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1. A spin-refrigerated cavity quantum electrodynamic sensor

   https://doi.org/10.1038/s41467-024-54333-8  

   Wang, Hanfeng; Tiwari, Kunal_L; Jacobs, Kurt; Judy, Michael; Zhang, Xin; Englund, Dirk_R; Trusheim, Matthew_E (November 2024, Nature Communications)

   Abstract Quantum sensors based on solid-state defects, in particular nitrogen-vacancy (NV) centers in diamond, enable precise measurement of magnetic fields, temperature, rotation, and electric fields. Cavity quantum electrodynamic (cQED) readout, in which an NV ensemble is hybridized with a microwave mode, can overcome limitations in optical spin detection and has resulted in leading magnetic sensitivities at the pT-level. This approach, however, remains far from the intrinsic spin-projection noise limit due to thermal Johnson-Nyquist noise and spin saturation effects. Here we tackle these challenges by combining recently demonstrated spin refrigeration techniques with comprehensive nonlinear modeling of the cQED sensor operation. We demonstrate that the optically-polarized NV ensemble simultaneously provides magnetic sensitivity and acts as a heat sink for the deleterious thermal microwave noise background, even when actively probed by a microwave field. Optimizing the NV-cQED system, we demonstrate a broadband sensitivity of 576 ± 6 fT/$$\sqrt{{{{\rm{Hz}}}}}$$ $\sqrt{\mathrm{Hz}}$around 15 kHz in ambient conditions. We then discuss the implications of this approach for the design of future magnetometers, including near-projection-limited devices approaching 3 fT/$$\sqrt{{{{\rm{Hz}}}}}$$ $\sqrt{\mathrm{Hz}}$sensitivity enabled by spin refrigeration. 

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2. Quantum control and Berry phase of electron spins in rotating levitated diamonds in high vacuum

   https://doi.org/10.1038/s41467-024-49175-3  

   Jin, Yuanbin; Shen, Kunhong; Ju, Peng; Gao, Xingyu; Zu, Chong; Grine, Alejandro_J; Li, Tongcang (June 2024, Nature Communications)

   Abstract Levitated diamond particles in high vacuum with internal spin qubits have been proposed for exploring macroscopic quantum mechanics, quantum gravity, and precision measurements. The coupling between spins and particle rotation can be utilized to study quantum geometric phase, create gyroscopes and rotational matter-wave interferometers. However, previous efforts in levitated diamonds struggled with vacuum level or spin state readouts. To address these gaps, we fabricate an integrated surface ion trap with multiple stabilization electrodes. This facilitates on-chip levitation and, for the first time, optically detected magnetic resonance measurements of a nanodiamond levitated in high vacuum. The internal temperature of our levitated nanodiamond remains moderate at pressures below 10⁻⁵Torr. We have driven a nanodiamond to rotate up to 20 MHz (1.2 × 10⁹rpm), surpassing typical nitrogen-vacancy (NV) center electron spin dephasing rates. Using these NV spins, we observe the effect of the Berry phase arising from particle rotation. In addition, we demonstrate quantum control of spins in a rotating nanodiamond. These results mark an important development in interfacing mechanical rotation with spin qubits, expanding our capacity to study quantum phenomena. 

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3. Diamond surface functionalization via visible light–driven C–H activation for nanoscale quantum sensing

   https://doi.org/10.1073/pnas.2316032121  

   Rodgers, Lila_V H; Nguyen, Suong T; Cox, James H; Zervas, Kalliope; Yuan, Zhiyang; Sangtawesin, Sorawis; Stacey, Alastair; Jaye, Cherno; Weiland, Conan; Pershin, Anton; et al (March 2024, Proceedings of the National Academy of Sciences)

   Nitrogen-vacancy (NV) centers in diamond are a promising platform for nanoscale NMR sensing. Despite significant progress toward using NV centers to detect and localize nuclear spins down to the single spin level, NV-based spectroscopy of individual, intact, arbitrary target molecules remains elusive. Such sensing requires that target molecules are immobilized within nanometers of NV centers with long spin coherence. The inert nature of diamond typically requires harsh functionalization techniques such as thermal annealing or plasma processing, limiting the scope of functional groups that can be attached to the surface. Solution-phase chemical methods can be readily generalized to install diverse functional groups, but they have not been widely explored for single-crystal diamond surfaces. Moreover, realizing shallow NV centers with long spin coherence times requires highly ordered single-crystal surfaces, and solution-phase functionalization has not yet been shown with such demanding conditions. In this work, we report a versatile strategy to directly functionalize C–H bonds on single-crystal diamond surfaces under ambient conditions using visible light, forming C–F, C–Cl, C–S, and C–N bonds at the surface. This method is compatible with NV centers within 10 nm of the surface with spin coherence times comparable to the state of the art. As a proof-of-principle demonstration, we use shallow ensembles of NV centers to detect nuclear spins from surface-bound functional groups. Our approach to surface functionalization opens the door to deploying NV centers as a tool for chemical sensing and single-molecule spectroscopy. 

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4. 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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5. Impact of microwave phase noise on diamond quantum sensing

   https://doi.org/10.1103/PhysRevResearch.6.043148  

   Berzins, Andris; Saleh_Ziabari, Maziar; Silani, Yaser; Fescenko, Ilja; Damron, Joshua T; Barry, John F; Jarmola, Andrey; Kehayias, Pauli; Richards, Bryan A; Smits, Janis; et al (November 2024, Physical Review Research)

   Precision optical measurements of the electron-spin precession of nitrogen-vacancy (NV) centers in diamond form the basis of numerous applications. The most sensitivity-demanding applications, such as femtotesla magnetometry, require the ability to measure changes in GHz spin transition frequencies at the sub-millihertz level, corresponding to a fractional resolution of better than ${10}^{-12}$. Here we study the impact of microwave (MW) phase noise on the response of an NV sensor. Fluctuations of the phase of the MW waveform cause undesired rotations of the NV spin state. These fluctuations are imprinted in the optical readout signal and, left unmitigated, are indistinguishable from magnetic-field noise. We show that the phase noise of several common commercial MW generators results in an effective $\mathrm{pT}{\mathrm{s}}^{1/2}$-range noise floor that varies with the MW carrier frequency and the detection frequency of the pulse sequence. The data are described by a frequency-domain model incorporating the MW phase-noise spectrum and the filter-function response of the sensing protocol. For controlled injection of white and random-walk phase noise, the observed NV magnetic noise floor is described by simple analytic expressions that accurately capture the scaling with pulse sequence length and the number of $\pi$pulses. We outline several strategies to suppress the impact of MW phase noise and implement a version, based on gradiometry, that realizes a $>10$-fold suppression. Our study highlights an important challenge in the pursuit of sensitive diamond quantum sensors and is applicable to other qubit systems with a large transition frequency. Published by the American Physical Society2024 

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