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Haomin's Commentary: Introduction to the Clock Synchronization Features of the Washington DC Metropolitan Quantum Network (DC-QNet)
2025-04-18
Original link: Appl. Phys. Lett. 125, 164004 (2024), RESEARCH ARTICLE | OCTOBER 15 2024, https://doi.org/10.1063/5.0225082
Original title: Clock synchronization characterization of the Washington DC metropolitan quantum network (DC-QNet)
▎Research Team Composition
Telecommunications Science Laboratory (LTS), National Institute of Standards and Technology (NIST) Information Technology Laboratory, U.S. Naval Observatory, U.S. Naval Research Laboratory (NRL), NIST Physical Measurement Laboratory, Joint Quantum Institute at the University of Maryland, Tes Research Company in La Jolla, California, Department of Physics at the University of Maryland, NIST Communications Technology Laboratory, NASA Goddard Space Flight Center, U.S. Army Combat Capabilities Development Command Army Research Laboratory (ARL), Department of Electrical and Computer Engineering at the University of Illinois at Chicago, Computing Physics Company in Springfield, Virginia, Institute for Research in Electronics and Applied Physics (IREAP) at the University of Maryland.
▎Article Abstract
In the quantum network protocol system, quantum interference measurement and high-precision flight time determination play important roles in quantum detection, both highly dependent on high-precision clock synchronization. The Washington DC metropolitan (DC-QNet) quantum network research test platform conducted in-depth studies on high-precision clock synchronization, comparing and analyzing the design architecture, implementation paths, and performance characteristics of the White Rabbit Precision Time Protocol (WR-PPT) and the Active Electronics Stabilization (ELSTAB) optical two-way time-frequency transfer (OTWTFT) methods. Experimental data show that the active electronics stabilization method achieved sub-picosecond time deviation (TDEV) over a 53 km fiber link within an integration time range of 1 second to 10⁵ seconds; while the White Rabbit Precision Time Protocol achieved 10 picosecond-level TDEV over a 128 km transmission distance within the same integration period. This paper details the sources of environmental fluctuations during clock synchronization and proposes on-site compensation strategies. The study found that factors such as temperature-induced propagation delay drift, dispersion effects, polarization state drift, and optical power fluctuations are key causes of clock synchronization errors. This work marks a critical step toward the practical application of metropolitan quantum networks, and the continued development of related compensation methods will accelerate the transition of the quantum internet from laboratory validation to engineering applications.
▎Main Text Introduction
The strategic value of quantum networks lies in three core areas: theoretically absolutely secure quantum key distribution, distributed quantum sensing and computing systems, and attack-resistant secure clock synchronization networks. High-precision time-frequency synchronization technology is a fundamental element supporting quantum networking, playing an important role in key processes such as indistinguishability in entanglement distribution, quantum entangled state distribution, and quantum entanglement swapping. At the current stage, high-precision clock synchronization mainly supports the construction of point-to-point quantum communication systems; in the future, as network scale expands, the demand for high-precision time synchronization protocols will dynamically change. Core requirements include seamless integration with existing telecom optical networks, co-transmission of quantum and classical optical signals on the same fiber, scalable network architectures, better environmental robustness, and enhanced security mechanisms to resist quantum attacks targeting time synchronization channels.
Since the single-photon pulse duration in ultrafast lasers varies from nanoseconds to femtoseconds, the preliminary goal of this study is to achieve a time deviation (TDEV) on the order of 1E-11 seconds at 1 second integration time to meet the needs of long-distance node entangled photon transmission. This can be realized through optical two-way time-frequency transfer (OTWTFT), mainly via White Rabbit Precision Time Protocol (WR-PTP) and Active Electronics Stabilization (ELSTAB) based OTWTFT.
Figure 1. (a) Active Electronics Stabilization (ELSTAB) and White Rabbit Precision Time Protocol (WR-PTP) time and frequency transfer methods are deployed as two independent networks in DC-QNet. E0 to E2 indicate the positions of ELSTAB modules. W0 to W5 indicate the positions of WR-PTP switches. In testing, WR-PTP communication is conducted via optical switches (link connected to a point on the star) or directly connected at the hub (link starting from a point on the pentagon); (b) Fractional frequency instability model of atomic clocks in DC-QNet and optical two-way time and frequency transfer (OTWTFT) methods, including measurement noise ranges of WR-PTP and ELSTAB devices.
