Researchers at Hiroshima University have developed a groundbreaking experimental approach to detect the Unruh effect, a phenomenon that links the theories of relativity and quantum mechanics. This advancement, published in Physical Review Letters on July 23, 2025, provides a tangible method for exploring the quantum vacuum and could accelerate the development of advanced quantum sensing technologies.
The Unruh effect, a theoretical prediction, states that an observer experiencing uniform acceleration perceives the quantum vacuum not as empty space, but as a thermal bath. This counterintuitive concept has been notoriously difficult to verify experimentally due to the immense accelerations, on the order of 10^20 m/s², typically required, which are beyond current technological capabilities. However, the Hiroshima University team has devised a method that circumvents this hurdle by utilizing the circular motion of metastable fluxon-antifluxon pairs within coupled annular Josephson junctions.
Advances in superconducting microfabrication have enabled the creation of extremely small-radius circuits, generating effective accelerations high enough to produce a detectable Unruh temperature of a few kelvin. This novel experimental setup translates the subtle quantum fluctuations induced by circular acceleration into a clear, macroscopic signature: a voltage jump across the superconducting circuit. By statistically analyzing the distribution of these voltage jumps, researchers can accurately measure the Unruh temperature, providing direct evidence of the effect.
The team plans further investigations into the decay processes of these fluxon-antifluxon pairs, including the role of macroscopic quantum tunneling, to refine the detection accuracy. Beyond its immediate implications for fundamental physics, this research holds considerable promise for technological advancement. The highly sensitive detection capabilities developed could significantly contribute to the field of advanced quantum sensing technologies.
Quantum sensing, a rapidly advancing field, is poised to revolutionize various sectors by offering unprecedented measurement precision. The insights gained from studying the Unruh effect could spur innovations in quantum metrology and the development of sophisticated quantum detectors. The work was supported by JSPS KAKENHI Grants and the HIRAKU-Global Program, funded by MEXT's “Strategic Professional Development Program for Young Researchers.” This advancement not only opens new avenues in our understanding of spacetime and quantum reality but also exemplifies how dedicated inquiry can bridge theoretical predictions with experimental realization, fostering a deeper appreciation for the intricate tapestry of the universe.