Operating single quantum emitters with a compact Stirling cryocooler

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Review of Scientific Instruments 86, 013113 (2015) and may be found at https://doi.org/10.1063/1.4906548.The development of an easy-to-operate light source emitting single photons has become a major driving force in the emerging field of quantum information technology. Here, we report on the application of a compact and user-friendly Stirling cryocooler in the field of nanophotonics. The Stirling cryocooler is used to operate a single quantum emitter constituted of a semiconductor quantum dot (QD) at a base temperature below 30 K. Proper vibration decoupling of the cryocooler and its surrounding enables free-space micro-photoluminescence spectroscopy to identify and analyze different charge-carrier states within a single quantum dot. As an exemplary application in quantum optics, we perform a Hanbury-Brown and Twiss experiment demonstrating a strong suppression of multi-photon emission events with g(2)(0) < 0.04 from this Stirling-cooled single quantum emitter under continuous wave excitation. Comparative experiments performed on the same quantum dot in a liquid helium (LHe)-flow cryostat show almost identical values of g(2)(0) for both configurations at a given temperature. The results of this proof of principle experiment demonstrate that low-vibration Stirling cryocoolers that have so far been considered exotic to the field of nanophotonics are an attractive alternative to expensive closed-cycle cryostats or LHe-flow cryostats, which could pave the way for the development of high-quality table-top non-classical light sources.BMBF, 03V0630, Entwicklung einer Halbleiterbasierten Einzelphotonenquelle für die Quanteninformationstechnologie (QSOURCE)DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Advanced in-situ electron-beam lithography for deterministic nanophotonic device processing

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Review of Scientific Instruments 86, 073903 (2015) and may be found at https://doi.org/10.1063/1.4926995.We report on an advanced in-situ electron-beam lithography technique based on high-resolution cathodoluminescence (CL) spectroscopy at low temperatures. The technique has been developed for the deterministic fabrication and quantitative evaluation of nanophotonic structures. It is of particular interest for the realization and optimization of non-classical light sources which require the pre-selection of single quantum dots (QDs) with very specific emission features. The two-step electron-beam lithography process comprises (a) the detailed optical study and selection of target QDs by means of CL-spectroscopy and (b) the precise retrieval of the locations and integration of target QDs into lithographically defined nanostructures. Our technology platform allows for a detailed pre-process determination of important optical and quantum optical properties of the QDs, such as the emission energies of excitonic complexes, the excitonic fine-structure splitting, the carrier dynamics, and the quantum nature of emission. In addition, it enables a direct and precise comparison of the optical properties of a single QD before and after integration which is very beneficial for the quantitative evaluation of cavity-enhanced quantum devices.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, Bauelement

Two-photon interference from remote deterministic quantum dot microlenses

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 110, 011104 (2017) and may be found at https://doi.org/10.1063/1.4973504.We report on two-photon interference (TPI) experiments using remote deterministic single-photon sources. Employing 3D in-situ electron-beam lithography, we fabricate quantum-light sources at specific target wavelengths by integrating pre-selected semiconductor quantum dots within monolithic microlenses. The individual single-photon sources show TPI visibilities of 49% and 22%, respectively, under pulsed p-shell excitation at 80 MHz. For the mutual TPI of the remote sources, we observe an uncorrected visibility of 29%, in quantitative agreement with the pure dephasing of the individual sources. Due to its efficient photon extraction within a broad spectral range (>20 nm), our microlens-based approach is predestinated for future entanglement swapping experiments utilizing entangled photon pairs emitted by distant biexciton-exciton radiative cascades.DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeEC/FP7/615613/EU/External Quantum Control of Photonic Semiconductor Nanostructures/EXQUISIT

Generation of maximally entangled states and coherent control in quantum dot microlenses

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 112, 153107 (2018) and may be found at https://doi.org/10.1063/1.5020242.The integration of entangled photon emitters in nanophotonic structures designed for the broadband enhancement of photon extraction is a major challenge for quantum information technologies. We study the potential of quantum dot (QD) microlenses as efficient emitters of maximally entangled photons. For this purpose, we perform quantum tomography measurements on InGaAs QDs integrated deterministically into microlenses. Even though the studied QDs show non-zero excitonic fine-structure splitting (FSS), polarization entanglement can be prepared with a fidelity close to unity. The quality of the measured entanglement is only dependent on the temporal resolution of the applied single-photon detectors compared to the period of the excitonic phase precession imposed by the FSS. Interestingly, entanglement is kept along the full excitonic wave-packet and is not affected by decoherence. Furthermore, coherent control of the upper biexcitonic state is demonstrated.DFG, SFB 787, Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeBMBF, 03V0630TIB, Entwicklung einer Halbleiterbasierten Einzelphotonenquelle für die Quanteninformationstechnologi

Single-photon emission at a rate of 143 MHz from a deterministic quantum-dot microlens triggered by a mode-locked vertical-external-cavity surface-emitting laser

