Scientists have measured temperatures as low as 4.9 kelvins — about 268 degrees Celsius below zero — by tracking the motion of hydrogen molecules with light instead of using a conventional thermometer.
Physicists at Nicolaus Copernicus University in Toruń used the technique to measure hydrogen between 4.9 and 7.8 kelvins.
Unlike conventional thermometers, the method does not need to be calibrated against another thermometer. It therefore acts as a primary thermometer, with the temperature determined directly from fundamental physical constants.
The same experiment allowed the researchers to calculate the gas pressure and test predictions of quantum electrodynamics to the tenth significant digit.
Temperatures around five kelvins are important for research into superconductors, quantum sensors and liquefied gases, but they are particularly difficult to measure accurately.
At these temperatures, conventional thermometers can disturb the system being measured. Their readings also depend on calibration against increasingly precise standards, creating a chain of comparisons that ultimately limits how directly the temperature can be known.
The Toruń team instead used the hydrogen molecules themselves.
Molecules moving towards a beam of light absorb a slightly different frequency from those moving away from it. This Doppler effect broadens a spectral line.
The faster the molecules move, the broader the line becomes. By measuring that width, the researchers could determine the temperature from the molecular motion.
The calculation relies on fundamental constants including the mass of the hydrogen molecule, the transition frequency, the speed of light and the Boltzmann constant.
The frequency was referenced using an optical frequency comb to UTC time at the Astrogeodynamic Observatory in Borowiec. This allowed the temperature to be determined without first comparing it with another thermometer.
Making the measurement work at just a few kelvins was the main experimental challenge.
The hydrogen spectral line is extremely weak, while a significant proportion of the gas freezes as the temperature falls. The researchers placed the hydrogen inside a 69-centimetre cavity between highly reflective mirrors.
Light circulated inside the cavity for an average of 26 microseconds, travelling a total distance of about eight kilometres through the gas. The long optical path amplified the tiny absorption signal enough to make it measurable.
The entire cavity also had to be cooled uniformly. The mirrors, copper housing and mechanism controlling its length all had to remain at almost the same temperature.
Even a warmer section of the chamber could have changed the distribution of molecular velocities and distorted the measurement. The experimental setup also reduced cryostat vibrations, while temperature fluctuations were kept below 0.001 kelvin.
The technique produced measurements between 4.9 and 7.8 kelvins. Above six kelvins, the uncertainty was less than 0.02 kelvin. At 4.9 kelvins, it rose to 0.18 kelvin as the hydrogen vapour pressure and optical signal fell sharply.
The results were broadly consistent with measurements from a commercial diode sensor.
The spectrum also revealed other physical properties. By determining the number of hydrogen molecules per unit volume, the researchers calculated the gas pressure and reconstructed part of the hydrogen phase diagram across more than three orders of magnitude in pressure.
The position of the spectral line also provided a test of quantum electrodynamics, with the results agreeing with theoretical predictions to the tenth significant digit.
The study was carried out by a 16-member team from the Institute of Physics at Nicolaus Copernicus University in Toruń and published in Nature Physics. (PAP)
kmp/ bar/
tr. RL
