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Physical Review Letters

Pulsed Squeezed Light

Richard E. Slusher, Philippe Grangier, Arthur LaPorta, Bernard Yurke, Mary J. Potasek

Published Nov 30, 1987• 283 citations
Publisher

Abstract

Squeezed light in the form of a train of pulses, each approximately 100 ps in width, has been generated by parametric down-conversion in KTiOP${\mathrm{O}}_{4}$ crystals. The measured noise reduction in the quiet quadrature of the field is 0.6 dB below the shot-noise limit. The quadrature noise components are measured with a balanced homodyne detector using a pulse train for the local oscillator. The noise reduction extends over a broad spectrum and has important implications for precision measurement at very short time scales.

DOI
10.1103/physrevlett.59.2566

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Squeezed light in the form of a train of pulses, each approximately 100 ps in width, has been generated by parametric down-conversion in KTiOPO4{\mathrm{O}}_{4}O4​ crystals. The measured noise reduction in the quiet quadrature of the field is 0.6 dB below the shot-noise limit. The quadrature noise components are measured with a balanced homodyne detector using a pulse train for the local oscillator. The noise reduction extends over a broad spectrum and has important implications for precision measurement at very short time scales.
A scheme is proposed, in which the pulse train of squeezed light is generated via forward four-wave mixing through a nonlinear medium with third-order polarizability, coherently pumped by a train of pulses of modelocked laser. One of the quadrature noise compoments of the squeezed light is measured with a balanced homodyne detector using a pulse train as the local oscillation. The theoretical results show that the fairly high-intensity output of squeezed light can be achieved, and the noise-power spectrum of the light is almost completely squeezed and exhibits the form of a train of pulses, but the shape of squeezed spectrum is reversed with respect to that of the pulse train of the input light.
Measurements of quantum-noise reduction have been performed over a broad frequency range up to 0.5 GHz using pulses of squeezed light generated by parametric amplification in KTiOPO4{\mathrm{KTiOPO}}_{4}KTiOPO4​ crystals. Both balanced-homodyne and direct-difference photodetection schemes were employed, and 0.5 dB of noise reduction was routinely obtained. A continuous-mode traveling-wave theory including the effects of dispersion and loss has been employed to model the experimental results.
Squeezed light with partial coherence is generated via optical parametric amplification (OPA) in a KTiOPO4 crystal that is pumped by the second harmonic of a Q-switched multimode Nd:YAG laser.1 More than 1 dB of squeezing is observed when a proper mode of the downconverted light is homodyned with the 380-ns multilongitudinal-mode local oscillator (LO) pulse. In such a system, large OPA gain is easily obtained in a single-crystal signal-pass setup. The detection of squeezed light is, however, not straightforward. In the usual implementation of a balanced homodyne detection scheme,2 the vacuum-noise level, as measured on a radio frequency spectrum analyzer, is determined by the average LO power. If the LO is derived from a Q-switched laser, as is the case in our experiment, the associated peak power can easily saturate the detectors and the subsequent electronics. To overcome this limitation, we have implemented a hybrid frequency/time domain measurement scheme in which the vacuum noise level is determined by the peak (instead of the average) LO power. The LO average power requirement is thus relaxed and the detector saturation problem avoided.
We demonstrate the methods for increasing the observed squeezing level in the generation of squeezed states of light at 1064 nm with periodically poled KTiOPO4 (PPKTP) crystal. We analyze the technical limits to the reduction of noise in the squeezed quadrature theoretically, including the intra-cavity loss of the optical parametric amplifier (OPA) cavity, the normalized amplitude, the detection frequency, and the mode matching on the balanced homodyning stage. To observe a high degree of squeezing, we improve the quality of mode matching into the OPA cavity and the mode cleaning (MC) cavity experimentally. By optimizing mode matching of the light into cavities, the finesse of the cavities would be higher in practice and the desirable spatial mode can be realized. As the intensity noise of the laser light reach the shot noise limit above the frequency of 1 MHz, which is sufficient for our experiment, so the MC cavity is incorporated to filter higher-order transverse modes of the local oscillator (LO) beam for the balanced homodyne detection. The experimental and theoretical results show that it is essential to optimize the mode matching efficiency to generate and detect high degree of squeezing otherwise an inefficiency mode matching will throw away the squeezing and transform the squeezed field into a vacuum field.
A transient noise measurement scheme is described to observe pulsed squeezing generated by means of parametric downconversion of a Q-switched laser. In the usual implementation of balanced-homodyne detection, the vacuumstate noise level is determined by the average local oscillator power. However, if the local oscillator is derived from a pulsed laser, as is the case in our experiments, the associated peak power can easily saturate the detectors along with the subsequent electronics. To overcome this limitation we have implemented a hybrid frequency-time-domain measurement scheme in which the vacuum-state noise level is determined by the peak, instead of the average, local oscillator power, thus avoiding the saturation problem.