Hybrid quantum circuits combine two or more physical systems, with the goal of harnessing the advantages and strengths of the different systems in order to better explore new phenomena and potentially bring about novel quantum technologies. This article presents a brief overview of the progress achieved so far in the field of hybrid circuits involving atoms, spins, and solid-state devices (including superconducting and nanomechanical systems). How these circuits combine elements from atomic physics, quantum optics, condensed matter physics, and nanoscience is discussed, and different possible approaches for integrating various systems into a single circuit are presented. In particular, hybrid quantum circuits can be fabricated on a chip, facilitating their future scalability, which is crucial for building future quantum technologies, including quantum detectors, simulators, and computers.

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Hybrid quantum circuits: Superconducting circuits interacting with other quantum systems

Quantum dots embedded in photonics nanostructures provide unprecedented control over the interaction between light and matter. This review gives an overview of the theoretical principles involved, as well as applications ranging from high-precision quantum electrodynamics experiments to quantum-information processing.

Interfacing single photons and single quantum dots with photonic nanostructures

This work was supported by the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the National Fundamental Research Program. We acknowledge financial support by the State of Bavaria and the German Ministry of Education and Research (BMBF) within the projects Q.com-H and the Chist-era project SSQN. N. G. acknowledges support from the Danish Research Council for Technology and Production.

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On-Demand Single Photons with High Extraction Efficiency and Near-Unity Indistinguishability from a Resonantly Driven Quantum Dot in a Micropillar.

3.1. Fundamental Characterization 6672 3.2. Photocurrent Action Spectroscopy 6673 3.3. Optical Characterization 6673 3.4. Characterization of Electron Transfer 6673 4. Electron Transport in Metal Oxide Films 6674 4.1. Mechanism of Photoinduced Carrier Transport 6674 4.2. Characterization of Diffusion Length 6675 4.3. One-Dimensional (1-D) Transport Architectures 6675 4.4. Electrolyte Interactions 6676 5. Recent Trends in Liquid-Junction Solar Cells 6676 5.1. Dye Sensitized Solar Cells 6677 5.2. Quantum Dot Sensitized Solar Cells 6678 5.3. Carbon Nanostructure Based Photochemical Solar Cells 6679

Beyond Photovoltaics: Semiconductor Nanoarchitectures for Liquid-Junction Solar Cells

Efficient interfaces between photons and quantum emitters form the basis for quantum networks and enable optical nonlinearities at the single-photon level. We demonstrate an integrated platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to diamond nanodevices. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum-optical switch controlled by a single color center. We control the switch using SiV metastable states and observe optical switching at the single-photon level. Raman transitions are used to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two entangled SiV centers.

https://science.sciencemag.org/content/sci/354/6314/847.full.pdf

An integrated diamond nanophotonics platform for quantum optical networks

Using a long-lived quantum-dot spin qubit coupled to a GaAs-based photonic crystal cavity, researchers demonstrate complete quantum control of an electron spin qubit. By cleverly controlling the charge state of the InAs quantum dot using laser pulses, optical initialization, control and readout of an electron spin are achieved.

Quantum control of a spin qubit coupled to a photonic crystal cavity

Solid-state quantum emitters have shown strong potential for applications in quantum information, but the spectral inhomogeneity of these emitters poses a significant challenge. We address this issue in a cavity–quantum dot system by demonstrating cavity-stimulated Raman spin flip emission. This process avoids populating the excited state of the emitter and generates a photon that is Raman shifted from the laser and enhanced by the cavity. The emission is spectrally narrow and tunable over a range of at least 125 GHz, which is two orders of magnitude greater than the natural linewidth. We obtain the regime in which the Raman emission is spin dependent, which couples the photon to a long-lived electron spin qubit. This process can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic-crystal quantum network. Cavity-stimulated Raman spin-flip emission is demonstrated by coupling a negatively charged InAs/GaAs quantum dot to a photonic crystal defect cavity. The emission is spectrally narrow and tunable over a range of about 125 GHz. The process can be made spin selective by tuning the scattered photons to be in resonance with the cavity.

Cavity-stimulated Raman emission from a single quantum dot spin

Efficient charge transport is demonstrated in TiO2/PbS quantum dot solar cells where the PbS absorber (∼1.1 eV band gap) is deposited by dip coating and ethanedithiol ligand exchange, with power efficiencies above 3% at AM1.5. An increase in power efficiency occurs as the device temperature is lowered to 170 K, with a open-circuit voltage of 0.66 V, short-circuit current density of 28.6 mA/cm2 and fill factor of 42.4%. This remarkable temperature dependence is due to a large increase in charge transport between the PbS quantum dots with decreasing temperature.

High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperature

We have fabricated ultrathin photovoltaic cadmium telluride (CdTe) film solar cells from colloidal nanorod solutions with a power conversion efficiency of 5.0% and internal quantum efficiency approaching unity near the band edge. Sintering of the CdTe nanorod films was necessary to facilitate grain growth and enhanced optical absorption. By analyzing electrode dependence, capacitance-voltage, temperature dependence, and current-voltage characteristics, the device performance is shown to be dominated by the formation of a p-CdTe/Al Schottky junction. The reduced need for material and cheaper processing make this an attractive technology for solar power generation.

CdTe Schottky diodes from colloidal nanocrystals

The integration of InAs/GaAs quantum dots into nanophotonic cavities has led to impressive demonstrations of cavity quantum electrodynamics. However, these demonstrations are primarily based on two-level excitonic systems. Efforts to couple long-lived quantum dot electron spin states with a cavity are only now succeeding. Here we report a two-spin–cavity system, achieved by embedding an InAs quantum dot molecule within a photonic crystal cavity. With this system we obtain a spin singlet–triplet Λ-system where the ground-state spin splitting exceeds the cavity linewidth by an order of magnitude. This allows us to observe cavity-stimulated Raman emission that is highly spin-selective. Moreover, we demonstrate the first cases of cavity-enhanced optical nonlinearities in a solid-state Λ-system. This provides an all-optical, local method to control the spin exchange splitting. Incorporation of a highly engineerable quantum dot molecule into the photonic crystal architecture advances prospects for a quantum network.

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Lily Yang

Papers

Quantum control of a spin qubit coupled to a photonic crystal cavity

Cavity-stimulated Raman emission from a single quantum dot spin

High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperature

CdTe Schottky diodes from colloidal nanocrystals

Spin–cavity interactions between a quantum dot molecule and a photonic crystal cavity