The working group deals with the development and investigation of quantum information on a solid-state basis.[1] The workhorse is the NV centre in diamond, but other centres like the ST1 or L2 centre investigated by the Solid-state Colour Centres group are also being studied.[2]
The production of colour centres in diamond and in particular the NV centre has been significantly improved in recent years. Today, it is possible to specifically produce near-surface NV centres with a conversion rate of more than 80%, which also exhibit extreme charge stability.[3] Deterministic ion implantation, which is being developed in the Räcke group and in collaboration with the Leibniz Institute of Surface Engineering e.V., Leipzig (IOM), has proven to be the method of choice.[4-6] This breakthrough allows the fabrication of scalable registers of qubits with high quality. NV centres have very long coherence times even at room temperature and all the necessary properties such as readout and initialisation to be able to use them as qubits.[1] These properties make it possible to build a scalable quantum computer that works at room temperature and is miniturisable. Our long-term goal is to combine a colour center-based coprocessor in a chip with a classical computer.[1] This work is flanked by collaborative projects such as BMBF CoGeQ and EU Mastro. The NV centre is surrounded by several nuclei with their own spin.[7] These have a much smaller magnetic field and thus interact with each other and the environment only on very long time scales. This is in contrast to NV centres with a strong magnetic field. Due to this fact, the NV centres can be used to act as mediators between the nuclear spins. The NV centres are only used to read out and couple the nuclear spins. For this we use techniques developed by the Wunderlich group. In particular, to put the environment into a defined state.[8]
The main qubits in our concept are therefore the nuclear spins. These can then also be connected as a compound to logical qubits and allow e.g. error-corrected computation. However readout and connection take place using NV centres. And this gives us is an additional advantage: A spin accumulates a global phase during a rotation of e.g. 2π, which is actually not measurable. In relation to another spin, however, this global phase becomes visible. If one uses an NV centre with several nuclear spins, a CNOT between the nuclei can be realised simply by rotating one NV centre over 2π.[1] This allows to create stable two qubits gates and is a big plus compare to the realisation of two qubits gates of superconducting platforms like transmons; they need up to 19 single qubits to create e.g. one Toffoli gate. In photonic platforms the creation of a two qubits gates is a problem at all. In particular, arithmetic calculations can be easily handled with NVs. It is also easy possible to build an AI system with qubits.
These and other "tricks" are being investigated in the research group. Many detailed problems are still open and allow a variety of questions for theses, which correspond to the future job description of a quantum engineer or quantum computer scientist. Just ask, we are happy about your interest.
Selected Publications
[1] S. Pezzagna, J. Meijer. Quantum computer based on color centers in diamond. Appl. Phys. Rev. 8, 011308 (2021). DOI: 10.1063/5.0007444.
[2] T. Lühmann, N. Raatz, R. John, M. Lesik, J. Rodiger, M. Portail, D. Wildanger, F. Kleissler, K. Nordlund, A. Zaitsev, J.-F. Roch, A. Tallaire, J. Meijer, S. Pezzagna. Screening and engineering of colour centres in diamond. J. Phys. D: Appl. Phys. 51, 483002 (2018). DOI: 10.1088/1361-6463/aadfab.
[3] T. Lühmann, R. John, R. Wunderlich, J. Meijer, S. Pezzagna. Coulomb-driven single defect engineering for scalable qubits and spin sensors in diamond. Nat. Commun. 10, 4956 (2019). DOI: 10.1038/s41467-019-12556-0.
[4] P. Räcke, D. Spemann, J.W. Gerlach, B. Rauschenbach, J. Meijer. Detection of small bunches of ions using image charges. Sci. Rep. 8, 9781 (2018). DOI: 10.1038/s41598-018-28167-6.
[5] P. Räcke, R. Wunderlich, J.W. Gerlach, J. Meijer, D. Spemann. Nanoscale ion implantation using focussed highly charged ions. New J. Phys. 22, 083028 (2020). DOI: 10.1088/1367-2630/aba0e6.
[6] I.P. Radko, M. Boll, N.M. Israelsen, N. Raatz, J. Meijer, F. Jelezko, U.L. Andersen, A. Huck. Determining the internal quantum efficiency of shallow-implanted nitrogen-vacancy defects in bulk diamond. Opt. Express 24, 27715 (2016). DOI: 10.1364/OE.24.027715.
[7] F. Dolde, I. Jakobi, B. Naydenov, N. Zhao, S. Pezzagna, C. Trautmann, J. Meijer, P. Neumann, F. Jelezko, J. Wrachtrup. Room-temperature entanglement between single defect spins in diamond. Nat. Phys. 9, 139 (2013). DOI: 10.1038/nphys2545.
[8] R. Wunderlich, J. Kohlrautz, B. Abel, J. Haase, J. Meijer. Optically induced cross relaxation via nitrogen-related defects for bulk diamond 13C hyperpolarization. Phys. Rev. B 96, 220407(R) (2017). DOI: 10.1103/PhysRevB.96.220407.
