So far, successful focused ion beam (FIB) based fabrication of photonic structures with quantum dots (QDs) has been limited to cases with above 1 µm thick cap, usually in a form of a distributed Bragg reflector of a vertical cavity, which simultaneously protects the active region from the destructive influence of the ion beam. Here, we propose optimized xenon-plasma FIB (Xe-PFIB) technology as a fast and cost-efficient solution alternative to the commonly used combination of electron beam lithography and etching. We demonstrate a 3D processing of GaAs-based photonic microstructures with InGaAs QDs emitting close to the telecom O-band for cylindrical mesas with different cap thicknesses (50-650 nm) obtained by using two approaches: (i) Xe-PFIB for both reducing the cap thickness as well as the in-plane microstructure size, and (ii) wet chemical etching for cap layer removal and subsequent Xe-PFIB for the in-plane milling. The latter appeared more efficient when judging by photoluminescence intensity. Utilizing an additional protecting layer of platinum or carbon was also tested. Eventually, we for the first time show successful FIB-based fabrication of photonic microstructures with bright emission from single QDs capped with only 200 nm layer, which indicates the prospects of this technology for processing of efficient QD-based single-photon sources for quantum communication.

Spin-controlled lasers are highly interesting photonic devices and have been shown to provide ultra-fast polarization dynamics in excess of 200 GHz. Another class of modern semiconductor lasers are high-beta emitters which benefit from enhanced light-matter interaction due to strong mode confinement in low-mode-volume microcavities. We combine the advantages of both laser types to demonstrate spin-lasing in high-beta microlasers for the first time. For this purpose, we realize bimodal high-beta quantum dot micropillar lasers for which the mode splitting and the polarization-oscillation frequency can be engineered via the pillar cross-section. The microlasers show very pronounced spin-lasing effects with polarization oscillation frequencies up to 15 GHz.

In this work, we determine the temperature dependence of refractive indices of In0.53Al0.1Ga0.37As and Al0.9Ga0.1As semiconductor alloys at telecommunication wavelengths in the range from room temperature down to 10 K. For that, we measure the temperature-dependent reflectance of two structures: with an Al0.9Ga0.1As/GaAs distributed Bragg reflector (DBR) designed for 1.3 µm and with an In0.53Al0.1Ga0.37As/InP DBR designed for 1.55 µm. The obtained experimental results are compared to DBR reflectivity spectra calculated within the transfer matrix method to determine refractive index values. We further show that changes due to the thermal expansion of the DBR layers are negligible for our method.

Spin‐controlled lasers are highly interesting photonic devices and have been shown to provide ultrafast polarization dynamics in excess of 200 GHz. In contrast to conventional semiconductor lasers their temporal properties are not limited by the intensity dynamics, but are governed primarily by the interaction of the spin dynamics with the birefringent mode splitting that determines the polarization oscillation frequency. Another class of modern semiconductor lasers are high‐ β emitters, which benefit from enhanced light–matter interaction due to strong mode confinement in low‐mode‐volume microcavities. In such structures, the emission properties can be tailored by the resonator geometry to realize for instance bimodal emission behavior in slightly elliptical micropillar cavities. This attractive feature is utilized to demonstrate and explore spin‐lasing effects in bimodal high‐ β quantum dot micropillar lasers. The studied microlasers with a β ‐factor of 4% show spin‐laser effects with experimental polarization oscillation frequencies up to 15 GHz and predicted frequencies up to about 100 GHz, which are controlled by the ellipticity of the resonator. These results reveal appealing prospects for very compact, ultrafast, and energy‐efficient spin‐lasers and can pave the way for future purely electrically injected spin‐lasers enabled by short injection path lengths.

We report on BB84 quantum key distribution tests employing a benchtop plug&play quantum-dot based single-photon source operating at O-band wavelengths. We perform a detailed characterization and exploit optimized temporal filters to maximize the tolerable losses.

