On a table lies a handful of round magnets. As long as no one touches them, they lie in disarray. But send a short current pulse through them, and each disk flips: the north pole swaps with the south. Neighboring disks push each other with their fields, like people in a crowded queue, and the whole handful arranges into a pattern that depends on which way the magnetic wind blew and how warm it is around.

This is exactly how the new computing device works, assembled by physicists from iron disks smaller than a millimeter. The current provides the push, the external field sets the direction, and heat adds randomness: at room temperature, the disks do not always flip identically. By connecting these disks into chains, the authors obtained a circuit that mimics the behavior of the AND logic gate, and from several gates they built a one-bit full adder — a device that adds two bits with a carry, just like in a conventional processor.

The key difference from conventional electronics is reversibility. In a typical chip, the signal goes only one way: knowing the input, you can determine the output, but not vice versa. Here, if you fix the result, the magnetic network itself 'remembers' which inputs could have led to it. This is like a conversation where you can hear not only the answer but also the question.

This ability to move from effect to cause makes probabilistic computers suitable for tasks that require exploring many possibilities — for example, factoring large numbers, which underpins modern encryption. The magnetic implementation outperforms previous attempts in speed, tunability, and energy efficiency: the disks barely heat up, and they can be controlled with weak pulses.

The handful of magnets from the table turned out to be not a toy but a draft of a new type of computation. The first assembled adder is like Edison's first light bulb: dim, short-lived, but showing the way forward.