The Silicon Vacancy in Silicon Carbide: a promising qubit in a technological material

Lead Research Organisation: Heriot-Watt University
Department Name: Sch of Engineering and Physical Science


How small can one shrink an electronic memory? The ultimate limit in storage can be reached by encoding information in a single particle, for example a single electron or a single atomic nucleus. There are several ways to encode a bit of information into an electron. For example, one could label "1" the presence of the particle, and "0" its absence. Or, one could use an intrinsic property of quantum objects called "spin", which makes the particle behave as a tiny magnet. In this case, "1" would be encoded, for example, as spin pointing to the North Pole and "0" as spin pointing to the South Pole of the single particle magnet.
Using single particles to encode information can give advantages that go beyond miniaturization. Electron spins obeys the laws of quantum mechanics. In quantum physics, the spin of an electron is not required to point either "North" or "South", like a magnetic needle, but it can be "North" AND "South" at the same time. Or, while a bit in a computing device is either in the "0" or "1" state, a quantum bit can be both at the same time. This is much more than a bizarre curiosity: in the last few decades, we have learnt that the laws of quantum mechanics can be exploited to perform tasks impossible for classical physics, such as secure communication, faster computing or precise sensing.

The goal of this project is to measure and control single spins in silicon carbide, a material consisting of a lattice of silicon and carbon atoms. A silicon atom missing in this lattice creates a defect which hosts a single electronic spin that can be measured and manipulated by laser and radiofrequency pulses. Basically, this system behaves as a single atom trapped in silicon carbide. Why silicon carbide? In addition to hosting spins with great properties, silicon carbide is a technological material routinely used by the semiconductor industry to manufacture transistors and other microelectronic components. The availability of established recipes for growth, doping and nano-fabrication can lead to practical quantum devices.
Control of single spins in silicon carbide is still in its infancy. We learned only recently how electrons are arranged in these defects. Only in the past year, control of a single spin in silicon carbide was demonstrated. There is plenty of information missing, and we can improve the efficiency of our control tools by learning more about the structure of these defects.
This project will characterize the electronic structure of these defects by studying how they absorb and emit light. By operating at very low temperatures, the noise related to atomic vibrations which would mask the optical signal will be frozen out. The knowledge about light emission and absorption, and its relation to the spin trapped in the defect, will enable us to realizing exciting schemes that use single spins to encode and decode information for future technologies.

Planned Impact

Due to a combination of outstanding spin properties, large-scale commercial availability of high-purity wafers and established fabrication techniques, silicon carbide offers a great opportunity for real-world quantum devices. Using this platform, the vision of an integrated chip, comprising optically-active spins in nanophotonic structures and electronic circuitry for charge control and spin manipulation can become a reality. While this project deals with the basic physics of spin qubits in SiC, this is the first step required to develop this vision and to generate impact.

Economic and Commercial Impact
In the short term, Norstel AB, a leading supplier of high-purity SiC wafers and industrial partner of this project, will gain the valuable benefits. The material they provide will be characterized using state-of-the-art single defect spectroscopy techniques, applied for the first time to SiC only in 2015.
In the medium term, this research will generate knowledge on SiC defects, which will have impact on the semiconductor community. SiC plays a critical role in several industrial sectors, including aerospace, electronics, industrial furnaces and wear-resistant mechanical parts. In electronics, SiC is the material of choice for sensors operating in harsh environments (aerospace, oil and gas, and geothermal exploration). In the UK, several companies are active on SiC power electronics, for example Dynex Semiconductors, Rolls Royce, BAE Systems. Moreover, Raytheon recently opened a SiC foundry in Glenrothes (Scotland). My research output can positively affect international industries active both in material growth and in device processing, among others Cree Inc, United Silicon Carbide Inc, Infineon, ROHM, Norstel, Dow Corning, NovaSiC. I will make sure that knowledge is transferred to the engineering community and to the semiconductor industry sector through the strategies outlined in "Pathways to Impact".

Societal Impact
In the longer term (>10 years), quantum technology with a technologically mature material such as SiC can pave the way to real-world quantum devices. In particular, I envision SiC quantum devices embedding integrated microcavities for efficient spin-photon interfacing as nodes for quantum communication networks. Highly coherent spins will function as a long-term quantum memory and near-infrared photons will distribute quantum information among the nodes.
A second application is in the field of sensing. Single spins are the smallest possible magnetic field sensors, providing high measurement sensitivity at the ultimate limits of spatial resolution. Nanoscale magnetic field sensors could benefit the information technology sector, allowing mapping of currents in microelectronic circuits or magnetic storage devices and enabling non-invasive high-resolution probing of failures. Or they could have a great impact on biomedical and pharmaceutical research and development, by offering a tool for nanoscale magnetic resonance imaging of molecules.
Finally PhD students and post-docs working on the project will acquire skills in areas like laser physics, optics, electron spin resonance, high-speed electronics, cryogenics, programming and data analysis, which can be employed in academia and high-tech industries.

