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.