Integrated Quantum Photonics with Silicon Carbide: Challenges and Prospects

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Integrated Quantum Photonics with Silicon Carbide: Challenges and Prospects

Daniil M. Lukin, Melissa A. Guidry, and Jelena Vučković

PRX Quantum 1, 020102 – Published 15 December 2020

Abstract

Optically addressable solid-state spin defects are promising candidates for storing and manipulating quantum information using their long coherence ground-state manifold; individual defects can be entangled using photon-photon interactions, offering a path toward large-scale quantum photonic networks. Quantum computing protocols place strict limits on the acceptable photon losses in the system. These low-loss requirements cannot be achieved without photonic engineering, but are attainable if combined with state-of-the-art nanophotonic technologies. However, most materials that host spin defects are challenging to process: as a result, the performance of quantum photonic devices is orders of magnitude behind that of their classical counterparts. Silicon carbide ( $Si C$ ) is well suited to bridge the classical-quantum photonics gap, since it hosts promising optically addressable spin defects and can be processed into $Si C$ -on-insulator for scalable, integrated photonics. In this paper, we discuss recent progress toward the development of scalable quantum photonic technologies based on solid-state spins in silicon carbide, and discuss current challenges and future directions.

Received 17 August 2020
Accepted 2 November 2020

DOI:https://doi.org/10.1103/PRXQuantum.1.020102

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Quantum information architectures & platforms Quantum information processing Quantum information with solid state qubits

Optical materials & elements

Cavity resonators Cryogenics Optical techniques

Quantum Information, Science & TechnologyCondensed Matter, Materials & Applied PhysicsAtomic, Molecular & Optical

Authors & Affiliations

Daniil M. Lukin, Melissa A. Guidry, and Jelena Vučković ^*

E. L. Ginzton Laboratory, Stanford University, Stanford, California 94305, USA

^*jela@stanford.edu

Popular Summary

Defects in crystals are usually undesirable imperfections that arise during crystal growth and processing. Some defects, however, have properties that make them useful for quantum computing. Defects like some color centers in silicon carbide and diamond have the ability to store quantum information for a long time in their ground states; furthermore, they can efficiently encode their spin state information onto photons in order to transmit the information over long distances. However, it is challenging to efficiently collect photons emitted by the defects, and nanoscale photonic engineering is necessary to strengthen the light-matter interaction to enable quantum computation.

Silicon carbide simultaneously hosts high-quality color centers and can be used to fabricate large-scale monolithic photonic device networks, which makes it a unique quantum material for scalable quantum photonic computation. In this paper, we discuss the challenges and prospects of developing a fully integrated color-center-based quantum photonic computer.

We offer a brief review of the state of the art of quantum photonics in silicon carbide, compare it to other leading solid-state platforms (such as diamond), and outline the current leading approaches for implementing color-center quantum technologies. Finally, we discuss how the industry-compatible silicon carbide-on-insulator platform can pioneer integrated color-center quantum technologies (including quantum simulation and quantum computation) in the next 5–10 years.

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Vol. 1, Iss. 2 — December - December 2020

