Highly indistinguishable photons from deterministic quantum-dot microlenses utilizing three-dimensional in situ electron-beam lithography.

M Gschrey,L Krüger,S Burger,M Seifried,J -H Schulze,F Schmidt,S Reitzenstein,A Strittmatter,P Schnauber,T Heindel,B Wohlfeil,R Schmidt,S Rodt,A Thoma

doi:10.1038/ncomms8662

M Gschrey, L Krüger + Show 12 more

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https://doi.org/10.1038/ncomms8662

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Abstract

The success of advanced quantum communication relies crucially on non-classical light sources emitting single indistinguishable photons at high flux rates and purity. We report on deterministically fabricated microlenses with single quantum dots inside which fulfil these requirements in a flexible and robust quantum device approach. In our concept we combine cathodoluminescence spectroscopy with advanced in situ three-dimensional electron-beam lithography at cryogenic temperatures to pattern monolithic microlenses precisely aligned to pre-selected single quantum dots above a distributed Bragg reflector. We demonstrate that the resulting deterministic quantum-dot microlenses enhance the photon-extraction efficiency to (23±3)%. Furthermore we prove that such microlenses assure close to pure emission of triggered single photons with a high degree of photon indistinguishability up to (80±7)% at saturation. As a unique feature, both single-photon purity and photon indistinguishability are preserved at high excitation power and pulsed excitation, even above saturation of the quantum emitter.

Highlights

Background correction in correlation measurementsFor the corrections applied to the raw data we took into account coincidences arising from the detection of photons from the quantum dots (QDs) emission and the laser background or avalanche photo-diodes (APDs) dark counts, respectively
We apply our deterministic quantum device concept by integrating pre-selected single QDs into monolithic microlenses using in situ three-dimensional (3D) electron-beam lithography in a low-temperature cathodoluminescence (CL) system[26]
After spin-coating the sample with the electron-beam sensitive resist polymethyl methacrylate (PMMA)[27] the QD luminescence is characterized via CL spectroscopy mapping at a cryogenic temperature of 5 K (a)

Summary

Introduction

For the corrections applied to the raw data we took into account coincidences arising from the detection of photons from the QD emission and the laser background or APD dark counts, respectively. The laser background was evaluated from a comparison of the APD count-rates close to the emission line for above-band excitation with the count-rates for quasi-resonant excitation. By using this method we evaluate the contribution from the laser background to be 250–750 Hz for the Hanbury-Brown and Twiss correlation measurements and 75–180 Hz for the TPI correlation measurements. Together with an APD dark count-rate of 50 Hz the entire uncorrelated coincidences per time-bin are given by N 1⁄4 ðn_ Laser þ n_ DarkÞ Á ðn_ APD1 þ n_ APD2Þ Á tbin Á tint with n_ Laser; n_ Dark being the laser background rate and dark count-rate, n_ APD1 þ n_ APD2 the count-rates on the two APDs, tbin the time-bin width, and tint the overall integration time

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Results

Conclusion