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We demonstrate optical spin polarization of the neutrally-charged silicon-vacancy defect in diamond (SiV 0 ), an S = 1 defect which emits with a zero-phonon line at 946 nm. The spin polarization is found to be most efficient under resonant excitation, but non-zero at below-resonant energies. We measure an ensemble spin coherence time T2>100 µs at low-temperature, and a spin relaxation limit of T1 > 25 s. Optical spin state initialization around 946 nm allows independent initialization of SiV 0 and NV − within the same optically-addressed volume, and SiV 0 emits within the telecoms downconversion band to 1550 nm: when combined with its high Debye-Waller factor, our initial results suggest that SiV 0 is a promising candidate for a long-range quantum communication technology.Point defects in diamond have attracted considerable interest owing to their application for quantum information processing, communication, and metrology. The most-studied defect, the negatively-charged nitrogenvacancy (NV − ) center, possesses efficient optical spin polarization and spin-state dependent fluorescence, enabling its exploitation as an ultra-sensitive nano-scale magnetic field sensor [1][2][3]. However, the zero phonon line (ZPL) of NV − accounts for only a few percent of its total emission [4], leading to low efficiency in coherent photonic applications. The negatively-charged siliconvacancy (SiV − ) center has also received significant interest as its high Debye-Waller factor (≈ 0.8 [5]) makes it an attractive candidate for long-range quantum computation and communication. However, the exceptional optical properties of SiV − are not matched by its spin properties, where a large spin-orbit coupling in the ground state enables phonon-assisted spin-state depopulation, resulting in spin-lattice relaxation-limited coherence lifetimes of 40 ns even at 5 K [6]: efforts are ongoing to overcome this limitation by strain engineering, but currently liquid helium temperatures and below are required to access and readout SiV − spin states [7].The neutrally-charged silicon-vacancy (SiV 0 ) has a ground state electron spin S = 1. Unlike the NV center, where the nitrogen remains covalently bonded to three carbon atoms and the nitrogen-vacancy axis forms a C 3v symmetry axis, the silicon atom in SiV adopts a bond-center location, with a D 3d axis formed by the 1 1 1 joining the split-vacancy [ Fig. 1(a)]. SiV 0 has been characterized both by electron paramagnetic resonance (EPR) [8,9] and optical absorption/photoluminescence (PL) [10]. Similarly to SiV − , the neutral charge state also has a high Debye-Waller factor, with the majority of its photons emitted at the primary zero-phonon line (ZPL) at 946 nm (1.31 eV) [ Fig. 1(b)]: this transition has been shown to occur between a 3 A 2g ground state (GS) We have studied SiV 0 in two samples grown by chemical vapor deposition (CVD); silicon was introduced by adding silane to the process gasses during growth. Sample A was grown on a {1 0 0}-oriented high pressure high temperature (HPHT) substrate, ir...

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The neutrally-charged silicon vacancy in diamond is a promising system for quantum technologies that combines high-efficiency, broadband optical spin polarization with long spin lifetimes (T2 ≈ 1 ms at 4 K) and up to 90 % of optical emission into its 946 nm zero-phonon line. However, the electronic structure of SiV 0 is poorly understood, making further exploitation difficult. Performing photoluminescence spectroscopy of SiV 0 under uniaxial stress, we find the previous excited electronic structure of a single 3 A1u state is incorrect, and identify instead a coupled 3 Eu − 3 A2u system, the lower state of which has forbidden optical emission at zero stress and so efficiently decreases the total emission of the defect: we propose a solution employing finite strain to form the basis of a spin-photon interface. Isotopic enrichment definitively assigns the 976 nm transition associated with the defect to a local mode of the silicon atom.

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Awakening from sleep reflects a profound transformation in neural activity and behavior. The thalamus is a key controller of arousal state, but whether its diverse nuclei exhibit coordinated or distinct activity at transitions in behavioral arousal state is unknown. Using fast fMRI at ultra-high field (7 Tesla), we measured sub-second activity across thalamocortical networks and within nine thalamic nuclei to delineate these dynamics during spontaneous transitions in behavioral arousal state. We discovered a stereotyped sequence of activity across thalamic nuclei and cingulate cortex that preceded behavioral arousal after a period of inactivity, followed by widespread deactivation. These thalamic dynamics were linked to whether participants subsequently fell back into unresponsiveness, with unified thalamic activation reflecting maintenance of behavior. These results provide an outline of the complex interactions across thalamocortical circuits that orchestrate behavioral arousal state transitions, and additionally, demonstrate that fast fMRI can resolve sub-second subcortical dynamics in the human brain.

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