One Seriously Accurate


Working for the National Institute of Standards and Technology (NIST), Andrew Ludlow is fascinated by clocks, but not the kind we’re used to. Atomic clocks have continued to make incredible strides and he’s part of a NIST group that shows the continued evolution of this device, recently creating the most accurate one to date. In fact, his clock is ten times more accurate than the previous clock and 10 billion times more stable over time than your standard wristwatch, according to a paper published by his team in the journal Science.

But, first, why is an atomic clock so important?

“Experimentally, it’s only been possible recently to build a clock by taking atoms and manipulating them,” he says. “If we can improve the clock, it can help in navigating performance in areas of synchronization, like radio telescopy. Changes in timekeeping can result in a clock being in different gravitational fields and measuring small special temporal variations—that could be interesting for geodesic measures. We could possibly measure the gravitational potential of earth with sub-centimeter precision.”

The technique of lasers enters the picture for cooling atoms. “They’re coming from incredibly hot temperatures and you’re cooking to millions of degree Kelvin,” he says. “The best cesium clocks today all use some form of laser cooling on atoms that are hot and moving around to measure to their ticking.”

NIST's ultra-stable ytterbium lattice atomic clock. Image: Burrus/NIST



As described in NIST’s Tech Beat: “Each of NIST’s ytterbium clocks relies on about 10,000 rare-earth atoms cooled to 10 microkelvin [10 millionths of a degree above absolute zero] and trapped in an optical lattice—a series of pancake-shaped wells made of laser light. Another laser that ‘ticks’ 518 trillion times per second provokes a transition between two energy levels in the atoms. The large number of atoms is key to the clocks' high stability.”

Ludlow adds that they’re able to manipulate lasers in ways that make them extremely stable in frequency. “Our ability in one particular laser system allowed us to achieve these results and on some level, for almost any optical clock, it requires very advanced control of laser systems.”

Tech Beat also notes: “Along the way, NIST scientists have made several improvements to both clocks, including the development of an ultra-low-noise laser used to excite the atoms, and the discovery of a method to cancel disruptive effects caused by collisions between atoms.”

But Ludlow says, regardless of any recent success, it’s important to note that it has all been part of decades of work by many, with current clocks being benchmarked using older clocks. “But it’s true that there have been leaps in how clocks would perform by using optical,” he says. “One of the things that made this measurement possible where demonstrated was a new level of stability. By having two or more atomic clocks, it’s a huge advantage. It’s not even simple to put together one but how else can you measure how precise a clock is if you don’t have another to compare it to?”

Ludlow traces his own path to atomic clocks to the university level. “Growing up, I was that kid who read Scientific American but it wasn’t until undergrad that this path started. I went to BYU as a chemistry major, took a couple of physics classes and enjoyed it enough to switch majors. A young physics professor named Scott Ferguson was an atomic physicist and his primary research was trying to study and explore ultra-cold plasma and cooling atoms.”

Ludlow ended up volunteering in his lab and it was through that experience that he was first exposed to some of the general principles related to atomic clocks. “I started appreciating the possibilities and that led to deciding to do my graduate studies in atomic physics. From that point on, I found atomic clocks and latched onto it.”

His Ph.D. research at JILA, a joint institute of the NIST and the University of Colorado-Boulder, focused on exploring different experiments in atomic physics based on strontium—and one of the applications is it can be used for atomic clocks. “I helped develop a strontium-based optical clock, finished up my Ph.D., and came to NIST. I’m hoping our present work with atomic clocks continues to progress.”

Eric Butterman is an independent writer.

Growing up, I was that kid who read Scientific American but it wasn’t until undergrad that this path started.

Andrew Ludlow, NIST


October 2013

by Eric Butterman,