

Scientists have built a clock which is 37 times more precise than the existing international standard.

The quantum-logic clock, which detects the energy state of a single aluminum ion, keeps time to within a second every 3.7 billion years. The new timekeeper could one day improve GPS or detect the slowing of time predicted by Einstein's theory of general relativity.

"It could it be a real contender for the next frequency standard, or next timekeeper," said physicist Chin-wen (James) Chou of the National Institute of Standards and Technology in Boulder, lead author of a study to appear in a forthcoming Physical Review Letters.

Chou's team is one of several racing to build an atomic clock that can replace the current international standard, the cesium fountain clock. The cesium clock loses one second every 100 million years. Chou's is not the first quantum-logic clock, but his uses aluminum and magnesium ions, which makes it twice as precise as its predecessors that used aluminum and beryllium.

To keep time, quantum-logic clocks measure the vibration frequency of UV lasers. Unfortunately, the best lasers we can build veer off their normal frequency by about one tick every hour, Chou said. To keep the laser's timekeeping precise, its vibration must be anchored to something much more stable.

That anchor is the vibration of an electrically charged aluminum atom, which vibrates at 1.1 Petahertz, or 1.1 quadrillion times a second.

The first step in measuring the ion's vibration is to hit it with UV lasers, which are tuned to the charged atom's rate of vibration. The aluminum ion can be in either a low- or high-quantum energy state.

"If the laser frequency is right on the ion frequency, then the ion will change state, but if the laser frequency is off a little bit, then the ion doesn't change state as efficiently," Chou said. "This efficiency is a signal that tells us, this signal is off by so much, and we should steer the frequency so it stays on the frequency of the aluminum ion."

But they can't tune the laser frequency to the aluminum ion state unless they can actually detect that state. To do that, the group couples the aluminum ion to a magnesium ion. A separate set of laser beams shine on the pair. If the aluminum ion changes state, then both ions start to move.

Detecting that motion requires a third set of lasers to focus on the magnesium ion. If the magnesium ion is in motion, it emits a photon of light. Otherwise, it stays dark.

"That’s the beauty of it, we can see just one ion emitting light," Chou said.

In a weird twist, the team can't actually tell how many times the clock ticks per second. That's because the definition of a second is currently based on the cesium fountain clock, which simply can't measure the precision of a more precise machine. It works using a similar principle as the aluminum clock, but uses the vibration of a cesium atom to anchor the frequency of a microwave source.

The clock could help resolve questions about the universal physical constants, such as the speed of light in a vacuum, or Planck's constant, an important value in quantum physics.

Physical constants are supposedly fixed over time, but some theories suggest they may vary slightly, he said. "Optical clocks are one of the candidates that might be able to see that really tiny variation over time," he said.

Global positioning devices also rely on extremely precise atomic clocks, so "if we have better and better clocks then we can tell our position, to a better and better precision," Chou said.

And the clocks could also show the effects of general relativity by detecting how much gravity warps time.

There's no plan to adopt the aluminum-ion clock as the formal international standard yet. To do so, the clock ticks need be transmitted around the world. That is normally done with optical cables, but those can only transmit such a stable frequency for around 60 miles, Chou said.

Image: Chou with the quantum clock, J. Burrus/NIST

Citation: C.-W. Chou, D.B. Hume, J.C.J. Koelemeij, D.J. Wineland, and T. Rosenband. C.-W. Chou, D.B. Hume, J.C.J. Koelemeij, D.J. Wineland, and T. Rosenband. 2010. Frequency Comparison of Two High-Accuracy Al+ Optical Clocks. Physical Review Letters. *

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