In this work, we present a stand-alone and fiber-coupled quantum-light source. The plug-and-play device is based on an optically driven quantum dot delivering single photons via an optical fiber. The quantum dot is deterministically integrated in a monolithic microlens which is precisely coupled to the core of an optical fiber via active optical alignment and epoxide adhesive bonding. The rigidly coupled fiber-emitter assembly is integrated in a compact Stirling cryocooler with a base temperature of 35 K. We benchmark our practical quantum device via photon auto-correlation measurements revealing g(2)(0) = 0.07 ± 0.05 under continuous-wave excitation and we demonstrate triggered non-classical light at a repetition rate of 80 MHz. The long-term stability of our quantum light source is evaluated by endurance tests showing that the fiber-coupled quantum dot emission is stable within 4% over several successive cool-down/warm-up cycles. Additionally, we demonstrate non-classical photon emission for a user-intervention-free 100-hour test run and stable single-photon count rates up to 11.7 kHz with a standard deviation of 4%.

We demonstrate metamaterial metal-based bolometers, which take advantage of resonant absorption in that a spectral and/or polarization filter can be built into the bolometer. Our proof-of-principle gold-nanostructure-based devices operate around 1.5 \mum wavelength and exhibit room-temperature time constants of about 134 \mus. The ultimate detectivity is limited by Johnson noise, enabling room-temperature detection of 1 nW light levels within 1 Hz bandwidth. Graded bolometer arrays might allow for integrated spectrometers with several octaves bandwidth without the need for gratings or prisms and for integrated polarization analysis without external polarization optics

Optimized light–matter coupling in semiconductor nanostructures is a key to understand their optical properties and can be enabled by advanced fabrication techniques. Using in situ electron beam lithography combined with a low-temperature cathodoluminescence imaging, we deterministically fabricate microlenses above selected InAs quantum dots (QDs), achieving their efficient coupling to the external light field. This enables performing four-wave mixing microspectroscopy of single QD excitons, revealing the exciton population and coherence dynamics. We infer the temperature dependence of the dephasing in order to address the impact of phonons on the decoherence of confined excitons. The loss of the coherence over the first picoseconds is associated with the emission of a phonon wave packet, also governing the phonon background in photoluminescence (PL) spectra. Using theory based on the independent boson model, we consistently explain the initial coherence decay, the zero-phonon line fraction, and the line shape of the phonon-assisted PL using realistic quantum dot geometries.

Integrated single-photon sources with high photon-extraction efficiency are key building blocks for applications in the field of quantum communications. We report on a bright single-photon source realized by on-chip integration of a deterministic quantum dot microlens with a 3D-printed multilens micro-objective. The device concept benefits from a sophisticated combination of in situ 3D electron-beam lithography to realize the quantum dot microlens and 3D femtosecond direct laser writing for creation of the micro-objective. In this way, we obtain a high-quality quantum device with broadband photon-extraction efficiency of (40 ± 4)% and high suppression of multiphoton emission events with g(2)(τ = 0) < 0.02. Our results highlight the opportunities that arise from tailoring the optical properties of quantum emitters using integrated optics with high potential for the further development of plug-and-play fiber-coupled single-photon sources.

Spectrally-tunable quantum light sources are key elements for the realization of long-distance quantum communication. A deterministically fabricated single-photon source with a photon extraction efficiency of η =(20 ± 2) %, a maximum tuning range of ΔE =  2.5 meV and a minimum g(2)(τ = 0) = 0.03 ± 0.02 is presented. The device consists of a single pre-selected quantum dot (QD) monolithically integrated into a microlens that is bonded onto a piezoelectric actuator via gold thermocompression bonding. Here, a thin gold layer simultaneously provides strain transfer and acts as a backside mirror for the QD-microlens to maximize the photon extraction efficiency. The QD-microlens structure is patterned via 3D in-situ electron-beam lithography (EBL), which allows us to pre-select and integrate suitable QDs based on their emission intensity and energy with a spectral accuracy of 1 meV for the final device. Together with strain fine-tuning, this enables the scalable realization of single-photon sources with identical emission energy. Moreover, we show that the emission energy of the source can be stabilized to µeV accuracy by closed-loop optical feedback. Thus, the combination of deterministic fabrication, spectral-tunability and high broadband photon-extraction efficiency makes the QD-microlens single-photon source an interesting building block for the realization of quantum communication networks.

We present an efficient broadband single-photon source which is fabricated by a flip-chip gold-bonding technique and in-situ electron beam lithography. The device comprises a single InGaAs quantum dot that is centered at the bottom of a monolithic mesa structure and located above a gold mirror for enhanced photon-extraction efficiency. We show a photon-extraction efficiency of ηext=(18±2) % into a numerical aperture of 0.4 and a high suppression of multi-photon events from this source with g(2)(0)=0.015±0.009. Our deterministic device with a backside gold mirror can be combined with electrical contacts and piezo-tuning capabilities in future refinements, which represents an important step towards a spectrally tunable plug-and-play quantum-light source with broadband enhancement for photonic quantum networks.

No abstract

User-friendly single-photon sources with high photon-extraction efficiency are crucial building blocks for photonic quantum applications. For many of these applications, such as long-distance quantum key distribution, the use of single-mode optical fibers is mandatory, which leads to stringent requirements regarding the device design and fabrication. We report on the on-chip integration of a quantum dot (QD) microlens with a 3D-printed micro-objective in combination with a single-mode on-chip fiber coupler. The practical quantum device is realized by the deterministic fabrication of the QD-microlens via in situ electron-beam lithography and the 3D two-photon laser writing of the on-chip micro-objective and fiber chuck. A QD with a microlens is an efficient single-photon source, whose emission is collimated by the on-chip micro-objective. A second polymer microlens is located at the end facet of the single-mode fiber and ensures that the collimated light is efficiently coupled into the fiber core. For this purpose, the fiber is placed in an on-chip fiber chuck, which is precisely aligned to the QD-microlens thanks to the sub-micrometer processing accuracy of high-resolution two-photon direct laser writing. The resulting quantum device has a broadband photon extraction efficiency, a single-mode fiber-coupling efficiency of 22%, a measured single-photon flux of 42 kHz (8.9 kHz) under cw (pulsed) optical excitation, which corresponds to 1.5 MHz (0.3 MHz) at the single-mode fiber output, and a multi-photon probability in terms of g(2)(0) = 0.00±0.000.04 (0.13 ± 0.05) under cw (pulsed) optical excitation. The stable design of the developed fiber-coupled quantum device makes it highly attractive for integration into user-friendly plug-and-play quantum applications.