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Optics Express

Influence of exciton dynamics on the interference of two photons from a microcavity single-photon source

Anthony J. Bennett, D. C. Unitt, Andrew J. Shields, P. Atkinson, D. A. Ritchie

Published Jan 1, 2005• 46 citations
Publisher

Abstract

One- and two-photon interference visibilities observed with the exciton emission from a quantum dot microcavity single-photon source are sensitive to the excitation conditions. In particular, the coherence time of the source is reduced with increasing pump power or excitation of the barrier layers. Furthermore, the two-photon interference visibility is affected by a long lived population of the biexciton state in the dot. This suggests that two-photon interference may be improved by controlling the exciton dynamics in the dot or by improved temporal resolution of the detection set-up.

DOI
10.1364/opex.13.007772

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One- and two-photon interference visibilities observed with the exciton emission from a quantum dot microcavity single-photon source are sensitive to the excitation conditions. In particular, the coherence time of the source is reduced with increasing pump power or excitation of the barrier layers. Furthermore, the two-photon interference visibility is affected by a long lived population of the biexciton state in the dot. This suggests that two-photon interference may be improved by controlling the exciton dynamics in the dot or by improved temporal resolution of the detection set-up.
A well controlled, triggered source of entangled photons is desirable for many applications in quantum information processing. The two-photon cascade from a biexciton state, two electrons and holes, confined in a quantum dot can be such a source of polarisation-entangled photons provided the two decay paths from the biexciton carry no “which-path” information. In recent years many techniques have been employed to make the two optical decay paths from the biexciton state indistinguishable. A number of these techniques will be discussed and entangled-photon emission from a single quantum dot with a high fidelity in the expected Bell-state from the cascade will be demonstrated. This source of entangled photons allows optical interferometry beyond the limits imposed by the photon wavelength. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The effect of the exciton fine-structure splitting on our entangled photon source will be shown. Surprisingly the entanglement is found to persist despite relatively large separations between the intermediate energy levels of up to 4µeV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We distinguish between the first-order coherence time, and a parameter defined as the cross-coherence time, this is illustrated in the figure.
The visibility of two-photon interference is limited by the indistinguishability of the photons. In the cascaded emission of a three-level system, such as a single quantum dot, the indistinguishability of each photon in the pair is primarily affected by two main factors: the temporal correlation between paired photons and dephasing. Investigating the individual effects of these factors on photon indistinguishability is challenging, as both factors affect it simultaneously. In this study, we investigate the temperature-dependent two-photon interference visibility of the biexciton and exciton photons emitted from a single quantum dot under two-photon resonant excitation, while keeping the temporal correlation between the paired photons intact. Finally, we simultaneously extract the coherence times of the biexciton and exciton photons as a function of temperature.
The observation of quantum mechanical interference in the exciton photoluminescence excitation (PLE) process in a semiconductor quantum dot is reported. A simple PLE spectroscopy revealed in a single InGaAs quantum dot a dynamic and continuous evolution of a resonance profile from a peak into a dip and to near transparency caused by an increase of excitation power density of only a few 100 W/cm2. A one-photon process via exciton states and a two-photon process via biexciton states, both of which return to the radiative exciton state, interfere quantum mechanically, resulting in a progressive decrease in exciton absorption under increasing excitation.
We report a joint experimental and theoretical study of the interference properties of a single-photon source based on a In(Ga)As quantum dot embedded in a quasiplanar GaAs microcavity. Using resonant laser excitation with a pulse separation of 2 ns, we find near-perfect interference of the emitted photons, and a corresponding indistinguishability of $\mathcal{I}=(99.6{\phantom{\rule{0.16em}{0ex}}}_{\ensuremath{-}\phantom{\rule{0.16em}{0ex}}1.4}^{+\phantom{\rule{0.16em}{0ex}}0.4})%$. For larger pulse separations, quasiresonant excitation conditions, increasing pump power, or with increasing temperature, the interference contrast is progressively and notably reduced. We present a systematic study of the relevant dephasing mechanisms and explain our results in the framework of a microscopic model of our system. For strictly resonant excitation, we show that photon indistinguishability is independent of pump power, but strongly influenced by virtual phonon-assisted processes which are not evident in excitonic Rabi oscillations.
The strong confinement of semiconductor excitons in a quantum dot gives rise to atomlike behavior. The full benefit of such a structure is best observed in resonant excitation where the excited state can be deterministically populated and coherently manipulated. Because of the large refractive index and device geometry it remains challenging to observe resonantly excited emission that is free from laser scattering in III/V self-assembled quantum dots. Here we exploit the biexciton binding energy to create an extremely clean single photon source via two-photon resonant excitation of an InAs/GaAs quantum dot. We observe complete suppression of the excitation laser and multiphoton emissions. Additionally, we perform full coherent control of the ground-biexciton state qubit and observe an extended coherence time using an all-optical echo technique. The deterministic coherent photon pair creation makes this system suitable for the generation of time-bin entanglement and experiments on the interaction of photons from dissimilar sources.
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.