Nuclear quadrupole resonance spectroscopy with a femtotesla diamond magnetometer.

Radio frequency (RF) magnetometers based on nitrogen vacancy centers in diamond are predicted to offer femtotesla sensitivity, but previous experiments were limited to the picotesla level. We demonstrate a femtotesla RF magnetometer using a diamond membrane inserted between ferrite flux concentrators. The device provides ~300-fold amplitude enhancement for RF magnetic fields from 70 kHz to 3.6 MHz, and the sensitivity reaches ~70 fT√s at 0.35 MHz. The sensor detected the 3.6-MHz nuclear quadrupole resonance (NQR) of room-temperature sodium nitrite powder. The sensor's recovery time after an RF pulse is ~35 µs, limited by the excitation coil's ring-down time. The sodium-nitrite NQR frequency shifts with temperature as -1.00±0.02 kHz/K, the magnetization dephasing time is T2*=887±51 µs, and multipulse sequences extend the signal lifetime to 332±23 ms, all consistent with coil-based studies. Our results expand the sensitivity frontier of diamond magnetometers to the femtotesla range, with potential applications in security, medical imaging, and materials science.

Temperature Sensitivity of 14N-V and 15N-V Ground-State Manifolds.

We measure electron- and nuclear-spin transition frequencies in the ground state of nitrogen-vacancy (N-V) centers in diamond for two nitrogen isotopes (14N-V and 15N-V) over temperatures ranging from 77 to 400 K. Measurements are performed using Ramsey interferometry and direct optical readout of the nuclear and electron spins. We extract coupling parameters Q (for 14N-V), D, A‖, A⊥, and γe/γn, and their temperature dependences for both isotopes. The temperature dependences of the nuclear-spin transitions within the ms=0 spin manifold near room temperature are found to be 0.52(1) ppm/K for 14N-V(|mI = -1⟩ ↔ |mI = +1⟩) and -1.1(1) ppm/K for 15N-V(|mI = -1/2⟩ ↔ |mI = +1/2⟩). An isotopic shift in the zero-field splitting parameter D between 14N-V and 15N-V is measured to be ~ 120 kHz. Residual transverse magnetic fields are observed to shift the nuclear-spin transition frequencies, especially for 15N-V. We have precisely determined the set of parameters relevant for the development of nuclear-spin-based diamond quantum sensors with greatly reduced sensitivity to environmental factors.

Demonstration of diamond nuclear spin gyroscope.

We demonstrate the operation of a rotation sensor based on the nitrogen-14 (14N) 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 14N nuclear spin precession. This measurement protocol suppresses the sensitivity to temperature variations in the 14N 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).

Diamond magnetometer enhanced by ferrite flux concentrators.

Magnetometers based on nitrogen-vacancy (NV) centers in diamond are promising room-temperature, solid-state sensors. However, their reported sensitivity to magnetic fields at low frequencies (≾1 kHz) is presently ≿10 pT s1/2, precluding potential applications in medical imaging, geoscience, and navigation. Here we show that high-permeability magnetic flux concentrators, which collect magnetic flux from a larger area and concentrate it into the diamond sensor, can be used to improve the sensitivity of diamond magnetometers. By inserting an NV-doped diamond membrane between two ferrite cones in a bowtie configuration, we realize a ~250-fold increase of the magnetic field amplitude within the diamond. We demonstrate a sensitivity of ~0.9 pT s1/2 to magnetic fields in the frequency range between 10 and 1000 Hz. This is accomplished using a dual-resonance modulation technique to suppress the effect of thermal shifts of the NV spin levels. The magnetometer uses 200 mW of laser power and 20 mW of microwave power. This work introduces a new degree of freedom for the design of diamond sensors by using structured magnetic materials to manipulate magnetic fields.

Two-dimensional nuclear magnetic resonance spectroscopy with a microfluidic diamond quantum sensor.

Quantum sensors based on nitrogen-vacancy centers in diamond have emerged as a promising detection modality for nuclear magnetic resonance (NMR) spectroscopy owing to their micrometer-scale detection volume and noninductive-based detection. A remaining challenge is to realize sufficiently high spectral resolution and concentration sensitivity for multidimensional NMR analysis of picoliter sample volumes. Here, we address this challenge by spatially separating the polarization and detection phases of the experiment in a microfluidic platform. We realize a spectral resolution of 0.65 ± 0.05 Hz, an order-of-magnitude improvement over previous diamond NMR studies. We use the platform to perform two-dimensional correlation spectroscopy of liquid analytes within an effective ∼40-picoliter detection volume. The use of diamond quantum sensors as in-line microfluidic NMR detectors is a major step toward applications in mass-limited chemical analysis and single-cell biology.

Diamond Magnetic Microscopy of Malarial Hemozoin Nanocrystals.

Magnetic microscopy of malarial hemozoin nanocrystals is performed by optically detected magnetic resonance imaging of near-surface diamond nitrogen-vacancy centers. Hemozoin crystals are extracted from Plasmodium falciparum-infected human blood cells and studied alongside synthetic hemozoin crystals. The stray magnetic fields produced by individual crystals are imaged at room temperature as a function of the applied field up to 350 mT. More than 100 nanocrystals are analyzed, revealing the distribution of their magnetic properties. Most crystals (96%) exhibit a linear dependence of the stray-field magnitude on the applied field, confirming hemozoin's paramagnetic nature. A volume magnetic susceptibility of 3.4 × 10-4 is inferred with use of a magnetostatic model informed by correlated scanning-electron-microscopy measurements of crystal dimensions. A small fraction of nanoparticles (4/82 for Plasmodium falciparum-produced nanoparticles and 1/41 for synthetic nanoparticles) exhibit a saturation behavior consistent with superparamagnetism. Translation of this platform to the study of living Plasmodium-infected cells may shed new light on hemozoin formation dynamics and their interaction with antimalarial drugs.

Electric-field-induced symmetry breaking of angular momentum distribution in atoms.

We report the experimental observation of alignment to orientation conversion in the 7D3/2 and 9D3/2 states of Cs in the presence of an external dc electric field and without the influence of magnetic fields or atomic collisions. Initial alignment of angular momentum states was created by two-step excitation with linearly polarized laser radiation. The appearance of transverse orientation of angular momentum was confirmed by the observation of circularly polarized light. We present experimentally measured signals and compare them with the results of a detailed theoretical model based on the optical Bloch equations. The effect is odd under time reversal and should be taken into account in ever more sensitive searches for an electron electric dipole moment.