US Military Wants Smaller and More Stable Atomic Clocks
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The U.S. military wants you … to design a better atomic clock.

The Defense Advanced Research Projects Agency (DARPA), the branch of the U.S. Department of Defense tasked with developing new technologies for the military, recently announced a new program called Atomic Clocks with Enhanced Stability (ACES). The program aims to design an atomic clock that is 1,000 times more stable than current models, which are so precise that they are capable of maintaining perfect time for billions of years, neither gaining nor losing 1 second during that time.

Atomic clocks are used to keep track of time in places where a tiny fraction of a second makes a huge difference. For instance, telecommunications towers employ them to synchronize data packets to within microseconds; if their clocks were off, the bits would pile up like cars in a traffic jam, and calls would get dropped. GPS satellites use them to time the signals that bounce between the satellites and the receivers to pinpoint specific locations. [5 of the Most Precise Clocks Ever Made]

"Every nanosecond you're off, you're out by 3 feet [0.9 meters]," said John Kitching, a group leader at the National Institute of Standards and Technology and an expert on small atomic clocks. "So, if you're out [by a] microsecond, you're off by a mile."

Ordinarily, atomic clocks resynchronize regularly — for example, cellphone towers will check their clocks against those in GPS satellites and adjust for any discrepancies. But they can't do that if the GPS signal gets lost. GPS signals are weak enough that they can be jammed or interfered with, sometimes even inadvertently by a passerby with a cellphone, Kitching said. This could cause a satellite to go offline, either by accident or design. You can even lose a GPS signal by walking into a building or a canyon. (You may have noticed that when you're inside a building, your phone's mapping app is usually using the local Wi-Fi.)

This is one reason the military wants to build more stable clocks — they want ones that stay synchronized even if they are out of contact with GPS systems for extended periods of time.

As part of the ACES program, the Department of Defense wants to have atomic clocks that are small enough to fit inside a wallet and that can run on a quarter of a watt. That second parameter will likely be the bigger challenge, Kitching told Live Science.

"The smallest atomic clocks fit into a deck of cards, but they run on about 10 watts," he said. "That's not much if you're plugging it into a wall, but an ordinary lithium-ion battery will run for about 10 minutes."

Power is such a problem because of the way atomic clocks work, Kitching said. In an atomic clock, the "pendulum" is an atom, usually of an alkali metal like rubidium or cesium. The metal is put into a tiny vacuum chamber, surrounded by a piece of silicon. Then, both are sandwiched between pieces of glass. The metal is warmed up, and some of its atoms separate, forming a vapor.

Then, a laser beam is fired through the metal. Lasers operate at a specific frequency, though they can be tuned up or down a small amount, he added. The laser beam hits the atoms, which vibrate at a specific frequency. Meanwhile, a photodetector picks up the beam as it exits the vacuum chamber. As the laser is tuned, the light starts to match the frequency of the atoms' vibrations, reaching a state called resonance. When it matches up, the photodetector picks up a stronger signal and turns that into an electrical pulse. The pulse goes to an oscillator that feeds back to the laser to keep it precisely tuned. Kitching said. All this takes power to run. [Video: How to Build the Most Accurate Atomic Clocks]

Even the most precise atomic clocks will drift, and the most sophisticated ones in labs like the NIST are operated at extremely low temperatures and are cooled with room-size laser beams. Both of these factors mean it will be challenging to make atomic clocks wallet-size and less power-hungry, said Kitching.

Robert Lutwak, DARPA's program manager for the atomic clock project, agreed that fulfilling all the requirements set out by the agency will not be easy. "NIST has a fairly unique mission — to demonstrate the highest possible accuracy in a laboratory setting. As such, they "pull out all of the stops" to achieve the optimum performance without regards for cost, size, weight, or power, and without need for robust continuous operation over time, temperature, vibration, shock, or other real-world environments," he told Live Science in an email. 

The ACES program will have a budget of up to $50 million and will include three phases, according to DARPA. The teams chosen to take part in the first phase of the program will build their clocks in a laboratory and have to show that the parts operate together as an atomic clock with better stability than existing models. The teams chosen to continue the program will be asked to pack their clocks into a space smaller than 2 cubic inches (33 cubic centimeters). The final stage will involve demonstrating that the atomic clock can fit into a space less than 3 cubic inches (49 cubic cm), along with all the associated electronics.

An earlier DARPA program that lasted from 2000 to 2009 managed to shrink atomic clocks by a factor of 100 and create ones that were stable by a factor of 1 in 10 billion each second (meaning they will drift one second every 317 years). "The goals of the ACES program are to advance these by at least an order of magnitude," Lutwak said.

On Feb. 1, DARPA will host an event to provide additional details about the ACES program.

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