Solid-state spin systems such as nitrogen-vacancy colour centres in diamond are promising for applications of quantum information, sensing and metrology. However, a key challenge for such solid-state systems is to realize a spin coherence time that is much longer than the time for quantum spin manipulation protocols. Here we demonstrate an improvement of more than two orders of magnitude in the spin coherence time (T 2 ) of nitrogen-vacancy centres compared with previous measurements: T 2 E0.6 s at 77 K. We employed dynamical decoupling pulse sequences to suppress nitrogen-vacancy spin decoherence, and found that T 2 is limited to approximately half of the longitudinal spin relaxation time over a wide range of temperatures, which we attribute to phonon-induced decoherence. Our results apply to ensembles of nitrogen-vacancy spins, and thus could advance quantum sensing, enable squeezing and many-body entanglement, and open a path to simulating driven, interactiondominated quantum many-body Hamiltonians.

We present an experimental study of the longitudinal electron-spin relaxation time (T1) of negatively charged nitrogen-vacancy (NV) ensembles in diamond. T1 was studied as a function of temperature from 5 to 475 K and magnetic field from 0 to 630 G for several samples with various NV and nitrogen concentrations. Our studies reveal three processes responsible for T1 relaxation. Above room temperature, a two-phonon Raman process dominates; below room temperature, we observe an Orbach-type process with an activation energy of 73(4) meV, which closely matches the local vibrational modes of the NV center. At yet lower temperatures, sample dependent cross-relaxation processes dominate, resulting in temperature independent values of T1 from milliseconds to minutes. The value of T1 in this limit depends sensitively on the magnetic field and can be tuned by more than 1 order of magnitude.

We report measurements of the optical properties of the 1042 nm transition of negatively-charged Nitrogen-Vacancy (NV) centers in type 1b diamond. The results indicate that the upper level of this transition couples to the m_s=+/-1 sublevels of the {^3}E excited state and is short-lived, with a lifetime <~ 1 ns. The lower level is shown to have a temperature-dependent lifetime of 462(10) ns at 4.4 K and 219(3) ns at 295 K. The light-polarization dependence of 1042 nm absorption confirms that the transition is between orbitals of A_1 and E character. The results shed new light on the NV level structure and optical pumping mechanism.Comment: 5 pages, 4 figure

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Electron spin resonance (ESR) describes a suite of techniques for characterizing electronic systems with applications in physics, chemistry, and biology. However, the requirement for large electron spin ensembles in conventional ESR techniques limits their spatial resolution. Here we present a method for measuring ESR spectra of nanoscale electronic environments by measuring the longitudinal relaxation time of a single-spin probe as it is systematically tuned into resonance with the target electronic system. As a proof of concept, we extracted the spectral distribution for the P1 electronic spin bath in diamond by using an ensemble of nitrogen-vacancy centres, and demonstrated excellent agreement with theoretical expectations. As the response of each nitrogen-vacancy spin in this experiment is dominated by a single P1 spin at a mean distance of 2.7 nm, the application of this technique to the single nitrogen-vacancy case will enable nanoscale ESR spectroscopy of atomic and molecular spin systems.

