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

Broadband Excitation by Chirped Pulses: Application to Single Electron Spins in Diamond

Pulsed excitation of broad spectra requires very high field strengths if monochromatic pulses are used. If the corresponding high power is not available or not desirable, the pulses can be replaced by suitable low-power pulses that distribute the power over a wider bandwidth. As a simple case, we use microwave pulses with a linear frequency chirp. We use these pulses to excite spectra of single NV-centers in a Ramsey experiment. Compared to the conventional Ramsey experiment, our approach increases the bandwidth by at least an order of magnitude. Compared to the conventional ODMR experiment, the chirped Ramsey experiment does not suffer from power broadening and increases the resolution by at least an order of magnitude. As an additional benefit, the chirped Ramsey spectrum contains not only `allowed' single quantum transitions, but also `forbidden' zero- and double quantum transitions, which can be distinguished from the single quantum transitions by phase-shifting the readout pulse with respect to the excitation pulse or by variation of the external magnetic field strength.Comment: 13 pages, 7 figure

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

La valeur du yuan:Les paradoxes du taux de change d’équilibre

La sous-évaluation de la monnaie chinoise vis-à-vis du dollar est souvent dénoncée comme l’une des causes des déséquilibres mondiaux. Les autorités américaines soutiennent que la Chine manipule son taux de change tandis que la majorité des travaux empiriques estiment que le yuan est sous-évalué de 15 à 30 %. Cet article montre que les modèles de taux de change d’équilibre généralement utilisés ne sont pas pertinents pour une économie en développement telle que l’économie chinoise. La Chine souffre d’un sous-emploi massif ; un taux de change sous-évalué au regard des critères traditionnels lui permet d’impulser la croissance vigoureuse nécessaire pour résorber son chômage. Plus généralement, le taux de change optimal d’un pays émergent ne peut être défini que relativement à une stratégie de croissance : plus le pays désire une croissance vigoureuse, plus il est prêt à faire un effort d’épargne, plus il doit pratiquer un taux de change faible. La première partie de l’article décrit les points saillants de la croissance chinoise depuis 1994, où la Chine est passée en change fixe avec le dollar : croissance élevée mais sous-emploi massif, faible inflation, accumulation massive de réserves, forts taux d’épargne et d’investissement. La deuxième partie présente et critique les évaluations du taux de change d’équilibre du yuan selon différentes méthodes. La troisième développe une maquette où le taux de change d’un pays en développement est volontairement fixé à un bas niveau pour permettre une croissance vigoureuse, de sorte que sous-évalué du point de vue des critères usuels, il est optimal du point de vue de la stratégie de croissance ; enfin, la quatrième partie montre la difficulté du choix d’un régime de change pour la Chine

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

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