National Physical Laboratory (NPL)

Are the constants constant?


Source: National Physical Laboratory (NPL)

Scientists at the National Physical Laboratory (NPL) and at Physikalisch-Technische Bundesanstalt (PTB) in Germany have been able to improve the constraints on time-variation of fundamental constants by making measurements of two optical clock transitions in the same atom, ytterbium.

Fundamental constants of nature are the pillars of modern physics, and measurement scientists are in the process of redefining the standard units of measurement, the SI units, to be directly related to these fundamental constants. Recent developments in the measurement of time and frequency are allowing scientists to test these constants and see if they do, indeed, live up to their name.

The current definition of the second is based on a microwave frequency in caesium atoms. New types of atomic clock operate at optical frequencies using laser light, rather than microwaves, and have been demonstrated to have much improved levels of stability and accuracy over microwave clocks. NPL is developing optical atomic clocks using a number of different atoms and ions, one of which is the ytterbium ion.

The tick rate of an atomic clock is what is known as a 'transition frequency'. This is the frequency of electromagnetic radiation absorbed or released as electrons orbiting the atom move between two different energy states. The ytterbium ion is presently unique in that it has two of these transitions that are being used as optical frequency standards.

These two transitions also have very different sensitivities to variation in the fine structure constant, which relates to the strength of the electromagnetic force. If the fine structure constant were changing over time, as has been proposed by some cosmologists and astrophysicists, one of these transition frequencies would get smaller and one would get larger. This means that by repeatedly measuring the frequencies, we can test the theory that the fine structure constant is changing over time.

Now, NPL has made measurements of the optical clock transitions against the SI second, along with the first ever measurement of the ratio between the two optical clock transitions in the same atom. Combining these results with data from PTB's ytterbium ion optical clock, as well as a wide variety of other optical clocks worldwide, has allowed a new limit to be placed on present-day time-variation of the fine structure constant at -0.7 (2.1) × 10-17 year-1. This means that we can rule out changes at the level of 2 parts in
100 000 000 000 000 000 per year. These measurements have also resulted in a near-threefold improvement in the constraint on the present-day time-variation of the proton-to-electron mass ratio at 0.2 (1.1) × 10-16 year-1.

As well as testing the laws of physics, this new measurement is vital for a future redefinition of the second. In addition, the improvements in atomic clocks and frequency measurement also have practical applications including improved navigation, timing and synchronisation.

The results have been published this week in Physical Review Letters and covered in Physics Viewpoint:

Frequency Ratio of Two Optical Clock Transitions in 171Yb+ and Constraints on the Time Variation of Fundamental Constants
R. M. Godun, P. B. R. Nisbet-Jones, J. M. Jones, S. A. King, L. A. M. Johnson, H. S. Margolis, K. Szymaniec, S. N. Lea, K. Bongs, and P. Gill
Phys. Rev. Lett., 113, 210801 (2014)

This work was funded by the Department for Business, Innovation and Skills (BIS), as part of the National Measurement System electromagnetics and time programme and also by the European Metrology Research Programme (EMRP), and the European Space Agency.

Find out more about Fundamental constants and tests of physical theories

Find out more about Optical Frequency Standards

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