This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Lett. 107, 041105 (2015) and may be found at https://doi.org/10.1063/1.4927429.We report on the realization of a quantum dot (QD) based single-photon source with a record-high single-photon emission rate. The quantum light source consists of an InGaAs QD which is deterministically integrated within a monolithic microlens with a distributed Bragg reflector as back-side mirror, which is triggered using the frequency-doubled emission of a mode-locked vertical-external-cavity surface-emitting laser (ML-VECSEL). The utilized compact and stable laser system allows us to excite the single-QD microlens at a wavelength of 508 nm with a pulse repetition rate close to 500 MHz at a pulse width of 4.2 ps. Probing the photon statistics of the emission from a single QD state at saturation, we demonstrate single-photon emission of the QD-microlens chip with g(2)(0) < 0.03 at a record-high single-photon flux of (143 ± 16) MHz collected by the first lens of the detection system. Our approach is fully compatible with resonant excitation schemes using wavelength tunable ML-VECSELs, which will optimize the quantum optical properties of the single-photon emission in terms of photon indistinguishability.BMBF, 03V0630, Entwicklung einer Halbleiterbasierten Einzelphotonenquelle für die Quanteninformationstechnologie (QSOURCE)DFG, 43659573, SFB 787: Halbleiter - Nanophotonik: Materialien, Modelle, BauelementeDFG, 192635911, GRK 1782: Funktionalisierung von HalbleiternDFG, 223848855, SFB 1083: Struktur und Dynamik innerer Grenzfläche

Observation of resonance fluorescence and the Mollow triplet from a coherently driven site-controlled quantum dot

This work was funded by project SIQUTE (contract EXL02) of the European Metrology Research Programme (EMRP). The EMRP is jointly funded by the EMRP participating countries within EURAMET and the European Union. Support was provided from the Villum Foundation via the VKR Centre of Excellence NATEC.Resonant excitation of solid state quantum emitters has the potential to deterministically excite a localized exciton while ensuring maximally coherent emission. In this work, we demonstrate the coherent coupling of an exciton localized in a lithographically positioned, site-controlled semiconductor quantum dot to an external resonant laser field. For strong continuous-wave driving we observe the characteristic Mollow triplet and analyze the Rabi splitting and sideband widths as a function of driving strength and temperature. The sideband widths increase linearly with temperature and the square of the driving strength, which we explain via coupling of the exciton to longitudinal acoustic phonons. We also find an increase of the Rabi splitting with temperature, which indicates a temperature-induced delocalization of the excitonic wave function resulting in an increase of the oscillator strength. Finally, we demonstrate coherent control of the exciton excited state population via pulsed resonant excitation and observe a damping of the Rabi oscillations with increasing pulse area, which is consistent with our excitonx2013;photon coupling model. We believe that our work outlines the possibility to implement fully scalable platforms of solid state quantum emitters. Such scalability is one of the key prerequisites for more advanced, integrated nanophotonic quantum circuits.PostprintPeer reviewe

Cascaded emission of linearly polarized single photons from positioned InP/GaInP quantum dots

This content may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This material originally appeared in Appl. Phys. Lett. 103, 191113 (2013) and may be found at https://doi.org/10.1063/1.4828354.We report on the optical characterization of site-controlled InP/GaInP quantum dots (QDs). Spatially resolved low temperature cathodoluminescence proves the long-range ordering of the buried emitters, revealing a yield of ∼90% of optically active, positioned QDs and a strong suppression of emitters on interstitial positions. The emission of single QDs shows a pronounced degree of linear polarization along the [0,−1,1] crystal axis with an average degree of polarization of 94%. Photon correlation measurements of the emission from a single QD indicate the single-photon character of the exciton and biexciton emission lines as well as the cascaded nature of the photon pair

In-situ cathodolumineszence lithography for the deterministic fabrication of deterministic quantumdevices

