Scalable Timing Coordination of Bell State Analyzers in Quantum Networks

TL;DR

The paper addresses the challenge of scalable timing coordination for optical Bell State Analyzers in multi-hop quantum networks, focusing on achieving simultaneous photon arrival at BSAs for reliable entanglement swapping and introducing the concept of photonic synchronization domains (PSDs). It analyzes memoryless versus memory-assisted architectures, showing that, in memoryless networks, timing adjustments can be confined to the BSA locus and the quantum channel, while quantum memories decouple this timing from inter-link operations to enable hop-by-hop control. It further explores cycles and multiple paths, proposing strategies that keep synchronization localized to links and nodes, and discusses how memory time can expand scalability and resource management. The results provide practical guidelines for constructing scalable, long-distance quantum networks using BSA-based timing control and memory-enabled enhancements. $ ext{Key concepts include }$ BSA-driven synchronization, HOM interference, $\Delta$ for arrival-time mismatch, all-photonic repeater graphs, and optical delay line considerations.

Abstract

The optical Bell State Analyzer (BSA) plays a key role in the optical generation of entanglement in quantum networks. The optical BSA is effective in controlling the timing of arriving photons to achieve interference. It is unclear whether timing synchronization is possible even in multi-hop and complex large-scale networks, and if so, how efficient it is. We investigate the scalability of BSA synchronization mechanisms over multiple hops for quantum networks both with and without memory in each node. We first focus on the exchange of entanglement between two network nodes via a BSA, especially effective methods of optical path coordination in achieving the simultaneous arrival of photons at the BSA. In optical memoryless quantum networks, including repeater graph state networks, we see that the quantum optical path coordination works well, though some possible timing coordination mechanisms have effects that cascade to adjacent links and beyond, some of which was not going to work well of timing coordination. We also discuss the effect of quantum memory, given that end-to-end extension of entangled states through multi-node entanglement exchange is essential for the practical application of quantum networks. Finally, cycles of all-optical links in the network topology are shown to may not be to synchronize, this property should be taken into account when considering synchronization in large networks.

Figures (9)

• Figure 1: An optical delay line (ODL) is a common tool for adjusting the arrival time of single photons or laser pulses in a simple configuration; the slide (center) can be moved under programmatic control to lengthen or shorten the path from one fiber (yellow) to the other. Using multiple such devices in a single photonic synchronization domain is a challenge. The red and blue arrows represent the optical paths of classical control and quantum channels. Details are given in the text.
• Figure 2: Schematic diagram of a quantum network for realizing entanglement exchange between two network nodes using support nodes (BSA)
• Figure 3: A laser pulse (pump light) is fired from the support node, generating Bell pair photons at the network nodes. One photon from each Bell pair at the network nodes is then fired towards the support node, causing entanglement swapping at the BSA. However, naively, a time discrepancy $\Delta$ occurs, so adjusting the path of the laser pulses can reduce $\Delta$ to zero.
• Figure 4: One photon from each Bell pair at the network nodes is directed towards the support node to initiate entanglement swapping at the BSA; the optical path of Bell pair photons is compensatedto to reduce $\Delta$ to zero.
• Figure 5: The BSA actively measures the firing timing. The command for measuring the firing timing is represented by a red dashed arrow. By staggering the timing at which one photon from each Bell pair is fired from the source to the BSA, $\Delta$ is reduced to zero.