In situ quantum verification of polarization-stabilized optical channels

TL;DR

This work integrates ancilla-assisted process tomography (AAPT) with classical polarization tracking to validate deployed quantum channels in real time. By reconstructing the full CPTP quantum map $\mathcal{E}_{\mathcal{Q}}$ via Bayesian AAPT and comparing it to a unitary reference from the classical tracker, the method reveals nonunitary noise outside the tracker’s reach and tracks channel fluctuations with a sliding-window approach. The experiments on a three-node quantum local-area network show that in situ AAPT provides meaningful quantum validation alongside traditional tracking, with potential for enhanced multiplexing and faster tomography through brighter sources and parallel processing. The approach offers a practical tool for quantum-classical coexistence networks, enabling diagnostic insight into quantum channel performance under realistic deployment conditions.

Abstract

The active stabilization of polarization channels is a task of growing importance as quantum networks move to deployed demonstrations over existing fiber infrastructure. However, the uniquely strict requirements for high-fidelity qubit transmission complicate the extent to which classical solutions may apply to future quantum networks, particularly in terms of recognizing noise sources present in low-flux, nonunitary channels. Here we introduce a novel in situ benchmarking approach that augments a classical polarization tracking system, limited to unitary correction, with simultaneously transmitted quantum light for ancilla-assisted process tomography of the full quantum map. Implemented in a local-area quantum network, our method uses the reconstructed map both to validate the classical compensation and to expose noise sources it fails to capture. A sliding measurement window that continuously updates the estimated quantum process further increases sensitivity to rapid channel fluctuations. Our results should unlock new opportunities for in situ channel characterization in quantum-classical coexistence networks.

Figures (4)

• Figure 1: (a) Fiber lightpaths between labs in the subbasement of the MSEE building at Purdue University. (b) Schematic of the Alice--Bob link (duplicated for Alice--Charlie) including the parallel fiber link for classical communications. WR: White Rabbit; SNSPD: superconducting nanowire single-photon detector; SPAD: single-photon avalanche diode; PA: polarization analyzer; DWDM: dense wavelength division multiplexing filter; ATT: attenuator; PC: manual polarization controller; EPC: electronic polarization controller; PS: pulse shaper; BS: fiber beamsplitter; PBS: fiber polarizing beamsplitter.
• Figure 2: State fidelity measured at various integration times with absolute values of the "Laser ON" Bayesian mean density matrices at 10 s integration. The orange line corresponds to the classical tracking sources being on during measurement with the teal representing the classical source being off. The ancilla is measured locally without a multiplexed channel.
• Figure 3: Quantum link verification between Alice and Bob with exemplar Choi matrices. The teal curves show the reference fidelity $\mathcal{F}_\mathcal{R}$ measured from the classical tracking system. The orange symbols with uncertainty bars show the fidelity $\mathcal{F}_\mathcal{Q}$ measured through segmented AAPT, in which each tomography is computed from 16 distinct measurements; the width of each symbol corresponds to the total measurement time (163 s). The blue lines show AAPT computed via a sliding 16-projector window advanced in 10 s steps across the acquisition stream. The Choi matrix magnitudes $|\Phi|$ correspond to the times marked by the dashed black lines, where the segmented AAPT Choi result is either that immediately to the right or intersected by the relevant vertical line. The measured loss of the link is 0.38 dB.
• Figure 4: Quantum link verification between Alice and Charlie with highlighted Choi matrices. The teal lines show the fidelity $\mathcal{F}_\mathcal{R}$ measured from the classical polarization references. The orange symbols with uncertainty bars show the fidelity $\mathcal{F}_\mathcal{Q}$ measured through AAPT with the length corresponding to the total measurement time (163 seconds). The blue lines show AAPT computed via a sliding 16-projector window advanced in 10 s steps across the acquisition stream. Example Choi matrices correspond to the times marked by the dashed black line, with the segmented AAPT result denoting the symbol immediately to the right or in the middle of the line. Here the measured loss of the link is 4.02 dB.