Empty space is not as empty as it seems. According to quantum theory, the vacuum is filled with particles that constantly pop in and out of existence. In 1976, physicist Bill Unruh showed that if an observer moves with acceleration, this 'quantum soup' turns into a genuinely warm bath for them — the Unruh effect. But accelerations achievable in the lab are far too small to notice even the slightest temperature rise. Direct measurements are still lacking.
A new study offers a workaround. Instead of accelerating a detector in ordinary space, the authors use a 'temporal' version of the effect. It is related to how massless fields become entangled between the past and future light cones. For such an effect to appear, you need a detector whose frequency changes according to a special law, as if it were moving through curved time. They set out to realize this on a superconducting circuit — a fluxonium. This is a tiny loop of aluminum on a silicon substrate, cooled to near absolute zero. In it, two nearly degenerate ground states and one excited state naturally arise, forming a Λ-system (three levels where transitions are allowed between the upper level and each of the lower ones, but not between the lower ones).
By controlling the magnetic flux through the loop, the frequency of the excited state is changed on the fly — in real time, rather than through accelerated motion. As a result, the detector accumulates a geometric phase (a shift that depends only on the path taken in parameter space, not on the speed of motion). Calculations show that over 530 nanoseconds, the population of the ground state shifts by about 10%. This is a thousand times more noticeable than what previous two-level detectors could achieve.
Such a leap in sensitivity is not just a technical trick. It turns the Unruh effect from a mathematical curiosity into something you can 'feel' on a tabletop. If the experiment succeeds, it will be the first time a fundamental property of the quantum vacuum has manifested itself in a man-made circuit.
as written by the authors arXiv:2607.24836
The Unruh effect predicts that an accelerating observer perceives the Minkowski vacuum as a thermal bath, yet direct detection remains experimentally inaccessible. Its timelike counterpart, arising from the entanglement of massless fields between the future and past light cones, offers a more feasible route but requires a detector whose transition frequency follows a specific conformal-time scaling. We propose and analyze a practical implementation of such a detector using superconducting fluxonium circuits, which naturally provide two quasi-degenerate ground states and a tunable excited state, forming an effective $$\Lambda$$-system. By modulating the excited-state transition frequency in Minkowski time, the detector accumulates a geometric phase associated with the timelike Unruh effect. Open-system simulations predict $$\sim 10\%$$ shift in the ground-state population within $$530$$ ns, representing a three-order-of-magnitude sensitivity enhancement over two-level Unruh-DeWitt detectors. These results establish a realistic quantum-circuit platform for experimentally probing the timelike Unruh effect and, more broadly, for testing fundamental nature of quantum fields using engineered quantum systems.
The Unruh effect is a close relative of Hawking radiation. In both cases, a horizon (of acceleration or a black hole) divides space so that vacuum fluctuations become real particles.
In Paul Anderson's novel 'Tau Zero', a starship accelerates so hard that the crew sees the cosmic microwave background shift into visible light — a poetic illustration of how acceleration changes the perception of the vacuum.