Coherently and Incoherently Pumped Telecom C-Band Single-Photon Source with High Brightness and Indistinguishability.

Long-range, terrestrial quantum networks require high-brightness single-photon sources emitting in the telecom C-band for maximum transmission rates. For solid-state quantum emitters, the underlying pumping process, i.e., coherent or incoherent excitation schemes, impacts several photon properties such as photon indistinguishability, single-photon purity, and photon number coherence. These properties play a major role in quantum communication applications, the latter in particular for quantum cryptography. Here, we present a versatile telecom C-band single-photon source that is operated coherently and incoherently using two complementary pumping schemes. The source is based on a quantum dot coupled to a circular Bragg grating cavity, whereas coherent (incoherent) operation is performed via the novel SUPER scheme (phonon-assisted excitation). In this way, high end-to-end-efficiencies (ηend) of 5.36% (6.09%) are achieved simultaneously with a small multiphoton contribution g(2)(0) of 0.076 ± 0.001 [g(2)(0) of 0.069 ± 0.001] for coherent (incoherent) operation.

Highly Indistinguishable Single Photons from Droplet-Etched GaAs Quantum Dots Integrated in Single-Mode Waveguides and Beamsplitters.

Integration of on-demand quantum emitters into photonic integrated circuits (PICs) has drawn much attention in recent years, as it promises a scalable implementation of quantum information schemes. A central property for several applications is the indistinguishability of the emitted photons. In this regard, GaAs quantum dots (QDs) obtained by droplet etching epitaxy show excellent performances, making the realization of these QDs into PICs highly appealing. Here, we show the first implementation in this direction, realizing the key passive elements needed in PICs, i.e., single-mode waveguides (WGs) with integrated GaAs-QDs and beamsplitters. We study the statistical distribution of wavelength, linewidth, and decay time of the excitonic line, as well as the quantum optical properties of individual emitters under resonant excitation. We achieve single-photon purities as high as 1 - g(2)(0) = 0.929 ± 0.009 and two-photon interference visibilities of up to VTPI = 0.953 ± 0.032 for consecutively emitted photons.

A Unipolar Quantum Dot Diode Structure for Advanced Quantum Light Sources.

Triggered, indistinguishable single photons are crucial in various quantum photonic implementations. Here, we realize a novel n+-i-n++ diode structure embedding semiconductor quantum dots: the gated device enables spectral tuning of the transitions and deterministic control of the charged states. Blinking-free single-photon emission and high two-photon indistinguishability are observed. The line width's temporal evolution is investigated across over 6 orders of magnitude time scales, combining photon-correlation Fourier spectroscopy, high-resolution photoluminescence spectroscopy, and two-photon interference (visibility of VTPI,2ns = (85.8 ± 2.2)% and VTPI,9ns = (78.3 ± 3.0)%). Most of the dots show no spectral broadening beyond ∼9 ns time scales, and the photons' line width ((420 ± 30) MHz) deviates from the Fourier-transform limit by a factor of 1.68. The combined techniques verify that most dephasing mechanisms occur at time scales ≤2 ns, despite their modest impact. The presence of n-doping implies higher carrier mobility, enhancing the device's appeal for high-speed tunable, high-performance quantum light sources.

Fully On-Chip Single-Photon Hanbury-Brown and Twiss Experiment on a Monolithic Semiconductor-Superconductor Platform.

Fully integrated quantum photonic circuits show a clear advantage in terms of stability and scalability compared to tabletop implementations. They will constitute a fundamental breakthrough in integrated quantum technologies, as a matter of example, in quantum simulation and quantum computation. Despite the fact that only a few building blocks are strictly necessary, their simultaneous realization is highly challenging. This is especially true for the simultaneous implementation of all three key components on the same chip: single-photon sources, photonic logic, and single-photon detectors. Here, we present a fully integrated Hanbury-Brown and Twiss setup on a micrometer-sized footprint consisting of a GaAs waveguide embedding quantum dots as single-photon sources, a waveguide beamsplitter, and two superconducting nanowire single-photon detectors. This enables a second-order correlation measurement on the single-photon level under both continuous-wave and pulsed resonant excitation. The presented proof-of-principle experiment proves the simultaneous realization and operation of all three key building blocks and therefore a major step towards fully integrated quantum optical chips.

Bragg grating cavities embedded into nano-photonic waveguides for Purcell enhanced quantum dot emission.

In the present work, we demonstrate the fabrication and optical properties of Bragg grating cavities that are directly integrated into ridge waveguides along with the Purcell enhanced emission from integrated quantum dots. Measured Q-factors up to 4600 are observed in combination with resonances of the fundamental mode within a ± 0.11 nm range along the full fabricated chip. The measured Purcell enhancement up to a factor of 3.5 ± 0.5 shows the potential utility for state-of-the-art on-chip quantum optical experiments as realized in off-chip implementations. Our measurements are fully supported via FDTD simulations giving a theoretical Purcell enhancement up to a factor of 20 with a highly directional ßdir-factor of 70 %. The straightforward upscaling and robust design of the investigated Bragg grating cavity in combination with a substantial Purcell enhancement represents a major step towards large scale on-chip quantum photonic circuits.

