Femtosecond clock synchronization for ultraprecise satellite formations, radio astronomy and military applications

DARPA has projects for developing clocks that are 10,000 times more precise than the atomic clocks used for the global positioning system.

The Global Positioning System (GPS), the Internet and many defense-critical applications for example—demand exceptionally precise time and frequency standards. Today’s systems, however, rely on 1950s atomic physics technologies. Recent advances in optical atomic systems give promise to a new generation of optical atomic clocks and quantum metrology that stands to transform numerous DoD applications. The Quantum-Assisted Sensing and Readout (QuASAR) program is developing new quantum control and readout techniques to provide a suite of measurement tools that will be broadly applicable across disciplines, with likely applications relating to biological imaging, inertial navigation and robust global positioning systems. Recently the program demonstrated the world’s most accurate clock with a total uncertainty of 2 parts in 10^18 , or about 10,000 times better than GPS clocks. This means that if the clock began ticking at the Big Bang nearly 14 billion years ago it would be accurate to better than one second today.

Clocks of this caliber could lead to improved positioning and navigation, and enable novel imaging and geological sensing techniques.

DARPA’s Ultrafast Laser Science and Engineering (PULSE) program is developing the technological means for engineering improved spectral sources, such as ultra-fast optical lasers—advances that in turn could facilitate more efficient and agile use of the entire electromagnetic spectrum and generate improvements in existing capabilities such as geolocation, navigation, communication, coherent imaging and radar, and perhaps give rise to entirely new spectrum-dependent capabilities. Recent PULSE demonstrations include synchronization of clocks with femtosecond precision across kilometers of turbulent atmosphere, corresponding to a 1,000-fold improvement over what is possible using conventional radio-frequency techniques.

The high coherence and full time synchronization demonstrated here could enable applications from time distribution to long-baseline radio astronomy. If extended to moving platforms by appropriate compensation for Doppler shifts, this technique could similarly enable applications such as precise formation flying of phased satellite arrays.

Optica – Tight real-time synchronization of a microwave clock to an optical clock across a turbulent air path

For greater phase coherence at Fourier frequencies beyond the effective synchronization bandwidth of 100 Hz, the quartz–DRO pair could be replaced by an optical-frequency-divisiongenerated signal or a cryogenic sapphire oscillator.

(a) Conceptual multistatic synthetic aperture radar where an array of microwave oscillators are synchronized to a single master optical oscillator; LO, local oscillator. (b) The master site’s clock is based on a laser stabilized to an optical cavity (optical oscillator). The remote site’s clock is based on a combined quartz oscillator and DRO. This remote microwave clock is tightly synchronized to the optical clock over a folded 4 km long air path via O-TWTFT. The time and the frequency outputs from each clock are compared in a separate measurement to verify femtosecond time offsets and high phase coherence of the synchronized signals

Abstract – Tight real-time synchronization of a microwave clock to an optical clock across a turbulent air path

The ability to distribute the precise time and frequency from an optical clock to remote platforms could enable future precise navigation and sensing systems. Here, we demonstrate tight, real-time synchronization of a remote microwave clock to a master optical clock over a turbulent 4 km open-air path via optical two-way time–frequency transfer. Once synchronized, the 10 GHz frequency signals generated at each site agree to 10^−14 at 1 s and below 10^−17 at 1000 seconds. In addition, the two clock times are synchronized to 13 femtosecond over an 8-hour period. The ability to phase-synchronize 10 GHz signals across platforms supports future distributed coherent sensing, while the ability to time-synchronize multiple microwave-based clocks to a high-performance master optical clock supports future precision navigation / timing systems

SOURCE – Optica journal, Arati Prabhakar Director, Defense Advanced Research Projects Agency (DARPA) Before the Subcommittee on Emerging Threats and Capabilities Armed Services Committee, U.S. Senate

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