In a water supply system, water sometimes flows not as a continuous stream but in separate identical portions, as if the tap opens and closes, letting through exactly one drop each time. This is roughly how electric current behaves in superconducting contacts called Josephson junctions. There, electrons move in pairs, and under certain conditions the current can increase not smoothly but in jumps — steps.
The authors assembled a chain of 69 such contacts connected in parallel. They irradiated them with microwaves at a frequency of 19.325 billion oscillations per second (19.325 GHz). As a result, the first step appeared on the current-voltage characteristic — the current locked at a value of 426 billionths of an ampere (426 nA). This is like the operation of many tiny pumps: they act in unison and deliver strictly identical portions of liquid. The more pumps, the greater the total flow, but each portion remains unchanged.
Importantly, the quantization accuracy did not degrade as the number of contacts increased. Previously, to obtain a noticeable current, one had to connect many junctions, but errors accumulated. Here, 69 pieces worked like a single well-coordinated orchestra. This solves the problem of current magnitude for a practical quantum standard — a kind of ruler against which all ammeters in the world are calibrated.
So far, the achieved accuracy is insufficient for metrological standards. But the very fact of synchronous operation of many junctions in the regime of quantum phase slippage is a serious step. In the future, such an assembly could become the heart of a new standard, where current would be measured not by heating a wire or the force of a magnet, but by fundamental constants of nature. And then the water-supply analogy would gain a real basis: electric current would be measured by counting identical portions of charge, like drops from a tap.
The value of 426 nA seems tiny, but it is tens of times larger than what could be obtained in a single junction with current quantization.