Many quantum networking applications require efficient photonic interfaces to quantum memories which can be produced at scale and with high yield. Synthetic diamond offers unique potential for the implementation of this technology as it hosts color centers which retain coherent optical interfaces and long spin coherence times in nanophotonic structures. Here, we report a technique enabling wafer-scale processing of thin-film diamond that combines ion implantation and membrane liftoff, high-quality overgrowth, targeted color center implantation, and serial, high-throughput thermocompression bonding with yields approaching unity. The deterministic deposition of thin diamond membranes onto semiconductor substrates facilitates consistent integration of photonic crystal cavities with silicon-vacancy (SiV) quantum memories. We demonstrate reliable, strong coupling of SiVs to photons with cooperativities approaching 50. Furthermore, we show that photonic crystal cavities can be reliably fabricated across several membranes bonded to the same handling chip. Our platform enables modular fabrication where the photonic layer can be integrated with functionalized substrates featuring electronic control lines such as coplanar waveguides for microwave delivery. Finally, we implement passive optical packaging with sub-decibel insertion loss. Together, these advances pave the way to the scalable assembly of optically addressable quantum memory arrays which are a key building block for modular photonic quantum interconnects.

We present in situ tuning of both the absolute and relative frequency spacing of the modes in an optical microcavity by incorporating a wedged diamond membrane. We demonstrate THz continuous tuning of doubly-resonant Raman scattering.

The negatively charged tin-vacancy center in diamond (SnV-) is an emerging platform for building the next generation of long-distance quantum networks. This is due to the SnV-'s favorable optical and spin properties including bright emission, insensitivity to electronic noise, and long spin coherence times at temperatures above 1 K. Here, we demonstrate measurement of a single SnV- electronic spin with a singleshot readout fidelity of 87.4%, which can be further improved to 98.5% by conditioning on multiple readouts. In the process, we develop understanding of the relationship between strain, magnetic field, spin readout, and microwave spin control. We show that high-fidelity readout is compatible with rapid microwave spin control, demonstrating a favorable parameter regime for use of the SnV- center as a highquality spin-photon interface. Finally, we use weak quantum measurement to study measurement-induced dephasing; this illuminates the fundamental interplay between measurement and decoherence in quantum mechanics, and provides a universal method to characterize the efficiency of color-center spin readout. Taken together, these results overcome an important hurdle in the development of the SnV--based quantum technologies and, in the process, develop techniques and understanding broadly applicable to the study of solid-state quantum emitters.

171Yb3+ in YVO4 is a promising candidate for building quantum networks with good optical addressability, excellent spin properties and a secondary nuclear-spin quantum register. However, the associated long optical lifetime necessitates coupling to optical resonators for faster emission of single photons and to facilitate control of single 171Yb ions. Previously, single 171Yb ions were addressed by coupling them to monolithic photonic crystal cavities fabricated via lengthy focused ion beam milling. Here, we design and fabricate a hybrid platform based on ions coupled to the evanescently decaying field of a GaAs photonic crystal cavity. For the most strongly coupled ion close to the GaAs-YVO interface, we find a 64-fold reduction in lifetime corresponding to a Purcell enhancement of 179. For an ion with a Purcell enhancement of 21, we experimentally detect and demonstrate coherent optical control. The results show a promising route toward a quantum network with 171Yb-YVO4 using a highly scalable platform that can readily be applied to other quantum emitters in the near-infrared.

We demonstrate coherent spin physics of a single tin vacancy center (SnV − ) in diamond pillars including coherent population trapping and Rabi oscillations. This work constitutes progress towards scalable quantum networks using SnV − centers.

We report the high efficiency coupling of diamond waveguides to thin-film lithiumniobate photonics, allowing for the optical addressing and characterization of individual SiV - color centers.