The public and policy-makers
Through the activities outlined in "Pathways to Impact", the public will gain awareness of the latest results in quantum physics, the promises of quantum technology and the impact of my own work. This will set them in a position to make conscious informed choices regarding why these activities are worth the taxpayer support.
Description We have studied spin-active colour centres (in particular the silicon vacancy, VSi) in silicon carbide, at cryogenic temperature, both at the ensemble and single centre level. This work was carried out through a collaboration with leading international scientists in the field (Stuttgart, Seoul, Linkoping). The study of individual defects at cryogenic temperature is important because if can give access to atomic-like optical transitions, which are averaged out in ensemble experiments. These transitions reveal important information about the electronic structure of the system and their precise knowledge is crucial to interface photons with spins. Our work provided the first set of spectroscopic experiments at cryogenic temperature, revealing the presence of ultra-stable spin-selective optical transition. The outstanding spectral stability of these optical transitions has been explained by the group of Prof Adam Gali in Hungary and it can open the way to integrating these spin-defects in nano-photonic structures without degrading the quality of spin-photon interfacing.
Exploitation Route The determination of the optical transitions associated with the Si vacancy in SiC is crucial to implement spin-photon interfaces in a technologically-friendly semiconductor. This will be important for the quantum networking community, since it may lead to quantum repeaters implemented on wafer-scale semiconductor chips. In addition, our findings on the electronic structure of the Si vacancy are important for materials scientists, since we are providing them with new knowledge that was not previously available.
Sectors Electronics

Description EPSRC Early-Career Fellowship
Amount £1,190,000 (GBP)
Funding ID EPSRC EP/S000550/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 03/2019 
End 02/2024
Description NQIT Partnership Resource
Amount £44,719 (GBP)
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Academic/University
Country United Kingdom
Start 11/2018 
End 10/2019
Description Research Incentive Grant
Amount £9,858 (GBP)
Funding ID RIG007503 
Organisation Carnegie Trust 
Sector Charity/Non Profit
Country United Kingdom
Start 09/2018 
End 08/2019
Description Collaboration on high-purity SiC material 
Organisation Linkoping University
Department University Medical School Linkoping
Country Sweden 
Sector Academic/University 
PI Contribution I have characterized SiC samples provided my collaborators, using state-of-the-art quantum techniques
Collaborator Contribution My partners are world leaders in the development of SiC material and devices. They have provided several samples, including: - a-plane SiC with isolated Si vacancies - c-plane SiC sample with isolated divacancy - pin diode structures
Impact R Nagy et al, Quantum properties of dichroic silicon vacancies in silicon carbide, Physical Review Applied 9 (3), 034022 (2018) this collaboration is multi-disciplinary, involving quantum spintronics and materials science
Start Year 2017
Description Collaboration on low-temperature spectroscopy of single VSi defects in SiC 
Organisation University of Stuttgart
Country Germany 
Sector Academic/University 
PI Contribution The goal of this collaboration is to create a synergy between my expertise on low-temperature spectroscopy of defects and Stuttgart's expertise on spins in SiC. While I have extensive expertise on diamond, the SiC platform was new to me. I participated to experimental measurements in Stuttgart and my contribution has been extremely important in modelling spin dynamics for a S=3/2system with small splitting between the transitions.
Collaborator Contribution My partners in Stuttgart contribute by enabling me to perform experiments that I could not perform at Heriot-Watt, due to the lack of a dedicated cryogenic setup.
Impact One paper published: R. Nagy et al, "Quantum properties of dichroic silicon vacancies in silicon carbide", Physical Review Applied 9 (3), 034022 (2018) One paper under review in Nature Communications: r. Nagy et al, "High-fidelity spin and optical control of single silicon vacancy centres in silicon carbide", arxiv:1810.10296 (2018) One more paper is currently being written
Start Year 2017
Description Electric sensing with defects in SiC 
Organisation Durham University
Department Durham University ESRC Impact Acceleration Account
Country United Kingdom 
Sector Academic/University 
PI Contribution We are developing a technique to measure electric fields in SiC by looking at spin-active defects. This could be useful as a way to map electric fields in micro-electronic devices.
Collaborator Contribution Clas-SiC is interested in the technique we are developing, for device diagnostics. We have a joint project, funded by the Quantum Hub, where they provide SiC samples in-kind.
Impact None, yet
Start Year 2018
Title Experimental Control Architecture 
Description This is a python architecture to control data acquisition and analysis in our lab. It's currently not available, since it's still under development. Will be made available when ready. 
Type Of Technology Software 
Year Produced 2018 
Impact None, yet 
Title ReadPTU 
Description We have developed a software package for fast processing of photon times of arrival, for quantum optics applications 
Type Of Technology Software 
Year Produced 2018 
Open Source License? Yes  
Impact This code has made our measurements much faster 
Description Royal Society Summer Science Exhibition "Atomic Architects" 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact My group developed a Royal Society Summer Science Exhibition in London ("Atomic Architects"). While the exhibit was mostly about emitters in 2D materials, I did also advertise my work on SiC funded by EPSRC.
Year(s) Of Engagement Activity 2018
Description Royal Society theoMurphy meeting on "SiC quantum spintronics" 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact I was awarded by the Royal Society the possibility to organize a scientific workshop on themes of SiC quantum spintronics. This workshop attracted scientists from all over the world, but also representatives from industry.
Year(s) Of Engagement Activity 2018