As shown in Figure 1 (a), to accelerate research on quantum network metrology and protocols, Washington DC has built a metropolitan quantum network (DC-QNet) with 7 sites connected by underground and aerial fiber links. Figure 1 (b) shows the frequency instability of atomic clocks in DC-QNet using two OTWTFT methods. The White Rabbit switches (WRS) are configured in a star topology, with a central reference clock (W3) as the master clock synchronizing multiple WRS [Figure 1 (a)]. Signals between WRS are connected via two different fibers in a "single-fiber unidirectional" manner, with laser wavelengths unified at 1539.77 nm and 1541.35 nm for the round trip. Additionally, to enable co-transmission of quantum and communication signals, the research team adopted a "single-fiber bidirectional" coarse wavelength division multiplexing (CWDM) architecture to transmit communication signals, with return signals at 1270 nm and 1290 nm (O-band).
The research team used time-correlated single photon counting (TCSPC) modules to measure the phase difference between the 10 MHz outputs of two White Rabbit nodes. After subtracting the initial (<1 ns) offset between the two clocks, the peak-to-peak phase difference between the two WRS is less than 200 ps. Figure 2 (a) shows the maximum time interval error (MTIE) of the measured peak-to-peak time error, with MTIE less than 25 ps at an averaging time of 10 s. As shown in Figure 2 (b), the overlapping Allan deviation (OADEV) is below 1E-11 over averaging times from 1 to 10⁵ seconds. Figure 2 (c) shows that at an averaging time of 10 s, the lower limit of time deviation for the underground link is below 1E-12 s. Due to aerial fiber links being exposed to air and experiencing rapid temperature gradients, their round-trip time delays exhibit significant instability and fluctuations.
The path length between W3 and W4 is 64 km, of which 61 km is aerial fiber link. Figure 2 (d) shows the path delay measured by the arrival timestamp method for W3-W4, with an average path delay of 0.32 milliseconds, an average path delay change rate dT/dL of 1.3 ps/s, and a maximum path delay change rate of 21 ps/s. Figure 2 (f) shows the average one-way path delay of the 53 km underground fiber link between W2 and W3, approximately 0.25 milliseconds. In contrast, the underground fiber link has an average path delay change rate dT/dL of 0.2 ps/s and a maximum path delay change rate of 3.5 ps/s.
WR-PTP can be used to measure the delay of fiber optic links to adjust transmission, thereby coordinating the arrival times of quantum and classical messages. By time-aligning the one-way path delay measurements of WRS at W3 and W4, the delay between two adjacent fibers is maintained within a few hundred picoseconds. For adjacent fibers, WR-PTP can be used as a probe signal because these fibers are usually affected by the same delay variations. Results can be improved by reducing the path delay normalization window. Figure 2(g) shows the one-way path delay difference with a normalization window of less than 10 seconds.
Figure 2. (a) Maximum Time Interval Error (MTIE); (b) OADEV and (c) DC-QNet WR-PTP deployed time deviation TDEV. Clock errors are measured by the reference clock server (WRS); (d) One-way path delay of the W4-W3 link [see Figure 1(a)] and air temperature measured at the weather station. The shaded area indicates cloud coverage; (e) Uncompensated round-trip path delay of the W2-W3 link over four days (solid blue line) and temperature measured at the weather station near W0 (dashed orange line); (f) One-way path delay of the W4-W3 link (solid blue line) and its rate of change (dashed orange line); (g) Path delay difference between two adjacent fibers in the same cable; (f) and (g) both show measurements from the W4-W3 overhead fiber optic link.
▎ Commentary
In summary, the research team deployed and tested two optical time transfer methods—Electrical Stabilization (ELSTAB) and White Rabbit-PTP (WR-PTP)—in the Washington metropolitan quantum network (DC-QNet), analyzing their synchronization performance over long-distance fiber links and the impact of environmental noise. The ELSTAB method achieved sub-picosecond level TDEV (1.5E−12 @1 second) on a 53 km underground link, with long-term stability reaching the 1E-15 level. The WR-PTP method achieved TDEV at the 10 ps level on a 128 km link, suitable for synchronizing distributed quantum network nodes. Underground fiber links perform better than overhead links due to smaller temperature fluctuations, but overhead fiber links can still meet quantum protocol requirements through real-time compensation. When quantum signals (C-band) and classical signals (O-band) are co-transmitted in the same fiber, backward scattering noise must be suppressed through filtering.
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