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arXiv:1707.02886 · Physical review. B./Physical review. B

Intrinsic and environmental effects on the interference properties of a high-performance quantum dot single-photon source

Stefan Gerhardt, Jake Iles-Smith, Dara P. S. McCutcheon, Yu-Ming He, Sebastian Unsleber, Simon Betzold, Niels Gregersen, Jesper Mørk, Sven Höfling, Christian Schneider

Published May 18, 2018• 27 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.97.195432

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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 I=(99.6\ensuremath−1.4+0.4)\mathcal{I}=(99.6{\phantom{\rule{0.16em}{0ex}}}_{\ensuremath{-}\phantom{\rule{0.16em}{0ex}}1.4}^{+\phantom{\rule{0.16em}{0ex}}0.4})%I=(99.6\ensuremath−1.4+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.
Abstract We report a set of experiments designed to test the quantum indistinguishability of single photons emitted by a resonantly pumped InAs/GaAs quantum dot, embedded in a microcavity. We first performed high resolution interferometric measurements on the source to infer its coherence time. By comparison with the exciton lifetime, obtained by time‐resolved spectroscopy, we found that some quantum dots were emitting almost transform‐limited photons. The measured ratio of the coherence time to the pulse duration gives a lower bound on the dephasing time which agrees well with previous studies. We then directly measured the mean overlap between wave packets of consecutive photons emitted 2 ns apart, through a Hong–Ou–Mandel type two‐photon interference experiment. For the best dot, an overlap as high as 0.81 was inferred. This result suggests that quantum dots could be a practical single photon source to perform quantum information processing experiments. We stress the importance of the microcavity for identical photon generation.
We report on two-photon interference of highly indistinguishable single photons emitted by a quantum dot. Strictly resonant excitation with picosecond laser pulses has been used to prepare coherent states with a significantly increased coherence time $({T}_{2}\ensuremath{\sim}1\text{ns})$ and reduced lifetime $({T}_{1}\ensuremath{\sim}650\text{ps})$, as compared to a nonresonant excitation scheme. Indistinguishable photons, with visibilities greater than 70%, have been observed by measuring the Hong-Ou-Mandel dip without postselection of the interfering photons. Near-unity indistinguishable photons should be achievable by preventing fluctuations in the electrostatic environment in the vicinity of the dots, considered as an important source of decoherence.
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.
We report on two-photon interferences from highly indistinguishable single photons emitted by a quantum dot. Stricly resonant excitation with picosecond laser pulses allows coherent state preparation with a signifi- cantly increased coherence time (T2 \sim 1 ns) and reduced lifetime (T1 \sim 650 ps), as compared to a non-resonant excitation scheme. Building-up the Hong-Ou-Mandel dip without post-selection of the interfering photons, visi- bilities greater than 70 % have been observed. Near-unity indistinguishable photons could be achieved for every dot if charge noise is controlled. Indeed, the remaining decoherence mechanism is likely due to the fluctuating electrostatic environment of the dots.
For photonic quantum technology applications, sources capable of emitting photons with indistinguishability close to unity are essential. Ideally, these sources should not require demanding cooling systems. Here, we present temperature-dependent two-photon-interference measurements on photons produced by the radiative decay of the negative trion in a low-noise GaAs quantum dot, which are in quantitative agreement with theoretical calculations accounting for carrier-phonon interactions and coupling to excited states. While at at the lowest explored temperatures the emission linewidth reaches values only 6(2) % above the Fourier limit and the indistinguishability I between subsequently emitted photons reaches 0.966(6), the latter drops to 0.05(4) at 55 K. We show that this loss can be explained with the coupling with energetically close excited trion states and suggest that the photon indistinguishability at elevated temperatures can be increased by employing Purcell enhancement of the emission rate or by increasing the energy separation of the excited states. Using cavity-enhanced emission, we experimentally verify the first route and demonstrate an improvement in photon indistinguishability from 0.314(25) to 0.80(3) at 32 K, which - to our knowledge - is the highest reported value at such temperature.
We investigate the temperature dependence of photon coherence properties through two-photon interference (TPI) measurements from a single quantum dot (QD) under resonant excitation. We show that the loss of indistinguishability is related only to the electron-phonon coupling and is not affected by spectral diffusion. Through these measurements and a complementary microscopic theory, we identify two independent separate decoherence processes, both of which are associated with phonons. Below 10 K, we find that the relaxation of the vibrational lattice is the dominant contribution to the loss of TPI visibility. This process is non-Markovian in nature and corresponds to real phonon transitions resulting in a broad phonon sideband in the QD emission spectra. Above 10 K, virtual phonon transitions to higher lying excited states in the QD become the dominant dephasing mechanism, this leads to a broadening of the zero phonon line, and a corresponding rapid decay in the visibility. The microscopic theory we develop provides analytic expressions for the dephasing rates for both virtual phonon scattering and non-Markovian lattice relaxation.