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arXiv:quant-ph/0403224 · Physical Review A

Compact source of Einstein-Podolsky-Rosen entanglement and squeezing at very low noise frequencies

Julien Laurat, T. Coudreau, G. Keller, Nicolas Treps, Claude Fabre

Published Oct 25, 2004• 66 citations
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Abstract

We report on the experimental demonstration of strong quadrature Einstein-Podolsky-Rosen entanglement and squeezing at very low noise sideband frequencies produced by a single type-II, self-phase-locked, frequency degenerate optical parametric oscillator below threshold. The generated two-mode squeezed vacuum state is preserved for noise frequencies as low as $50\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}$. Designing simple setups able to generate nonclassical states of light in the kHz regime is a key challenge for high sensitivity detection of ultraweak physical effects such as gravitational wave or small beam displacement.

DOI
10.1103/physreva.70.042315

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We report on the experimental demonstration of strong quadrature Einstein-Podolsky-Rosen entanglement and squeezing at very low noise sideband frequencies produced by a single type-II, self-phase-locked, frequency degenerate optical parametric oscillator below threshold. The generated two-mode squeezed vacuum state is preserved for noise frequencies as low as 50kHz50\phantom{\rule{0.3em}{0ex}}\mathrm{kHz}50kHz. Designing simple setups able to generate nonclassical states of light in the kHz regime is a key challenge for high sensitivity detection of ultraweak physical effects such as gravitational wave or small beam displacement.
We report on the experimental demonstration of strong quadrature EPR entanglement and squeezing at very low noise sideband frequencies produced by a single type-II, self-phase-locked, frequency degenerate optical parametric oscillator below threshold. The generated two-mode squeezed vacuum state is preserved for noise frequencies as low as 50 kHz. Designing simple setups able to generate non-classical states of light in the kHz regime is a key challenge for high sensitivity detection of ultra-weak physical effects such as gravitational wave or small beam displacement.
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Einstein-Podolsky-Rosen (EPR) entanglement is a criterion that is more demanding than just certifying entanglement. We theoretically and experimentally analyze the low-resource generation of bipartite continuous-variable entanglement, as realized by mixing a squeezed mode with a vacuum mode at a balanced beam splitter, i.e., the generation of so-called vacuum-class entanglement. We find that in order to observe EPR entanglement the total optical loss must be smaller than 33.333.333.3 %. However, arbitrarily strong EPR entanglement is generally possible with this scheme. We realize continuous-wave squeezed light at 155015501550 nm with up to 9.99.99.9 dB of nonclassical noise reduction, which is the highest value at a telecom wavelength so far. Using two phase-controlled balanced homodyne detectors we observe an EPR covariance product of $0.502\ifmmode\pm\else\textpm\fi{}0.006<1$, where 111 is the critical value. We discuss the feasibility of strong Gaussian entanglement and its application for quantum key distribution in a short-distance fiber network.
We demonstrate the generation of broadband continuous-wave optical squeezing from 280 Hz-100 kHz using a below-threshold optical parametric oscillator (OPO). The squeezed state phase was controlled using a noise locking technique. We show that low frequency noise sources, such as seed noise, pump noise, and detuning fluctuations, present in optical parametric amplifiers, have negligible effect on squeezing produced by a below-threshold OPO. This low frequency squeezing is ideal for improving the sensitivity of audio frequency measuring devices such as gravitational-wave detectors.
As important members of nonclassical states of light, squeezed states and entangled states are basic resources for realizing quantum measurements and constructing quantum information networks. We experimentally demonstrate that the two types of nonclassical optical states can be generated from an optical parametric oscillator (OPO) involving a periodically poled KTiOPO4 crystal with a domain-inversion period of 51.7 μm, by changing the polarization of the pump laser. When a vertically polarized 671 nm laser is used to pump the OPO, the intra-cavity frequency-down-conversion with type-0 quasi-phase matching is realized and the output optical beam is a quadrature amplitude squeezed state of light at the wavelength of 1342 nm with the fluctuation of quadrature component of 3.17 dB below the quantum noise limit (QNL). If the pump laser is horizontally polarized, the condition of the type-II quasi-phase matching is satisfied and the output optical beam becomes Einstein-Podolsky-Rosen entangled state of light with correlation variances of both quadrature amplitude-sum and quadrature phase-difference of 2.2 dB below the corresponding QNL.
The bright Einsten-Podolsky-Rosen (EPR) beams with the quantum correlations between the quadrature-phase amplitudes of the spatially separated signal and idler beams have been experimentally generated from a cw nondegenerate optical parametric amplifier injected by seed waves with degenerate frequency but orthogonal polarization. The correlation degree of $0.853\ifmmode\pm\else\textpm\fi{}0.004$ between the quadrature-phase amplitudes of the output entangled beams is directly inferred from the measured quadrature-phase squeezing of the output vacuum squeezed-state light field formed from superposition of the original signal and idler modes. Our theoretical calculation and experimental measurements provide a reliable method to confirm the quadrature phase-squeezing and EPR correlation of bright light field.