A coin in your hand always lands either heads or tails: that's how the familiar world works. But a cloud of 370 ultracold atoms can behave differently: it can freeze into a state akin to a coin that is both heads and tails at the same time. A new study shows how to test this experimentally.
The idea rests on comparing two scenarios. In one, the quantum scenario, the system can 'tunnel' between two states, like a ball that passes through a hill rather than rolling over it. In the other (the classical one, like a coin) there are no transitions between states; the system simply freezes. The authors calculated that with a slow change of the external field, the quantum system will make errors less and less often, while the classical one will maintain a constant error rate. The difference between them is huge and measurable.
To test this, you need a setup with a Bose–Einstein condensate — a gas cooled to nearly absolute zero. Calculations show that with currently achievable parameters, the effect survives even with noise: the safety margin is 8.8 times larger than the minimum required. The key secret is the system's symmetry, which suppresses the main source of disturbance.
Such an experiment would serve as a bridge between a world where a coin always lands on one side and a world where it can hover in midair, choosing neither.
A cloud of 370 atoms at a temperature of billionths of a degree above absolute zero is like all the cars in a city simultaneously freezing at a traffic light, yet each one knowing it is both moving and standing still.