Polarization-Controlled Quantum Interference in a Metro-Scale Fiber Network

Abstract

We report the first demonstration of multi-photon, dual-state entanglement distribution over a metropolitan-scale commercial fiber network, implemented on the EPB-IonQ Bohr-IV quantum network in Chattanooga, TN, using an all-fiber-optic experimental platform. Employing a spatially degenerate, continuous-wave type-II SPDC bi-photon source and fully fiber-coupled linear optics, we generated a 4-photon entangled state. Through polarization projective measurements on two locally retained photons, we then probabilistically heralded the remaining two photons into either a Bell state (particle-particle entanglement) or a N00N state (mode-mode entanglement), which were then distributed to two spatially separated network nodes. Experimental verification confirmed successful entanglement distribution across the deployed network despite significant channel losses and limited source fidelity. These results highlight the versatility of our polarization-controlled multi-photon entanglement distribution over real-world telecom infrastructure and lay the groundwork for future upgrades, including higher-quality non-degenerate photon sources, White Rabbit timing synchronization for true multi-node entanglement, and active polarization control for enhanced fidelity and long-term stability.

Figures (8)

• Figure 1: The EPB-IonQ Bohr-IV quantum network architecture.
• Figure 2: A bird’s-eye view of the EPB-IonQ Bohr-IV quantum network architecture in downtown Chattanooga is shown in (a), with the segment utilized in this work highlighted in (b). Since Equipment Hubs A and B are located on Broad Street and Tenth Street, respectively, we label them as BQN and TQN. The deployed fiber loops between UTC-TQN and UTC-BQN are approximately 5 km and 10 km in length respectively.
• Figure 3: Characterization of the fully fiber-coupled spatially-degenerate bi-photon source through (a) HOM interference and (b) singlet Bell state verification, with the corresponding experimental results, represented by solid green and pink circles, shown in (c) and (d), respectively. Photons are detected using SNSPDs, and two-photon coincidences are recorded with a coincidence window of 8 ns. The solid pink line in (c) is a Gaussian fit, while the solid green and pink lines in (d) are theoretical curves for the two-photon coincidence probabilities $P_{HH}$ and $P_{HV}$, respectively.
• Figure 4: Characterization of the relative photon arrival times across the network. (a) Relative arrival times of the 4 photons, with one photon sent to TQN and another one to BQN, while the rest two are retained at UTC. (b)&(c) Zoomed-in views of the temporal correlation measurements between UTC–TQN and UTC–BQN, respectively. The relative delay between the two photons retained locally at UTC, represented by the purple line, is too small to be resolved in (a).
• Figure 5: (a) Experimental layout for the 4-photon fusion operation locally at UTC. (b) Experimental layout for the 4-photon fusion operation across the network. The solid green circles in (c)&(d) are the experimental results for (a)&(b) respectively. A dip in the 4-photon coincidence counts indicates successful Bell-pair fusion. (e) Verification of the presence of the 4-photon entangled state on the network after fusion, performed via 4-photon polarization projective measurements. The solid pink lines in (c)&(d) are Gaussian fits.