Collective Strong Coupling with Homogeneous Rabi Frequencies using a 3D Lumped Element Microwave Resonator

We design and implement 3D lumped element microwave cavities for the coherent and uniform coupling to electron spins hosted by nitrogen vacancy centers in diamond. Our design spatially focuses the magnetic field to a small mode volume. We achieve large homogeneous single spin coupling rates, with an enhancement of the single spin Rabi frequency of more than one order of magnitude compared to standard 3D cavities with a fundamental resonance at \SI{3}{GHz}. Finite element simulations confirm that the magnetic field component is homogeneous throughout the entire sample volume, with a RMS deviation of 1.54\%. With a sample containing $10^{17}$ nitrogen vacancy electron spins we achieve a collective coupling strength of \Omega=\SI{12}{MHz}, a cooperativity factor $C=27$ and clearly enter the strong coupling regime. This allows to interface a macroscopic spin ensemble with microwave circuits, and the homogeneous Rabi frequency paves the way to manipulate the full ensemble population in a coherent way.Comment: 3 figure

Electron spin phase relaxation of phosphorus donors in nuclear spin enriched silicon

We report a pulsed EPR study of the phase relaxation of electron spins bound to phosphorus donors in isotopically purified 29^Si and natural abundance Si single crystals measured at 8 K.Comment: 5 pages, 3 figure

Microscopic origin of light-induced ESR centers in undoped hydrogenated amorphous silicon

29Si hyperfine (hf) structures of light-induced electron-spin-resonance (LESR) centers of g=2.004 and 2.01 have been investigated in undoped hydrogenated amorphous silicon (a-Si:H) with different 29Si content (1.6, 4.7,9.1 at. %) by means of pulsed and multifrequency (3,11,34 GHz) ESR techniques. We have experimentally deconvoluted overlapping LESR signals using the difference in the spin-lattice relaxation time between the two signals. The deconvoluted 29Si hf structure of g=2.004 indicates that the wave function of the g=2.004 center spreads mainly over two Si atoms. Accordingly, we propose that the origin of g=2.004 is electrons trapped in antibonding states of weak Si-Si bonds rather than those trapped at positively charged dangling bonds. The isotropic hf splittings were estimated to be around 7 mT for g=2.004 and below 3 mT for g=2.01, which are in good agreement with characteristics of the antibonding and bonding states of the weak Si-Si bond. We suggest, from our 29Si hf data and other experimental findings, that the g=2.004 center is localized spatially more than conduction-band-tail electrons detected by photoluminescence

Fluorine-vacancy defects in fluorine-implanted silicon studied by electron paramagnetic resonance

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Enhancing fluorescence excitation and collection from the nitrogen-vacancy center in diamond through a micro-concave mirror

We experimentally demonstrate a simple and robust optical fibers based method to achieve simultaneously efficient excitation and fluorescence collection from Nitrogen-Vacancy (NV) defects containing micro-crystalline diamond. We fabricate a suitable micro-concave (MC) mirror that focuses scattered excitation laser light into the diamond located at the focal point of the mirror. At the same instance, the mirror also couples the fluorescence light exiting out of the diamond crystal in the opposite direction of the optical fiber back into the optical fiber within its light acceptance cone. This part of fluorescence would have been otherwise lost from reaching the detector. Our proof-of-principle demonstration achieves a 25 times improvement in fluorescence collection compared to the case of not using any mirrors. The increase in light collection favors getting high signal-to-noise ratio (SNR) optically detected magnetic resonance (ODMR) signals hence offers a practical advantage in fiber-based NV quantum sensors. Additionally, we compacted the NV sensor system by replacing some bulky optical elements in the optical path with a 1x2 fiber optical coupler in our optical system. This reduces the complexity of the system and provides portability and robustness needed for applications like magnetic endoscopy and remote-magnetic sensing.Comment: 6 pages, 8 figure

Strongly Coupled Diamond Spin Qubits by Molecular Nitrogen Implantation

Ionized nitrogen molecules ($^{15}$N$_{2}^+$) are used as efficient point sources for creating NV$^-$ pairs in diamond with nanoscale spatial separation and up to 55 kHz magnetic coupling strength. Co-implantation of $^{12}$C$^+$ increased the yield of pairs, and a $^{13}$C-depleted diamond allowed 0.65 ms coherence times to be obtained. Further coupling to a third dark spin provided a strongly coupled three spin register. These results mark an important step towards realization of multi-qubit systems and scalable NV$^-$ quantum registers.Comment: Accepted for publication in Physical Review B: rapid communication