Single photons are a fundamental element of most quantum optical technologies. The ideal single-photon source is an on-demand, deterministic, single-photon source delivering light pulses in a well-defined polarization and spatiotemporal mode, and containing exactly one photon. In addition, for many applications, there is a quantum advantage if the single photons are indistinguishable in all their degrees of freedom. Single-photon sources based on parametric down-conversion are currently used, and while excellent in many ways, scaling to large quantum optical systems remains challenging. In 2000, semiconductor quantum dots were shown to emit single photons, opening a path towards integrated single-photon sources. Here, we review the progress achieved in the past few years, and discuss remaining challenges. The latest quantum dot-based single-photon sources are edging closer to the ideal single-photon source, and have opened new possibilities for quantum technologies.

High-performance semiconductor quantum-dot single-photon sources.

Quantum light–matter interfaces are at the heart of photonic quantum technologies. Quantum memories for photons, where non-classical states of photons are mapped onto stationary matter states and preserved for subsequent retrieval, are technical realizations enabled by exquisite control over interactions between light and matter. The ability of quantum memories to synchronize probabilistic events makes them a key component in quantum repeaters and quantum computation based on linear optics. This critical feature has motivated many groups to dedicate theoretical and experimental research to develop quantum memory devices. In recent years, exciting new applications, and more advanced developments of quantum memories, have proliferated. In this review, we outline some of the emerging applications of quantum memories in optical signal processing, quantum computation and non-linear optics. We review recent experimental and theoretical developments, and their impacts on more advanced photonic quantum technologies based on quantum memories.

https://tandfonline.com/doi/pdf/10.1080/09500340.2016.1148212

Quantum memories: emerging applications and recent advances.

Single spins trapped in self-assembled quantum dots present rich opportunities for studying their quantum mechanical properties. This Review surveys their optical properties, and the techniques for initializing, manipulating and reading out single spin qubits in these structures.

Single spins in self-assembled quantum dots

Two-dimensional transition metal dichalcogenide semiconductors are intriguing hosts for quantum light sources due to their unique optoelectronic properties. Here, we report that strain gradients, either unintentionally induced or generated by substrate patterning, result in spatially and spectrally isolated quantum emitters in mono- and bilayer WSe2. By correlating localized excitons with localized strain variations, we show that the quantum emitter emission energy can be red-tuned up to a remarkable ∼170 meV. We probe the fine-structure, magneto-optics, and second-order coherence of a strained emitter. These results raise the prospect of strain-engineering quantum emitter properties and deterministically creating arrays of quantum emitters in two-dimensional semiconductors.

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Strain-Induced Spatial and Spectral Isolation of Quantum Emitters in Mono- and Bilayer WSe2

Piezoelectric and ferroelectric materials have shown great potential for control of the optical process in emerging materials. There are three ways for them to impact on the optical process in various materials. They can act as external perturbations, such as ferroelectric gating and piezoelectric strain, to tune the optical properties of the materials and devices. Second, ferroelectricity and piezoelectricity as innate attributes may exist in some optoelectronic materials, which can couple with other functional features (e.g., semiconductor transport, photoexcitation, and photovoltaics) in the materials giving rise to unprecedented device characteristics. The last way is artificially introducing optical functionalities into ferroelectric and piezoelectric materials and devices, which provides an opportunity for investigating the intriguing interplay between the parameters (e.g., electric field, temperature, and strain) and the introduced optical properties. Here, the tuning strategies, fundamental mechanisms, and recent progress in ferroelectric and piezoelectric effects modulating the optical properties of a wide spectrum of materials, including lanthanide-doped phosphors, quantum dots, 2D materials, wurtzite-type semiconductors, and hybrid perovskites, are presented. Finally, the future outlook and challenges of this exciting field are suggested.

Ferroelectric and Piezoelectric Effects on the Optical Process in Advanced Materials and Devices.

Combining semiconductor quantum dots and atomic systems allows the light emitted from a quantum dot to be temporarily stored. Here, scientists describe a hybrid semiconductor-atomic interface that can slow down a single photon emitted from a quantum dot by 15 times its temporal width. The findings are attractive for the implementation of quantum memories and quantum repeaters.

L. Wang

Papers

Hybrid semiconductor-atomic interface: slowing down single photons from a quantum dot