Der Einsatz von Quantenpunkten (QPen) in zukünftigen Anwendungen im Bereich der Quantenkommunikation und der Quanteninformation erfordert die Entwicklung von deterministischen Technologien, welche eine gezielte Integration von QPen mit definierten optischen Eigenschaften in nanophotonische Bauelemente ermöglicht. In dieser Arbeit werden Kathodolumineszenz-Spektroskopie und Elektronenstrahllithographie bei tiefen Temperaturen zur in-situ Kathodolumineszenzlithographie (KLL) vereint, um gezielt QPe auszuwählen und in Mikrolinsen zur Erhöhung der Sammeleffizienz zu integrieren. Hierzu werden verschiedene Elektronenstrahllacke auf ihre Kontrasteigenschaften, Schwellendosen, sowie die erreichbaren Strukturgrößen bei Tieftemperatur hin untersucht. Mit Hilfe des Lacks Polymethylmethacrylat (PMMA), welcher bei hohen Bestrahlungsdosen karbonisiert und als negativ-Lack verwendet werden kann, wird der KLL-Prozess anhand der Herstellung von Einzel-QP-Mesen validiert und die erfolgreiche Integration der selektierten QPe, die hohe Prozessausbeute, sowie die ausgezeichnete optische Qualität der fertigen Strukturen demonstriert. Weiterhin wird aus statistischen Untersuchungen die räumliche Genauigkeit des Prozesses auf 34nm bestimmt. Zusätzlich werden, um die Auskoppeleffizienz der Emission aus dem Halbleiter zu erhöhen, monolithische Linsenstrukturen ohne und mit rückseitigem Bragg-Reflektor (DBR) numerisch optimiert und mittels KLL in einem 3D-Lithographieverfahren hergestellt. Für Linsen ohne und mit DBR werden Auskoppeleffizienzen von 5,6 % und 23,3 % bestimmt. Als bemerkenswerte Eigenschaft können eine Einzelphotonenemission mit g(2)(0) < 0,01, welche von der Anregungsleistung unabhängig ist, sowie eine Ununterscheidbarkeit mit V = 80 % beobachtet werden. Den Abschluss bilden die numerische Berechnung und die Herstellung einer Resonatorstruktur, welche aus einer von zwei Bragg-Reflektoren umschlossenen Linse besteht. Das Verhalten der Resonanz in Abhängigkeit der Linsengeometrie wird analysiert und die Wechselwirkung zwischen QP und Resonator nachgewiesen. Die in dieser Arbeit etablierte Technologie legt somit, durch die gezielte Selektion von geeigneten Emittern und die präzise Herstellung von nanophotonischen Strukturen, einen wichtigen Grundstein für die deterministische Herstellung von komplexen und skalierbaren Bauelementen für die zukünftige Quantentechnologie.The use of semiconductor quantum dots (QDs) in future quantum communication and quantum information applications requires the development of a deterministic technology platform for the controlled integration of QDs with pre-selected optical properties into nanophotonic devices. In this thesis, a fabrication technique called in-situ cathodoluminescence lithography (CLL) is established, which combines the high spatial resolution of cathodoluminescence spectroscopy with the versatility of electron-beam lithography at cryogenic temperatures. CLL is used to deterministically select and integrate QDs into monolithically fabricated microlenses to increase the photon-extraction efficiency (PEE). Therefore, various commercial available electron-beam resists have been tested to gain access to their low-temperature properties, their threshold doses and the smallest achievable resolution therewith. With the help of polymethyl methacrylate (PMMA), that carbonizes at high exposure doses when it acts as negative resist, the CLL-process is demonstrated by the fabrication of single-QD mesa structures with high optical quality and yield. For further validation, a statistical analysis of the process accuracy is performed, featuring an overall spatial alignment accuracy of 34 nm. To further increase the PEE, structures based on monolithically integrated microlenses without and with a back-side bragg-reflector were numerically optimized and fabricated by means of 3D-CLL. For lenses without and with bragg-reflector a PEE of 5.6 % and 23.3 % respectively is achieved. As a remarkable feature, a power-independent single-photon emission with g(2)(0) < 0.01 and an indistinguishability of the emitted photons of up to V = 80 % is determined up to saturation power. Finally, resonator structures consisting of microlenses that are sandwitched between two DBRs are numerically simulated and fabricated. The behaviour of the resonance in dependence on the lens geometry is analyzed and the interaction between a QD and the resonator is demonstrated. Altogether, this novel technology platform paves the way for the systematic selection and integration of suitable quantum emitters into precisely defined nanophotonic devices and is therefore a vital component for the realization of complex and scalable quantum systems

Tools for the performance optimization of single-photon quantum key distribution

Quantum light sources emitting triggered single photons or entangled photon pairs have the potential to boost the performance of quantum key distribution (QKD) systems. Proof-of-principle experiments affirmed these prospects, but further efforts are necessary to push this field beyond its current status. In this work, we show that temporal filtering of single-photon pulses enables a performance optimization of QKD systems implemented with realistic quantum light sources, both in experiment and simulations. To this end, we analyze the influence of temporal filtering of sub-Poissonian single-photon pulses on the expected secret key fraction, the quantum bit error ratio, and the tolerable channel losses. For this purpose, we developed a basic QKD testbed comprising a triggered solid-state single-photon source and a receiver module designed for four-state polarization coding via the BB84 protocol. Furthermore, we demonstrate real-time security monitoring by analyzing the photon statistics, in terms of g(2)(0), inside the quantum channel by correlating the photon flux recorded at the four ports of our receiver. Our findings are useful for the certification of QKD and can be applied and further extended for the optimization of various implementations of quantum communication based on sub-Poissonian quantum light sources, including measurement-device-independent schemes of QKD as well as quantum repeaters. Our work represents an important contribution towards the development of QKD-secured communication networks based on quantum light sources.BMBF, 13N14876, Quantenkommunikations-Systeme auf Basis von Einzelphotonenquellen (QuSecure