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Conference on Lasers and Electro-Optics

EPR correlation and quantum noise squeezing of output light fields from a narrowband NOPA

Changde Xie, Yun Zhang, Hai Wang, Xiaoying Li, Jietai Jing, Kunchi Peng

Published Aug 29, 2005• 0 citations
Publisher

Abstract

Our calculation and experiment presented an indirect and undoubted path to demonstrate the quantum correlation between bright EPR beams, that bypassed the technical difficulty in these types of experiments. A system of nondegenerate optical parametric amplifiers (NOPA) in which the interconnected squeezed vacuum field, bright coherent-squeezed state light and EPR beams can be obtained simultaneously is also very interesting for practical applications.

DOI
10.1109/iqec.2000.907758

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Our calculation and experiment presented an indirect and undoubted path to demonstrate the quantum correlation between bright EPR beams, that bypassed the technical difficulty in these types of experiments. A system of nondegenerate optical parametric amplifiers (NOPA) in which the interconnected squeezed vacuum field, bright coherent-squeezed state light and EPR beams can be obtained simultaneously is also very interesting for practical applications.
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.
Twin beams generated from a nondegenerate optical parametric amplifier (NOPA) of gaing possess1 an intensity correlation that falls below the quantum limit by (2g – 1). This occurs whenever the input idler to the NOPA is in the vacuum state. Such input is practically advantageous because it makes the NOPA output intensities phase insensitive. When a non-vacuum idler input state is chosen, the NOPA output intensities become phase sensitive. For a proper input state and a suitable pump phase the output intensity correlation falls below the quantum limit by ( g + g − 1 ) 2 , which is 3 dB superior to the vacuum-idler-input result for g >> 1 and is equivalent to the quantum-noise reduction obtainable from a squeezed state generated with the same gain. Thus the sensitivity improvement achievable with NOPA twin beams by using direct detection is equivalent to that obtainable with squeezed states by using homodyne detection. The above equivalence also provides a new scheme for the generation of pulsed squeezed light that, in addition to being simple, automatically generates a matched local oscillator2 for homodyne detection of the squeezed pulses.
The theoretical limit of the noise correlation between the signal and idler outputs of a nondegenerate optical parametric amplifier (NOPA) with a coherent state signal and vacuum state idler input can be enhanced if a non-vacuum coherent state idler input is employed. By choosing a balanced signal and idler input, the noise correlation is $1/{({\root}g + {\root}{g-1})}^2$ , where g is the intensity gain of the NOPA, and that is superior to the prediced outputs with single signal input by approximately 3dB. The result is applicable to all the schemes that use the NOPA to produce a sub-shot noise light generation such as feed-back or feed-forward control.
Quantum fluctuations in a nondegenerate optical parametric amplifier (NOPA) are investigated experimentally with a squeezed state coupled into the internal idler mode of the NOPA. Reductions of the inherent quantum noise of the amplifier are observed with a minimum noise level 0.7 dB below the usual noise level of the amplifier with its idler mode in a vacuum state. With two correlated quantum fields as the amplifier's inputs and proper adjustment of the gain of the amplifier, it is shown that the amplifier's intrinsic quantum noise can be completely suppressed so that noise-free amplification is achieved. It is also shown that the NOPA, when coupled to either a squeezed state or a nonclassically correlated state, can realize quantum tapping of optical information.
The quantum fluctuations and correlations of the output signal and idler beams from a type-II polarization nondegenerate optical parametric amplifier (NOPA) inside an optical cavity are investigated theoretically, which is driven by the different kinds of quantized fields such as the two-mode thermal field, the phase-conjugate state, and a pair of EPR beams at the signal and idler frequency. Spectral line shapes due to quantum interferences between the input quantum fields and the generated down-converted subharmonic fields of NOPA are studied by scanning the length of the NOPA cavity. The entanglement degree of the EPR entangled beams injected into the NOPA as signal and idler beams can be increased or decreased, which are realized by controlling the relative phase between the pump beam and the injected beams for the NOPA. The results demonstrate coherent phenomena of NOPA in the quantum regime and show phase-sensitive manipulations of quantum entanglement for quantum information processing.
This paper presents formulae for Einstein-Podolsky-Rosen (EPR) entanglement generated from N nondegenerate optical parametric amplifiers (NOPAs) interconnected in a linear coherent feedback (CFB) chain in the idealized lossless scenario and infinite bandwidth limit. The lossless scenario sets the ultimate EPR entanglement (two-mode squeezing) that can be achieved by this linear chain of NOPAs while the infinite bandwidth limit simplifies the analysis but gives an accurate approximation to the EPR entanglement at low frequencies of interest. Two adjustable phase shifts are placed at the outputs of the system to achieve the best EPR entanglement by selecting appropriate quadratures of the output fields.