If you carry a cup of coffee slowly and steadily around a room and then return to the same spot, nothing happens to the drink. But in the quantum world, things are different: a particle that makes such a loop comes back not quite the same. It picks up an elusive 'extra'—the geometric phase, which physicists call the Berry phase. This addition depends only on the shape of the route, not on the time taken.
Until now, all discussions of the Berry phase were conducted as if our quantum coffee never cools down. But real systems always exchange heat and particles with their surroundings, like carrying a cup in a draft. In the new work, physicists have for the first time measured the full, complex Berry phase of an artificial atom on a superconducting chip, where losses were not an annoying hindrance but part of the design. It turned out that the phase has an 'imaginary' half that is directly related to how much the quantum state decays along the way.
The experiment was performed on a superconducting qubit — a tiny aluminum antenna on a silicon substrate, cooled to a few thousandths of a degree above absolute zero. Using microwave pulses, the qubit was 'driven' along closed trajectories in its internal parameter space, while a deliberately added resistor served as a controlled energy leak. By measuring the qubit's state at the end of each loop, the scientists extracted both parts of the phase, the real and the imaginary, and showed that they behave differently when the direction of the loop is reversed.
The main surprise is that the imaginary phase can be controlled by choosing the trajectory. By changing the loop along which the qubit moves, the physicists could enhance or suppress the damping without touching the lossy environment itself. This is like a skier who can race down the mountain quickly and breezily, or meander to stretch out the descent and stay warmer. For quantum technologies, such a trick is valuable: it allows operations on fragile states without fearing they will collapse prematurely.
The very idea of the geometric phase in quantum mechanics was only realized in 1984, although it was mathematically predicted as early as the 1950s. And in the classical world, an analog of the Berry phase is the Foucault pendulum, whose plane of swing rotates due to the Earth's rotation.
In Neal Stephenson's novel 'Anathem', quantum states are used for computations across parallel universes — geometric phases with controlled losses could serve as a 'steering wheel' for navigating between branches of reality.