Session P3b: Timekeeping for Quantum Networking and Other Science Applications

Challenges of Synchronization in Quantum Communication Systems
Christopher Spiess, Pritom Paul, Fabian Steinlechner, Fraunhofer IOF, Friedrich Schiller University
Location: Beacon B

Quantum key distribution allows for a secure means of transferring a key that bases on the physics of single quanta of light. Such quantum communication in its broader sense can have challenging requirements on synchronization as well. While for classical communication the network time protocol or the precision time protocol works greatly thanks to timing precisions down to nanoseconds, timing uncertainties of picoseconds are required in quantum communication.
A common synchronization scheme for quantum communication is to use classical laser pulses to share the frequency reference between the communicating parties. Alternatively, rubidium clocks or GPS references serve as a reliable carrier of the frequency. The timing offset is usually extracted from the arrival time of the single photons themselves. The method has the advantage of compensating for fluctuations in photon runtime. This is an important difference from classical time transfer schemes, where the true status of the clock is transferred by eliminating the impact of the transfer medium through, e.g., two-way time transfer. However, using the single photons as both data and timing carrier is widely accepted due to its simplicity and the reduced number of components that lowers the overall costs and complexity of the quantum network.
The time transfer with single photons becomes difficult when there is not sufficient signal to retrieve the timing status. If the signal threshold is not reached, quantum bits are lost due to timing errors. In this work we assess these limitations and relate it to the requirements of quantum key distribution systems. A good application is long-distance fiber links that generally experience low transmission. Here we show time transfer with single photons between the reference frame of sender and receiver that are separated by 80 km of deployed fiber. We achieve a residual time deviation of less than 1 ps in just 1000 seconds using simple quartz oscillators. Although we use intensities on the single photon level, we reach a stability comparable to the White Rabbit protocol that is based on classical laser light. This result is consistent with other publications that utilize rubidium clocks instead of quartz clocks.
Particularly challenging are quantum communication scenarios on a free space link where atmospheric turbulence leads to strong fluctuations of the link transmission. Synchronization with single photons is even more difficult between moving platforms such as links between aircrafts/satellites and a ground station due to strong signal fades. In classical communication, we can handle it, as the signal is amplified optically. However, any amplification of the quantum signal is a security loophole and not an option. As a result, scenarios involving links between satellites and ground stations often still require the use of classical time transfer methods.
While there are a few application scenarios that present challenging requirements for synchronization schemes, single photons can serve as a reliable source of time for most scenarios. This demonstrates a promising direction towards achieving a self-sufficient quantum communication system.