High-rate Scalable Entanglement Swapping Between Remote Entanglement Sources on Deployed New York City Fibers

Abstract

Entanglement swapping between photon pairs generated at physically separated nodes over telecommunication fiber infrastructure is an essential step towards the quantum internet, enabling applications such as quantum repeaters, blind quantum computing, distributed quantum computing, and distributed quantum sensing. However, successful networked entanglement swapping relies on generating indistinguishable pairs of photons and preserving them over deployed fibers. This has limited most previous demonstrations to laboratory settings or relied on sophisticated methods to maintain the necessary indistinguishability. Here, we demonstrate a scalable entanglement swapping experiment using naturally indistinguishable entanglement sources based on warm atomic vapor cells. Without sharing lasers or optical frequency references between nodes, nor the need for pulsing the sources, we achieve a swapping rate of nearly 500 pairs/s while maintaining the CHSH parameter above 2. Additionally, we demonstrate the scalability of our method by maintaining the quality of the entanglement swapping on 17.6-km of deployed fibers in NYC, relying on commercially available SPADs at the spoke nodes, SNSPDs at the hub and standard time-synchronization techniques. Our work paves the way for the practical deployment of large-scale hub-and-spoke quantum networks within cities and data centers.

Figures (5)

• Figure 1: Experimental layout of a three-node entanglement swapping experiment. Nodes S1 and S2 host the independent entanglement sources while H is where the entanglement swapping happens. Qu-LSR: Qunnect's Dual laser systems for the bi-chromatic sources. SRC: Atomic vapor cell sources. HWP: Half waveplate. SPAD: Single photon avalanche detector. TDC: Time-to-digital converter (time tagging device). Qu-APC: Qunnect's automated polarization compensating devices (INJ: Injector. CMP: Compensator). FBS: Fiber beamsplitter. FPBS: Fiber polarization beamsplitter . All three nodes are controlled by Cisco’s centralized Quantum Network Orchestrator.
• Figure 2: Path of fibers for network experiments. Inset shows hardware used at the three locations. WR: White Rabbit, TDC: time tagging device, SPAD: single photon avalanche detector, SNSPD: superconducting nanowire single photon detector, Qu-LSR: Qunnect dual‑laser pump system, SRC: entanglement source, CHSH: apparatus for performing CHSH type measurements, SWAP: Bell state measurement optics for swapping, Qu-APC: Qunnect's automated polarization compensating device.
• Figure 3: Experimental results for the "local" swapping experiment. a) and b) show interference fringes as the HWP at S1 and S2 are rotated. From the data we extract Bell's S parameter and the swapping rate. We do this many times, changing the region of interest used for analysis to obtain the plot in c) of the S parameter as a function of the swapping rate.
• Figure 4: Single spoke entanglement distribution stability results. a) shows the measured pair rate and b) shows the entanglment fidelity, parametrized by the Bell's S parameter, as a function of time.
• Figure 5: Experimental results for the network swapping experiment. a) and b) show interference fringes as the HWP at S1 and S2 are rotated. From the data we extract Bell's S parameter and the swapping rate. We do this many times, changing the region of interest used for analysis to obtain the plot in c) of the S parameter as a function of the swapping rate.