Exploring quantum geometry and quantum sensing with spin defects in diamond

Abstract

Recent years have witnessed rapid development in the field of quantum information science. Quantum technologies are promising to revolutionize many fields, ranging from fast computation and simulation to precise sensing and secure communication. While the performance of quantum devices in the current noisy intermediate-scale quantum (NISQ) era is still limited, exploring intriguing applications of various quantum systems has been an active research area. Among the many physical platforms, solid-state spin defects, such as nitrogen-vacancy (NV) center in diamond, have attracted a lot of attentions thanks to their good controllability and long coherence time even at room temperature.

In this thesis, we present our efforts in demonstrating promising quantum applications based on the NV system. With microwave and optical pulses, we have good control on the NV center system, which enables us to engineer the desired Hamiltonian and probe the information of quantum states. In particular, the geometry of a quantum state is characterized by the quantum geometric tensor (QGT), which can find applications in simulating topological material and quantum metrology. We experimentally measure the QGT of an engineered Hamiltonian in a single NV center using weak periodic modulation method. Based on it, we are able to reveal the existence of a tensor monopole that characterizes a tensor gauge field in parameter space. Furthermore, we find that the QGT plays a significant role in quantum multi-parameter estimation. It not only quantifies the precision limit when estimating parameters but also links to the attainability of precision bounds.

Thanks to the sensitivity studied above, NV centers have emerged as powerful quantum sensors to detect various signals, ranging from electromagnetic fields to temperature, with high sensitivity and spatial resolution. Detecting biological or chemical signals is more challenging, but a promising avenue is to transduce them into magnetic noise that the NV center is sensitive to. We demonstrate this strategy by measuring the rotational Brownian motion of magnetic molecules. Then, exploiting the dipolar interaction between NV centers and magnetic molecules, we design a hybrid sensor that is capable of detecting the SARS-CoV-2 virus RNA with ultrahigh sensitivity and low false negative rate. Finally, we will show that the charge state of NV centers is sensitive to surface modification of nanodiamond, which enables us to design alkali ion sensors with the help of chemical engineering.