Session N8: Nitrogen-Vacancy Centers

Berry phase magnetometry using a single electronic spin in diamond

We present a new approach for improving the sensitivity and dynamic-range of nitrogen-vacancy (NV) center magnetic field sensing using Berry phase. In the conventional Ramsey interferometry, an NV spin accumulates dynamic phase proportional to the Larmor frequency. This approach provides high sensitivity in exchange for the dynamic-range due to 2pi phase ambiguity. Our approach, in which the magnetic field is encoded in the Berry phase of the spin, can unwrap this ambiguity due to a chirped magnetometry curve. This work will provide a new modality not only for magnetometry but also for thermometry and electrometry using solid-state spins.   

Charged nanodiamonds in a Paul trap

Colloidal nanodiamonds were ionized with atmospheric electrospray and loaded into a Paul trap. Fluorescence from atom-like NV0 and NV- colour centres has been observed. The very low intrinsic absorption of bulk diamond is favourable for reducing the heating of cooled, trapped, nanodiamond ions from the surrounding blackbody radiation of the trapping apparatus. The isolated environment of the ion trap is also favourable for in-situ modification of nanodiamond to reduce absorption inducing defects through either physical or chemical processes. The presence or intentional introduction of high luminescence atom-like colour centre defects such as NV or SiV offer the prospect of direct laser cooling in nanodiamonds with low emissivity. Such laser cooled nano-ions are of interest for sympathetically cooling ions of similar charge/mass ratios that lack closed optical transitions, such as large biomolecules.   

Atomic-scale nuclear spin imaging using quantum-assisted sensors in diamond

Recent developments in materials fabrication and coherent control have brought quantum magnetometers based on electronic spin defects in diamond close to single nuclear spin sensitivity. These quantum sensors have the potential to be a revolutionary tool in proteomics, thus helping drug discovery: They can overcome some of the challenges plaguing other experimental techniques (x-ray and NMR) and allow single protein reconstruction in their natural conditions. While the sensitivity of diamond-based magnetometers approaches the single nuclear spin level, the outstanding challenge is to resolve contributions arising from distinct nuclear spins in a dense sample and use the acquired signal to reconstruct their positions. This talk describes a strategy to boost the spatial resolution of NV-based magnetic resonance imaging, by combining the use of a quantum memory intrinsic to the NV system with Hamiltonian engineering by coherent quantum control. The proposed strategy promises to make diamond-based quantum sensors an invaluable technology for bioimaging, as they could achieve the reconstruction of biomolecules local structure without the need to crystallize them, to synthesize large ensembles or to alter their natural environment.   

Selective control of nanoscale multi-spin systems in diamond using strong pulsed field gradients

Individual control of proximal spin qubits is a key challenge in building solid-state-based quantum network architecture. Here we demonstrate selective driving of an array of electronic spins associated with nitrogen-vacancy (NV) centers in diamond with high-fidelity and nanometer-scale precision by use of frequency encoding gradient technique. A uniform magnetic field gradient of 0.1 G/nm is generated over 1 um x 5 um at room temperature by sending electric currents through micrometer-scale parallel wires. This approach also enables modulation of gradient strength at 1 MHz, which allows us to selectively readout Larmor precession phase of proximal NV spins via phase encoding.   

Nanometer-scale probing of spin waves using single electron spins

We have developed a new approach to exploring magnetic excitations in correlated-electron systems [1], based on single electronic spins in atom-like defects diamond known as nitrogen-vacancy (NV) color centers. We demonstrate the power of this approach by detecting spin-wave excitations in a ferromagnetic microdisc with nanoscale spatial sensitivity over a broad range of frequencies and magnetic fields. We show how spin-wave resonances can be exploited for on-chip amplification of microwave magnetic fields, allowing strongly increased spin manipulation rates and single-spin magnetometry with enhanced sensitivity. Finally, we show the possibility to detect the magnetic spin noise produced by a thin ($\sim$ 30 nm) layer of a patterned ferromagnet. For the interpretation of our results, we develop a general framework describing single-spin stray field detection in terms of a filter function sensitive mostly to spin fluctuations with wavevector $\sim 1/d$, where $d$ is the NV-ferromagnet distance. Our results pave the way towards quantitative and non-perturbative detection of spectral properties in nanomagnets, establishing NV center magnetometry as an emergent probe of collective spin dynamics in condensed matter.\\[4pt] [1] arXiv:1410.6423v2 (2014).   