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Figure 1
Quantum photonics with optically addressable spins. (a) A suitable defect features a spin-selective optical transition, where a photon degree of freedom (i.e., polarization, frequency, or time bin) is entangled with a ground-state spin featuring a long coherence time. The defect’s optical lifetime is determined by the magnitude of its optical dipole moment, which in turn is dictated by the orbital structure of the excited and ground states. (b) A quantum photonic network consists of multiqubit registers, each consisting of an optically addressable electron spin strongly coupled to nearby nuclear spins. The registers are integrated together in a network via an efficient waveguide or fiber interface. The network is equipped with beamsplitters and switches (which may be one and the same depending on the implementation) for long-distance entanglement and circuit reconfigurability. Low losses at all stages (including efficient photon collection from the defect, low-loss photon propagation in the waveguide, and high detector efficiency) are essential for fault-tolerant computation, efficient quantum simulation, and long-distance quantum communications. (c) Because of the weak confinement of optical photons in dielectric structures, light from a quantum emitter does not couple efficiently to a simple dielectric waveguide. Instead, a nanophotonic cavity or a slow-light waveguide mode must be used to enhance the emission into the waveguide mode via the Purcell effect. Orbital graphic in (a) adapted from Ref. [35].
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Figure 2
Optically active spins in $Si C$ . (a) The 4H- $Si C$ crystal lattice, showing the inequivalent configurations of the divacancy, silicon vacancy ( $V_{Si}$ ), and chromium ion. (b) Patterning of $V_{Si}$ using a focused proton beam enables three-dimensional control over placement of single defects and ensembles. (c) Direct laser writing of single $V_{Si}$ for on-demand color-center generation in $Si C$ devices under test. (d) Stable, nearly lifetime-limited optical emission from the $V_{Si}$ under above-resonant excitation. (e) The demonstration of indistinguishable single-photon emission from the $V_{Si}$ . (f) The $V_{Si}$ remains spectrally stable even under high-amplitude, fast Stark shift modulation, enabling the observation of optical Floquet eigenstates. (g) Advanced epitaxy techniques available commercially for $Si C$ made possible the integration of high-quality divacancy color centers into $p$ - $i$ - $n$ junctions, enabling the demonstration of nearly 1 THz of Stark shift. (h) By depleting the charge-trap environment via reverse bias in the $p$ - $i$ - $n$ junction, linewidth narrowing from 1 GHz to 20 MHz is observed, approaching the transform limit. (i) Among the nascent optically addressable defects in $Si C$ , the chromium ion stands out for, among other characteristics, its high Debye-Waller factor of 75%. Reproduced from (b) Ref. [73], (c) Ref. [74], (d),(e) Ref. [52], (f) Ref. [54], (g),(h) Ref. [63], (i) Ref. [70].
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Figure 3
Timeline of $Si C$ photonics development. First demonstration of a $Si C$ photonic device using the Smart-Cut approach with 6H- $Si C$ [93]. Soon after, suspended resonators in 3C- $Si C$ -on- $Si$ were demonstrated [98]. Strong intrinsic absorption of low-quality Smart Cut and heteroepitaxial 3C films was hypothesized to limit the achievable $Q$ factors. Using thicker 3C- $Si C$ epilayers or thinning down bulk-crystal 4H- $Si C$ , enabled record $Q$ factors in 3C- $Si C$ [101, 102], ultra-high $Q$ photonic crystal cavities [33], and low-loss 4H- $Si C$ -on-insulator waveguides [32]. Recently, devices with $Q$ factors exceeding $10^{6}$ were shown, enabling the demonstration of optical parametric oscillation and microcomb formation [34]. Reproduced from Refs. [32, 33, 34, 93, 98, 102].
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Figure 4
$Si C$ quantum photonics with defects. (a) An array of nanophotonic cavities fabricated in 4H- $Si C$ -on-insulator, integrated with single $V_{Si}$ color centers [32]. (b) A cavity is tuned via gas condensation through the color-center resonance, thereby enhancing the optical transition by 120 times. (c) Nanophotonic cavities fabricated via the photoelectrochemical undercut technique [107], integrated with single divacancies. (d) Divacancy emission spectrum on and off resonance with the cavity, showing enhancement of 53 times. (e) Coherent spin control of the cavity-integrated divacancy. Reproduced from (a),(b) Ref. [32], (c)–(e) Ref. [107].
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Figure 5
Approaches to scaling-up spin-based quantum photonic technologies. (a) Diamond nanophotonic cavity with a single silicon-vacancy defect and an adiabatically coupled fiber interface [133]. (b) Free-space coupled Fabry-Pérot microcavity enhancing the emission of a NV center in diamond [134]. Inset: the concave mirror can also be fabricated directly on the tip of a fiber [135]. (c) Pick-and-place heterogeneous on-chip integration of diamond microchiplets containing silicon- and germanium-vacancy centers on top of aluminum nitride photonic waveguides [112] (image courtesy of Noel Wan). (d) A heterogeneous approach without pick-and-place can be realized by using a secondary layer of photonic interconnects to postselect working quantum nodes [32]. (e) A conceptual diagram demonstrating how the example photonic network shown in Fig. 1 could be realized in a fully monolithic platform. In order to account for nonunity fabrication yield, $N$ redundant nodes are fabricated in the place of one node, and a $N \times 1$ switch (composed of cascaded $2 \times 1$ switches) selects one working node. Reproduced from (a) Ref. [133], (b) Refs. [134, 135] (d) Ref. [32].
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Figure 6
Quantum information processing with spectrally reconfigurable defects. (a) By rapidly modulating an optical transition, engineering of exotic photon emission spectra from color centers was demonstrated [54]. Via optimized Stark modulation (insets), the $V_{Si}$ has been engineered to emit photons in a superposition of 4 (top) and 2 (bottom) colors. (Reproduced from Ref. [54]). Based on this approach one may design nearly arbitrary reconfigurable connectivity via spectral overlap, by independently controlling color centers interacting together via photon-photon interactions through the resonator. This proposal is illustrated in (b),(c) where nine emitters are prepared in Floquet states to create a specific connectivity via spectral overlap. (d) A concept figure of the possible experimental implementation of this approach.
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