Robust entanglement at room temperature is a necessary requirement for practical applications in quantum technology. We demonstrate the creation of bipartite- and tripartite-entangled quantum states in a small quantum register consisting of individual 13C nuclei in a diamond lattice. Individual nuclear spins are controlled via their hyperfine coupling to a single electron at a nitrogen-vacancy defect center. Quantum correlations are of high quality and persist on a millisecond time scale even at room temperature, which is adequate for sophisticated quantum operations.

The dynamics of single electron and nuclear spins in a diamond lattice with different 13 C nuclear spin concentration is investigated. It is shown that coherent control of up to three individual nuclei in a dense nuclear spin cluster is feasible. The free induction decays of nuclear spin Bell states and single nuclear coherences among 13 C nuclear spins are compared and analyzed. Reduction of a free induction decay time T * 2 and a coherence time T2 upon increase of nuclear spin concentration has been found. For diamond material with depleted concentration of nuclear spin, T * 2 as long as 30 µs and T2 of up to 1.8 ms for the electron spin has been observed. The 13 C concentration dependence of T * 2 is explained by Fermi contact and dipolar interactions with nuclei in the lattice. It has been found that T2 decreases approximately as 1/n, where n is 13 C concentration, as expected for an electron spin interacting with a nuclear spin bath. PACS numbers:Defect centers in diamond have attracted considerable interest recently owing to their application for quantum information processing, communication and metrology. [1,2,3, 4,5,6,7] Especially the nitrogen-vacancy (NV) center, with its strong and spin dependant optical transitions allows for single spin readout and exquisite coherent control which is crucial for quantum information applications. [1,2,3, 4,5] Owing to the high Debye temperature of diamond and weak coupling to acoustic phonons NV electron spins show long coherence time. It was e.g. proposed to build small quantum registers by exploiting the interaction between the electron spin and a small number of nuclear spins in the immediate vicinity. Five-qubit would be sufficient to perform all functions necessary for a node in a defect center based quantum repeater node. [4,5] Up to now coherent control, swapping of coherence and even entanglement between up to two nuclei and the electron spin was demonstrated.[3] To increase the size of the quantum register, more nuclei need to be coupled to the electron spin. The approach taken here is to increase the concentration of paramagnetic 13 C nuclei in the lattice. We systematically demonstrate coherent control of up to three nuclear spins being coupled to an NV center electron spin in 13 C isotopically enriched crystals, notwithstanding the fact that the electron decoherence time T 2 linearly scales with the 13 C concentration. Furthermore, our experiments provide experimental insight into long studied problem of single central spin coupled to a paramagnetic environment. [8,9,10] Owing to possibility to address individual electron spins in matrix with adjustable nuclear spin content we show the transition from diluted to dense spin bath (the situation relevant for spins in GaAs quantum dots).The quantum system used in the present work is the negatively charged NV center in diamond, which comprises a substitutional nitrogen atom with an adjacent vacancy. (Fig. 1(h)) The electron ground state of it is a spin triplet. Upon optical excitation the NV center shows stron...

No abstract

We report successful introduction of negatively charged nitrogen-vacancy (NV(-)) centers in a 5 nm thin, isotopically enriched ([(12)C] = 99.99%) diamond layer by CVD. The present method allows for the formation of NV(-) in such a thin layer even when the surface is terminated by hydrogen atoms. NV(-) centers are found to have spin coherence times of between T2 ~ 10-100 μs at room temperature. Changing the surface termination to oxygen or fluorine leads to a slight increase in the NV(-) density, but not to any significant change in T2. The minimum detectable magnetic field estimated by this T2 is 3 nT after 100 s of averaging, which would be sufficient for the detection of nuclear magnetic fields exerted by a single proton. We demonstrate the suitability for nanoscale NMR by measuring the fluctuating field from ~10(4) proton nuclei placed on top of the 5 nm diamond film.

We demonstrate an absolute magnetometer based on quantum beats in the ground state of nitrogen-vacancy centers in diamond. We show that, by eliminating the dependence of spin evolution on the zero-field splitting D, the magnetometer is immune to temperature fluctuation and strain inhomogeneity. We apply this technique to measure low-frequency magnetic field noise by using a single nitrogen-vacancy center located within 500 nm of the surface of an isotopically pure (99.99% 12C) diamond. The photon-shot-noise limited sensitivity achieves 38  nT/sqrt[Hz] for 4.45 s acquisition time, a factor of sqrt[2] better than the implementation which uses only two spin levels. For long acquisition times (>10  s), we realize up to a factor of 15 improvement in magnetic sensitivity, which demonstrates the robustness of our technique against thermal drifts. Applying our technique to nitrogen-vacancy center ensembles, we eliminate dephasing from longitudinal strain inhomogeneity, resulting in a factor of 2.3 improvement in sensitivity.

Some of the stable isotopes of silicon and carbon have zero nuclear spin, whereas many of the other elements that constitute semiconductors consist entirely of stable isotopes that have nuclear spins. Silicon and diamond crystals composed of nuclear-spin-free stable isotopes ( 28 Si, 30 Si, or 12 C) are considered to be ideal host matrixes to place spin quantum bits (qubits) for quantum-computing and -sensing applications, because their coherent properties are not disrupted thanks to the absence of host nuclear spins. The present paper describes the state-of-theart and future perspective of silicon and diamond isotope engineering for development of quantum information-processing devices.

A summary of photo- and electrochemical surface modifications applied on single-crystalline chemical vapour deposition diamond films is given. The covalently bonded formation of amine and phenyl linker molecular layers is characterized using X-ray photoelectron spectroscopy, atomic force microscopy (AFM), cyclic voltammetry and field-effect transistor characterization experiments. Amine and phenyl layers are very different with respect to formation, growth, thickness and molecular arrangement. We deduce a sub-monolayer of amine linker molecules on diamond with approximately 10% coverage of 1.510(15) cm(-2) carbon bonds. Amine is bonded only on initially H-terminated surface areas. In the case of electrochemical deposition of phenyl layers, multilayer properties are detected with three-dimensional nitrophenyl growth properties. This leads to the formation of typically 25 A thick layers. The electrochemical bonding to boron-doped diamond works on H-terminated and oxidized surfaces. After reacting such films with heterobifunctional cross-linker molecules, thiol-modified ss-DNA markers are bonded to the organic system. Application of fluorescence and AFM on hybridized DNA films shows dense arrangements with densities up to 10(13) cm(-2). The DNA is tilted by an angle of approximately 35 degrees with respect to the diamond surface. Shortening the bonding time of thiol-modified ss-DNA to 10 min causes a decrease in DNA density to approximately 10(12) cm(-2). Application of AFM scratching experiments shows threshold removal forces of approximately 75 and 45 nN for the DNA bonded to the phenyl and the amine linker molecules, respectively. First, DNA sensor applications using Fe(CN6) 3-/4- mediator redox molecules and DNA field-effect transistor devices are introduced and discussed.

No abstract