Sensitive nanoscale magnetic resonance imaging (MRI) of target spins using nitrogen--vacancy (NV) centers in diamond will require a quantitative understanding of dominant noise at the surface. We probe this noise by applying dynamical decoupling to shallow NVs at calibrated depths. Results support a model of NV dephasing by a surface bath of electronic spins having a correlation rate of 200 kHz, much faster than that of the bulk N spin bath. Our method of combining nitrogen delta--doping growth and nanoscale depth imaging paves a way for studying spin noise present in diverse material surfaces.The negatively charged nitrogen--vacancy (NV) center in diamond is a robust quantum sensor of magnetic fields [1--4]. Although an individual NV has the capability to detect small numbers of electronic [5--7] and nuclear spins external to diamond [8--10], its widespread application in spin imaging has been limited by the ability to form shallow NVs that retain spin coherence near the surface. Shallow spins with long coherence time, T 2 , are important because quantum phase accumulation between two electronic spin states of the NV provides signal transduction, and hence the minimum detectable magnetic dipole moment scales as δµ ∝ r 3 / T 2 , with r the NV--target spin distance [3,4]. At odds with this figure of merit is strong evidence that the diamond crystal surface adversely affects T 2 , reducing it from ~2 ms for bulk NVs [11,12] to less than 10 µs for few--nm deep NVs [6,13--16], but the origin of this decoherence is an outstanding question. We consider in this letter a model of surface spin induced decoherence, a theory which has emerged from experiments on other systems [20,21] where long coherence is a requirement, such as in superconducting circuits [17,18] and spin

Electron spins and photons are complementary quantum-mechanical objects that can be used to carry, manipulate and transform quantum information. To combine these resources, it is desirable to achieve the coherent coupling of a single spin to photons stored in a superconducting resonator. Using a circuit design based on a nanoscale spin-valve, we coherently hybridize the individual spin and charge states of a double quantum dot while preserving spin coherence. This scheme allows us to achieve spin-photon coupling up to the MHz range at the single spin level. The cooperativity is found to reach 2.3, and the spin coherence time is about 60ns. We thereby demonstrate a mesoscopic device suitable for nondestructive spin read-out and distant spin coupling.The methods of cavity quantum electrodynamics hold promise for an efficient use of the spin degree of freedom in the context of quantum computation and simulation (1). Realizing a coherent coupling between a single spin and cavity photons could enable quantum nondemolition readout of a single spin, quantum spin manipulation, and facilitate the coupling of distant spins (1,2,3,4). It could also be used in hybrid architectures in which single spins are coupled to superconducting quantum bits (5), or to simulate one-dimensional spin chains (6).The natural coupling of a spin to the magnetic part of the electromagnetic field is weak (7). In order to enhance it, one needs a large spin ensemble, typically of about 10 12 spins (8,9,10,11,12,13), but these ensembles lose the intrinsic non-linearity of a single spin 1/2.Alternatively, several theoretical proposals have been put forward to electrically couple single spins to superconducting resonators in a mesoscopic circuit (14,15,16,17), building on the exquisite accuracy with which superconducting circuits can be used to couple superconducting qubits and photons and manipulate them (18). One such approach is to engineer an artificial spin-photon interaction by using ferromagnetic reservoirs (15).Noteworthy, the spin/photon coupling is also raising experimental efforts in the optical domain (19,20,21,22,23), but the circuit approach presents the significant advantage of scalability.Recent experiments have demonstrated the coupling of double quantum dot charge states to coplanar waveguide resonators, with a coupling strength gcharge ≈ 2  10 -50 MHz (24,25,26,27,28). In Ref,(29), the spin blockade read-out technique in quantum dots (30) was combined with charge sensing with a microwave resonator (31). In contrast to this spinblockade scheme, here we use the ferromagnetic proximity effect in a coherent conductor to engineer a spin-photon coupling. Our scheme relies on the use of a non collinear spin valve geometry, which realizes an artificial spin orbit interaction (15). Specifically, we contact two non collinear ferromagnets on a carbon nanotube double quantum dot.Our device is shown in Fig. 1, A-C. Our resonator is similar to a previous experiment (27) with a coupling scheme adapted from (24). It is a Nb resonator with a qua...