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Long-distance fiber link poised to create powerful networks of optical clocks by Staff Writers Washington DC (SPX) Mar 18, 2020
An academic-industrial team in Japan has connected three laboratories in a 100-kilometer region with an optical telecommunications fiber network stable enough to remotely interrogate optical atomic clocks. This type of fiber link is poised to expand the use of these extremely precise timekeepers by creating an infrastructure that could be used in a wide range of applications such as communication and navigation systems. "The laser system used for optical clocks is extremely complex and thus not practical to build at multiple locations," said Tomoya Akatsuka, a member of the research team from telecommunications company Nippon Telegraph and Telephone Corporation (NTT). "With our network scheme, a shared laser would enable an optical clock to operate remote clocks with much simpler laser systems." In The Optical Society (OSA) journal Optics Express, researchers from NTT, the University of Tokyo, RIKEN, and NTT East Corporation (NTT East), all in Japan, report the new low-noise fiber link. "Optical clocks and optical fiber links have reached the stage where they can be put into practical use," said Akatsuka. "Our system is compatible with existing optical communication systems and will help accelerate practical applications. For example, because optical clocks are sensitive to gravitational potential, linked clocks could be used for highly sensitive detection of early signs of earthquakes."
Dealing with noise "Although optical clock networks that simply connect distant clocks have been demonstrated in Europe, our scheme is more challenging because operating remote clocks with the delivered light requires a more stable fiber link," said Akatsuka. "In addition, the country's urban environments tend to contribute more noise to fiber networks in Japan. To cope with that noise, we used a cascaded link that divides a long fiber into shorter spans connected by ultralow-noise laser repeater stations that incorporate planar lightwave circuits (PLCs)." Optical interferometers fabricated on a small PLC chip were key for enabling a fiber link with extremely low noise. These interferometers were used in laser repeater stations that copy the optical phase of the received light to a repeater laser that is sent to a next station with fiber noise compensation. Applying noise compensation for each short span makes the laser signal less susceptible to noise and thus more stable. "Optical interferometers fabricated on a PLC chip have an unprecedented stability and provide a compact, robust and ultralow-noise optical system," said Akatsuka. "This is very advantageous when constructing cascaded fiber links in noisy environments such as those found in Japan."
Connecting the laboratories The laser's 1397-nanometer wavelength is twice that of the laser used to create the most stable type of optical clock known as a strontium optical lattice clock. This means that the fiber network could be used to operate many distant strontium optical lattice clocks via a shared laser. The researchers are now preparing optical lattice clocks to demonstrate a clock network using this fiber link and are working to make electrical components of the system more practical.
Research Report: "Optical frequency distribution using laser repeater stations with planar lightwave circuits"
Pulsar-white dwarf binary system confirms general relativistic frame-dragging Washington DC (SPX) Jan 31, 2020 A century after it was first theorized, researchers have detected the effects of Lense-Thirring precession - an effect of relativistic frame-dragging - in the motion of a distant binary star system, a new study reports. The results of the twenty-year study confirm a prediction of Einstein's general theory of relativity. When a massive object rotates, general relativity predicts that it pulls the surrounding spacetime around with it, a phenomenon known as frame-dragging. This phenomenon cause ... read more
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