Ultra-high-precision frequency transmission technology enables optical lattice clock network concept

What is an optical lattice clock network?

An optical lattice clock is a type of atomic clock invented in 2001 by Hidetoshi Katori, then Associate Professor at the University of Tokyo. It uses a laser to create a lattice-shaped container that can hold atoms — an optical lattice. Single strontium atoms are trapped in this lattice and cooled to near absolute zero.
The resonance frequency of the light absorbed by these atoms is then measured to determine the length of one second. The resonance frequency of the many atoms in the lattice can be measured all at once and the mean value determined, allowing fast and accurate measurement of time.
The caesium atomic clock currently used to define the length of a second would be expected to gain or lose less than one second in 30 million years. However, an optical lattice clock achieves a far higher degree of precision, gaining or losing one second over 30 billion years.

The optical lattice clock network concept uses optical lattice clocks stationed across Japan for a next-generation communication infrastructure (Figure 1).

Ultra-high-precision frequency transmission technology underpins the optical lattice clock network

The optical lattice clock network requires technology that can accurately transmit the frequency of the optical lattice clock through the network over long distances. NTT worked with the University of Tokyo and NTT East to conduct a demonstration experiment using ultra-high-precision frequency transmission technology (Figure 2).
In the experiment, NTT East connected the University of Tokyo in Hongō with the NTT Atsugi R&D Center using around 120 km of new fiber-optic cables, as well as connecting Hongō to RIKEN research institute in Wakō using around 30 km of existing cables, creating a fiber-optic network totaling around 150 km in length.
In this experiment to measure transmission precision, we first sent an optical-frequency reference from Wakō to Hongō, and then sent it on to Atsugi using a single fiber cable. Another fiber cable was used to send it back to Hongō, meaning it traveled in a loop totaling around 240 km.

Three relay stations, A, B and C, separated the route into multiple sections. Devices called repeaters (optical frequency relays) were installed at Hongō, Atsugi, relay station A and relay station B (RL1 to RL5).
Fiber-optic cables used for transmission are affected by the expansion and contraction of the actual fibers due to daily changes in temperature, and by vibration caused by the surrounding environment, both of which result in reduced accuracy. The repeaters relay the optical frequency while preventing this loss of accuracy as far as possible. Routing the signal through multiple repeaters allows the frequency to be transmitted across long distances while retaining its precision.
The repeater system in this experiment used planar lightwave circuit (PLC) technology developed by NTT. PLC chips can be manufactured using the same process as LSI, allowing manufacturing to be automated and costs to be reduced. They are also made of the same glass material as fiber-optic cables, meaning they have low signal loss and ensure a high level of stability and detection sensitivity. They can also be made compact, with a module size of 120 × 90 × 10 mm³, including fiber-optic connectors and electrical wiring.

Specifically, the system in the next station returns some of the transmitted light, comparing it with the original light to detect a phase difference, before making the necessary additions to negate the difference and improve transmission precision. Accuracy improves according to the measurement time. In this experiment the error range was less than 3 × 10^-16 at an averaging time of one second, and less than 1 × 10^-18 at 2600 seconds: a high degree of optical frequency transmission precision. This meant we were able to achieve the required accuracy in less than an hour of measurement.

The experiment confirmed that it is possible to transmit the optical lattice clock frequency 200 km or more while preserving its accuracy. This means it is possible to create an optical lattice clock network that connects every prefecture of Japan.

What does the optical lattice clock network provide?

An optical lattice clock network facilitated by ultra-high-precision frequency transmission technology can be used by communications services that utilize time synchronization (Figure 3, top). Specifically, frequency converters can convert optical frequencies from the optical lattice clock network into electrical frequencies and supply them to communications systems such as base stations. This allows more accurate and stable time synchronization than is currently possible.

The system can also be used to measure elevation differences using relativistic effects. Einstein's theory of general relativity describes the phenomenon of time progressing more slowly around massive objects. In the context of the earth, time will pass faster the further you go from the center of the earth — the higher the elevation, the faster time passes.
If we were to use an ultra-high-precision optical lattice clock with a measurement error of 1 × 10^-18 or less, differences in elevation could be measured to the centimeter — better than current global navigation satellite system (GNSS) positioning measurements (Figure 3, bottom).

Future Developments

NTT plans to continue its research on ultra-high-precision frequency transmission, including optical lattice clock frequency comparison experiments over long distances. We also plan to develop repeaters that can be operated stably even when more relay points are included.
In the future, we intend to achieve ultra-high-precision frequency transmission on a scale of 1000 km in order to contribute to the creation of a national network of optical lattice clocks.

References

• Experimental demonstration of ultra-high precision optical frequency transfer via 240-km-long telecommunications fiber
  https://group.ntt/en/newsrelease/2020/03/18/200318a.html