Late August, physicists at the National Institute of Standards and Technology (NIST) in Gaithersburg (MD) published details of their research into an experimental atomic clock of unprecedented accuracy. Had it started to tick 13.8 billion years ago, generally accepted as the moment the universe began, the clock would now be out by just a single second. Its tick is ten times more stable than the best known results for previous atomic clocks. Compared to a quartz watch, the NIST clock is ten billion times more accurate!
Atomic clocks fall into two main categories. The oldest and most common use microwave beams. A caesium atom receives microwave energy and produces one pulse per second. The second category of optical lattice clocks uses lasers to excite the atoms. This is precisely the technology used by the NIST for its clock.
It is a major step in the evolution of next-generation atomic clocks under development worldwide.
A new definition of the second in sight?
The NIST atomic clock relies on some 10,000 atoms of ytterbium, a rare-earth element, which are cooled to slightly above absolute zero (-273.15°C). These atoms are trapped in an optical lattice: a series of pancake-shaped wells made of laser light. Another laser that “ticks” 518 trillion times per second provokes a transition between two energy levels in the atoms to achieve an even greater level of stability than with a caesium atom. In the light of this groundbreaking discovery, the international definition of the second and therefore universal time could well be revised. Currently, the second is defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom.”
“This breakthrough has the potential for significant impacts not only on timekeeping, but also on GPS and a broad range of sensors measuring quantities that have tiny effects on the ticking rate of atomic clocks, including gravity, magnetic fields and temperature,” says Dr Andrew Ludlow, a physicist at NIST and co-author of the research published in the American journal Science. “It is a major step in the evolution of next-generation atomic clocks under development worldwide.”