Significant attention has been recently focussed on the realization of high precision nanothermometry using the spin-resonance temperature shift of the negatively charged nitrogen-vacancy (NV − ) center in diamond. However, the precise physical origins of the temperature shift is yet to be understood. Here, the shifts of the center's optical and spin resonances are observed and a model is developed that identifies the origin of each shift to be a combination of thermal expansion and electron-phonon interactions. Our results provide new insight into the center's vibronic properties and reveal implications for NV − thermometry.PACS numbers: 63.20.kp, 61.72.jn, 76.70.hb The negatively charged nitrogen-vacancy (NV − ) center in diamond [1] is an important quantum technology platform for a range of new applications exploiting quantum coherence. Beyond quantum information processing, the prospect of employing the NV − center as a room temperature nanoscale electric and magnetic field sensor has attracted considerable interest [2][3][4][5][6][7]. Recently, the effects of temperature on the center's ground state spin resonance have been investigated [9], which enabled the influence of temperature on existing NV − metrology applications to be characterized and new thermometry applications to be proposed [8][9][10][11] and demonstrated [12][13][14]. However, the temperature shift of the center's spin resonance is not well understood and previous attempts at modelling the shift have been largely unsuccessful [9][10][11]. It is evident that the implementation of the NV − center as a nano-thermometer, magnetometer or electrometer requires a thorough understanding of the temperature shifts of its resonances, particularly if these implementations are designed for ambient conditions [15]. Here, the temperature shifts of the center's visible, infrared and spin resonances are observed and a model is developed that identifies the origin of each shift to be a combination of thermal expansion and electron-phonon interactions. This new insight reveals implications for NV − metrology.The NV − center is a C 3v point defect in diamond consisting of a substitutional nitrogen atom adjacent to a carbon vacancy that has trapped an additional electron (refer to Fig. 1a). As depicted in Fig. 1b, the oneelectron orbital level structure of the NV − center contains three defect orbital levels (a 1 , e x and e y ) deep within the diamond bandgap. Electron paramagnetic resonance (EPR) observations and ab initio calculations indicate that these defect orbitals are highly localized to the center [16][17][18][19][20]. Figure 1c shows the center's manyelectron electronic structure generated by the occupation of the three defect orbitals by four electrons [21,22], including the low-temperature zero phonon line (ZPL) energies of the visible (E V ∼1.946 eV) [23] and infrared (E IR ∼1.19 eV) [24][25][26] transitions. The energy separations of the spin triplet and singlet levels ( 3 A 2 ↔ 1 E and 1 A 1 ↔ 3 E) are unknown. As depicted in the inset of Fig...

The readout of negatively charged nitrogen-vacancy centre electron spins is essential for applications in quantum computation, metrology and sensing. Conventional readout protocols are based on the detection of photons emitted from nitrogen-vacancy centres, a process limited by the efficiency of photon collection. We report on an alternative principle for detecting the magnetic resonance of nitrogen-vacancy centres, allowing the direct photoelectric readout of nitrogen-vacancy centres spin state in an all-diamond device. The photocurrent detection of magnetic resonance scheme is based on the detection of charge carriers promoted to the conduction band of diamond by two-photon ionization of nitrogen-vacancy centres. The optical and photoelectric detection of magnetic resonance are compared, by performing both types of measurements simultaneously. The minima detected in the measured photocurrent at resonant microwave frequencies are attributed to the spin-dependent ionization dynamics of nitrogen-vacancy, originating from spin-selective non-radiative transitions to the metastable singlet state.

We propose solid-state gyroscopes based on ensembles of negatively charged nitrogen-vacancy (${\rm NV^-}$) centers in diamond. In one scheme, rotation of the nitrogen-vacancy symmetry axis will induce Berry phase shifts in the ${\rm NV^{-}}$ electronic ground-state coherences proportional to the solid angle subtended by the symmetry axis. We estimate sensitivity in the range of $5\times10^{-3} {\rm rad/s/\sqrt{Hz}}$ in a 1 ${\rm mm^3}$ sensor volume using a simple Ramsey sequence. Incorporating dynamical decoupling to suppress dipolar relaxation may yield sensitivity at the level of $10^{-5} {\rm rad/s/\sqrt{Hz}}$. With a modified Ramsey scheme, Berry phase shifts in the ${\rm ^{14}N}$ hyperfine sublevels would be employed. The projected sensitivity is in the range of $10^{-5} {\rm rad/s/\sqrt{Hz}}$, however the smaller gyromagnetic ratio reduces sensitivity to magnetic-field noise by several orders of magnitude. Reaching $10^{-5} {\rm rad/s/\sqrt{Hz}}$ would represent an order of magnitude improvement over other compact, solid-state gyroscope technologies.Comment: 3 figures, 5 page