Principal Investigator: Professor Joerg Wrachtrup
The Project
NV Centres
The principal aim of this project is development of a quantum processor based on the storage of light states into spin states on NV (Nitrogen Vacancy) defect centres in diamond.
Defects or impurities often determine the mechanical, electrical and optical properties of solids. This effect can be used to engineer materials with desirable attributes. For example, precise control of impurity content underpins much of modern silicon device technology. NV centres occur in diamond: a carbon atom is substituted by a nitrogen atom, accompanied by a vacancy at a nearest neighbour lattice position.
Qubits can be defined as single electronic or nuclear spin states. In this implementation, electronic spin states of NV-centres in diamond could be used as quantum memories. The optical transition between ground and excited electronic states allows coupling of spin degrees of freedom to the state of the electromagnetic field. This coupling gives access
to the spin state readout via spin-selective scattering of photons, constituting an interface between two physically distinct encodings of information. Thus spin states could be used as a robust memory for qubits read in and out of memory by photons.
Initially this project will focus on feasibility tests of NV defects in diamond as centres for quantum state storage. This will involve observation and optimisation of long phase-memory times, electromagnetically induced transparency (EIT) and slow group-velocity of signal pulses. Long term objectives include the mapping and retrieval of classical light states to spin states of NV-centres as a precursor to the storage and recall of non-classical light states necessary for quantum memories.
Creation of entangled states of single atoms and photons by interference
Quantum repeaters will form the basic building blocks of any kind of quantum communication network, allowing transmission of quantum information despite the decoherence mechanisms inherent in quantum channels. One of the key elements to a quantum repeater will be the creation and distribution of entanglement over large distances. This project aims to engineer entanglement between NV defect centres in diamond by joint measurements on emitted photons.
Two distant NV defect centres in diamond will be optically excited, and their fluorescence signals subsequently interfered prior to detection. As the fluorescence signal is dependent on the electronic spin state, detection of the fluorescence photons can entangle the electronic degrees of freedom of the NV centres. The entangled states will be characterised by local readout of the internal spin state of the NV defects.
Initial work in this project has focussed on production and characterisation of defect centres suitable for the entanglement experiments. Defect centres have been successfully produced with high positioning accuracy using ion implantation techniques in ultra-pure diamond [2-3]. Future work will focus on the implementation of the entanglement protocol and the potential for scaling the scheme to include a larger number of qubits. In addition, this system will be used to generate entangled states of flying qubits.
Nitrogen Vacancy Defects in Diamond
This aim of this project is to use clusters of nuclear spins in diamond for quantum simulations, with state readout via measurements on the single spins of electrons. The nuclear spin clusters are based on NV (Nitrogen Vacancy) defect centres in diamond.
Systems of coupled spins are seen in a wide variety of physical scenarios and are being investigated in a new light as physicists consider the connection between entanglement of individual quantum systems and global physical properties. Of particular interest in this workpackage is the magnetic ordering of spins in clusters of nitrogen vacancy defect centres in diamond. Defect centres interact via optical transition dipole moment coupling and magnetic dipole coupling. By varying the relative strength of these interactions, researchers at Stuttgart plan to show a phase transition between a ferromagnetic or anti-ferromagnetic ordered ground state of the defect cluster.
Excess substitutional nitrogen can act as a major source of decoherence in diamond, contributing to the dephasing of NV centres. To achieve long coherence times, the presence of substitutional nitrogen atoms must be minimised in the diamond host – except at the required NV defect centres. This is achieved by ion implantation of nitrogen atoms in ultra-pure diamond. During the early stages of this project, defect centre pairs separated by ~3 nm have been successfully generated by this method. These small separations within a cluster are required for strong coupling between adjacent centres.
In recent experiments, coherent manipulation of an individual electron spin associated with an NV centre was successfully used to gain insight into its local environment. This environment is effectively separated into a set of individual proximal 13C nuclear spins, which are coupled coherently to the electron spin, and the remainder of the 13C nuclear spins, which cause the loss of coherence. The proximal nuclear spins can be addressed and coupled individually because of quantum back-action from the electron, which modifies their energy levels and magnetic moments, effectively distinguishing them from the rest of the nuclei. These results open the door to coherent manipulation of individual isolated nuclear spins in a solid-state environment even at room temperature [1]. Future work will include experiments on two- and three-spin systems, and generation of a single electron entangled with a single nuclear spin.
List of Publications
QAP
[1] L. Childress, M.V. Gurudev Dutt, J.M. Taylor, A.S. Zibrov, F. Jelezko, J. Wrachtrup, P.R. Hemmer and M.D. Lukin Coherent Dynamics of Coupled Electron and Nuclear Spin Qubits in Diamond, Science 314 281 (2006)
[2] Ph. Tamarat, T. Gaebel, J.R. Rabeau, M. Khan, A.D. Greentree, H. Wilson, L.C.L. Hollenberg, S. Prawer, P. Hemmer, F. Jelezko, and J. Wrachtrup Stark Shift Control of Single Optical Centers in Diamond, Phys. Rev. Lett. 97, 083002 (2006)
[3] J. Wrachtrup, F. Jelezko, Processing quantum information in diamond, Journal of Physics: Condensed Matter 18 807-824 (2006) quant-ph/0510152
T. Gaebel, M. Domhan, I. Popa, C. Wittmann, P. Neumann, F. Jelezko, J.R. Rabeau, N. Stavrias, A.D. Greentree, S. Prawer, J. Meijer, J. Twamley, P.R. Hemmer and J. Wrachtrup Room-temperature coherent coupling of single spins in diamond, Nature Physics 2 408-413 (2006) quant-ph/0605038
F. Jelezko, J. Wrachtrup, Single defect centres in diamond: A review, PHYSICA. STATUS SOLIDI A 203 3207 (2006)
A. Batalov, C. Zierl, T. Gaebel et al, Coherence properties of photons emitted by single defect centers in diamond (2007) arXiv:0710.1442
Ph. Tamarat, N. B. Manson, R. L. McMurtie et al, The excited state structure of the nitrogen-vacancy center in diamond (2007) cond-mat/0610357
P. Neumann, N. Mizuochi, F. Rempp et al, Multipartite Entanglement Among Single Spins in Diamond, Science 320 1326 (2008)
Related work
J. Meijer, T. Vogel, B. Burchard, I.W. Rangelow, L. Bischoff, J. Wrachtrup, M. Domhan, F. Jelezko, W. Schnitzler, S.A. Schulz, K. Singer and F. Schmidt-Kaler Concept of deterministic single ion doping with sub-nm spatial resolution, Applied Physics A: Materials Science & Processing 83 (2): 321-327 (2006)
J.R. Rabeau, P. Reichart, G. Tamanyan, D.N. Jamieson, S. Prawer, F. Jelezko, T. Gaebel, I. Popa, M. Domhan and J. Wrachtrup Implantation of labelled single nitrogen vacancy centers in diamond using 15N, Appl. Phys. Lett. 88, 023113 (2006)