Temperature gets a new definition with a quantum device

A better and more reliable definition of temperature could come from a quantum device full of giant atoms.

While some countries measure temperatures in degrees Celsius and others use Fahrenheit, physicists everywhere use a unit called the kelvin. Zero kelvin marks the absolute coldest temperature allowed by the known laws of physics, so the kelvin is said to measure “absolute temperature”. But in practice, making sure that when you measure one kelvin, it’s actually one kelvin, is a laborious process.

“If you want to make an absolute temperature measurement, you buy a commercial temperature sensor that has been calibrated by another commercial temperature sensor that has been calibrated by another commercial temperature sensor and so on. And one of those sensors was sent to the National Standards Institute at some point.” [and Technology],” he says Noah Schlossberger at NIST in Colorado.

He and his colleagues have now built a device that uses quantum effects to measure kelvins, which researchers could use instead of having someone else calibrate their sensors.

The device is a small box made of metal and glass containing trapped rubidium atoms. Researchers push these atoms to extreme size, using lasers to move the outermost electrons unusually far from the nucleus, and to extreme temperatures, using lasers and electromagnetic fields to trap and cool the atoms to about half a millikelvin, at temperatures roughly 600,000 from room temperature.

As a result, the outermost electrons in rubidium atoms become extremely sensitive to even a small increase in temperature, “jumping” into another quantum state when exposed to one. These jumps are what make the device a great temperature sensor, as there are well-established mathematical models that can determine the temperature differences needed to make them – effectively allowing Kelvin to be redefined in those terms.

The International Bureau of Weights and Measures defines the kelvin in a similar way—as the product of several quantum constants—but in practice even institutions like NIST use non-quantum devices for calibration. The hope is that the new device will provide a quantum definition of kelvin where calibration would not be needed.

“Every rubidium atom in the world is exactly the same, and they will behave exactly the same in the same environment. I can rebuild the device on the other side of the world and it will be exactly the same,” says Schlossberger. He says this is particularly important for keeping high-precision devices like atomic clocks, which can only work at very low Kelvin temperatures, function properly.

But the new device is still a prototype, so it still has imperfections in things like how quantum states are detected. It is also too bulky to leave the lab and took more than six months to build. Scientists are now working to optimize its design to make it more practical and to increase its accuracy.

Schlossberger presented the work on March 16 in American Physical Society Global Physics Summit in Colorado.

topics: