Electronics Weekly
The work has been done at JILA – a joint venture laboratory of NIST and the University of Colorado Boulder – and the Technical University of Vienna, a team which has made three major nuclear clock announcements in the last year.
“A solid-state nuclear clock has a great potential to become a robust and portable timing device that is highly precise,” said ,” said JILA project lead Professor Jun Ye. “We are searching for the parameter space for a compact nuclear clock to maintain 10-18 fractional frequency stability for continuous operation.”
A thorium isotope, 229mTh (also ‘thorium-229m’) is the favoured atom because, due to a quirk of nature, it has nuclear resonances in the ultra-violet – within the reach of lasers – rather than at x-ray or gamma ray frequencies.
The JILA-Vienna team has been looking for the transition with the lowest temperature coefficient, cooling the atom to -123°C with liquid nitrogen, to -44°C with a dry ice and methanol, and comparing those results with measurements at 20°C.
Two effects were revealed:
As the crystal warmed, it expanded, altering the atomic lattice, and shifting electric field gradients at the nuclei. This gradient caused the thorium transition to split into multiple spectral lines, which shifted in different directions as the temperature changed.
At the same time, lattice expansion changed the charge density of electrons in the crystal, making the nuclei’s spectral lines move in the same direction.
For one particular resonance, these effects almost cancelled, and its frequency shifted only 62kHz across the measurement range, “at least 30 times smaller than in the other transitions”, according to JILA.
“This transition is behaving in a way that’s really promising for clock applications,” added JILA researcher Chuankun Zhang. “If we can stabilize it further, it could be a real game-changer in precision timekeeping.”
The next step is to find the point in this temperature range that has the lowest temperature sensitivity slope to ease stabilisation.
For more, read ‘Temperature Sensitivity of a Thorium-229 Solid-State Nuclear Clock‘ in Physical Review Letters.
