We report results important for the creation of a best-of-both-worlds quantum hybrid system consisting of a solid-state source of single photons and an atomic ensemble as quantum memory. We generate single photons from a GaAs quantum dot (QD) frequency matched to the Rb D2 transitions and then use the Rb transitions to analyze spectrally the quantum dot photons. We demonstrate lifetime-limited QD linewidths (1.42 GHz) with both resonant and nonresonant excitation. The QD resonance fluorescence in the low power regime is dominated by Rayleigh scattering, a route to match quantum dot and Rb atom linewidths and to shape the temporal wave packet of the QD photons. Noise in the solid-state environment is relatively benign: there is a blinking of the resonance fluorescence at MHz rates but negligible dephasing of the QD excitonic transition. We therefore demonstrate significant progress towards the realization of an ideal solid-state source of single photons at a key wavelength for quantum technologies.

Quantum memories matched to single photon sources will form an important cornerstone of future quantum network technology. We demonstrate such a memory in warm Rb vapor with on-demand storage and retrieval, based on electromagnetically induced transparency. With an acceptance bandwidth of δf ¼ 0.66 GHz, the memory is suitable for single photons emitted by semiconductor quantum dots. In this regime, vapor cell memories offer an excellent compromise between storage efficiency, storage time, noise level, and experimental complexity, and atomic collisions have negligible influence on the optical coherences. Operation of the memory is demonstrated using attenuated laser pulses on the single photon level. For a 50 ns storage time, we measure η 50 ns e2e ¼ 3.4ð3Þ% end-to-end efficiency of the fiber-coupled memory, with a total intrinsic efficiency η int ¼ 17ð3Þ%. Straightforward technological improvements can boost the end-to-end-efficiency to η e2e ≈ 35%; beyond that, increasing the optical depth and exploiting the Zeeman substructure of the atoms will allow such a memory to approach near unity efficiency. In the present memory, the unconditional read-out noise level of 9 × 10 −3 photons is dominated by atomic fluorescence, and for input pulses containing on average μ 1 ¼ 0.27ð4Þ photons, the signal to noise level would be unity. DOI: 10.1103/PhysRevLett.119.060502 Quantum networks built from optical fiber-linked quantum nodes [1] open manifold opportunities across a range of scientific and technological frontiers. For example, highspeed quantum cryptography networks can be used for unconditionally secure communication in metropolitan areas [2], and quantum networks can help realize large scale quantum computers and quantum simulators that will allow for exponential speed-up in solving complex problems [3,4]. Photonic quantum networks, in turn, require a scalable quantum node technology that allows for (i) storing quantum information in a quantum memory [5], and (ii) ondemand conversion of this information into single photons traveling along the network interconnects.To realize quantum nodes, a heterogeneous approach [6,7] is highly promising. Heterogeneous quantum nodes consist of a single photon source and a compatible quantum memory, where the systems may be completely different from each other and can be individually optimized. For the single photon source, self-assembled semiconductor quantum dots (QD) are arguably the best choice, as they allow for high speed on-demand photon generation with up to GHz emission rates and measured efficiencies [8-10] as high as 75%. These sources can emit indistinguishable single photons [9,11,12] or even polarization-entangled photon pairs [13,14], and the QD spin can be entangled with an emitted photon [15,16]. However, the quantum dot itself is not a good quantum memory, since the coherence times are limited by the comparably strong coupling to the solid-state environment. To make this exquisite source of single or entangled photons useful for quantum networks, the QD theref...

We report on a fast, bandwidth-tunable single-photon source based on an epitaxial GaAs quantum dot. Exploiting spontaneous spin-flip Raman transitions, single photons at 780 nm are generated on-demand with tailored temporal profiles of durations exceeding the intrinsic quantum dot lifetime by up to three orders of magnitude. Second-order correlation measurements show a low multi-photon emission probability (g 2 (0) ∼ 0.10 − 0.15) at a generation rate up to 10 MHz. We observe Raman photons with linewidths as low as 200 MHz, narrow compared to the 1.1 GHz linewidth measured in resonance fluorescence. The generation of such narrowband single photons with controlled temporal shapes at the rubidium wavelength is a crucial step towards the development of an optimized hybrid semiconductor-atom interface.

A hybrid system of a semiconductor quantum dot single photon source and a rubidium quantum memory represents a promising architecture for future photonic quantum repeaters. One of the key challenges lies in matching the emission frequency of quantum dots with the transition frequency of rubidium atoms while preserving the relevant emission properties. Here, we demonstrate the bidirectional frequency tuning of the emission from a narrow-linewidth (close-to-transform-limited) quantum dot. The frequency tuning is based on a piezoelectric strain-amplification device, which can apply significant stress to thick bulk samples. The induced strain shifts the emission frequency of the quantum dot over a total range of 1.15 THz, about three orders of magnitude larger than its linewidth. Throughout the whole tuning process, both the spectral properties of the quantum dot and its single-photon emission characteristics are preserved. Our results show that external stress can be used as a promising tool for reversible frequency tuning of high-quality quantum dots and pave the wave toward the realization of a quantum dot–rubidium atom interface for quantum networking.

A semiconductor quantum dot (QD) can generate highly indistinguishable single-photons at a high rate. For application in quantum communication and integration in hybrid systems, control of the QD optical properties is essential. Understanding the connection between the optical properties of a QD and the growth process is therefore important. Here, we show for GaAs QDs, grown by infilling droplet-etched nano-holes, that the emission wavelength, the neutral-to-charged exciton splitting, and the diamagnetic shift are strongly correlated with the capture zone-area, an important concept from nucleation theory. We show that the capture-zone model applies to the growth of this system even in the limit of a low QD-density in which atoms diffuse over µm-distances. The strong correlations between the various QD parameters facilitate preselection of QDs for applications with specific requirements on the QD properties; they also suggest that a spectrally narrowed QD distribution will result if QD growth on a regular lattice can be achieved. arXiv:1902.10145v3 [cond-mat.mes-hall]