stable reference cavity by an rf sideband technique, thus providing the possibility for tunability of the system with a single knob.
Squeezed states of the electromagnetic field are characterized by a reduction of fluctuations for one of two quadrature phase amplitudes below the level of fluctuations for the vacuum state. This reduction is a manifestly quantum or nonclassical feature which is of great interest in its own right but also potentially important for precision measurement with sensitivity beyond the shot-noise or vacuum-state limit. We describe three different experiments in which squeezed light has been generated in our laboratory involving parametric downconversion, intracavity frequency doubling, and optical bistability with two-state atoms. By employing the squeezed light produced by a subthreshold optical parametric oscillator, we have achieved improvements in sensitivity beyond the vacuum-state limit for the detection of both phase and amplitude changes of the electromagnetic field. An alternate route to sensitivity beyond the vacuum-state limit is through quantum nondemolition measurement, where we have suggested and are implementing a scheme involving parametric conversion and polarization mixing.
Parametric downconversion in media has, so far, proved to be the most effective way of generating squeezed light. The development of frequency tunable sources of such non-classical light opens up novel possibilities in atomic physics and spectroscopy. A brief overview of the results in this area of quantum optics is given, and a new experiment aimed on subshot-noise measurement of atomic orientation is described. We report a preliminary 5 dB quantum noise reduction observed directly around the 917 nm Cs line. We introduce a figure of merit for the medium utilized for squeezed light generation in an optical parametric oscillator and evaluate some available nonlinear media from this perspective.