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arXiv:0802.4114 · Physical Review A

Continuous monitoring can improve indistinguishability of a single-photon source

Shesha Raghunathan, Todd A. Brun

Published Mar 20, 2009• 4 citations
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Abstract

An engineering technique using continuous quantum measurement in conjunction with feed forward is proposed to improve indistinguishability of a single-photon source. The technique involves continuous monitoring of the state of the emitter, processing the noisy output signal with a simple linear estimation algorithm, and feed forward to control a variable delay at the output. In the weak-coupling regime, the information gained by monitoring the state of the emitter is used to reduce the time uncertainty inherent in photon emission from the source, which improves the indistinguishability of the emitted photons.

DOI
10.1103/physreva.79.033831

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An engineering technique using continuous quantum measurement in conjunction with feed forward is proposed to improve indistinguishability of a single-photon source. The technique involves continuous monitoring of the state of the emitter, processing the noisy output signal with a simple linear estimation algorithm, and feed forward to control a variable delay at the output. In the weak-coupling regime, the information gained by monitoring the state of the emitter is used to reduce the time uncertainty inherent in photon emission from the source, which improves the indistinguishability of the emitted photons.
An engineering technique using continuous quantum measurement together with a change detection algorithm is proposed to improve the probability of single photon emission for a quantum-dot based single-photon source. The technique involves continuous monitoring of the emitter, integrating the measured signal, and a simple change detection circuit to decide when to stop pumping. The idea is to pump just long enough such that the emitter +++ cavity system is in a state that can emit at most one photon with high probability. Continuous monitoring provides partial information on the state of the emitter. This technique is useful when the system is operating in the weak coupling regime, and the rate of pumping is smaller than, or comparable to, the emitter-cavity coupling strength, as can be the case for electrical pumping.
We propose a technique to improve the probability of single-photon emission with an electrically pumped quantum dot in an optical microcavity by continuously monitoring the dot's energy state and using feedback to control when to stop pumping. The goal is to boost the probability of single-photon emission while bounding the probability of two or more photons. We model the system by a stochastic master equation that includes postmeasurement operations. Ideally, feedback should be based on the entire continuous measurement record, but in practice, it may be difficult to do such processing in real time. We show that even a simple threshold-based feedback scheme using measurements at a single time can improve performance over deterministic (open-loop) pumping. This technique is particularly useful for strong dot-cavity coupling with lower rates of pumping, as can be the case for electrical pumping. It is also numerically tractable since we can perform ensemble averaging with a single master equation rather than averaging over a large number of quantum trajectories.
We propose a continuous-variable quantum interface that transfers an arbitrary unknown quantum state between a well-controllable source oscillator and a weakly controllable noisy target oscillator through an arbitrary Gaussian coupling. The unit fidelity of transfer is achieved by the preprocessing of the source state involving an ancillary system, collective measurement of the source after the coupling together with the ancilla, and by a subsequent feed-forward correction of the target. Particularly, these techniques allow the transfer of quantum states of light to arbitrarily noisy matter systems.
The indistinguishability of successively generated photons from a single quantum emitter is most commonly measured using two-photon interference at a beam splitter. Whilst for sources excited in the pulsed regime the measured bunching of photons reflects the full wave-packet indistinguishability of the emitted photons, for continuous wave (cw) excitation, the inevitable dependence on detector timing resolution and driving strength obscures the underlying photon interference process. Here we derive a method to extract full photon wave-packet indistinguishability from cw measurements by considering the relevant correlation functions. The equivalence of both methods is experimentally verified through a comparison of cw and pulsed excitation measurements on an archetypal source of photons, a single molecule.
A source of single photons allows secure quantum key distribution, in addition, to being a critical resource for linear optics quantum computing. We describe our progress on deterministically creating single photons from spontaneous parametric downconversion, an extension of the Pittman, Jacobs and Franson scheme [Phys. Rev A, v66, 042303 (2002)]. Their idea was to conditionally prepare single photons by measuring one member of a spontaneously emitted photon pair and storing the remaining conditionally prepared photon until a predetermined time, when it would be "deterministically" released from storage. Our approach attempts to improve upon this by recycling the pump pulse in order to decrease the possibility of multiple-pair generation, while maintaining a high probability of producing a single pair. Many of the challenges we discuss are central to other quantum information technologies, including the need for low-loss optical storage, switching and detection, and fast feed-forward control.
Solid-state sources capable of emitting single photons on demand are of great interest in quantum information applications. Ideally, such a source should emit exactly one photon into the collection optics per trigger, the emitted photons should be indistinguishable and the source should be electrically driven. Several design strategies addressing these requirements have been proposed. In the cavity-based source, light emission is controlled using resonant cavity quantum electrodynamics effects, whereas in the waveguide-based source, broadband electric field screening effects are employed to direct the light emission into the optical mode of interest. For all the strategies, accurate modeling and careful optical engineering is required to achieve high performance.