New research explores intersection of quantum theory and gravity

Professors Sheng Dong and Lu Zhengtian, leading a joint research group at the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), have conducted a pioneering study on the coupling effect between neutron spin and gravitational force. Their research, published in Physical Review Letters, utilized a high-precision xenon isotope magnetometer.

The primary objective of this study was to determine the coupling strength between neutron spin and gravity. The team achieved this by measuring the weight difference between the neutron’s spin-up and spin-down states. Their experimental findings indicated that the weight difference between these two states was extremely minute, measuring less than two sextillionths (<2×10-21). Consequently, this discovery established a new upper limit on the coupling strength associated with this phenomenon.

An article titled “Testing Gravity’s Effect on Quantum Spins” was published in Physics, drawing attention to this remarkable measurement research as an innovative exploration at the intersection of quantum theory and gravity.

Out of the four fundamental physical interactions in nature, gravity is the only one that has yet to be experimentally linked to a particle’s intrinsic spin. If a coupling between spin and gravity exists, particles in different spin states will exhibit incredibly slight discrepancies in energy and force within the Earth’s gravitational field.

Since the 1970s, researchers have been devising various classical and quantum measurement techniques to investigate the coupling phenomenon between spin and gravity, continually refining measurement precision. These experiments have also aimed to examine the fundamental spacetime symmetry within gravitational interactions and to identify axion-like particles that mediate monopole-dipole interactions.

The USTC team developed a highly stable and sensitive 129Xe-131Xe-Rb co-magnetometer, incorporating their self-designed atomic devices and spectroscopic measurement techniques. They also employed precision measurement methods to mitigate systematic errors in co-magnetometer systems.

The co-magnetometer functioned as a quantum compass, gauging the coherent effects between the two quantum spin states pointing upward and downward. The quantum axis of the system was aligned with the Earth’s rotation axis, corresponding to the direction of the North Star, with a precision exceeding 0.6 degrees. This meticulous alignment significantly minimized experimental errors arising from the Earth’s rotation.

The experimental outcomes succeeded in reducing the upper limit of the neutron’s spin-gravity coupling strength by a factor of 17 while enhancing the precision of various fundamental physical effects by an order of magnitude.

Source: University of Science and Technology of China

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