A team in Japan has proposed a clever method to finally detect the mysterious Unruh effect, a predicted “phantom heat” arising from acceleration in quantum vacuums. Credit: Stock
The findings resolve a long-standing problem in fundamental physics.
Scientists at Hiroshima University have created a practical and highly sensitive method for detecting the Unruh effect, a long-anticipated phenomenon that lies at the intersection of relativity and quantum theory. This new strategy not only advances the study of fundamental physics but also opens the door to future technological applications.
The work was recently published in the journal Physical Review Letters.
The Fulling-Davies-Unruh effect, often referred to simply as the Unruh effect, is a profound theoretical concept linking Albert Einstein’s Theory of Relativity with 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’,” explained Noriyuki Hatakenaka, professor emeritus at Hiroshima University.
A circulating fluxon–antifluxon pair in coupled annular Josephson junctions behaves as a detector. The pair decays due to Unruh-induced fluctuations, and the resulting event is observed as a voltage jump. By measuring the distribution of the corresponding switching currents, the Unruh effect can be detected. Credit: Haruna Katayama and Noriyuki Hatakenaka, Hiroshima University
This surprising prediction highlights the deep relationship between two cornerstones of modern physics. Experimentally verifying the Unruh effect would not only unite aspects of relativity and quantum mechanics but also offer valuable insights into the very structure of spacetime. However, achieving such verification has remained one of the most persistent and difficult challenges in physics.
Overcoming extreme acceleration limits
“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.
A team at Hiroshima University has introduced a new strategy for detecting 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,” explained Hatakenaka. Thanks to progress in superconducting microfabrication, it is now possible to build circuits with extremely small radii. These compact designs generate exceptionally high effective accelerations and result in an Unruh temperature of a few kelvin—sufficiently large to be measured with existing technology.
Voltage jumps as measurable signals
“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.
Future directions in quantum exploration
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.
Reference: “Circular-Motion Fulling-Davies-Unruh Effect in Coupled Annular Josephson Junctions” by Haruna Katayama and Noriyuki Hatakenaka, 23 July 2025, Physical Review Letters.
DOI: 10.1103/mn34-7bj5
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.”
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