Abdelghani Laraoui

High-resolution correlation spectroscopy of 13C spins near a nitrogen-vacancy centre in diamond

Spin complexes comprising the nitrogen-vacancy centre and neighbouring spins are being considered as a building block for a new generation of spintronic and quantum information processing devices. As assembling identical spin clusters is difficult, new strategies are being developed to determine individual node structures with the highest precision. Here we use a pulse protocol to monitor the time evolution of the (13)C ensemble in the vicinity of a nitrogen-vacancy centre. We observe long-lived time correlations in the nuclear spin dynamics, limited by nitrogen-vacancy spin-lattice relaxation. We use the host (14)N spin as a quantum register and demonstrate that hyperfine-shifted resonances can be separated upon proper nitrogen-vacancy initialization. Intriguingly, we find that the amplitude of the correlation signal exhibits a sharp dependence on the applied magnetic field. We discuss this observation in the context of the quantum-to-classical transition proposed recently to explain the field dependence of the spin cluster dynamics.

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.

Scalable Fabrication of High Purity Diamond Nanocrystals with Long-Spin-Coherence Nitrogen Vacancy Centers

The combination of long spin coherence time and nanoscale size has made nitrogen vacancy (NV) centers in nanodiamonds the subject of much interest for quantum information and sensing applications. However, currently available high-pressure high-temperature (HPHT) nanodiamonds have a high concentration of paramagnetic impurities that limit their spin coherence time to the order of microseconds, less than 1% of that observed in bulk diamond. In this work, we use a porous metal mask and a reactive ion etching process to fabricate nanocrystals from high-purity chemical vapor deposition (CVD) diamond. We show that NV centers in these CVD nanodiamonds exhibit record-long spin coherence times in excess of 200 μs, enabling magnetic field sensitivities of 290 nT Hz(-1/2) with the spatial resolution characteristic of a 50 nm diameter probe.

Imaging thermal conductivity with nanoscale resolution using a scanning spin probe

The ability to probe nanoscale heat flow in a material is often limited by lack of spatial resolution. Here, we use a diamond-nanocrystal-hosted nitrogen-vacancy centre attached to the apex of a silicon thermal tip as a local temperature sensor. We apply an electrical current to heat up the tip and rely on the nitrogen vacancy to monitor the thermal changes the tip experiences as it is brought into contact with surfaces of varying thermal conductivity. By combining atomic force and confocal microscopy, we image phantom microstructures with nanoscale resolution, and attain excellent agreement between the thermal conductivity and topographic maps. The small mass and high thermal conductivity of the diamond host make the time response of our technique short, which we demonstrate by monitoring the tip temperature upon application of a heat pulse. Our approach promises multiple applications, from the investigation of phonon dynamics in nanostructures to the characterization of heterogeneous phase transitions and chemical reactions in various solid-state systems.

Magnetometry of random ac magnetic fields using a single nitrogen-vacancy center

We report on the use of a single nitrogen-vacancy (NV) center to probe fluctuating ac magnetic fields. Using engineered currents to induce random changes in the field amplitude and phase, we show that stochastic fluctuations reduce the NV center sensitivity and, in general, make the NV response field-dependent. We also introduce two modalities to determine the field spectral composition, unknown a priori in a practical application. One strategy capitalizes on the generation of ac-field-induced coherence “revivals” while the other approach uses the time-tagged fluorescence intensity record from successive NV observations to reconstruct the ac field spectral density. These studies are relevant for magnetic sensing in scenarios where the field of interest has a nontrivial, stochastic behavior, such as sensing unpolarized nuclear spin ensembles at low static magnetic fields.

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.

Recursive polarization of nuclear spins in diamond at arbitrary magnetic fields

We introduce an alternate route to dynamically polarize the nuclear spin host of nitrogen-vacancy (NV) centers in diamond. Our approach articulates optical, microwave and radio-frequency pulses to recursively transfer spin polarization from the NV electronic spin. Using two complementary variants of the same underlying principle, we demonstrate nitrogen nuclear spin initialization approaching 80% at room temperature both in ensemble and single NV centers. Unlike existing schemes, our approach does not rely on level anti-crossings and is thus applicable at arbitrary magnetic fields. This versatility should prove useful in applications ranging from nanoscale metrology to sensitivity-enhanced NMR.

Imaging nuclear spins weakly coupled to a probe paramagnetic center

Optically-detected paramagnetic centers in wide-bandgap semiconductors are emerging as a promising platform for nanoscale metrology at room temperature. Of particular interest are applications where the center is used as a probe to interrogate other spins that cannot be observed directly. Using the nitrogen-vacancy center in diamond as a model system, we propose a new strategy to determining the spatial coordinates of weakly coupled nuclear spins. The central idea is to label the target nucleus with a spin polarization that depends on its spatial location, which is subsequently revealed by making this polarization flow back to the NV for readout. Using extensive analytical and numerical modeling, we show that the technique can attain high spatial resolution depending on the NV lifetime and target spin location. No external magnetic field gradient is required, which circumvents complications resulting from changes in the direction of the applied magnetic field, and considerably simplifies the required instrumentation. Extensions of the present technique may be adapted to pinpoint the locations of other paramagnetic centers in the NV vicinity or to yield information on dynamical processes in molecules on the diamond surface.

Long-distance excitation of nitrogen-vacancy centers in diamond via surface spin waves

Coherent communication over mesoscale distances is a necessary condition for the application of solid-state spin qubits to scalable quantum information processing. Among other routes under study, one possibility entails the generation of magnetostatic surface spin waves (MSSW) dipolarly coupled to shallow paramagnetic defects in wide-bandgap semiconductors. As an initial step in this direction, here we make use of room-temperature MSSWs to mediate the interaction between the microwave field from an antenna and the spin of a nitrogen-vacancy (NV) center in diamond. We show that this transport spans distances exceeding 3 mm, a manifestation of the MSSW robustness and long diffusion length. Using the NV spin as a local sensor, we find that the MSSW amplitude grows linearly with the applied microwave power, suggesting this approach could be extended to amplify the signal from neighboring spin qubits by several orders of magnitude.