Paramagnetic resonance of photoexcited N-V defects in diamond. II. Hyperfine interaction with the 14N nucleus

Fast Quantum State Tomography in the Nitrogen Vacancy Center of Diamond

Quantum state tomography is the procedure for reconstructing unknown quantum states from a series of measurements of different observables. Depending on the physical system, different sets of observables have been used for this procedure. In the case of spin qubits, the most common procedure is to measure the transverse magnetization of the system as a function of time. Here, we present a different scheme that relies on time-independent observables and therefore does not require measurements at different evolution times, thereby greatly reducing the overall measurement time. To recover the full density matrix, we use a set of unitary operations that transform the density operator elements into the directly measurable observable. We demonstrate the performance of this scheme in the electron-nuclear spin system of the nitrogen vacancy center in diamond.

Photoluminescence of diamond containing nitrogen vacancy defects as a sensor of temperature upon exposure to vacuum- and extreme-ultraviolet radiation

Upon excitation with vacuum-ultraviolet (VUV) and extreme-ultraviolet (EUV) radiation, diamond with nitrogen vacancies (DNV) emits strong photoluminescence (PL) in the wavelength region of 550-800 nm. The spectral profiles of the DNV in the PL spectra appear to be strongly dependent on the temperature of the diamond. Moreover, all PL spectra intersect at one isosbestic point, 570 nm; this result is evidence that the NV0 and NV- defects in diamond interconvert with each other upon VUV and EUV radiation. We suggest the use of PL spectra of DNV excited with VUV or EUV light to indicate the temperature for applications such as in nano-photolithography technology for the manufacture of semiconductor devices.

Efficient Implementation of a Quantum Algorithm in a Single Nitrogen-Vacancy Center of Diamond

Quantum computers have the potential to speed up certain problems that are hard for classical computers. Hybrid systems, such as the nitrogen-vacancy (NV) center in diamond, are among the most promising systems to implement quantum computing, provided the control of the different types of qubits can be efficiently implemented. In the case of the NV center, the anisotropic hyperfine interaction allows one to control the nuclear spins indirectly, through gate operations targeting the electron spin, combined with free precession. Here, we demonstrate that this approach allows one to implement a full quantum algorithm, using the example of Grover's quantum search in a single NV center, whose electron is coupled to a carbon nuclear spin.

Room Temperature Electrically Detected Nuclear Spin Coherence of NV Centres in Diamond

We demonstrate electrical detection of the ¹⁴N nuclear spin coherence of NV centres at room temperature. Nuclear spins are candidates for quantum memories in quantum-information devices and quantum sensors, and hence the electrical detection of nuclear spin coherence is essential to develop and integrate such quantum devices. In the present study, we used a pulsed electrically detected electron-nuclear double resonance technique to measure the Rabi oscillations and coherence time (T₂) of ¹⁴N nuclear spins in NV centres at room temperature. We observed T₂ ≈ 0.9 ms at room temperature, however, this result should be taken as a lower limit due to limitations in the longitudinal relaxation time of the NV electron spins. Our results will pave the way for the development of novel electron- and nuclear-spin-based diamond quantum devices.

Nitrogen vacancy centers in diamond as angle-squared sensors

Nitrogen-vacancy (NV) centers are defects in diamonds, which, due to their electronic structure, have been extensively studied as magnetic field sensors. Such field detection applications usually employ the NV centers to detect field components aligned with the direction of the internally-defined spin axis of the NV center. In this work we detect magnetic fields which are slightly misaligned with the NV center axis. In particular, we demonstrate that the NV center can measure the square of the angle between the magnetic field and the NV center axis with high sensitivity which diverges as the external field approaches a value pre-defined by the NV center's internal parameters, in agreement with predictions. These results show that NV centers could be used as sensitive transducers for making quantum nondemolition (QND) measurements on systems such as nanomechanical oscillators.

High-frequency pulsed ENDOR spectroscopy of the NV(-) centre in the commercial HPHT diamond

This work reports direct 94GHz ENDOR spectroscopy of the (14)N nuclei in the NV(-) centre in single-crystal diamond. Roadmaps of ENDOR frequencies were measured and hyperfine/quadrupole interaction parameters were obtained, with AX,Y=-2.7MHz, AZ=-2.2MHz and P=-4.8MHz. The sign and value of each parameter was calculated using spin Hamiltonian matrix diagonalization, first and second order perturbation theory and confirmed experimentally. Magnetic field magnitude was measured by (13)C ENDOR signal with 0.02% precision or 0.5mT. The orientation of quadrupole, hyperfine and fine structure tensors are the same within error of experiment, g-factor is isotropic.

Experimental protection of two-qubit quantum gates against environmental noise by dynamical decoupling

Hybrid systems consisting of different types of qubits are promising for building quantum computers if they combine useful properties of their constituent qubits. However, they also pose additional challenges if one type of qubits is more susceptible to environmental noise than the others. Dynamical decoupling can help to protect such systems by reducing the decoherence due to the environmental noise, but the protection must be designed such that it does not interfere with the control fields driving the logical operations. Here, we test such a protection scheme on a quantum register consisting of the electronic and nuclear spins of a nitrogen-vacancy center in diamond. The results show that processing is compatible with protection: The dephasing time was extended almost to the limit given by the longitudinal relaxation time of the electron spin.

Strain coupling of a nitrogen-vacancy center spin to a diamond mechanical oscillator

We report on single electronic spins coupled to the motion of mechanical resonators by a novel mechanism based on crystal strain. Our device consists of single-crystal diamond cantilevers with embedded nitrogen-vacancy center spins. Using optically detected electron spin resonance, we determine the unknown spin-strain coupling constants and demonstrate that our system resides well within the resolved sideband regime. We realize coupling strengths exceeding 10 MHz under mechanical driving and show that our system has the potential to reach strong coupling. Our novel hybrid system forms a resource for future experiments on spin-based cantilever cooling and coherent spin-oscillator coupling.

