Two-way quantum time transfer: a method for daytime space-Earth links
Randy Lafler, Mark Eickhoff, Scott C. Newey, Yamil Nieves, Kurt E. Stoltenberg, J. Frank Camacho, Mark A. Harris, Denis W. Oesch, A. Lewis, R. Nicholas Lanning
High-precision remote clock synchronization is crucial for many classical and quantum network applications. Evaluating options for space-Earth links, we find that traditional solutions may not produce the desired synchronization for low Earth orbits and unnecessarily complicate quantum networking architectures. Demonstrating an alternative, we use commercial off-the-shelf quantum photon sources and detection equipment to synchronize two remote clocks across our free-space testbed utilizing a method called two-way quantum time transfer (QTT). We reach picosecond-scale timing precision under very lossy and noisy channel conditions representative of daytime space-Earth links and software-emulated satellite motion. This work demonstrates how QTT is potentially relevant for daytime space-Earth quantum networking and/or providing high-precision timing in GPS-denied environments.
High-precision remote clock synchronization is crucial for many classical and quantum network applications. Evaluating options for space-Earth links, we find that traditional solutions may not produce the desired synchronization for low Earth orbits and unnecessarily complicate quantum networking architectures. Demonstrating an alternative, we use commercial off-the-shelf quantum photon sources and detection equipment to synchronize two remote clocks across our free-space testbed utilizing a method called two-way quantum time transfer (QTT). We reach picosecond-scale timing precision under very lossy and noisy channel conditions representative of daytime space-Earth links and software-emulated satellite motion. This work demonstrates how QTT is potentially relevant for daytime space-Earth quantum networking and/or providing high-precision timing in GPS-denied environments.
Precise clock synchronization is important for quantum networking, enhanced position, navigation, timing, and other applications that require clock synchronization better than GPS, but remains an ongoing challenge. The authors propose a solution using hardware native to quantum networking. This quantum-time-transfer technique can quickly achieve picosecond-level clock synchronization despite relatively low-performance quantum-photon sources and detection equipment. Furthermore, it is robust against the high loss and high noise channel conditions representative of daytime space-Earth links, and could provide high-precision secure timing in GPS-denied environments.
Accurate timekeeping is essential for scientific and technological advancements, particularly in areas of communication, networking, navigation, and high precision measurements. While many methods of time resolution are already established using classical resources, they fail to combine high precision outcomes with large-scale implementations. Additionally, numerous quantum networking architectures using satellite-assisted methods have demonstrated quantum communication on scales exceeding ground-based methods. For these reasons, we propose the use of a time synchronization method by which pairs of highly time-correlated photons are exchanged between clocks on satellites and clocks on Earth, al- lowing users to reconstruct the time offsets between their clocks. This is known as Quantum Clock Synchronization (QCS). We analyze global time synchronization, via numerical simulation, using a network of satellites equipped with quantum resources, such as entangled photon sources (SPDC) and single photon detectors. We find that for satellites in low-earth orbits, a configuration of 50 evenly-space satellites provides constant visibility and sub-nanosecond time synchronization to any location on Earth. Due to practical constraints, clocks available for use on satellites are often of lesser quality than those available for ground use. However, by allowing the satellites in our network to synchronize among themselves, we overcome this limitation, forming a highly precise “master clock” capable of global, high precision time synchronization. We then examine the amount of quantum security provided by polarization-entangled photon sources as a potential way to protect an experimental implementation of QCS from attacks such as signal spoofing. To do this, we identify regions on Earth where Bell violations between exchanged entangled photons can ensure quantum security at various confidence vii levels. Combined with similar insights gained from determining the network scale of our QCS protocol at the single-link level, we demonstrate the potential for highly precise, global, and potentially quantum-secure clock synchronization. Finally, we modify our QCS protocol to support frequency, or tick-rate, offsets in addition to constant time offsets. We motivate this by presenting some relativistic effects that can introduce frequency differences between clocks in space and on Earth. Provided that relative velocities between clocks are corrected for, we identify constant and variable clock defects through numerical simulations.
