Quantum two-way time transfer over a 250 km direct fiber-optic link

Utilizing energy-time entangled biphotons as the resource, quantum two-way time transfer (Q-TWTT) has shown superior transfer stability far below 1 picosecond level in numerous experimental demonstrations. In this article, we provide further insights into comparing this method with the classical time transfer method based on intensity modulation-direct detection (IM-DD) regarding their signal-to-noise ratio (SNR) performance against backscattering noise in long-distance fiber transmission. The exceptional SNR of Q-TWTT indicates its potential to pioneer new avenues for overcoming the transmission distance limitation in single-span fiber. In light of this, a groundbreaking achievement of Q-TWTT is demonstrated over a remarkable 250 km-long direct fiber-optic link. Despite a total transmission loss of 55 dB, time transfer stability of 4.9 ps at an averaging time of 40 s and 0.6 ps at 10240 s is achieved. This marks a significant advancement in the experimental demonstration of long-haul fiber quantum time transfer, which also offers a highly compatible and potentially quantum secure solution for constructing large-scale quantum networks.