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arXiv:2306.05676 · Physical Review A

Improving a quantum-dot-based single-photon source with continuous measurements

Anirudh Lanka, Todd A. Brun

Published Nov 14, 2023• 3 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.108.053711

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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.
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.
The master equation for the microcavity-quantum dot coupled system is established based on the quantum regression theorem,and the time-dependent evolution of the density matrix of the two-level system for a quantum dot in a microcavity is studied under pumping by laser pulses.The characteristics such as the number of emitted photons and the second-order corelation function are derived from the density matrixes.Parameters of the microcavity and the pumping conditions are optimized for single-photon sources.The relativity between the microcavity decay rate and coupling coefficient is critical for single photon emission.
A single photon source is an enabling technology in device-independent quantum communication, quantum simulation, linear optics-based and measurement-based quantum computing. These applications employ many photons and place stringent requirements on the efficiency of single photon creation. The scaling on efficiency is typically an exponential function of the number of photons. Schemes taking full advantage of quantum superposition also depend sensitively on the coherence of the photons, i.e. their indistinguishability. \nThis thesis reports a single-photon source with a high end-to-end efficiency. We employ gated quantum dots in an open, tunable microcavity. The gating provides control of the charge and electrical tuning of the emission frequency, the high-quality material and careful design of the system ensures low noise, and the tunability of the microcavity compensates for the lack of control in quantum dot position and emission frequency. Transmission through the top mirror is the dominant escape route for photons from the microcavity, and this output is well-matched to a single-mode fiber. With this design, we can create a single photon at the output of the final optical fiber on-demand with a probability of 57% and with an average two-photon interference visibility of 97.5%. Coherence persists in trains of thousands of photons with single photon creation at a repetition rate of 1 GHz.
Semiconductor quantum dots (QDs) coupled with optical microcavities provide an excellent platform for generating high-performance single-photon sources. However, the current microcavity structures are difficult to solve the tradeoff between single-photon purity and indistinguishability without additional filtering, due to challenges in suppressing broad phonon sideband spectrum and two-photon emission in the practical experiments. To address these issues, we have developed a novel asymmetric microcavity that supports two nondegenerate polarized modes, both resonant with a single quantum dot. One cavity mode is broadband and is used for excitation to reduce two-photon emission intensity, while the other is narrowband and is used for collection to enhance the indistinguishability of single-photon sources. This asymmetric microcavity also has the ability to overcome at least half of the photon efficiency loss under polarization filtering. Moreover, our model has employed an excitation pulse that resonates with cavity modes eliminating spectral distortion of intracavity fields, and efficiently achieving deterministic population inversion of QDs compared to an elliptical cavity. Our scheme paves the way to develop the excellent performance single-photon sources with higher brightness, near-unity purity, and indistinguishability without any additional filtering in practical experiments.
Abstract Cavity quantum electrodynamics is widely used in many solid-state systems for improving quantum emitter performances or accessing specific physical regimes. For these purposes it is fundamental that the non-classical emitter, like a quantum dot or an NV center, matches the cavity mode, both spatially and spectrally. In the present work, we couple single photons stemming from In(Ga)As quantum dots into an open fiber-based Fabry–Pérot cavity. Such a system allows for reaching an optimal spatial and spectral matching for every present emitter and every optical transition, by precisely tuning the cavity geometry. In addition to that, the capability of deterministically and repeatedly locating a single quantum dot enables to compare the behavior of the quantum emitter inside the cavity with respect to before it is placed inside. The presented open-cavity system shows full flexibility by precisely tuning in resonance different QD transitions, namely excitons, biexcitons and trions. A measured Purcell enhancement of 4.4 ± 0.5 is obtained with a cavity finesse of about 140, while still demonstrating a single-photon source with vanishing multi-photon emission probability.
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.