A New Quantum Secure Time Transfer System

TL;DR

This work addresses the vulnerability of classical clock synchronization to attacks by introducing a quantum secure time transfer (QSTT) protocol embedded in a hybrid quantum-key distribution plus post-quantum cryptography (QKD-PQC) architecture. The approach uses SPDC-entangled photons to generate both timing signals and QKD keys, encrypting the maximum feasible portion of timing data with information-theoretic security via QKD-OTP under a rate constraint $r_1 \le r_2$, while the remaining data are protected by an obfuscated PQC sequence and a Wegman-Carter MAC. Experimentally, the authors demonstrate satellite-like entangled-photon distribution over a short free-space link, implement partition-based scrambling of diff-time-tags, and achieve a sharp cross-correlation peak at zero delay after synchronization, with a measured timing jitter $\sigma \approx 0.69\ \text{ns}$ and a net QKD key rate of $289 \pm 56$ bits/s under QSTT usage (vs $664 \pm 102$ bits/s without QSTT). The results indicate a robust, information-theoretically secure time transfer mechanism suitable for future satellite networks, balancing security and key-rate efficiency. Overall, the paper advances QSTT by integrating quantum key generation, obfuscated post-quantum encryption, and partition-based data scrambling to harden timing data against adversarial attacks.

Abstract

High-precision clock synchronization is essential for a wide range of network-distributed applications. In the quantum space, these applications include communication, sensing, and positioning. However, current synchronization techniques are vulnerable to attacks, such as intercept-resend attacks, spoofing, and delay attacks. Here, we propose and experimentally demonstrate a new quantum secure time transfer (QSTT) system, subsequently used for clock synchronization, that largely negates such attacks. Novel to our system is the optimal use of self-generated quantum keys within the QSTT to information-theoretically secure the maximum amount of timing data; as well as the introduction, within a hybrid quantum/post-quantum architecture, of an information-theoretic secure obfuscated encryption sequence of the remaining timing data. With these enhancements, we argue that our new system represents the most robust implementation of QSTT to date.

Figures (5)

• Figure 1: Quantum signaling for timing and key generation. QKD-generated keys (at rate $r_2$) enable QKD-OTP encryption of a subset of diff-time-tags (provided $r_1 \le r_2$), while the remaining diff-time-tags are protected through obfuscated sequence encryption using multiple post-quantum cryptography solutions.
• Figure 2: The experimental setup with the QSTT protocol embedded within a wider hybrid QKD-PQC system. The optical setup has single photon detectors D1 through D4 at Alice detecting the horizontal ($H$), vertical ($V$), diagonal ($D$), or anti-diagonal ($A$) basis states, respectively (D5 through D8 similarly for Bob); and the post-processing unit, followed by encryption-decryption, MAC authentication of the final encrypted timing data, GPS free clock synchronization, generation of QKD keys. The entangled photon source is external to both Alice and Bob. In many configurations, the source is anticipated to be on board a satellite. The inset at the top right details the use of the three key sets as applied to encryption and authentication of different data (see main text).
• Figure 3: Coincidence counts as a function of a relative delay, $t'$, added to Bob’s time-tag array. Counts are shown with (orange) and without (blue) application of the synchronization protocol. For each $t'$, coincidences are counted only when Alice’s and Bob’s time-tags match within a 0.5 ns window. The data is accumulated over $40~$seconds.
• Figure 4: Net QKD key rate and QBER across $10$ QKD sessions with and without the QSTT protocol. The QKD key rate decreases under a secure synchronization protocol due to key consumption in QSTT, while the QBER remains stable. This trade-off highlights the balance between synchronization security and key generation efficiency in QKD systems. Since the key rates shown assume the asymptotic limit of data sampling, they are upper limits.
• Figure 5: Example illustration of the partitioning and shuffling process for one segment of $\boldsymbol{\Delta T_A}$. For this example, $n=16$ samples are divided into $2^{k}=4$ partitions (each of size 4), and then the partitions are permuted according to $\rho=(3,1,4,2)$. The left column shows the $\boldsymbol{\Delta T_A=}\{\Delta t_{1},\dots,\Delta t_{16}\}$, the centre column shows the four contiguous partitions before shuffling, and the right column shows the final reordered sequence after applying the permutation $\rho$.