Deterministic solid-state quantum light sources are key building blocks in photonic quantum technologies. While several proof-of-principle experiments of quantum communication using such sources have been realized, all of them required bulky setups. Here, we evaluate for the first time the performance of a compact and stand-alone fiber-coupled single-photon source emitting in the telecom O-band ($1321\,$nm) for its application in quantum key distribution (QKD). For this purpose, we developed a compact 19'' rack module including a deterministically fiber-coupled quantum dot single-photon source integrated into a Stirling cryocooler, a pulsed diode laser for driving the quantum dot, and a fiber-based spectral filter. We further employed this compact quantum light source in a QKD testbed designed for polarization coding via the BB84 protocol resulting in $g^{(2)}(0) = 0.10\pm0.01$ and a raw key rate of up to $(4.72\pm0.13)\,$kHz using an external laser for excitation. In this setting we investigate the achievable performance expected in full implementations of QKD. Using 2D temporal filtering on receiver side, we evaluate optimal parameter settings for different QKD transmission scenarios taking also finite key size effects into account. Using optimized parameter sets for the temporal acceptance time window, we predict a maximal tolerable loss of $23.19\,$dB. Finally, we compare our results to previous QKD systems using quantum dot single-photon sources. Our study represents an important step forward in the development of fiber-based quantum-secured communication networks exploiting sub-Poissonian quantum light sources.

TimmKupko,1 LucasRickert,1 FelixUrban,1 JanGroße,1 Nicole Srocka,1 SvenRodt,1 AnnaMusiał,2 KingaŻołnacz,3 PawełMergo,4 KamilDybka,5 WacławUrbańczyk,3 Grzegorz Sęk,2 SvenBurger,6 StephanReitzenstein,1 and TobiasHeindel1, ∗ Institute of Solid State Physics, Technische Universität Berlin, 10623 Berlin, Germany Department of Experimental Physics, Wroclaw University of Science and Technology, 50-370 Wrocław, Poland Department of Optics and Photonics, Wroclaw University of Science and Technology, 50-370 Wrocław, Poland Institute of Chemical Sciences, Maria Curie Sklodowska University, 20-031 Lublin , Poland Fibrain Sp. z o.o., 36-062 Zaczernie, Poland Zuse Institute Berlin, 14195 Berlin, Germany

We demonstrate experimental results based on time-resolved photoluminescence spectroscopy to determine the oscillator strength and the internal quantum efficiency (IQE) of InGaAs quantum dots (QDs). Using a strain-reducing layer, these QDs can be employed for the manufacturing of single-photon sources emitting in the telecom O-Band. The oscillator strength and IQE are evaluated by determining the radiative and non-radiative decay rates under the variation of the optical density of states at the position of the QD for InGaAs QDs emitting at wavelengths below 1 mu m. For this purpose, we perform measurements on a QD sample for different thicknesses of the capping layer realized by a controlled wet-chemical etching process. From numeric modeling of the radiative and non-radiative decay rates dependence on the capping layer thickness, we determine an oscillator strength of 24.6 +/- 3.2 and a high IQE of (85 +/- 10)% for the long-wavelength InGaAs QDs.

Quantum photonic circuits with integrated on-demand quantum emitters can act as building blocks for photonic gates and processors with enhanced quantum functionality. To scale up such quantum devices to larger and more powerful systems, eventually reaching the quantum advantage, the scalable integration of many emitters with identical emission wavelengths is of utmost importance. Here, we report on the deterministic integration of self-assembled quantum dots (QDs) in waveguide structures by means of in situ electron beam lithography (EBL). Applying external bias voltages to the p-i-n-doped and electrically contacted quantum circuits allows for spectral fine-tuning of the QDs via the quantum confined Stark effect. We achieve a tuning range of (0.40 ± 0.16) nm, which together with a spectral pre-selection accuracy of (0.2 ± 1.6) nm in the in situ EBL process is on average large enough to tune individual QDs into resonance. Thus, deterministic QD integration with spectral pre-selection in conjunction with Stark tuning of the QD emission wavelength is an attractive combination that has high potential to enable the scalable fabrication of integrated quantum photonic circuits in the future.