Robust, low-loss photonic packaging of on-chip nanophotonic circuits is a key enabling technology for the deployment of integrated photonics in a variety of classical and quantum technologies including optical communications and quantum communications, sensing, and transduction. To date, no process has been established that enables permanent, broadband, and cryogenically-compatible coupling with sub-dB losses from optical fibers to nanophotonic circuits. Here we report a technique for reproducibly generating a permanently packaged interface between a tapered optical fiber and nanophotonic devices with a record-low coupling loss < 1 dB per facet at near-infrared wavelengths ( 730 nm) that remains stable from 300 K to 30 mK. We further demonstrate the compatibility of this technique with etched lithium niobate on insulator waveguides. The technique lifts performance limitations imposed by scattering as light transfers between photonic devices and optical fibers, paving the way for scalable integration of photonic technologies at both room and cryogenic temperatures.

On-chip photonic quantum circuits with integrated quantum memories have the potential to radically progress hardware for quantum information processing. In particular, negatively charged group-IV color centers in diamond are promising candidates for quantum memories, as they combine long storage times with excellent optical emission properties and an optically-addressable spin state. However, as a material, diamond lacks many functionalities needed to realize scalable quantum systems. Thin-film lithium niobate (TFLN), in contrast, offers a number of useful photonic nonlinearities, including the electro-optic effect, piezoelectricity, and capabilities for periodically-poled quasi-phase matching. Here, we present highly efficient heterogeneous integration of diamond nanobeams containing negatively charged silicon-vacancy (SiV) centers with TFLN waveguides. We observe greater than 90\% transmission efficiency between the diamond nanobeam and TFLN waveguide on average across multiple measurements. By comparing saturation signal levels between confocal and integrated collection, we determine a $10$-fold increase in photon counts channeled into TFLN waveguides versus that into out-of-plane collection channels. Our results constitute a key step for creating scalable integrated quantum photonic circuits that leverage the advantages of both diamond and TFLN materials.

The negatively charged tin-vacancy (SnV-) center in diamond is a promising solid-state qubit for applications in quantum networking due to its high quantum efficiency, strong zero phonon emission, and reduced sensitivity to electrical noise. The SnV- has a large spin-orbit coupling, which allows for long spin lifetimes at elevated temperatures, but unfortunately suppresses the magnetic dipole transitions desired for quantum control. Here, by use of a naturally strained center, we overcome this limitation and achieve high-fidelity microwave spin control. We demonstrate a pi-pulse fidelity of up to 99.51+/0.03%$ and a Hahn-echo coherence time of T2echo = 170.0+/-2.8 microseconds, both the highest yet reported for SnV- platform. This performance comes without compromise to optical stability, and is demonstrated at 1.7 Kelvin where ample cooling power is available to mitigate drive induced heating. These results pave the way for SnV- spins to be used as a building block for future quantum technologies.

We present a study on the resonant excitation and electrical Stark tuning of negatively charged tin-vacancy (SnV - ) defect centers in diamond.

We demonstrate coherent optical control of single Yb 3+ ions in YVO 4 coupled evanescently to a GaAs photonic crystal cavity.

Quantum networks rely on the efficient coupling of coherent quantum memories to photonic links. Tin-vacancy centers (SnV) have emerged as promising candidates for the implementation of an optically interfaced quantum memory based on their long spin coherence times at temperatures above 1K. To enhance their interaction with photons SnV centers need to be integrated with high-quality photonic structures. In this work, we report the enhancement of coherent emission of SnV centers by coupling them to a nanophotonic waveguide resonator. We observe strong intensity enhancement of the photon emission when the cavity is resonant with the color center. We demonstrate strong enhancement of the radiative recombination rate of SnV centers resulting in their predominant emission via the coherent zero-phonon line and into the cavity mode. These results are a significant step toward color-center-based quantum information processing applications without the need for dilution refrigerators.

