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  1. Abstract

    Extending the coherence lifetime of a qubit is central to the implementation and deployment of quantum technologies, particularly in the solid state where various noise sources intrinsic to the material host play a limiting role. This study examines theoretically the coherent spin dynamics of a hetero‐spin system formed by a spin featuring a non‐zero crystal field and in proximity to a paramagnetic center . An analysis of the energy level structure of the dyad shows this system exhibits apair of levels separated by a magnetic‐field‐insensitive energy gap, which can be exploited to create long‐lived zero‐quantum coherences. It is found that these coherences are selectively sensitive to “local”—as opposed to “global”—magnetic field fluctuations, suggesting these spin dyads can serve as a nanoscale gradiometer for precision magnetometry. On the other hand, the distinct response of either spin species to electric or thermal stimuli allows one to implement alternative sensing protocols for magnetic‐noise‐free electrometry and thermometry.

     
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  4. Disorder and many body interactions are known to impact transport and thermalization in competing ways, with the dominance of one or the other giving rise to fundamentally different dynamical phases. Here we investigate the spin diffusion dynamics of 13 C in diamond, which we dynamically polarize at room temperature via optical spin pumping of engineered color centers. We focus on low-abundance, strongly hyperfine-coupled nuclei, whose role in the polarization transport we expose through the integrated impact of variable radio-frequency excitation on the observable bulk 13 C magnetic resonance signal. Unexpectedly, we find good thermal contact throughout the nuclear spin bath, virtually independent of the hyperfine coupling strength, which we attribute to effective carbon-carbon interactions mediated by the electronic spin ensemble. In particular, observations across the full range of hyperfine couplings indicate the nuclear spin diffusion constant takes values up to two orders of magnitude greater than that expected from homo-nuclear spin couplings. 
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  5. Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13 C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13 C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal. 
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