Why electron-nuclear spin clusters for Quantum Sensing?

Control over the microscopic and quantum mechanical structure of multi-electron-nuclear spin clusters can achieve the holy grail of quantum information science (QIS): control over polarization transfer, spin coherence and relaxation times in water, the universal juice of biology and an adversarial environment for quantum sensing. Strong and asymmetric coupling between two or more electrons can be designed to excite the intended electron-electron-nucleus triple flip quantum transition that underlies quantum sensing and/or multi-electron dynamic nuclear polarization (ME-DNP) induced NMR signal enhancements to determining the electronic structure property of target nuclear spins, molecular distances, and motion.

What is the knowledge gap?

The microscopic origin of ME-DNP is not well understood, making it difficult to tune and exploit for quantum sensing. For example, whether the single-line radical, BDPA, exhibits one-electron Overhauser Effect s or multi-electron DNP by the Thermal Mixing Effect, or whether diamond P1 and NV centers are isolated or clustered relative to w_n of the nearby ¹³C nuclear spin is an open question. In fact, the Han lab in collaboration with multiple expert groups in the field discovered that a large fraction of P1 centers in diamond used for sensing are exchange coupled. This is an important insight as these structures must be considered when designing optimal quantum sensors, perhaps by materials design exploiting DNP characterization, and can be utilized for longer range quantum sensing. At low magnetic field and cryogenic temperatures of ~100 K, the dipolar coupling between electrons lead to spectral overlap, prohibiting individual electron spin manipulation. Conventional DNP at high magnetic field relies on the rapid transfer of nuclear spin polarization from nuclei near the electron to the bulk sample via nuclear spin diffusion to build up the total NMR signal. However, a read-out solely based on bulk nuclear polarization obscures the observation of the underlying microscopic process of coupled many-electron-nuclear spin flip-flop transitions.

Han Lab Strategy?

The strategy of the Han lab is to design novel coupled electron-nuclear spin clusters, and to perform DNP-enhanced NMR near the optical limit that permits the initialization at pure spin state in steady state, facilitates quantum operation and permits the study of spin dynamics and physics in the absence of certain thermal noise. The design, triaging and optimization of coupled electron-nuclear spin clusters to achieve long-range quantum sensing requires dual NMR and EPR manipulation, detection, and characterization tools at high magnetic fields. The goal–and the Han lab already has exciting leads to this end–is to utilize the knowledge gained in these extreme limits to develop quantum sensing applications in water and under biologically relevant conditions.

Unique Han Lab Tools and Concepts:

• Unique dual EPR/DNP instruments at high field at variable cryogenic temperatures < 7 K for diagnosis of DNP mechanisms under DNP conditions. A new ~2K setup will bring us to “Optical Limit”.
• Unique approach to advance DNP by dissecting and designing many-electron-nuclear spin clusters for DNP and QIS studies supported by dual EPR/DNP diagnosis and quantum mechanical spin dynamics simulation.