We report on the realization of an array of 28 × 28 mesas with site-controlled InGaAs quantum dots acting as single-photon sources for potential applications in photonic quantum technology. The site-selective growth of quantum dots is achieved by using the buried stressor approach where an oxide aperture serves as the nucleation site in the center of each mesa. Spectroscopic maps demonstrate the positioning of quantum dots with an inhomogeneous broadening of the ensemble emission of only 15.8 meV. Individual quantum dots are characterized by clean single-quantum-dot spectra with narrow exciton, biexciton, and trion lines, with a best value of 27 μeV and an ensemble average of 120 μeV. Beyond that, Hanbury Brown and Twiss and Hong-Ou-Mandel measurements validate the quantum nature of emission in terms of high single-photon purity and photon indistinguishability with a g(2)(0) value of (0.026 ± 0.026) and a post-selected two-photon interference visibility V = (87.1 ± 9.7)% with an associated coherence time of τc = (194 ± 7) ps.

A user-friendly fibre-coupled single-photon source operating at telecom wavelengths is a key component of photonic quantum networks providing long-haul ultra-secure data exchange. To take full advantage of quantum-mechanical data protection and to maximize the transmission rate and distance, a true quantum source providing single-photons on demand is highly desirable. We tackle this great challenge by developing a ready to use semiconductor quantum dot (QD)-based device that launches single photons at a wavelength of 1.3 um directly into a single-mode optical fibre. In our approach the QD is deterministically integrated into a nanophotonic structure to ensure efficient on-chip coupling into a fibre. The whole arrangement is integrated into a 19" compatible housing to enable stand-alone operation by cooling via a compact Stirling cryocooler. The realized source delivers single photons with multiphoton events probability as low as 0.15 and single-photon emission rate up to 73 kHz into a standard telecom single-mode fibre.

The cover image presents the first stand-alone telecom quantum light source launching single photons directly into a single-mode optical fiber. It includes a semiconductor quantum dot (QD) which is excited by an integrated laser and cooled by compact Stirling cooler at 40 K. The advanced quantum device includes all filter elements to suppress intensive laser light and to direct single photons at a wavelength of 1.3 µm to the output. For further details see article number 2000018 by Stephan Reitzenstein and co-workers.

Neuromorphic computing has received considerable attention as promising alternatives to classical von Neumann computing architectures. An attractive concept in this field is reservoir computing which is based on coupled non-linear elements to enable for instance ultra-fast pattern recognition. We focus on the development of microlasers in a dense regular array for the implementation of photonic reservoir computing based on the diffractive coupling. The coupling relies on injection locking of microlasers and sets stringent requirements on the spectral homogeneity of the array, which needs to be on the order of the achievable locking range. We realize CaAs/AlCaAs micropillar arrays with InGaAs quantum dots as active medium. To achieve the high spectral homogeneity on the order of 100 mu eV, as determined by injection locking experiments, the emission energy of each individual micropillar is adjusted to compensate for local inhomogeneities of order similar to 1.3 meV in the underlying microcavity structure. The realized micropillar arrays have a spectral inhomogeneity as low as 190 mu eV for an 8 x 8 array and down to 118 mu eV for a 5 x 5 sub-array. The arrays have high potential to enable the implementation of powerful photonic reservoir computing, which can be extended to a reservoir of hundreds of microlasers in the future.

