We demonstrate by pulse EPR that two electron spin qubits in bulk ZnO, the Mn 2+ and the Co 2+ spin qubits, which have respectively long (T2 (6K) = 178 µs) and short (T2 (1.7K) = 9 µs) transverse spin coherence time T2 at low temperature, have however very short and similar Rabi oscillation damping times, on the order of TR ≈ 250 ns at low temperature. A detailed study of Mn 2+ spin qubits has shown that the main contribution to the Rabi oscilation damping rate is temperature independent and proportional to the Rabi frequency. This main contribution to the damping rate during coherent microwave manipulation of spins is interpreted as due to the changes of the dipolar couplings induced by the long microwave pulse used in this kind of EPR nutation experiment. Strategies are suggested for overcoming this problem of Rabi oscillation overdamping in future spin based quantum computers.

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-We present a study by ferromagnetic resonance at microwave Q band of two sheets of cobalt nanoparticles obtained by annealing SiO2 layers implanted with cobalt ions. This experimental study is performed as a function of the applied magnetic field orientation, temperature, and dose of implanted cobalt ions. We demonstrate that each of those magnetic sheet of cobalt nanoparticles can be well modelled by a nearly two dimensional ferromagnetic sheet having a reduced effective saturation magnetization, compared to a regular thin film of cobalt. The nanoparticles are found superparamagnetic above around 210 K and ferromagnetic below this blocking temperature. Magnetostatic calculations show that a strong magnetic field gradient of around 0.1 G/nm could be produced by a ferromagnetic nanostripe patterned in such magnetic sheet of cobalt nanoparticles. Such a strong magnetic field gradient combined with electron paramagnetic resonance may be relevant for implementing an intermediate scale quantum computer based on arrays of coupled electron spins, as previously reported (Eur. Phys. J. B (2014) 87, 183). However, this new approach only works if no additional spin decoherence is introduced by the spin waves exitations of the ferromagnetic nanostructure. We thus suggest theoretically some possible magnetic anisotropy engineering of cobalt nanoparticles that could allow to suppress the spin qubit decoherence induced by the unwanted collective excitation of their spins.Magnetic nano-objects have many potential applications. Magnetic nanoparticles (NPs) can be used as contrats agent in the diagnosis and treatment of cancer [1], magnetic nanostripes can be used as a medium for efficient classical data transmission and processing [2], and magnetic nanodots can be used as storage elements for high density magnetic data recording [3]. The present (a) 1-2/ benzid@unistra.fr (b) 3/ dominique.muller@icube.unistra.fr (c) 1/ turek@unistra.fr (d) 1/ tribollet@unistra.fr : corresponding author work reports on a new potential application of metal magnetic nanoparticles embedded in dielectric matrix. This is related to quantum information processing and electron paramagnetic resonance (EPR) spectroscopy. It was recently theoretically demonstrated [4] that the strong magnetic field gradient produced by a ferromagnetic nanostripe combined with the microwave pulses delivered by a pulsed electron paramagnetic resonance spectrometer, could constitute two of the three key elements constituting the hardware of a potential small scale spin based quantum computer, the third key elep-1

The effect of thermal excitation on paramagnetic defects in natural sedimentary quartz irradiated with different doses of gamma radiation was studied using electron spin resonance (ESR) spectroscopy. We report a variation in the activation energy and the frequency factor for [AlO4/h+]0 and [TiO4/M+]0 paramagnetic defects with the gamma dose, for a dose range investigated between ∼100 Gy and ∼40 000 Gy. Our results indicate that both [AlO4/h+]0 and [TiO4/M+]0 defects are less thermally stable above 1 kGy–2 kGy than below this dose range. The correlation between the two kinetic parameters (activation energy and frequency factor) satisfies the Meyer–Neldel rule. A linear correlation was found between the amplitude of the ESR signals of [AlO4/h+]0 and [TiO4/M+]0 paramagnetic defects corresponding to different doses after the application of thermal treatments in the pulse annealing procedure. We propose a mechanism involving the exchange of the cation, assigned mainly to Li+ here, between the two defects. Under irradiation, the cation is removed from [AlO4/M+]0 (forming [AlO4/h+]0) to [TiO4]− (forming [TiO4/M+]0), while under heating, the reverse mechanism takes place. The migration energy of the cation from one defect to another was found to be about 51 meV, corresponding to a temperature of about 325 °C.