Ultracold lab fridge modified for faster, more efficient cooling

By modifying a refrigerator commonly used in both research and industry, researchers at the US National Institute of Standards and Technology (NIST) have drastically reduced the time and energy required to cool materials to within a few degrees above absolute zero. The scientists are now working to commercialise their device with an industrial partner, while their results have been published in the journal Nature Communications.

From stabilising qubits to maintaining the superconducting properties of materials and keeping NASA’s James Webb Space Telescope cool enough to observe the heavens, ultracold refrigeration is essential to the operation of many devices and sensors. For decades, the pulse tube refrigerator (PTR) has been the workhorse device for achieving temperatures as cold as the vacuum of outer space, by cyclically compressing (heating) and expanding (cooling) high-pressure helium gas.

Unfortunately the PTR is rather power-hungry, consuming more electricity than any other component of an ultralow-temperature experiment. Indeed, when NIST researcher Ryan Snodgrass and his colleagues took a closer look at the refrigerator, they found that manufacturers had built the device to be energy efficient only at its final operating temperature of 4 K, or four degrees above absolute zero. These refrigerators are thus extremely inefficient at higher temperatures — a big issue because the cooldown process begins at room temperature.

During a series of experiments, Snodgrass and colleagues discovered that at room temperature, the helium gas was under such high pressure that some of it was shunted through a relief valve instead of being used for cooling. By changing the mechanical connections between the compressor and the refrigerator, the team ensured that none of the helium would be wasted, greatly improving the efficiency of the refrigerator.

In particular, the researchers continually adjusted a series of valves that control the amount of helium gas flowing from the compressor to the refrigerator. They found that if they allowed the valves to have a larger opening at room temperature and then gradually closed them as cooling proceeded, they could reduce the cooldown time to 25–50% of what it is now. Currently, scientists must wait a day or more for new quantum circuits to be cold enough to test.

Since the progress of scientific research can be limited by the time it takes to reach cryogenic temperatures, the faster cooldown provided by this technology could impact many fields, including quantum computing and other areas of quantum research. The technology developed by the NIST team could also allow scientists to replace large pulse tube refrigerators with much smaller ones, which require less supporting infrastructure, Snodgrass said.

The need for these refrigerators is expected to greatly expand as research on quantum computing, along with its reliance on cryogenic technology, continues to grow. The modified PTR would thus result in massive savings, with the scientists saying their prototype could annually save an estimated 27 million watts of power, US$30 million in global electricity consumption and enough cooling water to fill 5000 Olympic swimming pools. In addition to supporting a burgeoning quantum economy, the device would also expedite research because scientists would no longer have to wait days or weeks for qubits and other quantum components to cool.

Image caption: Photograph of the two-stage, dual-inlet PTR studied, which is the same type that is typically used to precool dilution refrigerators. Image has been modified from the original and appears courtesy of the study authors under CC BY 4.0