New Laser Technique Could Redefine Absolute Zero and the Kelvin

The kelvin (K) is a unit of temperature, one of the seven base units, along with the Celsius and Fahrenheit degrees. Absolute zero on the Kelvin scale is defined as being equivalent to zero kelvin (0 K). The magnitude of the kelvin unit is precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water.

Now, a group of French physicists were able to perform the first direct measurement of the Boltzmann constant, using a technique known as laser spectroscopy, whose accuracy could help in creating a new and improved definition of the kelvin unit of temperature.

The Boltzmann constant (k or kB) is the physical constant relating temperature to energy, in fact a bridge between macroscopic and microscopic physics, relating the kinetic energy of an ensemble of microscopic particles, like gas molecules, to its temperature.

Only one technique can, so far, determine the constant to an accuracy of about 2 parts-per-million (ppm), but the new one - currently less accurate, but easily improvable - could surpass the present degree of accuracy.

This promised accuracy is welcomed by the Paris-based International Committee for Weights and Measures (CIPM), which is planning to redefine the kelvin in 2011 using kB. They want to define the kelvin and other SI units in terms of each other and the fundamental constants; more specifically, they want to define the absolute temperature involving a time unit, the second, which is known to an extremely high degree of accuracy of about one part in 1016.

The new alternative way of measuring kB to ppm accuracy, the laser spectroscopy technique, was developed by Christian Chardonnet and colleagues at Universit� Paris 13 - Institut Galil�e, and is based on the fact that the thermal motion of a molecule - ammonia in Chardonnet's experiment - smears out peaks in its optical absorption spectrum in a process called thermal broadening.

This phenomenon is determined by kB, but also by the pressure and temperature of the gas and the frequency of the light being absorbed, so one only needs to measure the width of the broadening as a function of pressure at a fixed temperature and frequency, to determine kB to an accuracy of about two parts in ten thousand.

Although not completely reliable yet, the researchers say this applications could be improved to 1 ppm.