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Optics Letters

Optimized real-time homodyne detection of bright pico-second twin beams by digital signal processing

Hong-Yi Yan, Wen Zhen Zhao, Xueshi Guo, Xiaoying Li

Published Feb 18, 2025• 1 citation
Publisher

Abstract

Twin beams are useful tools in quantum technologies. However, due to their asymmetry in photon number distribution, a scheme needs to be optimized in order to fully exploit their quantum advantage. Here, we demonstrate a detection scheme for bright pico-second twin beams generated from a fiber optical parametric amplifier, in which the electronic gain of the detecting process is optimized in real time by digital signal processing without accurately calibrating the detection channels. Twin beams of -7.5 dB intensity difference squeezing (IDS) (the highest in an optical fiber system) is generated at a brightness of about 0.1 W in peak power. Our result paves the way for applications of twin beams in time-varying channels with asymmetric loss or noise.

DOI
10.1364/ol.549639

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Twin beams are useful tools in quantum technologies. However, due to their asymmetry in photon number distribution, a scheme needs to be optimized in order to fully exploit their quantum advantage. Here, we demonstrate a detection scheme for bright pico-second twin beams generated from a fiber optical parametric amplifier, in which the electronic gain of the detecting process is optimized in real time by digital signal processing without accurately calibrating the detection channels. Twin beams of -7.5 dB intensity difference squeezing (IDS) (the highest in an optical fiber system) is generated at a brightness of about 0.1 W in peak power. Our result paves the way for applications of twin beams in time-varying channels with asymmetric loss or noise.
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Recently we analyzed an optical-phase feed-forward scheme to produce a tunable squeezed-light beam from the twin beams generated by means of nondegenerate parametric amplification [ Opt. Lett.16, 755 ( 1991)]. Here we present a wideband model of this scheme. The noise characteristics of the homodyne detector that measures squeezing properties of the phase-controlled output signal beam from the parametric amplifier are analyzed in detail. To minimize the squeezed-noise power at an arbitrarily given sideband frequency, we calculate the optimum phase feed-forward transfer function. This feed-forward transfer function is realizable through the use of a bandpass filter and an electronic amplifier. For certain frequency profiles of the gain and phase of the parametric amplifier, the squeezed-noise power spectral density can be minimized over the entire bandwidth of the electronic detector that is used in homodyne detection.
We report generation of pulsed twin beams of light through optical parametric amplification in a fiber Sagnac loop. By pumping the Sagnac loop with picosecond pulses at a wavelength near the zero-dispersion wavelength of the fiber, we achieve phase-matched nondegenerate four-wave mixing with gain. For a gain of 2.2, the intensity noises of the amplified signal and the generated idler (conjugate) pulses are found to be correlated by 5.0 dB, and the subtracted noise drops below the shot-noise limit by 1.1 dB (2.6 dB when corrected for losses). We have investigated the gain dependence of the quantum-noise reduction as well as of the intensity noises of the amplified signal and idler pulses. As the gain increases, we observe the onset of excess noise on the idler pulses.
nonlinearity in an optical fiber can generate optical pulses with continuous variable quantum correlation. However, pairwise correlation of the generated pulses in this system has not been demonstrated. Here we report a time-domain measurement of an intensity difference squeezed state generated in a fiber. With a fast response differential detection system, we show the generated twin-beam pulses are pairwisely correlated, and -3.8-dB (-8.1 dB after detection losses correction) intensity difference squeezing degree is measured in the time domain. Our result is beneficial for generating multi-mode entangled state by time-division multiplexing in fiber system.