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arXiv:1403.8001 · Physical Review B

Volume dependence of excitonic fine structure splitting in geometrically similar quantum dots

Yong-Heng Huo, Vlastimil Křápek, Armando Rastelli, Oliver G. Schmidt

Published Jul 30, 2014• 31 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevb.90.041304

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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.
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.
Single semiconductor quantum dots, which are considered as two-level systems in the artificial atom model, are promising structures for the realization of integrated devices such as single photon sources for quantum information applications. However, contrary to genuine atoms, they are condensed matter systems which suffer from the coupling to their solid environment, leading to a degradation of the coherence of the emitted photons. One possibility to avoid this coupling, thereby improving the coherence of the photons until the one imposed by the radiative limit T2 = 2T1, is to perform strictly resonant excitation of a quantum dot at low temperature. Using an experimental setup that spatially decouples the excitation and detection paths, we showed that a fine control of the electrostatic environment related to the nanostructures residual doping remains a major challenge to answer the widely encountered problem of the inhibition of the resonance fluorescence of a quantum dot. Meanwhile, thanks to quantum optics experiments and high resolution optical spectroscopy, we demonstrated that the resonant Rayleigh scattering regime where quantum dots emit single photons with the laser coherence time, paves the way to the development of "ultra-coherent" sources of single photons with high degrees of indistinguishability. In this regime where the laser tailors the coherence of the photons and where the radiative limit becomes a secondary requirement, two photons emitted successively by the same quantum dot are indistinguishable on time scales never obtained before for a solid nano-emitter (up to ten nanoseconds, comparing to the carriers lifetime and decoherence time of about a hundred of picoseconds).
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.
Single-photon interference is observed on the ultranarrow long-term stable exciton resonance of an individual semiconductor quantum dot. This interference is related to the fine-structure splitting and allows direct conclusions about the coherence properties of the exciton. When selectively addressing a particular dot by quasiresonant phonon-assisted excitation, despite a rapid orientation relaxation on a 1-ps time scale, coherence is partly maintained. No significant further decoherence occurs when the ground state is reached until the exciton recombines radiatively (approximately 300 ps).
The ability of individual photonic qubits to interact via interference lies at the heart of many quantum networking applications. For their development, sources of highly coherent, indistinguishable photons are therefore crucial. Here, we investigate the noise sources that affect InAs/InP quantum dots and limit their coherence. We show that the droplet epitaxy growth mode leads to a quiet environment with 96% of exciton transitions having a coherence time above 100 ps, even under non-resonant excitation. Further, Hong–Ou–Mandel interference reveal a corrected two-photon interference visibility of 98.6±1.6% for these quantum dots, showing their potential for quantum networking applications.
We investigate the indistinguishability of single photons generated from strain-free GaAs/AlGaAs quantum dots using pulsed resonance fluorescence techniques. In pulsed two-photon interference measurements we observe a single photon indistinguishability with a raw visibility of 95%. This can be traced back to the short intrinsic lifetime of excitons and trions confined in the GaAs quantum dots and demonstrates that for this material system the generation of single photons is possible with near-unity indistinguishability even without Purcell enhancement.