Professor Chen Yan's team from the University of Science and Technology of China has discovered and utilized the "beat frequency effect" in the harmonics of cardiac mechanical activity for the first time, successfully overcoming the large-scale interference caused by respiratory motion under far-field conditions. Without the need for any model training, they have achieved high-precision non-contact monitoring of human cardiac activity using millimeter wave radar technology. This research achievement marks a new stage in non-contact cardiac monitoring technology, providing innovative solutions for early prevention and long-term monitoring of cardiovascular diseases. Recently, relevant research results were published in Nature Communications. Long term continuous monitoring of cardiac activity is crucial for early detection of diseases. However, existing cardiac monitoring technologies are mostly contact based measurements. For example, traditional electrocardiogram devices require multiple electrodes to be attached to the body surface, while wearable devices are often based on photoplethysmography. Due to insufficient comfort and sensitivity to the usage environment, these methods are difficult to achieve long-term continuous monitoring of cardiac activity and may miss the optimal time for diagnosis and treatment of cardiovascular diseases. In recent years, millimeter wave radar technology has been used for monitoring cardiac activity, demonstrating the advantages of non-contact, convenience, and high accuracy, but facing the major challenge of "respiratory spectrum leakage". Due to the respiratory amplitude (centimeter level) being much larger than the heartbeat amplitude (sub millimeter level), respiratory harmonics cause significant spectral leakage in the heartbeat frequency band, resulting in a severe decrease in signal-to-noise ratio and limiting the accuracy of cardiac activity monitoring. This time, the research team discovered two important physical phenomena through systematic analysis and successfully solved this problem. Firstly, it was observed that respiratory harmonics attenuate faster than heartbeat harmonics, especially in the high frequency range, where the impact of respiratory interference is significantly reduced; Secondly, it was found that there is a "beat frequency effect" in the heartbeat harmonics, which means that the superposition of high-order heartbeat harmonics will produce beat frequency characteristics consistent with the heartbeat cycle, with a frequency equal to the difference between adjacent harmonic frequencies. Based on these two phenomena, the research team shifted the heartbeat feature extraction frequency band from the fundamental frequency to the high-order harmonic frequency band, effectively eliminating the interference of respiratory harmonics and significantly improving monitoring accuracy. In a large-scale hospital scenario involving 6222 participants and a daily life scenario involving 21 nights, they achieved median errors of 26.1 milliseconds and 34.1 milliseconds, fully verifying the medical application value of the relevant technology. (New Society)