Nitrogen vacancy (NV) centers in diamond have been used as ultrasensitive magnetometers to perform nuclear magnetic resonance (NMR) spectroscopy of statistically polarized samples at 1–100 nm length scales. However, the spectral linewidth is typically limited to the kHz level, both by the NV sensor coherence time and by rapid molecular diffusion of the nuclei through the detection volume which in turn is critical for achieving long nuclear coherence times. Here we provide a blueprint supported by detailed theoretical analysis for a set-up that combines a sensitivity sufficient for detecting NMR signals from nano- to micron-scale samples with a spectral resolution that is limited only by the nuclear spin coherence, i.e. comparable to conventional NMR. Our protocol detects the nuclear polarization induced along the direction of an external magnetic field with near surface NV centers using lock-in detection techniques to enable phase coherent signal averaging. Using the NV centers in a dual role of NMR detector and optical hyperpolarization source to increase signal to noise, and in combination with Bayesian inference models for signal processing, nano/microscale NMR spectroscopy can be performed on sample concentrations in the micromolar range, several orders of magnitude better than the current state of the art.
In this work, we demonstrate initialization and readout of nuclear spins via a negatively charged silicon-vacancy (SiV) electron spin qubit. Under Hartmann-Hahn conditions the electron spin polarization is coherently transferred to the nuclear spin. The readout of the nuclear polarization is observed via the fluorescence of the SiV. We also show that the coherence time of the nuclear spin (6 ms) is limited by the electron spin-lattice relaxation due to the hyperfine coupling to the electron spin. This work paves the way towards realization of building blocks of quantum hardware with an efficient spin-photon interface based on the SiV color center coupled to a long lasting nuclear memory.Recent advances with color centers in diamond based on IV-group elements [1-4] hold promise to provide an efficient interface between photons and spin qubits. These color centers possess a high Debye-Waller factor (larger than 0.5) [5,6], which implies a high flux of coherent photons, and furthermore an exceptional spectral stability owing to the inversion symmetry of the defects [7,8]. Both of these properties are crucial to realize long distant entanglement based on light-matter interface [9][10][11] which forms an essential building block for scalable quantum processors and quantum repeaters. However, these color centers are not free of constraints. A main drawback is the limited coherence time of the electron spin which is induced by a fast phonon mediated relaxation process between the orbital branches of the ground state [12]. Different attempts to overcome this problem were recently demonstrated, including the application of high strain [13] or freezing of the specimen to millikelvin temperatures [14]. Of these two methods, the former leads to symmetry distortion that affects the optical properties, while the latter requires dilution refrigerators, which are expensive and offer only limited cooling capability. Another route to beat this limitation is to use a defect of this family only as a spin-photon interface and readout gate, while storing the information on a long living nuclear memory. However, to realize such a hybrid approach several problems have to be tackled. Among them are the initialization of the nuclear memory and its readout. The simple polarization technique utilizing level anticrossing and optical pumping commonly used for NV center coupled to 13 C or ST1 centers [15,16] is not applicable to the systems of SiV family. In this letter, we report deterministic polarization of a small nuclear spin ensemble via a dynamic nuclear polar- * petr.siyushev@uni-ulm.de ization protocol. By measuring the Larmor precession in different magnetic fields we identify these nuclei as 13 C. From this ensemble, we choose one nuclear spin with coupling strength in the order of few hundred kHz. This 13 C spin is used to demonstrate nuclear magnetic resonance and Rabi oscillations via nuclear spin polarization readout protocol. For the experiment, a 111 -oriented diamond sample containing ingrown SiV center was chosen and placed ...
Current metrological bounds typically assume full control over all particles that are involved in the protocol. Relaxing this assumption we study metrological performance when only limited control is available. As an example, we measure a static magnetic field when a fully controlled quantum sensor is supplemented by particles over which only global control is possible. We show that even for a noisy quantum sensor, a protocol that maps the magnetic field to a precession frequency can achieve transient super-Heisenberg scaling in measurement time and Heisenberg scaling in the particle number. This leads to an estimation uncertainty that approaches that achievable under full control to within a factor independent of the particle number for a given total time. Applications to hybrid sensing devices and the crucial role of the quantum character of the sensor are discussed.
APPROACHING THE HEISENBERG LIMIT WITHOUT ENTANGLEMENT PREPARATIONBefore stating the main results, we briefly recapitulate the achievable uncertainties under unconstrained metrology to make it available for later comparison with our schemes. arXiv:1905.12582v3 [quant-ph] 3 Apr 2020
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