Electronic properties and metrology applications of the diamond NV- center under pressure

The negatively charged nitrogen-vacancy (NV-) center in diamond has realized new frontiers in quantum technology. Here, the optical and spin resonances of the NV- center are observed under hydrostatic pressures up to 60 GPa. Our results motivate powerful new techniques to measure pressure and image high-pressure magnetic and electric phenomena. Additionally, molecular orbital analysis and semiclassical calculations provide insight into the effects of compression on the electronic orbitals of the NV- center.

Approach to dark spin cooling in a diamond nanocrystal

Using a Hartman-Hahn protocol, we demonstrate spin polarization transfer from a single, optically polarized nitrogen-vacancy (NV) center to the ensemble of paramagnetic defects hosted by an individual diamond nanocrystal. Owing to the strong NV-bath coupling, the transfer takes place on a short, microsecond time scale. Upon fast repetition of the pulse sequence, we observe strong polarization transfer blockade, which we interpret as an indication of spin bath cooling. Numerical simulations indicate that the spin bath polarization is nonuniform throughout the nanoparticle, averaging approximately 5% over the crystal volume, but reaching up to 25% in the immediate vicinity of the NV. These observations may prove relevant to the planning of future bath-assisted magnetometry tests.

Nitrogen-vacancy-assisted magnetometry of paramagnetic centers in an individual diamond nanocrystal

Semiconductor nanoparticles host a number of paramagnetic point defects and impurities, many of them adjacent to the surface, whose response to external stimuli could help probe the complex dynamics of the particle and its local, nanoscale environment. Here, we use optically detected magnetic resonance in a nitrogen-vacancy (NV) center within an individual diamond nanocrystal to investigate the composition and spin dynamics of the particle-hosted spin bath. For the present sample, a ∼45 nm diamond crystal, NV-assisted dark-spin spectroscopy reveals the presence of nitrogen donors and a second, yet-unidentified class of paramagnetic centers. Both groups share a common spin lifetime considerably shorter than that observed for the NV spin, suggesting some form of spatial clustering, possibly on the nanoparticle surface. Using double spin resonance and dynamical decoupling, we also demonstrate control of the combined NV center-spin bath dynamics and attain NV coherence lifetimes comparable to those reported for bulk, Type Ib samples. Extensions based on the experiments presented herein hold promise for applications in nanoscale magnetic sensing, biomedical labeling, and imaging.

Optical and spin coherence properties of nitrogen-vacancy centers placed in a 100 nm thick isotopically purified diamond layer

We have studied optical and spin properties of near-surface nitrogen-vacancy (NV) centers incorporated during chemical vapor phase growth of isotopically purified (12)C single-crystal diamond layers. The spectral diffusion-limited line width of zero-phonon luminescence from the NV centers is 1.2 ± 0.5 GHz, a considerable improvement over that of NV centers formed by ion implantation and annealing. Enhanced spin dephasing times (T(2)* ≈ 90 μs, T(2) ≈ 1.7 ms) due to the reduction of (13)C nuclear spins persist even for NV centers placed within 100 nm of the surface.

Temperature dependence of the nitrogen-vacancy magnetic resonance in diamond

The temperature dependence of the magnetic-resonance spectra of nitrogen-vacancy (NV-) ensembles in the range of 280-330 K was studied. Four samples prepared under different conditions were analyzed with NV- concentrations ranging from 10 ppb to 15 ppm. For all samples, the axial zero-field splitting (ZFS) parameter D was found to vary significantly with temperature, T, as dD/dT=-74.2(7) kHz/K. The transverse ZFS parameter E was nonzero (between 4 and 11 MHz) in all samples, and exhibited a temperature dependence of dE/(EdT)=-1.4(3)x10{-4} K-1. The results might be accounted for by considering local thermal expansion. The temperature dependence of the ZFS parameters presents a significant challenge for diamond magnetometers and may ultimately limit their bandwidth and sensitivity.

Observation of coherent oscillation of a single nuclear spin and realization of a two-qubit conditional quantum gate

Rabi nutations of a single nuclear spin in a solid have been observed. The experiments were carried out on a single electron and a single 13C nuclear spin of a single nitrogen-vacancy defect center in diamond. The system was used for implementation of quantum logical NOT and a conditional two-qubit gate (CROT). Density matrix tomography of the CROT gate shows that the gate fidelity achieved in our experiments is up to 0.9, good enough to be used in quantum algorithms.

Observation of coherent oscillations in a single electron spin

Rabi nutations and Hahn echo modulation of a single electron spin in a single defect center have been observed. The coherent evolution of the spin quantum state is followed via optical detection of the spin state. Coherence times up to several microseconds at room temperature have been measured. Optical excitation of the spin states leads to decoherence. Quantum beats between electron spin transitions in a single spin Hahn echo experiment are observed. A closer analysis reveals that beats also result from the hyperfine coupling of the electron spin to a single 14N nuclear spin. The results are analyzed in terms of a density matrix approach of an electron spin interacting with two oscillating fields.

Hydrogen incorporation in diamond: the nitrogen-vacancy-hydrogen complex

We report the identification of the nitrogen-vacancy-hydrogen complex in a freestanding nitrogen-doped isotopically engineered single crystal diamond synthesized by chemical vapor deposition. The hydrogen atom is located in the vacancy of a nearest-neighbor nitrogen-vacancy defect and appears to be bonded to the nitrogen atom maintaining the trigonal symmetry of the center. The defect is observed by electron paramagnetic resonance in the negative charge state in samples containing a suitable electron donor (e.g., substitutional nitrogen N(0)(S)).