Quantum Sensing With Diamond Nitrogen-vacancy Centers Under High Pressures

The nitrogen vacancy center(NV)in diamond has developed rapidly in the field of quantum sensing.It can detect physical quantities such as magnetic field,stress,temperature,electric field,etc.,with ultra-high sensitivity and spatial resolution,and can still work stably under extreme conditions.However,the properties of NV center and quantum sensing applications under extreme pressure have not been systematically studied.The purpose of this thesis is to apply NV quantum sensor to high-pressure experimental research,and to study the properties of NV in high-pressure environment.The core technology of NV as a quantum sensor is the optically detected magnetic resonance(ODMR)based on the principle of electron magnetic resonance(EMR).This technique requires the polarization and readout of quantum states with lasers,and the manipulation of quantum states with microwaves and radio frequencies.A high pressure environment is achieved with a diamond anvil cell(DAC).Optical and electrical measurement techniques developed in DAC technology facilitate loading lasers and microwaves into our NV experiments.We use a self-built confocal fluorescence microscope system to observe the ODMR signal under high pressure in the DAC.To fully exploit the quantum nature of diamond NV-based probes,their spin dynamics and the quantum control of the spin states under high pressure are urgently needed.In this work,we demonstrate coherent driving,spin relaxation,and spin dephasing measurements for ensemble NV centers up to 32.8 GPa.With this well-controlled in situ quantum sensor,we investigate the pressure-induced magnetic phase transition of a micron-size permanent magnet Nd2Fe14B sample inside a diamond anvil cell,with a spatial resolution of 2 μm,and a sensitivity of 20(?).We have successfully demonstrated that diamond NV center,a unique in-situ quantum sensor for high-pressure science,can be optically polarized and read out under megabar pressure.We achieved the ODMR signal of NV electron spin to 143.3 GPaunder high pressure,and observed the response of NV electron spin ODMR signal to magnetic field and the rabi oscillation signal of NV electron spin under pressure of 80 GPa.We find that maintaining of hydrostatic pressures,rather than the change of NV optical properties,is the key challenge for diamond quantum sensing at ultrahigh pressures.We propose and demonstrate a high-pressure NMR(and NQR)scheme enabled by diamond NV centers.The first critical step of this scheme is hyperpolarization of the target nuclear spins with NV centers,followed by standard NMR sequence to control the evolution of the nuclear spins,and a second critical step is detection of the nuclear spin states with nearby NV quantum sensors.We experimentally demonstrate high-pressure NMR of 14N nuclear spins in a microdiamond up to 16.6 GPa.NMR spectra reveal that both the 14N nuclear quadrupole and hyperfine coupling terms deceases with increasing pressures,indicating the electron probability density of NV centers shifts away from the nitrogen nucleus under pressures.The tens of kilohertz width of the NMR spectra is attributed to the fast relaxation time of the NV electron spins and can be suppressed with high-purity diamond and advanced correlation method.This scheme could be generalized to measure other parameters such as temperature,pressure and their gradients under extreme conditions.This will be beneficial for frontier research of condensed matter physics and geophysics.We demonstrate ODMR of NV centers and extend the working pressure of diamond quantum sensing to megabar region.These results provide new clues to the understanding of NV center and contribute to quantum sensing under extreme conditions.The results of NMR under high pressure deepen our understanding of the electronic orbitals of diamond NV center,and shine a new light on the highpressure NMR and NQR.