PORTLAND, Ore. -- Quantum clocks were recently harnessed to outperform current atomic clocks by 10 times at the National Institute of Standards and Technology (NIST). By coupling the quantum states of trapped ions (electrically charged atoms), their natural vibration frequencies were synchronized to 17 digits of accuracy-the most ever measured. Based on aluminum and mercury ions, the new clocks stay accurate to within 1 second every billion years, compared with 80 million years for the current atomic-clock standard at NIST.
According to NIST, such ultra-accurate clocks are useful for synchronizing telecommunications networks, space navigation, satellite positioning and deep-space communications, and could enable new types of gravity sensors for exploring underground natural resources here on Earth.
Physicists also hope to use the new clocks to plumb cosmological mysteries, such as whether the ultimate yardstick in nature--the fine-structure constant (a dimensionless quantity that characterizes the strength of electromagnetic interactions)--is changing over time. It was necessary to design two new clocks--one from aluminum ions and the other from mercury ions--because one clock was needed to test the accuracy of the other. After one year of constant monitoring, the mercury-based clock was about 20 percent more accurate than the aluminum-based clock. However, the aluminum-ion quantum clock is the more desirable, since aluminum is immune to magnetic and thermal perturbations from the environment.
"For over 20 years, we thought that aluminum clocks would be better than the cesium atoms we use today, but we were unable to build an optical clock based on aluminum," said Till Rosenband, the NIST architect of the new quantum clocks. "The innovation that enabled our success was to couple the quantum states of two different ions, then use the second ion to synchronize our laser."
Today's atomic clocks use optics to generate ultraprecise clock "ticks" with a laser that first cools cesium atoms by slowing down their thermal motion (so that only their natural vibration frequency is left), then synchronizes laser pulses with that frequency. Unfortunately, although aluminum atoms can be cooled with a laser, it is impossible to synchronize a laser with an aluminum atom's vibration. The solution, discovered by Rosenband's team at NIST, was to couple the quantum states of the aluminum ion with a nearby beryllium and synchronize a laser to its vibration frequency, thereby achieving an optical clock with the accuracy of the aluminum ion's natural vibrational modes.
"Because aluminum and beryllium atoms are electrically charged, we were able to couple their movements, essentially transferring the quantum state of the aluminum ion to the beryllium ion, then synchronize our laser with the beryllium ion to achieve the accuracy of aluminum through the two ions' shared motion," said Rosenband. The ultrahigh accuracy of the new aluminum quantum clock, and its immunity to magnetic fields and thermal fluctuations, could also enable physicists to answer the question of whether the fundamental constants in the universe, such as the fine-structure constant, are changing over time.
Physicists have proposed that the fine-structure constant is changing, as a solution to several cosmological puzzles. After a year of testing, however, NIST has determined that the fine-structure constant is not changing by any more than 1.6 quadrillionth (a millionth of a billionth) of 1 percent per year, with an uncertainty of 2.3 quadrillionth of 1 percent per year. It could be changing more slowly than that, but the NIST scientists have concluded that it is constant, or more precisely, "consistent, with no change."
Funding for the project was provided by the Office of Naval Research, the Disruptive Technology Office and other sources.