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1. Precise high-fidelity electron–nuclear spin entangling gates in NV centers via hybrid dynamical decoupling sequences

   https://doi.org/10.1088/1367-2630/ab9bc0  

   Dong, Wenzheng; Calderon-Vargas, F. A.; Economou, Sophia E. (July 2020, New Journal of Physics)

   Abstract Color centers in solids, such as the nitrogen-vacancy center in diamond, offer well-protected and well-controlled localized electron spins that can be employed in various quantum technologies. Moreover, the long coherence time of the surrounding spinful nuclei can enable a robust quantum register controlled through the color center. We design pulse sequence protocols that drive the electron spin to generate robust entangling gates with these nuclear memory qubits. We find that compared to using Carr-Purcell-Meiboom-Gill (CPMG) alone, Uhrig decoupling sequence and hybrid protocols composed of CPMG and Uhrig sequences improve these entangling gates in terms of fidelity, spin control range, and spin selectivity. We provide analytical expressions for the sequence protocols and also show numerically the efficacy of our method on nitrogen-vacancy centers in diamond. Our results are broadly applicable to color centers weakly coupled to a small number of nuclear spin qubits. 

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2. Characterizing the magnetic noise power spectrum of dark spins in diamond

   https://doi.org/10.1088/1367-2630/adb2b8  

   Williams, Ethan_Que; Ramanathan, Chandrasekhar (February 2025, New Journal of Physics)

   Abstract The coherence times of solid-state spin qubits are often limited by the presence of a spin bath. Characterizing the spectrum of the local magnetic field fluctuations of the bath is key to understanding spin qubit decoherence. Here we use pulsed electron paramagnetic resonance (pEPR) based noise spectroscopy to measure the magnetic noise power spectra for ensembles of P1 (substitutional nitrogen) centers in diamond that typically form the bath for NV (nitrogen-vacancy) centers. The Carr-Purcell-Meiboom-Gill (CPMG) dynamical decoupling experiments on the P1 centers were performed on a low [N] CVD (chemical vapor deposition) sample and a high [N] HPHT (high-temperature, high-pressure) sample at 89 mT. We characterize the NV centers of the latter sample using the same 2.5 GHz pEPR spectrometer. All power spectra show two distinct features, a broad component that is observed to scale as approximately 1/ω^{0.7-1.0}, and a prominent peak at the 13C Larmor frequency. The behavior of the broad component is consistent with an inhomogeneous distribution of Lorentzian spectra due to clustering of P1 centers, which has recently been shown to be prevalent in HPHT diamond. We develop techniques utilizing harmonics of the CPMG filter function to improve characterization of high-frequency signals, which we demonstrate on the 13C nuclear Larmor frequency. At 190 mT this is 2.04 MHz, 5.7 times higher than the CPMG modulation frequency (<357 kHz, hardware-limited). Understanding the properties of the bath allow us to either exploit it as a quantum resource or optimize decoupling performance, while also informing sample fabrication technologies. The techniques are applicable to ac magnetometry for nanoscale nuclear magnetic resonance and chemical sensing. 

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3. Demonstration of diamond nuclear spin gyroscope

   https://doi.org/10.1126/sciadv.abl3840  

   Jarmola, Andrey; Lourette, Sean; Acosta, Victor M.; Birdwell, A. Glen; Blümler, Peter; Budker, Dmitry; Ivanov, Tony; Malinovsky, Vladimir S. (October 2021, Science Advances)

   We demonstrate the operation of a rotation sensor based on the nitrogen-14 ( 14 N) nuclear spins intrinsic to nitrogen-vacancy (NV) color centers in diamond. The sensor uses optical polarization and readout of the nuclei and a radio-frequency double-quantum pulse protocol that monitors 14 N nuclear spin precession. This measurement protocol suppresses the sensitivity to temperature variations in the 14 N quadrupole splitting, and it does not require microwave pulses resonant with the NV electron spin transitions. The device was tested on a rotation platform and demonstrated a sensitivity of 4.7°/ s (13 mHz/ Hz ), with a bias stability of 0.4 °/s (1.1 mHz). 

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4. Photon-mediated interactions between quantum emitters in a diamond nanocavity

   https://doi.org/10.1126/science.aau4691  

   Evans, R. E.; Bhaskar, M. K.; Sukachev, D. D.; Nguyen, C. T.; Sipahigil, A.; Burek, M. J.; Machielse, B.; Zhang, G. H.; Zibrov, A. S.; Bielejec, E.; et al (September 2018, Science)

   Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically mediated interactions. Such controlled interactions will be crucial in developing cavity-mediated quantum gates between spin qubits and for realizing scalable quantum network nodes. 

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5. Spin coherent quantum transport of electrons between defects in diamond

   https://doi.org/10.1515/nanoph-2019-0144  

   Oberg, Lachlan M.; Huang, Eric; Reddy, Prithvi M.; Alkauskas, Audrius; Greentree, Andrew D.; Cole, Jared H.; Manson, Neil B.; Meriles, Carlos A.; Doherty, Marcus W. (August 2019, Nanophotonics)

   Abstract The nitrogen-vacancy (NV) color center in diamond has rapidly emerged as an important solid-state system for quantum information processing. Whereas individual spin registers have been used to implement small-scale diamond quantum computing, the realization of a large-scale device requires the development of an on-chip quantum bus for transporting information between distant qubits. Here, we propose a method for coherent quantum transport of an electron and its spin state between distant NV centers. Transport is achieved by the implementation of spatial stimulated adiabatic Raman passage through the optical control of the NV center charge states and the confined conduction states of a diamond nanostructure. Our models show that, for two NV centers in a diamond nanowire, high-fidelity transport can be achieved over distances of order hundreds of nanometers in timescales of order hundreds of nanoseconds. Spatial adiabatic passage is therefore a promising option for realizing an on-chip spin quantum bus. 

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