Probing nuclear spin dynamics near solid-state atom-like systems via photon statistics

Interaction between an electronic spin and its surrounding nuclear spin environment is a major source of decoherence in most solid-state spin qubits. Recently, optical pumping techniques were used to monitor and control the nuclear bath surrounding such solid state systems. We develop a detailed theoretical model for nuclear spin diffusion of $^{13}$C spin bath near an individual Nitrogen Vacancy (NV$^-$) center in diamond subject to coherent population trapping (CPT) and explain suppression of bath dynamics due to the presence of an electronic dark state. We then develop a random walk model reminiscent of velocity selective coherent population trapping (VSCPT) to understand the anomalous diffusion of nuclear spins in the presence of an electronic dark state. Using this model, we propose a method for probing of non-equilibrium dynamics of the nuclear spin bath by analyzing photon statistics of NV fluorescence.   

Towards quantum superposition of a levitated nanodiamond with a NV center

Creating large Schr\"odinger's cat states with massive objects is one of the most challenging goals in quantum mechanics. We have previously achieved an important step of this goal by cooling the center-of-mass motion of a levitated microsphere from room temperature to millikelvin temperatures with feedback cooling. To generate spatial quantum superposition states with an optical cavity, however, requires a very strong quadratic coupling that is difficult to achieve. We proposed to optically trap a nanodiamond with a nitrogen-vacancy (NV) center in vacuum, and generate large spatial superposition states using the NV spin-optomechanical coupling in a strong magnetic gradient field. The large spatial superposition states can be used to study objective collapse theories of quantum mechanics. We have optically trapped nanodiamonds in air and are working towards this goal.   

Measurement of topological invariants with spin qubits in diamond

We present our measurements on topological invariants using spin qubits. The ground states of nitrogen-vacancy (NV) color centers in diamond are used as an ideal qubit whose states can be fully controlled by microwave frequency detuning, amplitude and relative phase. Manipulating these parameters on a closed manifold, we study the robustness of topological invariants against surrounding paramagnetic spins at room temperature. We also discuss how these measurements can be extended to qutrit or multi-qubit systems.   

NV magnetic imaging of topological spin patterns in magnetic multilayers

Scanning diamond microscopes with an atom-like nitrogen-vacancy (NV) color center near the probe tip have recently emerged as a leading tool for the study of nanoscale magnetism in a broad range of systems. We report on the development of a new approach for positiong a single NV centre at a few nanometres from the sample of interest. This is achieved by fabricating our magnetic device at the top of a polished quartz fiber, whose distance from a diamond nanopillar containing NV centers is then controlled via an atomic force microscope feedback. We employ this method for the investigation of thin ferromagnetic Co/Pt multilayers, where interfacial spin-orbit coupling is expected to stabilize complex topologically protected spin textures. The few-nanometers real-space extension of an isolated skyrmion structure in thin magnetic films makes its detection via standard spectroscopic techniques challenging, suggesting how NV magnetometry can be a unique candidate for the study of novel mesoscopic magnetism.   

High-sensitivity single NV magnetometry by spin-to-charge state mapping

Nitrogen-Vacancy (NV) centers in diamond are atom-like quantum system in a solid state matrix whom its structure allows optical readout of the electronic spin. However, the optimal duration of optical readout is limited by a singlet state lifetime making single shot spin readout out of reach. On the other side, the NV center charge state readout can be extremely efficient (up to 99\% fidelity) by using excitation at 594 nm. We will present a new method of spin readout utilizing a spin-depending photoionization process to map the electronic spin state of the NV onto the its charge state. Moreover, pre-selection on the charged state allows to minimize data acquisition time. This scheme improves single NV AC magnetometry by a factor of 5 and will benefit other single NV center experiments as well.