  Magnetocardiography (MCG) has emerged as a sensitive and precise method to
diagnose cardiovascular diseases, providing more diagnostic information than
traditional technology. However, the sensor limitations of conventional MCG
systems, such as large size and cryogenic requirement, have hindered the
widespread application and in-depth understanding of this technology. In this
study, we present a high-sensitivity, room-temperature MCG system based on the
negatively charged Nitrogen-Vacancy (NV) centers in diamond. The magnetic
cardiac signal of a living rat, characterized by an approximately 20 pT
amplitude in the R-wave, is successfully captured through non-invasive
measurement using this innovative solid-state spin sensor. To detect these
extremely weak biomagnetic signals, we utilize sensitivity-enhancing techniques
such as magnetic flux concentration. These approaches have enabled us to
simultaneously achieve a magnetometry sensitivity of 9 $\text{pT}\cdot
\text{Hz}^{-1/2}$ and a sensor scale of 5 $\text{mm}$. By extending the sensing
scale of the NV centers from cellular and molecular level to macroscopic level
of living creatures, we have opened the future of solid-state quantum sensing
technologies in clinical environments.
