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arXiv:1504.07345 · Scientific Reports

Quantum information tapping using a fiber optical parametric amplifier with noise figure improved by correlated inputs

Xueshi Guo, Xiaoying Li, Nannan Liu, Z. Y. Ou

Published Jul 26, 2016• 19 citations

Abstract

One of the important functions in a communication network is the distribution of information. It is not a problem to accomplish this in a classical system since classical information can be copied at will. However, challenges arise in quantum system because extra quantum noise is often added when the information content of a quantum state is distributed to various users. Here, we experimentally demonstrate a quantum information tap by using a fiber optical parametric amplifier (FOPA) with correlated inputs, whose noise is reduced by the destructive quantum interference through quantum entanglement between the signal and the idler input fields. By measuring the noise figure of the FOPA and comparing with a regular FOPA, we observe an improvement of 0.7 ± 0.1 dB and 0.84 ± 0.09 dB from the signal and idler outputs, respectively. When the low noise FOPA functions as an information splitter, the device has a total information transfer coefficient of Ts+Ti = 1.5 ± 0.2, which is greater than the classical limit of 1. Moreover, this fiber based device works at the 1550 nm telecom band, so it is compatible with the current fiber-optical network for quantum information distribution.

DOI: 10.1038/srep30214

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One of the important functions in a communication network is the distribution of information. It is not a problem to accomplish this in a classical system since classical information can be copied at will. However, challenges arise in quantum system because extra quantum noise is often added when the information content of a quantum state is distributed to various users. Here, we experimentally demonstrate a quantum information tap by using a fiber optical parametric amplifier (FOPA) with correlated inputs, whose noise is reduced by the destructive quantum interference through quantum entanglement between the signal and the idler input fields. By measuring the noise figure of the FOPA and comparing with a regular FOPA, we observe an improvement of 0.7 ± 0.1 dB and 0.84 ± 0.09 dB from the signal and idler outputs, respectively. When the low noise FOPA functions as an information splitter, the device has a total information transfer coefficient of Ts+Ti = 1.5 ± 0.2, which is greater than the classical limit of 1. Moreover, this fiber based device works at the 1550 nm telecom band, so it is compatible with the current fiber-optical network for quantum information distribution.

We experimentally demonstrate an improvement of

0.7\pm 0.1

dB in the noise figure in a low noise fiber-optical parametric amplifier (FOPA) with the injection of correlated quantum fields generated from spontaneous four-wave mixing in fibers. The noise performance of the amplifier is characterized with homodyne detectors. The experimental result shows that the noise levels in the two output ports (signal and idler) of the amplifier are both

1.0\pm 0.1

dB lower than a regular FOPA with vacuum at the unused input port. In addition, we demonstrate the scheme can be used as a quantum information tap with a combined information transfer efficiency (signal and tap) as

T_{sig}+T_{tap} =1.47\pm 0.2

, which is larger than the classical limit of 1. This low noise FOPA can be applied to the low noise amplification in the measurement of weak signals.

We study two squeezed-state-dependent schemes of the quantum optical tap for information encoded in quantum-correlated fields from a two-mode entangled source. We find that, by using two independent squeezed beam splitters, the quantum information encoded in entangled dual beams can be divided into four parts without noise increase. We calculate the corresponding transfer coefficients to confirm that quantum optical tapping is achieved. As a variation to the original tapping schemes, we consider parametric amplifiers to replace beam splitters as tapping devices and find that they work equally well and have the advantage of tolerance to external losses. We also analyze the scalability of the schemes by cascading two outputs into the next stage and find that the output signals of the first stage can be further split noiselessly by the second stage. This shows that the scheme is cascadable and scalable for efficient quantum information distribution in an optical network.

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 demonstrate that a phase-insensitive parametric amplifier, when coupled to a quantum correlated source, can be used as a quantum information tap for noiseless three-way signal splitting. We find that the output signals are amplified noiselessly in two of the three output ports, while the other can more or less keep its original input size without adding noise. This scheme can be cascaded and scaled up for efficient information distribution in an optical network. Furthermore, we find that this scheme satisfies the criteria for a nonideal quantum nondemolition (QND) measurement and thus can serve as a QND measurement device. With two readouts correlated to the input, we find this scheme also satisfies the criterion for a sequential QND measurement.

Optical parametric amplifiers (OPAs) are promising to overcome the wavelength coverage and noise limitations in conventional optical amplifiers based on rare-earth doping and semiconductor gain. However, the high power requirement remains a major obstacle to the widespread use of OPAs. Integrated OPAs can in principle improve the pump efficiency with tight mode confinement; however, challenges associated with propagation loss, limited nonlinearity, and susceptibility to nanoscale fabrication imperfections prevent them from competing with conventional bulk and fiber-based OPAs. Here, we demonstrate a highly efficient integrated OPAs with continuous-wave net gain. The pump efficiency is improved by over one order of magnitude. Phase-sensitive gain of 23.5 dB is demonstrated, significantly exceeding previous integrated OPAs, using only 110 mW pump power and no cavity enhancement. This is achieved with parametric down-conversion in thin-film lithium niobate waveguides using the adapted poling technique to maintain the coherence of nonlinear interactions. Moreover, the high parametric gain exceeds fibre-chip-fibre losses, leading to appreciable net gain up to 10 dB. The 3 dB bandwidth is approximately 120 nm, covering telecommunication S-, C-, and Lbands. Quantum-limited noise performance is confirmed through the measurement of output field fluctuation below the classical limit. We further demonstrate that signalto-noise ratio in noisy optical communications can be increased by leveraging this efficient integrated OPA. Our work marks a significant step towards ideal optical amplifiers with strong amplification, high efficiency, quantum-limited noise, large bandwidth, and continuous-wave operation, unlocking new possibilities for next-generation photonic information processing systems.