Magnetic strong coupling in a spin-photon system and transition to classical regime

We study the energy level structure of the Tavis-Cumming model applied to an ensemble of independent magnetic spins $s=1/2$ coupled to a variable number of photons. Rabi splittings are calculated and their distribution is analyzed as a functin of photon number $n_{\rm max}$ and spin system size $N$. A sharp transition in the distribution of the Rabi frequency is found at $n_{\rm max}\approx N$. The width of the Rabi frequency spectrum diverges as $\sqrt{N}$ at this point. For increased number of photons $n_{\rm max}>N$, the Rabi frequencies converge to a value proportional to $\sqrt{n_{\rm max}}$. This behavior is interpreted as analogous to the classical spin resonance mechanism where the photon is treated as a classical field and one resonance peak is expected. We also present experimental data demonstrating cooperative, magnetic strong coupling between a spin system and photons, measured at room temperature. This points towards quantum computing implementation with magnetic spins, using cavity quantum-electrodynamics techniques.Comment: Received 8 April 2010; revised manuscript received 17 June 2010; published 14 July 201

Entrapment of magnetic micro-crystals for on-chip electron spin resonance studies

On-chip Electron Spin Resonance (ESR) of magnetic molecules requires the ability to precisely position nanosized samples in antinodes of the electro-magnetic field for maximal magnetic interaction. A method is developed to entrap micro-crystals containing spins in a well defined location on a substrate's surface. Traditional cavity ESR measurements are then performed on a mesoscopic crystal at 34 GHz. Polycrystalline diluted Cr$^{5+}$ spins were entrapped as well and measured while approaching the lower limit of the ESR sensitivity. This method suggests the feasibility of on-chip ESR measurements at dilution refrigerator temperatures by enabling the positioning of samples atop an on-chip superconducting cavity.Comment: to appear in Journal of Applied Physic

Tunable multi-photon Rabi oscillations in an electronic spin system

We report on multi-photon Rabi oscillations and controlled tuning of a multi-level system at room temperature (S=5/2 for Mn2+:MgO) in and out of a quasi-harmonic level configuration. The anisotropy is much smaller than the Zeeman splittings, such as the six level scheme shows only a small deviation from an equidistant diagram. This allows us to tune the spin dynamics by either compensating the cubic anisotropy with a precise static field orientation, or by microwave field intensity. Using the rotating frame approximation, the experiments are very well explained by both an analytical model and a generalized numerical model. The calculated multi-photon Rabi frequencies are in excellent agreement with the experimental data

Multi-photon Rabi oscillations in high spin paramagnetic impurity

We report on multiple photon monochromatic quantum oscillations (Rabi oscillations) observed by pulsed EPR (Electron Paramagnetic Resonance) of Mn$^{2+}$ (S=5/2) impurities in MgO. We find that when the microwave magnetic field is similar or large than the anisotropy splitting, the Rabi oscillations have a spectrum made of many frequencies not predicted by the S=1/2 Rabi model. We show that these new frequencies come from multiple photon coherent manipulation of the multi-level spin impurity. We develop a model based on the crystal field theory and the rotating frame approximation, describing the observed phenomenon with a very good agreement.Comment: International Conference: Resonance in Condensed Matter Altshuler 10

Generalised joint regression for count data: a penalty extension for competitive settings

We propose a versatile joint regression framework for count responses. The method is implemented in the R add-on package GJRM and allows for modelling linear and non-linear dependence through the use of several copulae. Moreover, the parameters of the marginal distributions of the count responses and of the copula can be specified as flexible functions of covariates. Motivated by competitive settings, we also discuss an extension which forces the regression coefficients of the marginal (linear) predictors to be equal via a suitable penalisation. Model fitting is based on a trust region algorithm which estimates simultaneously all the parameters of the joint models. We investigate the proposal’s empirical performance in two simulation studies, the first one designed for arbitrary count data, the other one reflecting competitive settings. Finally, the method is applied to football data, showing its benefits compared to the standard approach with regard to predictive performance

Multiphoton coherent manipulation in large-spin qubits

Large spin Mn2+ ions (S=5/2) diluted in a non-magnetic MgO matrix of high crystalline symmetry are used to realize a six level system that can be operated by means of multi-photon coherent Rabi oscillations. This spin system has a very small anisotropy which can be tuned in-situ to reversibly transform the system between harmonic and non-harmonic level configurations. Decoherence effects are strongly suppressed as a result of the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. These results suggest new ways of manipulating, reading and resetting spin quantum states which can be applied to encode a qubit across several quantum levels.Comment: Published versio

Early initiation of antiretroviral therapy following in utero HIV infection is associated with low viral reservoirs but other factors determine subsequent plasma viral rebound

BACKGROUND: Early HIV diagnosis allows combination antiretroviral therapy (cART) initiation in the first days of life following in utero (IU) infection. The impact of early cART initiation on infant viral reservoir size in the setting of high-frequency cART non-adherence is unknown. METHODS: Peripheral blood total HIV DNA from 164 early treated (day 0-21 of life) IU HIV-infected South African infants was measured using droplet digital PCR at birth and following suppressive cART. We evaluated the impact of cART initiation timing on HIV reservoir size and decay, and on the risk of subsequent plasma viraemia in cART-suppressed infants. FINDINGS: Baseline HIV DNA (median 2.8 log10 copies/million PBMC, range 0.7 - 4.8) did not correlate with age at cART initiation (0-21 days) but instead with maternal antenatal cART use. In 98 infants with plasma viral suppression on cART, HIV DNA half-life was 28 days. However, the probability of maintenance of plasma aviraemia was low (0.46 at 12 months) and not influenced by HIV DNA load. Unexpectedly, longer time to viral suppression was associated with protection against subsequent viral rebound. CONCLUSIONS: With effective prophylaxis against mother-to-child transmission, cART initiation timing in the first 3 weeks of life is not critical to reservoir size