We experimentally demonstrate an entanglement-based Quantum-Secure Time Transfer (QSTT) system in an emulated low Earth orbit satellite-to-ground channel using a type-0 Sagnac-based entangled-photon source. Our new QSTT system delivers a finite Quantum Key Distribution (QKD) key rate of approximately 3bitss−1 with a 10−10 security parameter, providing a 30-fold increase in the QKD key rate compared to the state-of-the-art entanglement-based QKD system delivered by the Micius satellite. Beyond this high key rate outcome, novel to our QSTT system is a GPS-free clock synchronization, optimized use of QKD, embedded post-quantum security, and obfuscation of the system configuration via use of a pre-shared key. Collectively, these enhancements deliver the most efficient and secure deployment of QSTT to date, and point the way forward to high-accuracy ultra-secure time transfer in space.
A satellite-based scheme to perform clock synchronization between ground stations spread across the globe using quantum resources was proposed in [Phys. Rev. A 107, 022615 (2023).] based on a quantum clock synchronization (QCS) protocol. Such a scheme could achieve synchronization up to the picosecond level over distances of thousands of kilometers. Nonetheless, the implementation of this QCS protocol has yet to be demonstrated experimentally in situations where the satellite velocities cannot be neglected, as is the case in many realistic scenarios. In this work, we develop analytical and numerical tools to study the effect of the relative velocity between the satellite and ground stations on the success of the QCS protocol. We conclude that the protocol can still run successfully if the data-acquisition window is chosen appropriately. As a demonstration, we simulate the synchronization outcomes for cities across the continental United States using a single satellite in a low earth orbit, low-cost entanglement sources, portable atomic clocks, and avalanche detectors. We conclude that, after including the effect of relative motion, subnanosecond- to picosecond-level precision can still be achieved over distance scales of $\ensuremath{\approx}4000$ km. Such high-precision synchronization is currently not achievable over long distances ($\ensuremath{\gtrsim}100\phantom{\rule{4.pt}{0ex}}\text{km}$) with standard classical techniques including the GPS. The simulation tools developed in this work are in principle applicable to other means of synchronizing clocks using entangled photons, which are expected to form the basis of future quantum networks like the quantum internet, distributed quantum sensing, and the quantum GPS.
The Global Positioning System (GPS) provides uninterrupted position and timing data across the globe with a precision of up to 40 ns, but is insufficient for advanced applications such as quantum communication, and is susceptible to jamming and spoofing attacks. This article introduces a protocol for synchronizing clocks using a constellation of satellites that relies on the exchange of entangled photons. Simulation shows that 50 low-Earth-orbit satellites bearing off-the-shelf atomic clocks can distribute time globally with a precision that is 2 to 4 orders of magnitude higher than that of GPS. Additionally, the use of entangled photons provides an extra layer of quantum security.
Precision synchronization is vital for robust long-distance quantum networking over fiber and free-space channels for which high-fidelity entanglement swapping between separate sources via an optical Bell state measurement requires temporal overlap of photonic qubits arriving from either source. This challenge is particularly distinct in satellite-based entanglement distribution in which relative motion, channel effects, and propagation delay must be addressed. This work presents a precision synchronization method for free space entanglement distribution, and reports on risk reduction testing in a quantum networking testbed at MIT Lincoln Laboratory. Primary consideration is for a dual-uplink architecture in which photons from entanglement sources at two ground locations interact in an optical Bell-state measurement implemented on a satellite in a low-earth orbit. The control approach uses independent entanglement sources at each ground location supplemented with a synchronization signal for feedback control from a timing discriminant measured at the spacecraft. The approach is being implemented in a laboratory testbed using 1-GHz repetition rate 1550-nm band entanglement sources generating ~10-MHz source entanglement rates with few-ps photon pulse lengths. The paper describes both fundamental architectural considerations and practical implementation details.