Instead of an ice bath, a warm gas blanket. That is how the cryogenic current comparator at Japan's National Metrology Institute is now cooled. Previously, this instrument, which compares electric currents to within billionths of a percent, required immersion in liquid helium. Today, it operates inside a closed-cycle refrigerator, where it is cooled not by a liquid but by a gas.
The challenge was that a refrigerator without liquid helium generates vibrations. These vibrations disrupt precise measurements, much as shaky hands hinder a surgeon. The engineers found a solution: they placed the instrument in a sealed chamber and filled it with low-pressure gaseous helium. The gas transfers cold from the refrigerator walls to the instrument, but it does not transmit vibrations as strongly as a solid body.
The result surprised even the authors themselves. At a pressure of about 20 pascals, the measurement noise matched that of liquid helium. For the quantum resistance standard, an uncertainty of 1 nanoohm per ohm was achieved in just 100 seconds of averaging. It is as if the new scale delivered the same accuracy as the old apothecary's balance, but without the fragile glass bath.
Now the system consumes less than a liter of gaseous helium per cycle. That is equivalent to one milliliter of liquid helium: hundreds of times less than before. Given the global helium shortage, such savings are critical. Since September 2025, the instrument has been used for calibration services, and in the future, similar setups could operate in any laboratory without relying on supplies of liquid helium.
The helium blanket turned out to be just as good as the ice bath. And crucially, it allows quantum standards to step out of cryogenic bunkers into ordinary laboratories.
Gaseous helium at 4.2 K remains a gas at pressures below 100 kPa, so it can be used as a heat-exchange medium without the risk of condensation. A consumption of less than 1 liter of gas per cycle is equivalent to just 1 milliliter of liquid helium, which is hundreds of times less than traditional cryostats.
In Arkady and Boris Strugatsky's novel 'Monday Starts on Saturday', the magical institute NIICHAVO faces the problem of resource shortages for experiments. Here, science solves the helium shortage with an elegant engineering solution, reminiscent of how characters in science fiction circumvent environmental constraints.