The Adelaide University research team in front of the Australian navy vessel hosting the portable atomic clock for the study. [Image: Adelaide University]
The world’s most accurate timepieces live in cushy environments in standards laboratories. But a close competitor has proven successful at sailing the high seas.
Researchers in Australia put their newest laser-cooled optical atomic clock on an ocean voyage to test its stability and portability (Optica, doi: 10.1364/OPTICA.584095). The device, based on an ultra-narrow transition in ytterbium atoms, demonstrated high accuracy and frequency even after multiple days aboard a moving naval vessel. Such devices could provide precision support to navigation systems without the need for global satellite positioning networks.
Movable atomic clocks
Both civilian and military applications have been awaiting the development of optical clocks that aren’t confined to a stable, stationary lab. Typically, the sedentary timepieces require a complex vacuum system with multiple cooling and trapping lasers and isolation from vibrations and temperature fluctuations. Some earlier efforts at making movable optical clocks took several days to transport and set up in the field.
Andre Luiten and his colleagues at the University of Adelaide had taken their two previous efforts at portable atomic clocks (and a third from a US laboratory) on a New Zealand ship. Their experiences with those devices, especially one that used the ytterbium-174 isotope, informed the current work.
According to Luiten, the previous Yb clock was much simpler because it relied on a relatively narrow atomic transition in the green part of the spectrum, with heated ytterbium-174 vapor inside a sealed glass cell.
Inside the new clock, an oven containing ytterbium-171 atoms sprays the particles into an ultra-low vacuum system. “In this new case, we are using a blue laser to cool and collimate the atoms so that they are traveling in a very narrow beam of slow atoms in that vacuum,” Luiten says. “Then, instead of performing spectroscopy on a relatively narrow atomic transition in the green, we instead perform spectroscopy on an exceedingly narrow atomic transition in the yellow part of the spectrum.” The 1S0 → 3P0 transition, only 10 mHz wide, exists in ytterbium-171 but not in the heavier isotope.
“The previous experiment taught us a lot about how to deploy complex quantum technology in the real world,” Luiten says. “Further, both clocks contained optical frequency combs that were necessary to convert their optical output into a useful electronic signal that can be used in conventional timing systems. Finally, we actually use one of the older green clocks as a key component of the yellow clock to produce the yellow light that is probing the atoms in the new clock.”
A unique timekeeper
According to Luiten, the Adelaide team’s biggest challenge was building every component of the atomic clock from scratch because little off-the-shelf equipment exists for such a project. “We have pushed everything to its limits,” he says.
The performance of the ytterbium-171 optical clock is roughly a factor of 4 times better than the best commercial atomic clocks, though not quite as good as its counterparts at the US National Institute of Standards and Technology (NIST) and other labs around the world. But the team’s goal was to make the clock portable, not set accuracy records. Luiten says his team is now working on a simplified optical clock that will be essentially immune to inertial effects on moving vessels.
