The solid-state quantum photonics (SSQP) group is one of the world leaders in the fabrication of quantum light sources based on Gallium arsenide (GaAs) quantum dots. Specifically, we are focusing on high quality entangled photon pair sources – from wafer growth over device fabrication to quantum optical investigations. This is facilitated by a very diverse set of sophisticated materials science and quantum optical laboratories as well as usage of world-class clean room facilities at the IFW Dresden. One of our specialties is the fabrication of quantum dots embedded into nanomembranes, which allows precise placement of quantum dots to any planar surface and enables the creation of high yield semiconductor quantum light sources.
Current research interests include the realization of quantum repeater segments by combining our entangled photon sources with Diamond-based quantum memories and telecom wavelength long-range transport in the framework of the QR.X project of the German Federal Ministry of Education and Research (BMBF). The SSQP is member of the QR.X consortium and contributes to the ct.qmat excellence cluster for topological and complex quantum matter.
High-quality quantum dots are fabricated using in-situ droplet etching process during the epitaxial growth of a GaAs/AlGaAs heterostructure inside our molecular beam epitaxy machine, see also figure 1. The fabricated heterostructures are typically characterized using micro-photoluminescence spectroscopy as well as atomic force and electron beam microscopy to ensure consistent quantum dot quality. In a second step the quantum dots are processed into optical micro devices – such as quantum dot nanomembranes attached to Gallium phosphide solid immersion lenses or planar optical cavities - using optical and electron beam lithography as well as dry and wet chemical etching processes. This advanced processing is facilitated by the excellent clean room facilities of the IFW Dresden. Consequently, the produced quantum dot devices are investigated in our quantum optical laboratories. Possible investigations include deterministic GHz-clocked production of entangled photon pairs using resonant two-photon excitation, polarization resolved single photon correlation as well as polarization resolved high-resolution excitation spectroscopy. To this end instruments such as closed-cycle cryostats, high-resolution spectrometers, superconducting nanowire single photon detectors, time-resolved single photon counting electronics and quantum optical interferometers (e.g. Hong-Ou-Mandel) as well as a variety of continuous wave and pulsed laser sources are employed.
Using the described methods and facilities we were able to publish a number of research studies that document significant advances in the field of semiconductor-based quantum light sources. We were able to show that using droplet etched GaAs quantum dot devices deterministic and on demand GHz-clocked maximally entangled photon emission is possible . Some core results are of this publication are illustrated in figure 2. We were able to demonstrate heralded and on demand preparation of specific spin qubits in GaAs quantum dots using a polarization resolved quasi-resonant excitation scheme . Experimental optimization of the evanescent field coupling between quantum dot nanomembrane and Gallium phosphide solid immersion lens yielded significantly improved coupling of quantum dot emission to single mode fibers . Some core results of the experimental optimization of the far-field pattern to single mode fibers is shown in figure 3.
W. Nie, N. L. Sharma, C. Weigelt, C. Hopfmann, and O. G. Schmidt
Appl. Phys. Lett. 119, 244003 (2021)
We present an efficient experimental method to optimize the combined extraction efficiencies and the far-field emission patterns of solid state-based single and entangled photon pair sources for efficient coupling to single mode fibers. This method is demonstrated for emitters based on droplet etched GaAs quantum dot nanomembranes attached to gallium phosphide solid immersion lenses using an adhesive layer of poly(methyl methacrylate). By varying the thickness of the latter, the optimization of both the extraction efficiency and the far-field emission pattern for single mode fiber coupling is facilitated. The applied method of far-field characterization is validated by benchmarking it against direct measurements of the single mode fiber coupling efficiency. Using this scheme, devices with a more than 150-fold enhanced free-space intensity compared to an unprocessed sample as well as a fiber coupling efficiency of 64% are achieved. In addition, the optimized device has been employed for on-demand generation of maximally entanglement photon pairs using two-photon excitation of the quantum dot bi-exciton exciton cascade. This universal approach for experimental optimization can be applied to other photonic nanostructures, including circular Bragg grating and micropillar cavities as well as monolithic microlenses.
C. Hopfmann, W. Nie, N. L. Sharma, C. Weigelt, F. Ding, and O. G. Schmidt
We present a 1 GHz-clocked, maximally entangled and on-demand photon pair source based on droplet etched GaAs quantum dots using two-photon excitation. By employing these GaP microlens-enhanced devices in conjunction with their substantial brightness, raw entanglement fidelities of up to 0.95±0.01 and postselected photon indistinguishabilities of up to 0.93±0.01, the suitability for quantum repeater based long range quantum entanglement distribution schemes is shown. Comprehensive investigations of a complete set of polarization selective two-photon correlations facilitate an innovative method to determine the extraction and excitation efficiencies directly, as opposed to commonly employed indirect techniques. Additionally, time-resolved analysis of Hong-Ou-Mandel interference traces reveal an alternative approach to the investigation of pure photon dephasing.
C. Hopfmann, N. L. Sharma, W. Nie, R. Keil, F. Ding, and O. G. Schmidt
Phys. Rev. B 104, 075301 (2021)
We present a comprehensive study on heralded spin preparation employing excited state resonances of droplet-etched GaAs quantum dots. This achievement will facilitate future investigations of spin qubit based quantum memories using the GaAs quantum dot material platform. By observation of excitation spectra for a range of fundamental excitonic transitions, the properties of different quantum dot energy levels, i.e., shells, are revealed. The innovative use of polarization-resolved excitation and detection in the context of quasiresonant excitation spectroscopy of quantum dots greatly simplifies the determination of the spin preparation fidelities—irrespective of the relative orientations of laboratory and quantum dot polarization eigenbases. By employing this method, spin preparation fidelities of quantum dot ground states of up to 85% are found. Additionally, the characteristic nonradiative decay time is investigated as a function of ground state, excitation resonance, and excitation power level, yielding decay times as low as 29 ps for s−p shell exited state transitions. Finally, by time-resolved correlation spectroscopy it is demonstrated that the employed excitation scheme has a significant impact on the electronic environment of quantum dot transitions and their apparent brightness.