Open AccessJournal Article10.1103/PHYSREVA.100.023840

Quantum state correction using a measurement-based feedforward mechanism

- 26 Aug 2019

- Vol. 100, Iss: 2, pp 023840

TL;DR: In this article, a state comparison amplifier for optical coherent states, which features an active measurement and feed-forward mechanism to correct for errors made during the initial amplification, is presented.

Abstract: One of the weaknesses of quantum optical state postselection schemes is the low success probability. Typically there is a trade-off between amplifier properties such as success probability and output state fidelity. However, here we present a state comparison amplifier for optical coherent states, which features an active measurement and feedforward mechanism to correct for errors made during the initial amplification. The simple and relatively low latency mechanism allows us to correct for a binary phase alphabet. We demonstrate a significant simultaneous improvement in the amplifier characteristic parameters: output state fidelity, correct state fraction, and success probability. This demonstrates that nondeterministic quantum amplification can be enhanced significantly by measurement and feedforward.

1. What are the contributions mentioned in the paper "Quantum state correction using a measurement-based feedforward mechanism" ?

In this paper, a measurement and feed-forward mechanism was applied to the SCAMP, which allowed the key characteristic parameters of this nondeterministic amplifier to improve.

2. What are the future works mentioned in the paper "Quantum state correction using a measurement-based feedforward mechanism" ?

As SCAMP has been demonstrated with larger phase alphabets [ 17,19 ], a follow-on experiment would be to investigate the possibility of improving those higher-order alphabets with an adapted mechanism.

FIG. 5. Operation of the feedforward switch mechanism. Incoming photons from the first state comparison stage, beam splitter 1 (BS1), triggered the single-photon avalanche diode (SPAD). The electrical output of the detector was split to record photon arrival time and trigger the switch. The switch had two electrical inputs to set the second conditional guess. Channel 1 applied a voltage corresponding to the initial guess. Channel 2 applied a voltage corresponding to a change in initial guess by π . The electrical output was connected to the second lithium-niobate phase modulator (PM2).

FIG. 1. Schematic diagram of the state comparison amplifier with the measurement feedforward mechanism between two state comparison stages. The amplifier first makes a comparison between an input signal state and locally prepared initial guess state on beam splitter 1 (BS1). One output is monitored by detector D0. This detector sets the phase of the second conditional guess. When triggered, the feedforward switch applies a phase shift of π . Red lines and arrows indicate optical fiber, while blue indicates electrical connections. Phase modulators are denoted PM.

TABLE I. Postselection heralding conditions for the feedforward state comparison amplifier operating with measurement and feedforward active and inactive. When the feedforward mechanism is inactive, there is no correction to the second guess phase encoding (φ2) when the trigger detector records an event. When the feedforward mechanism is active, the second guess is altered by a phase shift of π , corresponding to a voltage of Vπ , relative to the initial guess.

FIG. 6. The interferometric visibility at the first state comparison stage and the conditional correct state fraction. This plot shows that the visibility of the first state comparison stage and the conditional correct state fraction are related. If the visibility of the first state comparison stage were to reduce, so would the conditional correct state fraction.

FIG. 7. The relative improvement of the feedforward operation of the state comparison amplifier over the nonfeedforward device. As the mean-photon number increases, so too does the relative improvement. This is due to an increase in the detection rate for a wrong initial guess.

FIG. 2. State comparison amplifier characteristic parameters when the feedforward mechanism is active and inactive. The parameters of interest are (a) fidelity, (b) correct state fraction, (c) success probability, and (d) intensity gain. For all subfigures, the experimental data are represented by the points, while the dashed lines of the same color are the corresponding theoretical predictions. It can be seen in (a)–(c) that when the feedforward is active, there is an improvement to the parameters. In (d), the diversion of experimentally measured gain and theory is due to the nonlinear response caused by dead time of the single-photon detectors at very high count rates. The errors correspond to less than 2% of the actual values, and hence the data-point values cover the error bars.

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Quantum state correction using a measurement-based feedforward mechanism

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