NIST boffins shrink atomic beam clocks to the size of a postage stamp

There’s a new atomic clock on the block at the US National Institute of Standards and Technology (NIST), and while it’s not as accurate as its predecessors, it has one big advantage: it’s small enough to fit in your pocket.

The chip-scale beam clock (CSBC) is small – only about the size of a postage stamp – and it relies on previously established technologies for measuring time through atoms developed at NIST.

Now wait, you might be thinking: NIST has had tiny atomic clocks since the early 2000s when it developed the chip-scale atomic clock or CSAC. That’s true, and this little device takes its inspiration from CSACs. What makes this one different is the process it uses to measure atomic resonance frequencies: atomic beams, like the giant, stationary atomic clocks that have long been used as the US standard for measuring time.

“CSAC is low-power and high-performance for its size. It’s an impressive device, but it drifts after running for a few thousand seconds,” said NIST physicist and CSBC paper co-author William McGee. CSACs tend to lose more time when the gas surrounding the atoms in their tiny chambers changes temperature. That’s a frequent concern given CSACs are often used in applications such as underwater oil and gas exploration, military navigation and telecommunications, all of which may see CSACs exposed to a variety of conditions.

The CSBC experiment, McGehee added, was about figuring out whether combining CSACs and beam clocks was even possible, and it appears it was.

A small beam scheme

Atomic beam clocks – the most recent being NIST-7, the US timekeeping standard from 1993 to 1999 – have often been replaced by more accurate, and larger, atomic fountain clocks.

Lest you think them obsolete, beam clocks are still useful for measuring time, with modern models only expected to experience a one-second drift every million years. Recent fountain clock designs have achieved a one-second drift within 100 million years – hyper-accurate, but not nearly as necessary in situations where smaller is better.

The microwave cavities in the beam clock must be very large, so they are large – the NIST-7 is more than eight feet long – but the microfabrication techniques used to build the CSACs have opened up the possibility of building tiny beam channels just 100 micrometers wide and 10mm long developed. in a device made from stacked layers of etched silicone and glass.

Rubidium is used as the source material, which is heated to excite the atoms and release them into the channels. To avoid errors from the gas molecules in the chamber, the team incorporated materials called non-evaporable getters, or NEGs, which can collect the gases as well as pull in the rubidium atoms. Small graphite rods are also included to collect stray atoms that might affect the measurements.

Despite its innovative design and the fact that it has been shown to work, the experimental CSBC is still not that accurate, and performs “at a level slightly worse than existing CSACs,” NIST said. The team still thinks it has a path forward, though.

“The presented beam clock approach has the potential to outperform existing chip-scale atomic clocks in both long-term stability and accuracy,” McGehee and his team wrote in their paper. Doing so would require eliminating the common causes of drift found in 15 months (41.45 Scaramuccis or 10.4 Trusses, for those who keep counting by The Reg units) CSBC ran for testing purposes.

While noting that the device has operated for more than a year “without degradation of the vacuum environment or saturation of the passive pumps,” the team noted that Doppler and Zeema are changing, end-to-end phase changes. of the cavity, collisional shifts and light shifts, among others, all contributed to the CSBC drift during the tests.

Future tests, the team said, will try to improve stability by incorporating micro-optical and thermal packaging that will increase the size of the standard CSAC (which NIST notes is about the size of a piece of sushi ) but hopefully make it more resistant to changes that affected its test performance.

If successful, the team believes their design could be used for quantum sensing, inertial sensing using atom interferometry, electrometry and the construction of “higher performance compact clocks using optical transitions,” the researchers predict. .

“Taking into account each of the common sources of drift … an ultimate fractional frequency stability at or below the level of [a CSAC] appears to be feasible with a chip-scale atomic beam clock,” they conclude. ®