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arXiv:2306.15207 · ACS Photonics

Efficient Photonic Integration of Diamond Color Centers and Thin-Film Lithium Niobate

Daniel Riedel, Hope Lee, Jason F. Herrmann, Jakob Grzesik, Vahid Ansari, Jean-Michel Borit, Hubert S. Stokowski, Shahriar Aghaeimeibodi, Haiyu Lu, Patrick J. McQuade, Nicholas A. Melosh, Zhi‐Xun Shen, Amir H. Safavi‐Naeini, Jelena Vuckovic A

Published Dec 4, 2023• 25 citations
arXivHTMLPDF

Abstract

On-chip photonic quantum circuits with integrated quantum memories have the potential to radically advance 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 the 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 the 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 the TFLN waveguide on average across multiple measurements. By comparing saturation signal levels between confocal and integrated collection, we estimate a more than 10-fold increase in photon emission channeled into TFLN waveguides versus that channeled 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.

DOI
10.1021/acsphotonics.3c00992

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On-chip photonic quantum circuits with integrated quantum memories have the potential to radically advance 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 the 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 the 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 the TFLN waveguide on average across multiple measurements. By comparing saturation signal levels between confocal and integrated collection, we estimate a more than 10-fold increase in photon emission channeled into TFLN waveguides versus that channeled 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.
Diamond photonics has enabled efficient interfaces for quantum memories and is predicted to be a critical component of quantum networks. However, scalable network architectures require spatial, temporal, and spectral control of photons, which relies on nonlinear and electro-optic functionalities that diamond alone cannot provide. Here, we demonstrate heterogeneous integration of a thin-film lithium niobate (TFLN) platform, which has strong chi-2 nonlinearity and electro-optic effects, with thin diamond films. We demonstrate high-Q diamond photonic crystal cavities (Q factors exceeding 5x10^4 at 735 nm) that are lithographically aligned with TFLN photonic backbone and critically coupled to it. This allows us to realize low-loss diamond-TFLN "escalators" (loss ~1 dB/coupler) that support efficient light transfer between them. At cryogenic temperatures (5K), we can collect photons emitted from silicon vacancies (SiVs) embedded within the diamond structure via the TFLN photonic circuit. This approach establishes a scalable route toward integrated photonic circuits for practical quantum networking and other technologies.
The realization of quantum networks capable of distributing quantum information over kilometer scale distances is key to maximizing the potential of quantum information technology. Implementing this technology requires both the ability to generate remote entanglement between quantum memory nodes, requiring an efficient spin-photon interface, and the ability to efficiently scale the classical optical control and multiplexing optics within a given node. In this work, we develop integrated photonics platforms in diamond and thin film lithium niobate to address both of these issues. Together, these platforms provide the necessary ingredients to create large scale quantum networking technology. For the quantum network nodes, we extend previous results combining diamond nanophotonic resonators with the silicon-vacancy defect as a quantum networking platform. We demonstrate, for the first time, quantum entanglement between SiV quantum memories across 35km in a deployed fiber network. We also develop additional functionalities within the SiV platform including the incorporation of deterministic strain to increase defect yield and deterministic single photon generation with the SiV. In order to enable the scalable control and multiplexing of individual SiV centers necessary for realizing practical quantum network nodes, we develop a state-of-the-art electro-optics platform operating at visible wavelengths using thin film lithium niobate. We develop an understanding of fabrication induced losses at visible wavelengths in the platform and a variety of key devices in the platform including high-bandwidth, low-drive voltage amplitude and phase modulators, as well as high-efficiency off chip couplers. Utilizing this platform we perform ultra-high efficiency electro-optic frequency shifting, achieving a 15 GHz frequency shift with over 50% efficiency. Finally, using our measured device performance, we discuss and model how these devices can effectively multiplex SiV-based quantum nodes. Thus through these two platforms we have demonstrated the necessary ingredients to realize large-scale quantum network nodes.
The integration of quantum emitters with integrated photonics enables complex quantum photonic circuits that are necessary for photonic implementation of quantum simulators, computers, and networks. Thin-film lithium niobate is an ideal material substrate for quantum photonics because it can tightly confine light in small waveguides and has a strong electro-optic effect that can switch and modulate single photons at low power and high speed. However, lithium niobate lacks efficient single-photon emitters, which are essential for scalable quantum photonic circuits. We demonstrate deterministic coupling of single-photon emitters with a lithium niobate photonic chip. The emitters are composed of InAs quantum dots embedded in an InP nanobeam, which we transfer to a lithium niobate waveguide with nanoscale accuracy using a pick-and-place approach. An adiabatic taper transfers single photons emitted into the nanobeam to the lithium niobate waveguide with high efficiency. We verify the single photon nature of the emission using photon correlation measurements performed with an on-chip beamsplitter. Our results demonstrate an important step toward fast, reconfigurable quantum photonic circuits for quantum information processing.
Integrated quantum photonics holds significant promise for scalable photonic quantum information processing, quantum repeaters, and quantum networks, but its development is hindered by the mismatch between materials hosting high-quality quantum emitters and those compatible with mature photonic technologies. Heterogeneous integration offers a potential solution to this challenge, yet practical implementations have been limited by inevitable insertion losses at material interfaces. Here, we present a self-aligned heterogeneous quantum photonic integration approach that can deterministically achieve near-unity coupling efficiency at the interface. To showcase our approach, we demonstrate Purcell enhancement of a silicon vacancy (SiV) center in diamond induced by a heterogeneous photonic crystal cavity defined by titanium dioxide (TiO2), as well as optical spin control and readout via a TiO2 photonic circuit. We further show that, when combined with inverse photonic design, our approach enables efficient and broadband collection of single photons from a color center into a heterogeneous waveguide. Our approach is not restricted to SiV centers or TiO2; it can be broadly applied to integrate diverse solid-state quantum emitters with thin-film photonic devices where conformal deposition is possible. Together, these results establish a practical route to scalable quantum photonic integrated circuits that combine high-quality quantum emitters with technologically mature photonic platforms.
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.