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Quantum Interfaces and Processors in Semiconductors

Diamond Quantum Photonics

The realization of scalable and reliable quantum information processing is a central challenge for the field; a promising candidate is an optical, solid-state platform. In our group, we investigate single color centers in diamond, which stand out as excellent quantum bits (qubits) due to their long coherence and short quantum gate times.

One of the important components for building a solid-state spin-based quantum network interconnected with photons is an efficient spin-photon interface. A cavity-based spin-photon interface provides a solution for enhancing the coherent emission of photons into the zero-phonon line (ZPL), and can additionally act as an ultrafast single photon source, which finds applications in a broad range of quantum technologies.

In one monolithic approach, chemical vapor deposition (CVD) grown negatively charged silicon-vacancy (SiV–) color centers are embedded in diamond photonic crystal cavities (Figure 1 (a-c)). We demonstrate strong enhancement of spontaneous emission rate of a single silicon-vacancy center in diamond, reaching a regime where the excited state lifetime is dominated by the spontaneous emission into the cavity mode (Figure 1 (d-f)).

Built upon this interface, frequency tunable single-photon emission from a single SiV can be demonstrated, by utilizing a cavity-enhanced Raman scattering process. We have achieved a tuning range of up to 100 GHz, as shown in Figure 1 (g), well beyond the spectral inhomogeneity of 30 GHz observed for SiVs embedded in nanostructures; such results pave the way toward the realization of photon-mediated many-body interactions between different SiVs.

A remaining challenge toward the development of integrated photonic circuits in diamond is the integration of color centers into large-scale optical networks: arrays of qubits need to be interconnected to form a quantum processor. Such optical quantum circuits will operate at the level of single or few photons, which require extremely efficient on-chip components and optical interfaces. In our work, we develop such scalable photonic circuits by exploring new avenues of fabrication and design. We first use inverse-design algorithms to realize optimized quantum photonic devices with non-intuitive physical features. We then explore scalable fabrication methods in the Stanford Nanofabrication Facilities, which enable the fabrication of these uncommon designs in nonstandard materials. Via this approach, we recently realized on-chip optical vertical couplers (Figure 2 A). Through continuous and discrete optimization stages, the inverse design approach finds a unique solution, while obeying fabrication constraints such as minimum feature size (Figure 2 B). This design improves the coupling efficiency between free-space and diamond waveguide modes by a factor of >25 in comparison with typical optical interfaces in diamond (Figure 2 C). Furthermore, the vertical couplers are broadband and robust against fabrication and alignment imperfections (Figure 2 D). Our current efforts are toward the development of large-scale quantum optical experiments involving several color centers entangled in a quantum circuit (Figure 2 E). Ultimately, we hope to make major progress in the development of universal quantum computers, quantum repeaters, and photonic quantum simulators.

Silicon Carbide Photonics

4H-Silicon Carbide (4H-SiC) offers unique potential for on-chip quantum photonics, as it hosts a variety of promising color centers and possesses strong second- and third-order optical nonlinearities. Our group developed a fabrication process for thin films of 4H-SiC-on-Insulator (SiCOI) which is compatible with industry-standard, CMOS nanofabrication; we are exploring efficient integration of optically addressable qubits in SiC into photonic circuits, as well as monolithic quantum frequency conversion to the telecommunications band.

Left: Silicon vacancy color centers in 4H-SiCOI. Right: SiCOI photonics with high quality factors demonstrate the potential of SiC for low-loss photonics applications.

Left: Silicon vacancy color centers embedded in SiC photonic crystal cavities. Right: Strong enhancement of emission (by a factor of 120) from a silicon vacancy in a SiC nanocavity to enable efficient spin-photon interfaces.

Left: Second-harmonic generation in 4H-SiC: a doubly-resonant scheme allows for high efficiency frequency conversion. Center: Conversion efficiency of 360%/Watt in a microring resonator. Right: Concept figure showing (i) integrated optically-entangled spins and (ii) monolithic generation and frequency conversion of quantum light.

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