National Institute of Standards and Technology (NIST) physicists have set a new world record for atomic clock stability using a pair of ytterbium-based timepieces. Stable down to quintillionths of a second difference between one tick of the clock and the next, the new system may prove invaluable in many areas of science, such as determining just how constant the fundamental universal constants really are, and helping in the search for dark matter.
We highlighted the accuracy capabilities of ytterbium a few years ago, when the NIST team work on a cesium-clock rival was in its infancy. At the time it was still lacking in stability, but now – with some judicious fiddling in the intermediate years, particularly around a tiny, but important deformation in the laser frequency that synchronizes with the ytterbium atoms – the new NIST design has really come into its own.
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"We eliminated a critical type of noise in the clock's operation, effectively making the clock signal stronger," said NIST physicist Andrew Ludlow. "This means we can reach a clock instability of 1.5 parts in a quintillion (1 followed by 18 zeros) in just a few thousand seconds. While this only slightly beats the record level of clock stability we demonstrated a few years ago, we get there 10 times faster."
Whilst the new ytterbium lattice "double clock" is the most stable clock in the world, another NIST atomic clock, based on strontium, is still the most precise timepiece ever made. The difference between the two is that where the strontium device is super-accurate in how near its time interval measurements are in relation to the frequency at which the atoms naturally oscillate between two energy states, the ytterbium units are much more capable of keeping accurate stability between each transition interval when that energy level changes.
In detail, as the ytterbium and strontium atomic clocks operate at frequencies high up on the light spectrum, a laser pulse is timed to resonate with the frequency of the transition between two energy levels in the atoms. The subsequent on/off measurement is then used as an atomic "tick" in the laser pulse to provide a timebase for the clock. However, any noise that results in a disturbance to the frequency of the pulse upsets the overall timekeeping precision.
This is because the ytterbium units compensate for certain laser frequency fluctuations that may not be captured or observed in between time intervals. In other words, the gaps between the laser pulses used to measure the atoms energy oscillations at specific frequencies (and, therefore, from one time increment to the next) means that there may be unwanted noise generated in this "dead time" between measurements that can upset the stability of the whole process.
To avoid this, the new ytterbium dual-clock system has zero dead time (and, as such, is dubbed the ZDT clock) and a correspondingly infinitesimal amount of dead-time noise, because it is able to continuously measure the energy states of atoms by constantly switching between two groups of ytterbium atoms contained in an optical lattice generated by one laser and probed by another.
Using one group of 5,000 atoms and another of 10,000, the device measures the reactions of both sets of atoms in response to a single laser switching continuously between them, and combines the results to produce a single accurate correction of the laser frequency resonance. And, because these measurements and adjustments take place at twice the speed of a single atomic clock, the new NIST unit reaches stability levels 10 times faster than previously possible. In fact, the performance is now so great that the researchers believe that further improvements to accuracy and stability will no longer be sought in laser systems of such devices, but in the atomic system of the clock itself.
According to the scientists, this new way of producing ultra-stable clocks with dual-atomic measurement atoms may eventually result in reducing the size of atomic clocks while making them less complex by making both atomic systems share a single vacuum unit and simplifying the laser systems – the result being that such a device could be made small enough to be portable beyond the laboratory.
The scientists believe that an advantage in such an accurate, but relatively small atomic clock could be realized in the possibility of distributing such units all around the Earth for relativistic geodesy (determining the shape of the Earth using gravity-based measurements) or launched into space for experiments on general relativity, to check just how constant the fundamental constants of nature really are, and to aid in the search for dark matter.
The results of this research were recently published in the journal Nature Photonics.