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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Interference of single photons from two separate semiconductor quantum dots

Edward B. Flagg, Andreas Müller, Sergey Vladimirovich Polyakov, Alexander Ling, Alan L. Migdall, Glenn S. Solomon

Published Feb 10, 2011• 31 citations
Publisher

Abstract

We demonstrate interference between discrete photons emitted by two different semiconductor quantum dots and quantify their degree of indistinguishability. The quantum dot emission energies are tuned into resonance by straining the samples. Upon interference on a beamsplitter, the photons are shown to be 18.1% indistinguishable, resulting in a coincidence detection rate below the classical limit. Post-selecting only those detections occurring within a short time of each other increases the measured indistinguishability to 47%. The photons are partially distinguishable due to dephasing of the exciton states, and post-selection is also affected by the detector response time.

DOI
10.1117/12.874853

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We demonstrate interference between discrete photons emitted by two different semiconductor quantum dots and quantify their degree of indistinguishability. The quantum dot emission energies are tuned into resonance by straining the samples. Upon interference on a beamsplitter, the photons are shown to be 18.1% indistinguishable, resulting in a coincidence detection rate below the classical limit. Post-selecting only those detections occurring within a short time of each other increases the measured indistinguishability to 47%. The photons are partially distinguishable due to dephasing of the exciton states, and post-selection is also affected by the detector response time.
We demonstrate and characterize interference between discrete photons emitted by two separate semiconductor quantum dot states in different samples excited by a pulsed laser. Their energies are tuned into resonance using strain. The photons have a total coalescence probability of 18.1% and the coincidence rate is below the classical limit. Postselection of coincidences within a narrow time window increases the coalescence probability to 47%. The probabilities are reduced from unity because of dephasing and the postselection value is also reduced by the detector time response.
The authors present steps toward the experimental realization of indistinguishable single photon sources based on independent unstrained GaAs quantum dots (QDs), which are embedded in planar cavities to improve the light collection efficiency. The emission lines of two QDs are brought into resonance and overlapped at a beam splitter. The coherence properties of the emitted photons are investigated by measuring the first-order field correlation function. Despite the fact that the short dephasing time of the selected QDs prevents us to observe quantum interference between the two photons, the approach could be applied to other QDs.
In quantum mechanics, particles in identical states are indistinguishable, giving rise to effects with no classical analog. For instance, the bosonic nature of light insures that upon interference two indistinguishable photons will coalesce into a single inseparable state. Through this coalescence, we demonstrate that photons produced from two separate quantum dots are indistinguishable. Further, we show that single photons created in a fundamentally different process - parametric down-conversion in a nonlinear crystal - can be manipulated to be indistinguishable from those from quantum dots. The quantum interference in both experiments occurs with a visibility reduced from unity because the quantum dot photons are not lifetime-limited due to the presence of pure dephasing. The measured visibility closely matches the theoretical visibility predicted for photons with the parameters of those measured here.
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.
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.