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Applied Physics Letters

Robust generation of bright two-color entangled optical beams from a phase-insensitive optical parametric amplifier

Xiaomin Guo, Jun-Jun Zhao, Yongmin Li

Published Feb 27, 2012• 11 citations
Publisher

Abstract

Bright two-color continuous variable (CV) entangled optical beams at 0.8 and 1.5 μm are generated by utilizing a phase-insensitive optical parametric amplifier with only 1.5 μm signal field injected. Without locking the relative phase between the signal and pump fields, the amplitude quadrature difference squeezing of 3.30 dB and phase quadrature sum squeezing of 3.35 dB are observed, which satisfy the entanglement criterion. The demonstrated scheme here eliminates the necessity of precise phase control between the signal and pump fields and enables robust generation of two-color CV entangled states.

DOI
10.1063/1.3690876

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Bright two-color continuous variable (CV) entangled optical beams at 0.8 and 1.5 μm are generated by utilizing a phase-insensitive optical parametric amplifier with only 1.5 μm signal field injected. Without locking the relative phase between the signal and pump fields, the amplitude quadrature difference squeezing of 3.30 dB and phase quadrature sum squeezing of 3.35 dB are observed, which satisfy the entanglement criterion. The demonstrated scheme here eliminates the necessity of precise phase control between the signal and pump fields and enables robust generation of two-color CV entangled states.
We present a scheme for generating and homodyne detecting of continuous-variable entanglement of bright optical beams with a large wavelength difference by utilizing an optical parametric oscillator (OPO) and an optical parametric amplifier (OPA) simultaneously. Entangled optical beams at 0.8 and 1.5 \ensuremath{\mu}m are generated from the OPA; the seed beams injected in the OPA as well as the local oscillators at the two wavelengths needed for homodyne detection are provided by the OPO. The entangler is a ring resonator involving a second-order nonlinear crystal that is pumped from two opposite directions. In one direction the pump power is above the oscillation threshold and the optical nonlinear resonator operates as an OPO. In the other direction the pump power is below the threshold and it operates as a phase-sensitive frequency nondegenerate optical parametric amplifier. Our scheme combines the advantages of both OPO and OPA quantum optical devices and opens another avenue for preparation and homodyne detection of high quality bright entangled light with a large wavelength difference.
We observed continuous-variable entanglement between two bright beams emitted above threshold by a phase-difference-locked ultrastable optical parametric oscillator. The amplitude-difference squeezing is -3 dB and the phase-sum one is -1.35 dB. The beams are classically phase-locked at a frequency difference of 161.8273240(5) MHz, continuously tunable by the OPO crystal temperature. Such unprecedented frequency-difference stability paves the way for implementing quantum frequency multiplexing and CV quantum information in the frequency domain.
We demonstrate experimentally quantum frequency up-conversion of a continuous variable entangled optical field via sum-frequency-generation process. The two-color entangled state initially entangled at 806 and 1518 nm with an amplitude quadrature difference squeezing of 3.2 dB and phase quadrature sum squeezing of 3.1 dB is converted to a new entangled state at 530 and 1518 nm with the amplitude quadrature difference squeezing of 1.7 dB and phase quadrature sum squeezing of 1.8 dB. Our implementation enables the observation of entanglement between two light fields spanning approximately 1.5 octaves in optical frequency. The presented scheme is robust to the excess amplitude and phase noises of the pump field, making it a practical building block for quantum information processing and communication networks.
We demonstrate the experimental generation of quadrature amplitude entanglement in the 1550 nm band by using a fiber optical parametric amplifier. The measured noise variances of the difference and sum of the quadrature amplitudes of the pulsed signal and idler twin beams fall below the shot noise limit by about 1 and 0.8 dB (4.2 and 3.6 dB after the correction for efficiency), respectively, showing that the inseparability criterion of Einstein-Podolsky-Rosen entanglement I<2 is satisfied. Our investigation reveals that the quality of the measured entanglement can be further improved by increasing the transmission efficiency of the twin beams and by optimizing the temporal mode matching of the two sets of homodyne detection systems.
We present the first measurement of squeezed-state entanglement between the twin beams produced in an optical parametric oscillator operating above threshold. In addition to the usual squeezing in the intensity difference between the twin beams, we have measured squeezing in the sum of phase quadratures. Our scheme enables us to measure such phase anticorrelations between fields of different frequencies. In the present measurements, wavelengths differ by approximately 1 nm. Entanglement is demonstrated according to the Duan et al. criterion [Phys. Rev. Lett. 84, 2722 (2000)] Delta2p- + Delta2q+ = 1.41(2) < 2. This experiment opens the way for new potential applications such as the transfer of quantum information between different parts of the electromagnetic spectrum.
Continuous-variable entangled optical beams at the degenerate wavelength of 0.8 μm or 1.5 μm have been investigated extensively, but separately. The two-color entangled states of these two useful wavelengths, with sufficiently high degrees of entanglement, still lag behind. In this work, we analyze the various limiting factors that affect the entanglement degree. On the basis of this, we successfully achieve 6 dB of two-color quadrature entangled light beams by improving the escape efficiency of the nondegenerate optical amplifier, the stability of the phase-locking servo system, and the detection efficiency. Our entangled source is constructed only from a single ring optical resonator, and thus is highly compact, which is suitable for applications in long-distance quantum communication networks.