US Military Wants Smaller and More Stable Atomic Clocks

                                

             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. 

Ordinarily, atomic clocks resynchronize regularly — for example, cellphone towers will check their clocks against those in GPS satellitesand 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.