The studies of many-body dynamics of interacting spin ensembles, as well as quantum sensing in solid state systems, are often limited by the need for high spin concentrations, along with efficient decoupling of the spin ensemble from its environment. In particular, for an ensemble of nitrogenvacancy (NV) centers in diamond, high conversion efficiencies between nitrogen (P1) defects and NV centers are essential, while maintaining long coherence times of an NV ensemble. In this work, we study the effect of electron irradiation on the conversion efficiency and the coherence time of various types of diamond samples with different initial nitrogen concentrations. The samples were irradiated using a 200 keV transmission electron microscope (TEM). Our study reveals that the efficiency of NV creation strongly depends on the initial conversion efficiency as well as on the initial nitrogen concentration. The irradiation of the examined samples exhibits an order of magnitude improvement in the NV concentration (up to ∼ 10 11 NV/cm 2 ), without degradation in their coherence times of ∼ 180 µs. We address the potential of this technique toward the study of many-body physics of NV ensembles and the creation of non-classical spin states for quantum sensing. The study of quantum many-body spin physics in realistic solid-state platforms has been a long-standing goal in quantum and condensed-matter physics. In addition to the fundamental understanding of spin dynamics, such research could pave the way toward the demonstration of non-classical spin states, which will be useful for a variety of applications in quantum information and quantum sensing. One of the leading candidates for such studies is the negatively charged nitrogen-vacancy (NV) center in diamond, having unique spin and optical properties, which make it useful for various sensing applications [1][2][3][4][5][6][7][8][9], as well as a resource for quantum information processing and quantum simulation [10][11][12].The current state-of-the-art is limited by the requirement of obtaining high spin concentrations while maintaining long coherence times. The sensitivity of magnetic sensing grows as the square-root of the number of spins [1,3], thus enhanced NV concentrations could improve magnetometric sensitivities. Furthermore, enhanced NV concentrations could lead to strong NV-NV couplings, which together with long coherence times, achieved using a proper dynamical decoupling protocol [13], could pave the way toward the study of many-body dynamics in the NV-NV interaction-dominated regime [10][11][12]. However, nitrogen defects not associated with vacancies (P1 centers) create randomly fluctuating magnetic fields that cause decoherence of the quantum state of the NV ensemble [14,15]. As a result, in most cases it would be beneficial to increase the concentration of NV centers while keeping the nitrogen concentration constant, i.e. improve the N to NV conversion efficiency.A common technique for improving the conversion efficiency is the irradiation of the sample with elec...

In this work we use fluorescence from nitrogen-vacancy defects in diamond to detect and explore other paramagnetic defects in the diamond, such as P1 defects, which are commonly undetectable through optical detection of magnetic resonance in standard conditions. Our method does not require overlap between the defects' resonances and therefore is applicable in a wide region of magnetic fields and frequencies, as verified by excellent fit to theoretical predictions. We propose a depolarization scheme of P1 defects to account for the observed data. To verify our results, we perform cavity-based detection of magnetic resonance and find a good agreement between the measured optically induced polarization and the value obtained theoretically from rate equations. The findings in this work may open the way to detection of paramagnetic defects outside of the diamond through the photoluminesence of nitrogen-vacancy defects, which might be useful for imaging in biology.

No abstract

We study magnetic field penetration into a thin film made of a superconducting niobium. Imaging of magnetic field is performed by optically detecting magnetic resonances of negatively charged nitrogen-vacancy defects inside a single crystal diamond, which is attached to the niobium film under study. The experimental results are compared with theoretical predictions based on the critical state model, and good agreement is obtained.

The coupling between defects in diamond and a superconducting microwave resonator is studied in the nonlinear regime. Both negatively charged nitrogen-vacancy and P1 defects are explored. The measured cavity mode response exhibits strong nonlinearity near a spin resonance. Data is compared with theoretical predictions and a good agreement is obtained in a wide range of externally controlled parameters. The nonlinear effect under study in the current paper is expected to play a role in any cavity-based magnetic resonance imaging technique and to impose a fundamental limit upon its sensitivity. PACS numbers: 42.50.Pq,81.05.ug,76.30.Mi Cavity quantum electrodynamics (CQED) [1] is the study of the interaction between photons confined in a cavity and matter. CQED has applications in a variety of fields, including magnetic resonance imaging and quantum computation [2]. The CQED interaction can be probed by measuring the response of a cavity mode. Commonly, the effect of matter on the response diminishes as the energy stored in the cavity mode under study is increased [3]. This nonlinear effect, which is the focus of the current study, imposes a severe limit upon the performance of a variety of CQED systems.In the current study we explore nonlinear CQED interaction between defects in a diamond crystal and a superconducting microwave cavity (resonator) having a spiral shape [4,5]. Two types of defects are investigated, a negatively charged nitrogen-vacancy NV − defect and a nitrogen 14 (nuclear spin 1) substitutional defect (P1). Strong coupling between defects in diamond and a superconducting resonator has been demonstrated at ultra-low temperatures [6][7][8][9][10][11][12], however the regime of nonlinear response was not addressed. In this study, we find that the cavity response becomes highly nonlinear near a CQED resonance. In addition, for the case of NV − defects, the response is strongly affected by applying opticallyinduced spin polarization (OISP). The experimental findings are compared with theory and good agreement is obtained.The experimental setup is schematically depicted in Fig. 1(a). Defects in a [100] type Ib diamond are created using 2.8 MeV electron irradiation with a dose of approximately 8 × 10 18 e/cm 2 , followed by annealing at 800 • C for 8 hours and acid cleaning, resulting in the formation of NV − defects with density of 1.23 × 10 17 cm −3 [13]. The diamond wafer is then placed on top of a sapphire wafer supporting a superconducting spiral resonator made of niobium [see Fig. 1(b)]. Externally ap-*These authors contributed equally to this work. FIG. 1: The experimental setup. (a) A loop antenna (LA) is coupled to the spiral resonator. Two multimode optical fibers are coupled to the diamond wafer. Fiber F1 is employed for delivering laser light of wavelength λL = 532 nm, and fiber F2 probes the emitted photoluminescence (PL). (b) The spiral resonator has 3 turns, an inner radius of 0.59 mm and an outer radius of 0.79 mm. (c) The resonance lineshape of the spiral's fundamental mode vs. tempera...