$^{171}$Yb$^{3+}$ in YVO$_4$ is a promising candidate for building quantum networks with good optical addressability, excellent spin properties and a secondary nuclear-spin quantum register. However, the associated long optical lifetime necessitates coupling to optical resonators for faster emission of single photons and to facilitate control of single $^{171}$Yb ions. Previously, single $^{171}$Yb ions were addressed by coupling them to monolithic photonic crystal cavities fabricated via lengthy focused ion beam milling. Here, we design and fabricate a hybrid platform based on ions coupled to the evanescently decaying field of a GaAs photonic crystal cavity. We experimentally detect and demonstrate coherent optical control of single $^{171}$Yb ions. For the most strongly coupled ions, we find a 64 fold reduction in lifetime. The results show a promising route towards a quantum network with $^{171}$Yb:YVO$_4$ using a highly scalable platform.

We present the coupling of tin-vacancy centers in diamond to a photonic crystal cavity resulting in a more than 10-fold enhancement of the spontaneous radiative recombination rate.

With a highly coherent, optically addressable electron spin, the nitrogen-vacancy (NV) center in diamond is a promising candidate for a node in a quantum network. A resonant microcavity can boost the flux of coherent photons emerging from single NV centers. Here, we present an open Fabry–Pérot microcavity geometry containing a single-crystal diamond membrane, which operates in a regime where the vacuum electric field is strongly confined to the diamond membrane. There is a field anti-node at the diamond–air interface. Despite the presence of surface losses, a finesse of F=11500 was observed. The quality (Q) factor for the lowest mode number is 120000; the mode volume V is estimated to be 3.9λ03, where λ0 is the free-space wavelength. We investigate the interplay between different loss mechanisms and the impact these loss channels have on the performance of the cavity. This analysis suggests that the surface waviness (roughness with a spatial frequency comparable to that of the microcavity mode) is the mechanism preventing the Q/V ratio from reaching even higher values. Finally, we apply the extracted cavity parameters to the NV center and calculate a predicted Purcell factor exceeding 150.

Quantum optical networks will enable distribution of quantum entanglement at long distances, with applications including interconnects between future quantum computers and secure quantum communications. I will present our recent work on developing quantum networking components based on rare-earth ions such as single optically addressable quantum bits based on ytterbium 171 in yttrium orthovanadate, microwave to optical transducers based on erbium doped crystals coupled to microwave and optical resonators, and on-chip telecom optical quantum memories.

We present a surface-passivation method that reduces surface-related losses by almost 2 orders of magnitude in a highly miniaturized GaAs open microcavity. The microcavity consists of a curved dielectric distributed Bragg reflector with radius of approximately 10 ?m paired with a GaAs-based heterostructure. The heterostructure consists of a semiconductor distributed Bragg reflector followed by an n-i-p diode with a layer of quantum dots in the intrinsic region. Free-carrier absorption in the highly -n-doped and highly -p-doped layers is minimized by our positioning them close to a node of the vacuum electromagnetic field. The surface, however, resides at an antinode of the vacuum field and results in significant loss. These losses are much reduced by surface passivation. The strong dependence on wavelength implies that the main effect of the surface passivation is to eliminate the surface electric field, thereby quenching below-band-gap absorption via a Franz-Keldysh-like effect. An additional benefit is that the surface passivation reduces scattering at the GaAs surface. These results are important in other nanophotonic devices that rely on a GaAs-vacuum interface to confine the electromagnetic field.

Building quantum networks requires efficient coupling of solid-state quantum emitters to photonic devices. Tinvacancy center (SnV) has attracted much interest for having long spin coherence times at temperatures above 1 K. Employing SnV as an optically addressable qubit requires integration with photonic structures to both route the emitted photons and enhance the light-matter interaction. We present incorporation of high-quality SnV centers with narrow linewidths in suspended diamond waveguides. Furthermore, we fabricate photonic crystal cavities in diamond with embedded SnV centers. We observe strong intensity enhancement of the photon emission when the cavity is resonant with the color center. Time-resolved photoluminescence measurements confirm that this effect is due to radiative Purcell enhancement of the spontaneous emission. Finally, we demonstrate Stark tuning of transition frequency of SnV centers, essential for multiemitter applications. These results are a significant step toward color-center-based quantum information processing applications without the need for dilution refrigerators.