An optical clock with neutral strontium (Sr) atoms is considered one of the top candidates for the definition of a “new” second. The probabilities have increased considerably, since its frequency will now be determined more accurately (probably by an order of magnitude). Scientists of the Physikalisch-Technische Bundesanstalt (PTB) have laid the foundation for this by measuring the influence of the most important uncertainty factor, namely the ambient temperature. To date, its influence could only be derived theoretically. Their results, which have been published in the latest issue of the scientific journal Physical Review Letters, might well spark interest in geodesy and in fundamental physical research, specifically in the question as to whether fundamental constants are really constant.
Optical clocks are deemed the clocks of the future because they could allow the International System of Units (SI) base unit, the second, which is already the most accurate of all SI base units, to be even more accurate. Its definition would no longer be based on the interaction of microwave radiation with caesium (Cs) atoms. Instead, it would be based on the interaction of optical radiation with Sr or other atoms or ions. But even before the new definition, optical clocks are useful (e.g., in geodesy where they can help determine Earth’s geoid, which is basically, the exact position of “sea level”) and even more accurate. Furthermore, optical clocks provide fundamental physicists with the potential to detect possible changes in the fundamental constants (e.g., the fine-structure constant).
Optical clocks are so accurate because optical radiation oscillates extremely fast, considerably faster than microwave radiation, which is currently used in Cs atomic clocks to “produce” the second. The faster the “pendulum” (the oscillating system) of a clock is, the finer the interval of time can, in principle, be broken down and, thus, the more stable and accurate the clock becomes. In an optical Sr clock, a cloud of neutral Sr atoms is cooled down in two steps by means of laser radiation until the atoms finally exhibit a speed of only a few centimetres per second. A so-called “optical lattice” ensures that the atoms are trapped and can virtually no longer move.
Unfortunately, Sr atoms react relatively strongly to changes in the ambient temperature; their atomic levels are then shifted energetically, which causes the clock to become inaccurate. This is the highest contribution to the uncertainty of this clock and PTB scientists have now succeeded in measuring it for the first time.
Photo: View of the ultra-high vacuum chamber where strontium atoms are cooled and stored. The blue fluorescent light in the upper third of the window is a cloud of cold strontium atoms (the drop-shaped formation below the blue fluorescent atom beam in the upper part of the vacuum window). (Credit: PTB)