Machine learning enhanced evaluation of semiconductor quantum dots.

A key challenge in quantum photonics today is the efficient and on-demand generation of high-quality single photons and entangled photon pairs. In this regard, one of the most promising types of emitters are semiconductor quantum dots, fluorescent nanostructures also described as artificial atoms. The main technological challenge in upscaling to an industrial level is the typically random spatial and spectral distribution in their growth. Furthermore, depending on the intended application, different requirements are imposed on a quantum dot, which are reflected in its spectral properties. Given that an in-depth suitability analysis is lengthy and costly, it is common practice to pre-select promising candidate quantum dots using their emission spectrum. Currently, this is done by hand. Therefore, to automate and expedite this process, in this paper, we propose a data-driven machine-learning-based method of evaluating the applicability of a semiconductor quantum dot as single photon source. For this, first, a minimally redundant, but maximally relevant feature representation for quantum dot emission spectra is derived by combining conventional spectral analysis with an autoencoding convolutional neural network. The obtained feature vector is subsequently used as input to a neural network regression model, which is specifically designed to not only return a rating score, gauging the technical suitability of a quantum dot, but also a measure of confidence for its evaluation. For training and testing, a large dataset of self-assembled InAs/GaAs semiconductor quantum dot emission spectra is used, partially labelled by a team of experts in the field. Overall, highly convincing results are achieved, as quantum dots are reliably evaluated correctly. Note, that the presented methodology can account for different spectral requirements and is applicable regardless of the underlying photonic structure, fabrication method and material composition. We therefore consider it the first step towards a fully integrated evaluation framework for quantum dots, proving the use of machine learning beneficial in the advancement of future quantum technologies.

Deterministic storage and retrieval of telecom light from a quantum dot single-photon source interfaced with an atomic quantum memory.

A hybrid interface of solid-state single-photon sources and atomic quantum memories is a long sought-after goal in photonic quantum technologies. Here, we demonstrate deterministic storage and retrieval of light from a semiconductor quantum dot in an atomic ensemble quantum memory at telecommunications wavelengths. We store single photons from an indium arsenide quantum dot in a high-bandwidth rubidium vapor-based quantum memory, with a total internal memory efficiency of (12.9 ± 0.4)%. The signal-to-noise ratio of the retrieved light field is 18.2 ± 0.6, limited only by detector dark counts.

Optical charge injection and coherent control of a quantum-dot spin-qubit emitting at telecom wavelengths.

Solid-state quantum emitters with manipulable spin-qubits are promising platforms for quantum communication applications. Although such light-matter interfaces could be realized in many systems only a few allow for light emission in the telecom bands necessary for long-distance quantum networks. Here, we propose and implement an optically active solid-state spin-qubit based on a hole confined in a single InAs/GaAs quantum dot grown on an InGaAs metamorphic buffer layer emitting photons in the C-band. We lift the hole spin-degeneracy using an external magnetic field and demonstrate hole injection, initialization, read-out and complete coherent control using picosecond optical pulses. These results showcase a solid-state spin-qubit platform compatible with preexisting optical fiber networks.

Two-photon interference in the telecom C-band after frequency conversion of photons from remote quantum emitters.

Efficient fibre-based long-distance quantum communication via quantum repeaters relies on deterministic single-photon sources at telecom wavelengths, potentially exploiting the existing world-wide infrastructures. For upscaling the experimental complexity in quantum networking, two-photon interference (TPI) of remote non-classical emitters in the low-loss telecom bands is of utmost importance. Several experiments have been conducted regarding TPI of distinct emitters, for example, using trapped atoms1, ions2, nitrogen vacancy centres3,4, silicon vacancy centres5, organic molecules6 and semiconductor quantum dots7,8. However, the spectral range was far from the highly desirable telecom C-band. Here, we exploit quantum frequency conversion to realize TPI at 1,550 nm with single photons stemming from two remote quantum dots. We thereby prove quantum frequency conversion9-11 as a bridging technology and a precise and stable mechanism to erase the frequency difference between independent emitters. On resonance, a TPI visibility of 29 ± 3% has been observed, limited only by the spectral diffusion processes of the individual quantum dots12,13. The local fibre network used covers several rooms between two floors of the building. Even the addition of up to 2 km of fibre channel shows no influence on the TPI visibility, proving the photon wavepacket distortion to be negligible. Our studies pave the way to establish long-distance entanglement distribution between remote solid-state emitters including interfaces with various quantum hybrid systems14-16.