A perfect diamond is transparent. What gives it colour are atomic sized defects in the lattices structure, aptly called colour centers. Such colour centers possess electronic and nuclear spin degrees of freedom that can be controlled and brought to interact by microwave and radiofrequency fields and whose states can be read out with the help of visible light. Thanks to the remarkably long coherence times of electron and nuclear spins they are excellent candidates for the realization of quantum information processing, quantum simulation and quantum sensing. In close collaboration with the experimental team at the Institute for Quantum Optics and on occasion the Institute of Organic Chemistry III we are exploring all these possibilities.

Quantum Information Processing: A fundamental requirement for quantum information processing is the ability to achieve quantum coherent interaction between qubits. For colour centers in diamond this poses several challenges, including the management of noise, the design of interactions and the creation of regular and sufficiently closely spaced colour centers. We are addressing these challenges with a variety of strategies.

• A. Bermudez, F. Jelezko, M.B. Plenio and A. Retzker. Electron-mediated nuclear-spin interactions between distant NV-centers. Phys. Rev. Lett. 107, 150503 (2011).
• J.-M. Cai, B. Naydenov, R. Pfeiffer, L. P. McGuinness, K. D. Jahnke, F. Jelezko, M. B. Plenio and A. Retzker. Robust dynamical decoupling with concatenated continuous driving. New. J. Phys. 14, 113023 (2012).
• A. Albrecht, A. Retzker, G. Koplovitz, F. Jelezko, S. Yochelis, Y. Nevo, O. Shoseyov,Y. Paltiel and M.B. Plenio. Self-assembling hybrid diamond-biological quantum devices. New. J. Phys. 16, 093002 (2014).
• A. Albrecht and M.B. Plenio. Filter design for hybrid spin gates. arXiv: 1504.03575 (2015).

Furthermore, we explore hybrid setups, combining the colour centers’ spin degrees of freedom with vibrational modes of the diamond structure, or exploiting the diamond spins as a quantum memory for superconducting qubits.

• A. Albrecht, A. Retzker, F. Jelezko and M.B. Plenio. Coupling of nitrogen vacancy centers in nanodiamonds by means of phonons. New. J. Phys. 15, 083014 (2013).
• J.-M. Cai, F. Jelezko, N. Katz, A. Retzker and M.B. Plenio. Long-lived driven solid-state quantum memory. New. J. Phys. 14, 093030 (2012).

Quantum Sensing: The sensitivity of electron spins to external magnetic fields can cause decoherence but may also be turned into an advantage as the electron spins can also be used as a sensor for these magnetic fields. We are devising measurement strategies that can detect minute fields down to those emanating from single nuclear and electronic spins in the presence of ambient noise. Our long-term goal in this direction is the application of such strategies in biological environments to unravel biological phenomena at the atomic level including the structure and function of proteins as well as the possible role of quantum effects in biology.

• J.M. Cai, F. Jelezko, M.B. Plenio and A. Retzker. Diamond based single molecule magnetic resonance spectroscopy. New J. Phys. 15, 013020 (2013)
• A. Ermakova, G. Pramanik, J.-M. Cai, G. Algara-Siller, U. Kaiser, T. Weil, Y.-K. Tzeng, H. C. Chang, L. P. McGuinness, M.B. Plenio, B. Naydenov, and F. Jelezko. Detection of a few metallo-protein molecules using color centers in nanodiamonds. Nano. Lett. 13, 3305 (2013).
• Z.-Y. Wang, J.M. Cai, A. Retzker and M.B. Plenio. All-optical magnetic resonance of high spectral resolution using a nitrogen-vacancy spin in diamond. New. J. Phys. 16, 083033 (2014).

For that task we also explore signal processing techniques such as matrix completion to reconstruct the sensing signal from incomplete data sets. Such approaches allow for tremendously reducing the experimental acquisition time. That way, multidimensional spectroscopy techniques, which ultimately allow to extract dynamical properties and the identification of protein structures, become feasible.

• A.C. Müller, X. Kong, J.-M. Cai, K. Melentijevic, A. Stacey, M. Markham, J. Isoya, S. Pezzagna, J. Meijer, J. Du, M.B. Plenio, B. Naydenov, L.P. McGuinness and F. Jelezko. Nuclear magnetic resonance spectroscopy with single spin sensitivity. Nature Commun. 5, 4703 (2014).
• M. Kost, J.-M. Cai and M. B. Plenio. Resolving single molecule structures with nitrogen-vacancy centers in diamond. arXiv: 1407.6262 (2014).

Polarizing nuclear spins by polarization exchange with the colour center allows for the reduction of the noise background in sensing and computation tasks. In addition such techniques can be used to (hyper-)polarize nuclear spins for magnetic resonance imaging (C-13 MRI) with significant signal gain. We analyze various such strategies and their application for sensing tasks in biological and medical applications.

• P. London, J. Scheuer, J.M. Cai, I. Schwarz, A. Retzker, M.B. Plenio, M. Katagiri, T. Teraji, S. Koizumi, J. Isoya, R. Fischer, L.P. McGuinness, B. Naydenov and F. Jelezko. Detecting and polarizing nuclear spins with nuclear double resonance on a single electron spin. Phys. Rev. Lett. 111, 067601 (2013).
• Q. Chen, I. Schwarz, F. Jelezko, A. Retzker and M.B. Plenio. Optical hyperpolarization of 13C nuclear spins in nanodiamond ensembles. arXiv: 1504.02368 (2015).

A new hybrid sensor concept, primarily suited for the detection of small forces and pressures, is proposed by combining a layer of nitrogen vacancy colour centers with piezomagnetic elements. A transduction of strain to magnetic fields, the latter straightforwardly detected by its impact on the colour center’s energy level splitting, allows for up to a thousand times sensitivity improvement as compared to direct strain detection methods. It is thus promising for life science applications as exploring minuscule forces in protein folding or DNA stretching on the nanoscale.

• J.M. Cai, F. Jelezko and M.B. Plenio. Hybrid sensor based on colour centres in diamond and piezoactive layers. Nature Commun. 5, 4065 (2014).

Quantum Simulation: The ability to sense small number spins with colour centers also allows us to explore entirely new architectures for scalable quantum simulators that can operate at room temperature. It consists of strongly-interacting nuclear spins attached to the diamond surface by its direct chemical treatment, or by means of a functionalized graphene sheet. The initialization, control and read-out of this quantum simulator can be accomplished with nitrogen-vacancy centers implanted in diamond. The system can be engineered to simulate a wide variety of interesting strongly-correlated models with long-range dipole-dipole interactions. The realization of this device is being pursued in close collaboration with the Institute for Quantum Optics.

• J.M. Cai, A. Retzker, F. Jelezko and M.B. Plenio. A large-scale quantum simulator on a diamond surface at room temperature. Nature Physics 9, 168-173 (2013)

Nanodiamonds for interferometric applications: Nanodiamonds comprising addressable colour centers form mesoscopic quantum systems. Interferometric applications are based on entangling the colour center spin with the particle motion, the latter crucially dependent on the diamond size and shape. This allows for analyzing quantum superposition of significantly large particles, studies of decoherence and the emergence of classicality, as well as addressing fundamental questions about the applicability range of quantum mechanics.

• A. Albrecht, A. Retzker and M.B. Plenio. Testing quantum gravity by nanodiamond interferometry with nitrogen-vacancy centers. Phys. Rev. A 90, 033834 (2014).