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.

multi-qubit systems are crucial for the advancement and application of quantum science. such systems require maintaining long coherence times while increasing the number of qubits available for coherent manipulation. For solid-state spin systems, qubit coherence is closely related to fundamental questions of many-body spin dynamics. Here we apply a coherent spectroscopic technique to characterize the dynamics of the composite solid-state spin environment of nitrogen-vacancy colour centres in room temperature diamond. We identify a possible new mechanism in diamond for suppression of electronic spin-bath dynamics in the presence of a nuclear spin bath of sufficient concentration. This suppression enhances the efficacy of dynamical decoupling techniques, resulting in increased coherence times for multispin-qubit systems, thus paving the way for applications in quantum information, sensing and metrology.

We report on the noise spectrum experienced by few nanometer deep nitrogen-vacancy centers in diamond as a function of depth, surface coating, magnetic field and temperature. Analysis reveals a double-Lorentzian noise spectrum consistent with a surface electronic spin bath in the low frequency regime, along with a faster noise source attributed to surface-modified phononic coupling. These results shed new light on the mechanisms responsible for surface noise affecting shallow spins at semiconductor interfaces, and suggests possible directions for further studies. We demonstrate dynamical decoupling from the surface noise, paving the way to applications ranging from nanoscale NMR to quantum networks.Nanoscale magnetic imaging and magnetic resonance spectroscopy, recently demonstrated using nitrogenvacancy (NV) color centers in diamond [1][2][3][4], are capable of yielding unique insights into chemistry, biology and physical sciences. The sensitivity and resolution of these techniques relies heavily on the NV coherence properties, which empirically are much worse for shallow NV centers than those deep within bulk diamond [5]. An understanding of the origin of surface related noise enables optimal decoupling or surface passivation to be performed. It is critical not only for improving NV applications in quantum sensing [6,7], quantum information processing [8], and photonics [9], but is also an outstanding problem in many solid-state quantum systems (e.g. [10,11]). Furthermore, overcoming noise at the diamond interface is a significant obstacle to realizing hybrid quantum systems with NV centers [12,13], which are expected to play an important role in realistic devices.For NV centers in bulk diamond, noise sources limiting coherence times have been identified with internal nuclear and electronic spin baths, and interactions with phonons [14,15]. Although additional noise sources related to the diamond surface, and affecting shallow NVs, have been observed [16], their origin is not currently well understood. This phenomenon is general and has been observed at various semiconductor interfaces, resulting in the development of several theoretical models, which are still without significant experimental confirmation [17,18]. Here we use shallow implanted NV centers as nanoscale sensors to perform spectroscopy of the diamond surface. We use dynamical decoupling techniques together with measurements of longitudinal (T 1 ) relaxation under varying conditions (surface coating, magnetic field, temperature) in order to characterize the surface-induced noise. The strength and frequency dependence of fluctuations as a function of the NV distance from the surface are investigated with nanometer precision. We directly measure the noise spectrum experienced by shallow NV centers, revealing an unexpected double-Lorentzian structure which indicates contributions from two distinct noise sources. We find that the low frequency noise experienced by shallow NVs is consistent with electronic spin impurities on the surface [ Fig. 1(a)], w...