DARPA’s Quantum-Assisted Sensing and Readout (QuASAR) team built two optical lattice clocks that use ultracold ytterbium atoms to measure the passage of time. Much like the ticking of a pocket watch, the ytterbium clocks tick off seconds by measuring the frequency of light absorbed by atoms as electrons in the ground state jump to an excited state. Each of the clocks relies on approximately 10,000 rare-earth ytterbium atoms cooled to ten millionths of a degree above absolute zero and trapped in an optical lattice made of laser light. Another laser provides the resonant energy necessary for the atoms to cycle between two energy levels a rate of 518 trillion times per second.
How can the Department of Defense take advantage of clocks that are precise to one second in a period comparable to the age of the universe? Currently, the QuASAR work is exploring the limits of just how precisely humans can measure time, but it could also practically impact high-performance timing applications such as GPS position and time dissemination. The QuASAR clocks offer performance that is 10,000 times better than the current atomic clocks used to support GPS satellites. This extreme stability could vastly extend the time between clock updates and may obviate attempts by an adversary to spoof GPS signals. Such clock precision could also enable new, more precise methods to measure gravity, magnetic fields and temperature.
Researchers from the National Institute of Standards and Technology (NIST), with funding from DARPA’s Quantum-Assisted Sensing and Readout (QuASAR) program, have built a pair of ytterbium atomic clocks that measure time with a precision that is approximately ten times better than the world’s previous best clocks, also developed under QuASAR. How good are they? The record-setting clocks are stable to within less than two parts per quintillion (1 followed by 18 zeros). They measure time so precisely that their readout would be equivalent to specifying the Earth’s diameter to less than the width of a single atom or the age of the known universe to less than one second.
The clocks’ precision performance raises a technical issue: it exceeds DoD’s ability to transfer time among devices for purposes of synchronization. It’s not currently feasible to embed atomic clocks in all of the devices that could benefit from ultraprecise timekeeping, so DoD uses various technologies to broadcast time from several master clocks. DARPA’s PULSE program, a companion program to QuASAR, is exploring how laser technology can be improved to facilitate optical time transfer among devices so that they can benefit from the improved timekeeping.
Defense applications, such as geo-location, navigation, communication, coherent imaging and radar, depend on the generation and transmission of stable, agile electromagnetic radiation. Improved radiation sources—for example, lower noise microwaves or higher flux x-rays—could enhance existing capabilities and enable entirely new technologies.
The Program in Ultrafast Laser Science and Engineering (PULSE) seeks the technological means for such improved radiation sources. Through precise spectral engineering in the optical domain, more efficient and agile use may be made of the entire electromagnetic spectrum. By generating and engineering waves in the optical domain, where engineers already exercise exquisite stability and control, these waveforms may be down or up-converted to the desired wavelength.
PULSE will also aim to apply this technology to enable synchronization, metrology and communications applications spanning the electromagnetic spectrum, from radio frequencies to x-rays. By building on established ultrafast laser techniques, PULSE seeks to:
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