Background and research objective

Quantum networks formed by many quantum nodes connected via quantum channels can be used as a foundation for quantum communications and large scale networked and/or distributed quantum computing [1,2]. Since only the photonic quantum channels are known to be practical, quantum nodes should serve as both universal quantum computing processors and quantum interfaces between photonic qubits and stationary qubits. This vision is shared by many researchers; the quantum internet alliance of the EU quantum flagship program is an example (http://quantum-internet.team/).

Among many quantum systems, isolated ions have been known as a promising system for efficient stationary qubits and quantum interfaces [3]. Spins in solids, such as donor spins and color center spins, were thought to impractical because it was commonly believed that spins associated to point defects in solids were effectively coupled to environmental noise, such as fluctuating electric fields from other defects and thermal noise at finite temperature. However, after several decades of research, people have been able to produce high purity single crystalline solids, which serve as semiconductor-vacuums, where long-lived spins with coherence times exceeding 1 second can be found [4–6]. Electronic spins coupled to nuclear spins can be used as a small size quantum registers for multi qubit operations, e.g., quantum error correction [7]. Among well-known color centers in diamond, the NV center is a leading contender whose fluorescence emission properties are strongly related to the electronic spin states. Recently, NV centers have been used to demonstrate entanglement of distant quantum nodes via photonic interference, whose success rate exceeds the decoherence rate [8].

While the NV center in diamond has been intensively investigated and successfully used for many proof-of-idea experiments for quantum information technology and quantum metrology [1,2], researchers have been looking for other candidate color centers in solids [9]. The reason is as follows. The NV center in diamond is merely one of several hundred color centers present in diamond, and diamond is merely one of many crystalline solid materials with potential application in quantum information technology and quantum metrology. Deep defect levels in large-bandgap can hardly be perturbed from transition to band edges, allowing stable and closed-cycle optical transitions. High purity single crystalline solids provide a diluted electronic spin bath and reproducible electronic structures of desired point defects. When abundant atoms forming crystalline solids have zero nuclear spins, well-isolated spin qubits can be formed in the so-called semiconductor vacuum [10]. These criteria can be satisfied not only by diamond but also by several wide-bandgap semiconductors, e.g. silicon carbide [9]. In addition, well-developed fabrication techniques will be helpful to develop practical quantum devices, which may also be suitable for miniaturization.

Since Koehl et al. published a seminal work in 2011 [11], silicon carbides have been examined for their potential in small-scale quantum devices. They demonstrated that color center ensembles in silicon carbide have long-lived spins and that optical spin detection is possible. Soon, two types of single color centers, namely divacancies and silicon vacancies, could be isolated and single qubit operations were demonstrated [12,13]. From these works, a vast amount of research has been done [1]. Examples are: vector magnetometry [14] and bright single photon sources with electrical pumping [15–17]. Among them, recent results have shown that silicon vacancies in silicon carbide show promising properties as a spin-to-photon interface: high optical contrast of spin readout, strong spin-dependent optical transition cycle, and long-lived spins [18,19]. In addition, a new report shows that electrical spin detection is also possible at room temperature [20]. So far, the fabrication of efficient quantum devices has been hindered due to the extreme hardness of diamond. However, because well-developed silicon carbide fabrication techniques already exist, including CMOS compatible methods and convenient doping thanks to lower activation energy of donors and acceptors [21], these results tell that color centers in silicon carbide may play an important role in quantum applications.

In order to pave the way for quantum applications based on point defects in semiconductor devices, our group will investigate various aspects of color centers in silicon carbide and related quantum devices, such as:

• Correlation between spin states and optical transitions of isolated point defects in wide bandgap semiconductors, such as silicon vacancies in silicon carbide. This research can lead to the development of efficient spin-to-photon quantum interfaces, a building block of quantum networks.

• Quantum registers consisting of electronic spins of point defects and nearby nuclear spins in wide bandgap semiconductors. This will allow to scale-up small size quantum processors based on easily fabricable material platforms. The widely used optical excitation and detection of spin qubits can be combined with the electrical control and detection methods by fabricating optoelectronic device, e.g., quantum LEDs. Expected advantages are the speed-up of the readout and the miniaturization.

• Charge state conversion mechanisms of point defects in silicon carbide. This should be well understood because the charge state determines the electronic structure and, thus, the optical transition energy and probability, paramagnetic properties, and the stability of optical transitions. Efficient control of charge states will allow the control of hyperfine interactions of nuclear spins, stable and indistinguishable photon emission, and ionization (charging and recharging process), which are necessary for efficient quantum registers, quantum interfaces, and electrical spin detection.

• Quantum metrology based on point defects in semiconductors working under ambient conditions. For example, several connected spin qubits in quantum registers, such as entangled spin probes, will be useful not only for quantum information processing but also quantum metrology, such as magnetometry. The phase of spin qubits can be used for quantum parameter estimation via interference methods, and the charge state of point defects is also sensitive to environmental parameter fluctuation such as electric fields, optical fields, temperature, and microscopic changes of the doping profile and Fermi-level fluctuation in semiconductor devices. Quantum probes in semiconductors can lead to new era for metrology.

• Quantum devices based on semiconductors. By utilizing mature fabrication techniques from silicon industry, we probably can devise electronic, optoelectronic, and photonic devices harnessing quantum systems. Furthermore, when electrical detection and excitation are well controlled, local measurement can be separately performed via electrical circuits from optical interface so that crosstalk between optical control and measurements can be prevented. The on-demand quantum light source may be feasible and combined with the optical interface as well.