In the last year, researchers climbed Mauna Loa volcano in Hawai’i and directed a laser towards a reflector placed on Haleakala peak in Maui. They successfully transmitted rapid pulses of laser light across 150 km through turbulent air.
Despite the faintness of the pulses, this achievement fulfilled a longstanding aspiration of physicists, which encompasses the ability to transmit highly accurate time signals through the atmosphere over long distances, utilising power levels that align with the requirements of future space-based missions.
Scientists, including those from NIST, have made significant progress in transferring time information to geosynchronous satellites 36,000 kilometres away. This method achieves femtosecond (a unit of time for a short duration often used to measure fast processes) precision, surpassing current satellite techniques by 10,000. It also ensures synchronisation resilience in atmospheric disturbances, even with minimal timing signal strength.
Enhanced coordination among remote devices opens intriguing possibilities. State-of-the-art optical atomic clocks offer exceptional precision, but comparing clocks across continents requires a communication method that can convey such accuracy over long distances. The researchers need to improve microwave-based techniques in this regard.
However, a new approach using geosynchronous satellites allows connections between optical clocks worldwide without limitations. This advancement supports redefining the International System of Units (SI) second based on a visual standard and enables fundamental physics measurements like exploring dark matter and general testing relativity.
The ability to synchronise widely separated sensors using optical atomic clocks opens up new possibilities, such as advancing very long baseline interferometry (VLBI) for improved black hole imaging. It distributed coherent sensing, which is unprecedented, would require real optical clock connections.
The team’s research demonstrated that the time-programmable frequency comb, an innovation in frequency comb technology, enabled the transmission and reception of high-frequency time signals. This novel frequency comb played a crucial role in achieving these results.
“We envision utilising sensor arrays to observe space and earth. The successful implementation of these arrays relies on the connection of precise optical clocks, and the obtained results suggest that they now possess the necessary tool for this purpose,” said Laura Sinclair, a NIST physicist.
The team experimented using their innovative frequency comb and a reflector positioned on two mountains 150 km apart to showcase the signal’s ability to reach a satellite without being lost in transit. By sending the light from the time programmable frequency comb to one mountain and receiving the reflection from the other, they demonstrated that the signal could penetrate through challenging atmospheric conditions, indicating its potential to overcome similar obstacles on its way to geosynchronous orbit.
The round trip was successful, achieving synchronisation even at the minimum signal strength required, known as the “quantum limit.” Previous research demonstrated that the time-programmable frequency comb could operate at this limit, where less than one photon in a billion reaches the target device. Remarkably, it functioned effectively with a laser power of only 40 microwatts, approximately 30 times less than a typical laser pointer.
“We aimed to test the system’s capabilities and have demonstrated that it can maintain a high level of performance with realistic transmission power and aperture size for future satellite systems,” Sinclair explained. “The system’s ability to operate effectively, even when receiving less than a billionth of the transmitted light and in the presence of rapidly changing light loss, is promising for establishing the timing infrastructure of future sensing networks.”
In the future, the NIST team will focus on reducing the system’s size, weight, and power requirements while adapting it for mobile platforms.
