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Journal of the Optical Society of America B

Generation of squeezed states of light by intracavity harmonic conversion

Harry J. Kimble, J. L. Hall

Published Jan 1, 1986• 2 citations
OpenAlex

Abstract

In recent years interest in measurement science has been peaked by the possibility of sensitivity beyond the standard quantum limit. Much discussion has centered on states of the radiation field in which the fluctuations in one quadrature are reduced at the expense of the other (squeezing). We report an investigation of the production of squeezed states of light for second harmonic generation within an optical cavity resonant at both fundamental and harmonic frequencies.1,2

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Physical Review A•Oct 1, 1993•5 citations
In recent years interest in measurement science has been peaked by the possibility of sensitivity beyond the standard quantum limit. Much discussion has centered on states of the radiation field in which the fluctuations in one quadrature are reduced at the expense of the other (squeezing). We report an investigation of the production of squeezed states of light for second harmonic generation within an optical cavity resonant at both fundamental and harmonic frequencies.1,2
We have generated squeezed states of light by the process of second harmonic conversion within an optical cavity resonant at both fundamental and harmonic frequencies. The experiment consists of a standing-wave cavity containing a crystal of lithium niobate doped with magnesium oxide and driven by the field from a frequency-stabilized Nd:YAG laser (incident power, ~10 mW). The total field reflected from the cavity illuminates a pair of high quantum efficiency photodiodes, whose individual photocurrents are summed. Observations of squeezing are made by analyzing the spectral density of fluctuations for this sum photo-current while the cavity detuning is scanned through resonance. An intensity regulator holds the reflected light intensity constant during the scan by control of the incident light level. Reductions in photocurrent fluctuations of 12% relative to the vacuum-state or shot-noise level are achieved for frequency offsets near 4 MHz. Since significantly higher degrees of squeezing are predicted by our theoretical analysis, we discuss those processes that currently limit the observed noise reduction, including phase noise produced by the intracavity crystal in the absence of nonlinear conversion.
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.
Squeezed states of light represent a manifestation of the nonclassical character of the electromagnetic field and hence are of great intrinsic interest in quantum optics. There are as well a number of applications in measurement science and optical communication associated with the possibility of sensitivity beyond the standard quan­tum limit. The experiments that we have undertaken attempt to explore the degree of achievable and detectable squeezing in three distinct physical systems. From a conceptual point of view each is in some sense ideal and should provide a large degree of squeezing. However, in practice each offers a potential set of advantages and disadvantages associated with technological constraints and with a lack of precise correspondence between experiment and existing theoretical models. The three experiments that we describe are as follows: (1) degenerate parametric oscillation in a doubly resonant cavity pumped by the frequency-doubled light from a Nd:YAG laser at 1.06 μm, (2) intracavity second harmonic generation in a doubly resonant cavity pumped at a fundamental wave­length of 515 nm with an Ar+ laser, and (3) optical bistability for a collection of two-level atoms within a high finesse interferometer of decay rate comparable to the inverse atomic lifetime.
We analyze the quantum-fluctuation properties of second-harmonic radiation generated in the high-efficiency conversion process used in a recent experiment by Hakuta, Marmet, and Stoicheff [Phys. Rev. Lett. 66, 596 (1991)]. We extend their work by studying the second-harmonic generation inside an optical cavity. The active medium inside the cavity is modeled by three-level atoms with degenerated upper levels (1s, 2p, and 2s states of atomic hydrogen) interacting with a strong constant electric field. An enhancement of the conversion efficiency and a reduction of the added noise are found as a result of an electromagnetically induced transparency. We show that the scheme is capable of producing light in a squeezed or coherent state close to a minimum-uncertainty state with a large mean amplitude.