Ensemble-NV-diamond Magnetometer Using Flux Concentration And Modulation Method

As a technology of critical importance,the magnetometry has attract many attentions.From the Hall sensor invented in the 20th century to the extensively studied atomic magnetometer at present,great efforts have been taken on the development of magnetometers.That makes it possilble to achieve magnetic field measurement with higher accuracy and to obtain more information from the results.Based on the magnetometers,many important applications in the field of biomagnetism,geomagnetic survey,non-destructive evaluation and susceptometry can be realized.These applications are closely related to the living and security of human beings.It is noteworthy that,the detection of low-frequency magnetic fields,such as the geomagnetic anomalies and the magnetic fields of organisms,is always important for these applications.Therefore,the development of magnetometers faces the enormous challenge of sensitivity enhancement in low-frequency domain.Quantum sensing uses quantum systems,quantum properties and quantum phenomenons to precisely measure macroscopic physical quantities.A quantum system that takes up a significant position in the field of quantum sensing is the nitrogenvacancy(NV)center in diamond,which is a solid-state spin system with outstanding physical properties.The NV center allows optical polarization and readout for its electronic spin state,and the electronic spin state is very sensitive to the magnetic field.Thus,plenty of works,which focus on magnetometry using single NV center and NV ensemble,have been proposed in recent years.The NV-diamond magnetometry for low-frequency magnetic field measurement can be significantly affected by the lowfrequency noises in solid-state spin system,such as the quasi-static dephasing noise in the diamond lattice atmosphere.Hence,the sensitivity of NV-diamond magnetometer in low-frequency domain is limited by the short dephasing time T2*of several hunderd nanoseconds.Besides,the temperature dependent fluorescence caused by the temperature dependent ground state zero-field splitting of NV center will also introduce lowfrequency noise that limit the sensitivity of NV-diamond magnetometer.In this thesis,we use NV ensemble as the magnetic sensing element and study the ensemble-NV-diamond magnetometer using flux concentration and modulation method.Our aim is to solve the problem that the sensitivity of NV-diamond magnetometer for low-frequency magnetic field measurement is limited by the T2*.At the same time,we study the ensemble-NV-diamond magnetometer based on the double-transiton method,for suppressing the impact of temperature drift on magnetic field measurement.The main content of this thesis includes the following parts:1.The demonstration of ensemble-NV-diamond magnetometer using flux concentration and modulation method.In this work,the low-frequency magnetic field is magnified and converted to high-frequency domain via the flux concentration and modulation method.The dynamical decoupling sequence is used to detect the high-frequency magnetic field and prolong the dephasing time.The breakthrough of T2*limit has been achieved.The sensitivity in low-frequency domain has been enhanced from 4.6 nT/Hz1/2 to 32 pT/Hz1/2.With further improvements,the magnetometer has potential to realize femtotesla sensitivity at room temperature in the future.Such magnetometer can be applied in the measurement of magnetocardiogram.2.The demonstration of ensemble-NV-diamond magnetometer based on the double-transiton method.In this work,both of the transitions between |ms=0>and|ms=±1>states are utilized with incomplete degeneracy of the |ms=±1>sublevels.Thus,the impact of temperature drift can be counteracted.The drift of magnetic field measurement result is about 7-folds lower than that of the conventional method.With further improvements,the magnetometer has potential to be applied in biomagnetism and space scientific research.