Nowadays, we are in the midst of the so-called second quantum technology revolution, which aims to bring paradigm-shifting solutions to critically important applications in every corner of human life. Examples include secure communication that detects any form of spying, efficient computation that is exponentially faster than any classical version, and quantum sensing that exploits the unique quantum properties to provide unprecedented sensitivity. To tackle these grand challenges, we need to develop groundbreaking technology covering all levels, from materials through devices to systems.

Quantum photonic integrated circuits (QPICs) are the technology of choice for various applications because of their compactness, energy efficiency, low cost, high performance and potential for up-scalability1. The basic building blocks for QPICs include quantum light sources, passive and active devices for light manipulation such as beam splitters, filters, wavelength conversion, modulation, etc., and quantum detectors. Despite limited commercial availability, silicon carbide on insulator (SiCOI) is emerging as the next-generation material platform for quantum technology2. As seen in Table 1, in comparison to the existing material platforms of silicon on insulator (SOI), silicon nitride (SiN) and lithium niobate on insulator (LNOI)3, SiCOI shows advantages in the following aspects: broad transparency with < 1 dB/cm waveguide loss demonstrated from ~ 400 nm to ~ 5000 nm; electro-optic, and thermal switching/modulation/tuning mechanisms demonstrated; second harmonic and other nonlinear light generation and color center-based single photon sources demonstrated. Some reported color centers in SiC work at telecommunication wavelength ranges and room temperature, and have long spin coherence times up to 5 s4, which holds promising potential applications in quantum communications, quantum sensing, and quantum computing.

Table 1 Comparison of SiCOI with the major material platforms of SOI, SiN and LNOI
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Among the basic building blocks, the quantum light source is pivotal. Depending on the applications, quantum light sources are versatile. One type is called single photon source, where a single photon is emitted from a single defect or atom, such as color centers and quantum dots. The other type is generated through the nonlinearity of the material, known as spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM), corresponding to second-order (χ2) and third-order (χ3) nonlinearity respectively5. SiC has the unique advantages of possessing both second-order and third-order nonlinearity, along with versatile color centers. The corresponding quantum light sources are schematically shown in Fig. 1.

Fig. 1: Schematic of three types of quantum light sources based on the SiCOI platform.
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a spontaneous parametric down-conversion (SPDC), b spontaneous four-wave mixing (SFWM) and c color centers. p pump, i idler, s signal, ZPL zero phonon line, ω angular frequency

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As an example of a quantum light source generated from nonlinearity, in a newly published paper in Light: Science & Applications by Anouar Rahmouni, Xiao Tang, Thomas Gerrits, Oliver Slattery and Lijun Ma from the National Institute of Standards and Technology, USA, and Ruixuan Wang, Jinwei Li and Qing Li from the Department of Electrical and Computer Engineering, Carnegie Mellon University, USA, proposes entangled photon pair generation based on third-order nonlinearity, i.e., four-wave mixing (FWM), in the integrated SiC platform6. For the first time, they have generated strongly correlated photon pairs at the telecom C-band wavelength by implementing spontaneous four-wave mixing in a compact microring resonator in the 4H SiCOI platform. The entangled source has been measured to have a maximum coincidence-to-accidental ratio greater than 600 for an on-chip photon pair rate of (9 ± 1) × 103 pairs/s and a pump power of 0.17 mW, a heralded g(2) (0) on the order of 10−3, and a visibility of two-photon interference fringe exceeding 99%. These results are comparable to those obtained from more mature nonlinear integrated platforms such as silicon, silicon nitride and aluminum gallium arsenide.

The above observation for the first time demonstrated entangled photon pair generation from an integrated 4H-SiCOI platform. Quantum information systems are very sensitive to noise, which can greatly reduce the fidelity of quantum information. This work achieved a high CAR (coincidence-to-accidental ratio) of photon pair generation (>600) and a visibility of two-photon interference fringe (>99%), experimentally demonstrating that the noise level in such an SiC platform satisfies the requirements of quantum information devices based on single photons.

Looking forward, the performance of entangled photon pair generation from spontaneous four-wave mixing of 4H-SiCOI is expected to improve as the 4H-SiC material and nanofabrication technology develop, broadening its quantum applications7. 4H-SiC possesses both second-order and third-order nonlinearity. Quantum light sources based on second-order nonlinearities such as second harmonic generation and sum frequency generation, are expected in the near future, which will also provide flexibility for wavelength conversion.

It is foreseeable that the potential of the SiCOI platform will be gradually released as material quality and nanofabrication technology advance. Among the widely available polytype of SiC, 3 C and 4H, 4H-SiC shows the best crystal quality benefitting from its extensive application in power electronics. Therefore the above observation is from this polytype. 3C-SiC possesses slightly different optical properties. For now, 3C-SiCOI has demonstrated better performance in electro-optical modulation than 4H-SiCOI8. This versatility incites competition among different polytypes and speeds up the technology maturity, which in general helps SiC emerge as a promising material platform for QPIC.