Mesoscopic Magnetic Resonance Detection Surpassing Quantum Limits

Getting insight into the nature is an eternal pursuit of human beings.However,measurement precision limits the extent to which we can understand the nature,thus a higher measurement precision is a permanent theme for scientific research.With the birth of quantum mechanics in the last century,people learned that a profound law un-derlies the nature called 'Heisenberg uncertainty principle',which limits how precise a measurement could be.In the following study,people found that there is a definite pre-cision limit for multiple independent quantum sensors,called the standard quantum limit(SQL).Most strikingly,quantum entanglement,taken as 'Spooky interaction' by Ein-stein to refute the Copenhagen interpretation of quantum mechanics,can be harnessed as a quantum resource to beat the SQL and arrive at the ultimate precision limit—the Heisenberg limit(HL).The surprising findings have attracted widespread attention and gradually formed a specialized physical branch as quantum metrology,an important part of the field of quantum precision measurements.This dissertation will be focused on these two quantum limits.There are various experimental systems researching on the field of quantum metrology such as trapped ions,cold thermal atoms,Bose-Einstein condensates(BEC),photons,and mechanical system.The solid-state spin system in diamond(the nitrogen-vacancy(NV)defect center)has been rapidly developed over the last decade,benefit-ting from its excellent properties:convenient initialization by laser illumination,op-tical readout of spin states,and long-lived spin coherence even under ambient condi-tions.Nevertheless,very little research based on NV centers is involved in quantum metrology.It has unique advantages in the field of microscale magnetic detection as an atomic-sized sensor.To date,the magnetic resonance detection of a single electron spin and a small ensemble of nuclear spins has been accomplished.However,its magnetic sensitivity is still relatively poor and far from the SQL.This dissertation is committed to improving its magnetic sensitivity up to the SQL,and meanwhile to surpass the energy resolution limit(ERL)of an individual NV center.Moreover,by leveraging multiple-qubit entanglement,its phase sensitivity is enhanced to the degree of beating the SQL and being close to the HL.It is indispensable to com-prehensively improve the basic performances of the solid-state spin system,including high-fidelity preparation of a pure state,high-fidelity non-local gates,and high-fidelity projective measurements.A great number of hard techniques are involved here.The preparation of a pure state demands real-time feedback for NV negative state prepara-tion,chopped laser sequence for a better spin polarization without destroying charge state,and exquisite optimal control and population shelving for polarization transfer to nuclear spins;the non-local gates require an accurate detection and description of local magnetic noise and an elaborate design of shaped pulse sequences by optimal control with the ability of resisting the magnetic noise and the control noise;the realization of the projective measurements wants a strong and highly stable magnetic field(?0.8 T)and high-frequency microwaves(?25 GHz)via the technique of repetitive readouts.Besides,the non-local gates for multiple qubits involve surrounding nuclear spins strongly coupled to the NV electron spin,including the attendant 14N nuclear spin and randomly distributed 13C nuclear spins.Hz-precision measurements for their coupling parameters are performed,and based on this the identity test of single NV-centers in diamond is conducted.In addition,we proposed a robust and integrated atomic-like clock based on ensemble NV centers for commercial applications.With this high-sensitivity and high-spatial-resolution magnetic sensor,we can ex-plore the principle and method of microscale magnetic resonance,and implement ex-perimental verification.Here,we choose the spatial scale from 50 nm to 10 ?m as our detection range called‘mesoscopic'or‘mesoscale'in the dissertation,which is totally blank for conventional magnetic resonance.Since mesoscopic magnetic resonance de-tection is still an emergent field,we first build a complete theoretical framework for detection methods and magnetic signal calculation.Based on the framework,we exper-imentally perform the principal verification of both mesoscopic paramagnetic magnetic resonance and nuclear magnetic resonance.In the future,we envision that the method and technology developed here could be extensively applied to the fields of cell biology and material sciences for the study of mesoscopic magnetic phenomena.