The phantom heat of empty space might soon be detectable

The work was recently published in Physical Review Letters.

The Fulling-Davies-Unruh effect, or simply the Unruh effect, is a striking theoretical prediction at the profound intersection of Albert Einstein's Theory of Relativity and Quantum Theory. "In quantum theory, even the vacuum seethes with tiny energy fluctuations, where particles and antiparticles briefly appear and vanish. Remarkably, the Unruh effect shows how these 'vacuum ripples' are perceived depends on the observer's motion. A stationary observer sees nothing, but an observer undergoing acceleration perceives them as real particles with a thermal energy distribution -- a 'quantum warmth'," said Noriyuki Hatakenaka, professor emeritus at Hiroshima University.

The counterintuitive result emphasizes the important connection between these two pillars of modern physics. If scientists could experimentally verify the Unruh effect, it would not only bridge the gap between general relativity and quantum mechanics but also provide profound insights into the nature of spacetime itself. Yet the experimental verification of the Unruh effect has been a long-standing and significant challenge in fundamental physics.

"The core problem has been the extraordinarily large accelerations -- on the order of 10²⁰ m/s² -- required to make this effect detectable, rendering its observation practically impossible with current technology at least in linear acceleration systems," said Haruna Katayama, assistant professor at Hiroshima University.

The researchers at Hiroshima University have proposed a promising approach to observe the Unruh effect. "Our work aims to overcome this fundamental hurdle by proposing a novel and feasible experimental method. We utilize the circular motion of metastable fluxon-antifluxon pairs within coupled annular Josephson junctions," said Hatakenaka. Advances in superconducting microfabrication allow the creation of circuits with extremely small radii, enabling immensely high effective accelerations and producing an Unruh temperature of a few kelvin -- high enough to be experimentally detectable with current technology.

"We have proposed a realistic, highly sensitive, and unambiguous method to detect the elusive Unruh effect. Our proposed system offers a clear pathway to experimentally observe this 'phantom heat' of acceleration for the first time," said Katayama. In their innovative setup, the "quantum warmth" induced by the circular acceleration causes fluctuations that trigger the splitting of the metastable fluxon-antifluxon pairs. Crucially, this splitting event manifests as a clear, macroscopic voltage jump across the superconducting circuit. This voltage jump serves as an undeniable and easily measurable signal, providing a direct and robust signature of the Unruh effect's presence. By statistically analyzing the distribution of these voltage jumps, the researchers can precisely measure the Unruh temperature with high accuracy.

"One of the most surprising aspects is that microscopic quantum fluctuations can induce sudden, macroscopic voltage jumps, making the elusive Unruh effect directly observable. Even more striking, the switching distribution shifts solely with acceleration while all other parameters remain fixed -- a clear statistical fingerprint of the Unruh effect itself," said Hatakenaka.

Looking ahead, Katayama said, "Our immediate next step is to conduct a detailed analysis of the decay processes of the fluxon-antifluxon pairs. This includes thoroughly investigating the role of macroscopic quantum tunneling, a quantum-mechanical phenomenon where particles can pass through potential barriers, which was not extensively explored in this initial work. Understanding these intricate decay mechanisms will be crucial for refining the experimental detection of the Unruh effect."

Their ultimate goal in this research is multifaceted. Beyond the immediate detection, they aim to investigate potential connections between this phenomenon and other quantum fields coupled to their detector. "By deepening our understanding of these novel quantum phenomena, we hope to contribute significantly to the search for a unified theory of all physical laws," said Hatakenaka.

The researchers note that the highly sensitive and broad-range detection capabilities developed in this research hold immense promise for paving the way for future applications, particularly in the field of advanced quantum sensing technologies. "We aspire for this work to open new avenues in fundamental physics and to inspire further exploration into the true nature of spacetime and quantum reality," said Katayama.

The research team includes Noriyuki Hatakenaka, professor emeritus in the Graduate School of Advanced Science and Engineering at Hiroshima University, and Haruna Katayama, assistant professor in the Graduate School of Advanced Science and Engineering at Hiroshima University.

This work was supported by JSPS KAKENHI Grants and by the HIRAKU-Global Program, which is funded by MEXT's "Strategic Professional Development Program for Young Researchers."