We present an effective method for direct fiber coupling of a quantum dot (QD) that is deterministically incorporated into a cylindrical mesa. For precise positioning of the fiber with respect to the QD-mesa, we use a scanning procedure relying on interference of light reflected back from the fiber end-face and the top surface of the mesa, applicable for both single-mode and multi-mode fibers. The central part of the fiber end-face is etched to control the required distance between the top surface of the mesa and the fiber core. Emission around 1260 nm from a fiber-coupled InGaAs/GaAs QD is demonstrated and its stability is proven over multiple cooling cycles. Moreover, a single photon character of emission from such system for a line emitting above 1200 nm is proven experimentally by photon autocorrelation measurements with an obtained value of the second order correlation function at zero time-delay well below 0.5.

Here we report on the fabrication and characterization of dense arrays of up to 900 microlasers for the implementation of a photonic hardware platform for reservoir computing [1, 2], a powerful concept of efficient neuromorphic data processing in which the input data gets transferred into a random and fixed recurrent network of coupled neural nodes. The network projects the input information into the high-dimensional reservoir space, which then can be processed simply by a trained readout system. This leads to interesting applications in the field of ultra-fast pattern recognition, or the prediction of chaotic temporal sequences [2]. To realize this interesting concept with nanophotonic nodes, we present a hardware platform based on a dense array of semiconductor micropillar lasers, which can be optically coupled and injected within a fully parallel concept. Our scheme is implemented by a diffractively multiplied external resonator, which redirects the emission of individual laser nodes in the array to their neighbouring nodes [3]. To enable this scheme the pitch of the laser array must be precisely adjusted, and most importantly their spectral homogeneity needs to be on the order of the emission linewidth of the cavity mode.

We report on the realization of a dense, large-scale array of 900 quantum dot micropillar cavities with high spectral homogeneity. We target applications in photonic information processing such as optical reservoir computing which can be implemented in large arrays of optically coupled microlasers. To achieve the required spectral homogeneity for the underlying optical injection locking, we calculate and set the diameter of each individual micropillar within the array during the fabrication process by taking the diameter-dependent emission wavelength of the microcavities into account. Using this kind of diameter adjustment, we improve the overall wavelength homogeneity in a 30 × 30 micropillar array by 64% and reduce the standard deviation of the resonance energy distribution by 26% from 352 μeV in the planar unprocessed sample to 262 μeV in the fabricated array. In addition, we present a detailed analysis of the device quality and the diameter control of the micropillar’s emission wavelength, which includes important information for the effective application of the developed fabrication method for the realization of highly homogeneous micropillar arrays in the future.

Atomic scale switches working at room temperature represent the ultimate level of device miniaturization. Using scanning tunneling microscopy, we find a bistable switching between two mirror-symmetric configurations of self-assembled magic rare earth silicide clusters on the Si(111) 7 × 7 surface. Density functional theory reveals an energy barrier of 1.3 eV between the two cluster configurations, suppressing the switching even at room temperature. However, intentional switching between the two states is possible in the presence of a close tunneling tip due to a tip-induced lowering of the energy barrier.

The formation, atomic structure, and electronic properties of Tb silicide layers on the Si(111) surface were studied using scanning tunneling microscopy as well as core-level and angle-resolved photoelectron spectroscopy. For Tb exposures around one monolayer, the formation of a hexagonal TbSi2 monolayer was found, while higher coverages led to the formation of a hexagonal Tb3Si5 multilayer with a 3×3R30° superstructure in the bulk layers. For the monolayer silicide, Si-2p core level spectra show a Fermi level position very close to the conduction band minimum of the silicon substrate, while the Fermi level shifts toward midgap in the multilayer case. The electronic structure of the monolayer is characterized by a Fermi surface consisting of electronlike ellipses around the M¯ points and a holelike state around the Γ¯ point. The effective masses of the band around the M¯ points are strongly anisotropic, with values around 1.45 m0 in the long direction and 0.16 m0 in the short direction of the ellipses. In the case of the multilayer, the ellipses around the M¯ points are less eccentric, and there are indications for Umklapp processes due to the 3×3R30° superstructure in the silicide bulk layers. The overall behavior of Tb is found to be similar to that of other trivalent rare earths on Si(111).