Silicon vacancy defects in 4H-silicon carbide semiconductor for quantum applications

Abstract

Secure transmission of information is a crucial element nowadays for industry and national security. Today, the only known way to establish provably secure communication is based on quantum key distribution in a quantum network. Currently, transmission rates and communication distances are limited by (unavoidable) optical loss in fibers and the fundamental quantum no-cloning theorem. In analogy to classical communication, improved network performance is obtained in a network based on multiple nodes that are connected by (quantum) repeaters. The information should be transferred with telecom wavelength photons to be compatible with existing classical fiber networks. The nodes and repeaters need to consist of a quantum system with good optical properties and long memory times, e.g. using a spin with high coherence. Additionally, such memories will be useful for computation and entanglement creation. A realistic quantum system should also be scalable and cheap in fabrication. The first demonstration of a solid state quantum repeater has been recently realized with the NV-centre in diamond. This demonstration showed that the NV-centre in diamond can be used in principle as a quantum repeater but it also brings drawbacks. The NV-centre in diamond has a good spin coherence but a low emission of photons inside the zero phonon line which can be used in a quantum network. A crucial challenge for color defects like the NV-centre in diamond is spectral stability. After a certain amount of time, a sanity check needs to be done to measure the wavelength of the resonant absorption lines. Any change in absorption wavelength requires adapting a multitude of experimental parameters, which is a show-stopper for long-term network reliability. These drawbacks decrease the transmission rate within a quantum network. If one would plan to realize a commercial quantum network, a tailored quantum system with all the mentioned desired properties would be needed. My first approach in this thesis was to use a semiconductor material like 4H-SiC with matured industrial fabrication knowledge (Chapter 2). 4H-SiC hosts a large variety of known quantum defects. I choose to analyze the silicon vacancy V1 centre because of the ZPL emission at 861 nm. It is known from the literature that this wavelength can be efficiently converted to commonly used telecom wavelengths (1530 - 1625 nm). I first analyzed the optical and spin properties in ensembles (Chapter 3). The spin measurement showed that one can coherently manipulate silicon vacancy V1 centre. Emission spectra of a single silicon vacancy V1 centres showed that ~ 40 % of the emission is guided into the ZPL (~ 3 % NV centre). I perform resonant excitation studies in Chapter 4 to investigate the optical properties. Surprisingly, the result showed spectrally stable optical transitions, which was not expected. The general opinion in the research community during this time was that only defects with inversion symmetry can show spectrally stable transitions. 4H-SiC is a piezo electric material which, by definition can not host inversion symmetry quantum defects. The physical origin of spectrally stable transitions for the V1 centre in 4H-SiC was found in the symmetry of the ground and excited state. Both states share the same symmetry and, more importantly, nearly the same dipole moment. The symmetry shields the optical transition frequencies of the silicon vacancy V1 centre against electric field fluctuations and causes spectrally stable optical transitions. For the research community, this discovery opened a new approach in identifying spectrally stable quantum defects in various materials. I analyzed additionally the spin coherence properties of a single silicon vacancy V1 centre and measured a spin coherence time up to 1 ms, which is comparable with the NV-centre in diamond. It has been shown for the NV-centre in diamond that nuclear spins can be used as quantum memory. In 4H-SiC, two types of isotopes exist, 29Si and 13C, that can also be exploited as a quantum memory. It turned also out that the symmetries of ground and excited states are responsible for very low inhomogeneous distribution of the V1 centre resonant absorption lines. In Chapter 5, this property is investigated, especially in view of experiments that require multiple indistinguishable single photon emitters. In my thesis I present the physical properties of a quantum system with excellent optical and spin properties. Additionally, the indistinguishable single photon emission of silicon vacancy V1 centre and mature fabrication knowledge in 4H-SiC make the systems scalable. All these properties combined makes the silicon vacancy V1 centre in 4H-SiC an excellent candidate for the realization of a quantum repeater network.