© 2026 Aqora Quantum S.A.S.

Terms Privacy Legal Notice

arXiv:2510.26034 · Physical Review Applied

In situ quantum verification of polarization-stabilized optical channels

Matthew L. Stevens, Noah I. Wasserbeck, Zachary Goisman, Arefur Rahman, John Michael Record, T. K. Truong, Ariq Haqq, Muneer Alshowkan, Brian T. Kirby, Nils T. Otterstrom, Joseph M. Lukens

Published Mar 2, 2026• 1 citation

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.

DOI: 10.1103/63wh-4x54

Related papers

In situ quantum verification of polarization-stabilized optical channels

Matthew L. Stevens, Noah I. Wasserbeck, Zachary Goisman +8 more

We demonstrate in situ benchmarking of classical polarization stabilization in deployed fiber-optic channels via coexistent ancilla-assisted process tomography, leveraging sliding windows to enhance temporal sensitivity to environmental fluctuations.

•Jan 1, 2026•0 citations

Coexistent Quantum Channel Characterization Using Spectrally Resolved Bayesian Quantum Process Tomography

Joseph C. Chapman, Joseph M. Lukens, Muneer Alshowkan +3 more

The coexistence of quantum and classical signals over the same optical fiber with minimal degradation of the transmitted quantum information is critical for operating large-scale quantum networks over the existing communications infrastructure. Here, we systematically characterize the quantum channel that results from simultaneously distributing approximate single-photon polarization-encoded qubits and classical light of varying intensities through fiber-optic channels of up to 15 km. Using spectrally resolved quantum process tomography with a Bayesian reconstruction method that we develop, we estimate the full quantum channel from experimental photon counting data, both with and without classical background. Furthermore, although we find the exact channel description to be a weak function of the pump polarization, we nevertheless show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel when the noise is dominated by Raman scattering. These results provide a basis for the future development of quantum repeater designs and quantum error-correcting codes for real-world channels and inform models used in the analysis and simulation of quantum networks.

arXiv:2208.14514•Physical Review Applied•Apr 10, 2023•24 citations

Deployed quantum link characterization via Bayesian ancilla-assisted process tomography

Arefur Rahman, Noah I. Wasserbeck, Zachary Goisman +5 more

The development of large-scale quantum networks requires reliable quantum channels, the quality of which can be quantified by the framework of quantum process tomography. In this work, we leverage ancilla-assisted process tomography (AAPT) and Bayesian inference to probe a 1.6 km deployed fiber-optic link. We send one of the two polarization-entangled photons at Alice in one building to Bob in another, exploiting the local qubit as an ancilla system to characterize the corresponding quantum channel. Monitoring over a 24 h period returns a steady process fidelity of 97.6(1)%, while controllable spectral filtering with passbands from 0.025 to 4.38 THz finds fidelities that first increase, and then level off with bandwidth, suggesting both stable operation with time and minimal polarization mode dispersion. To our knowledge, these results represent the first AAPT of a deployed quantum link, revealing a valuable tool for in situ analysis of entanglement-based quantum networks.

arXiv:2410.00892•Optics Letters•Mar 5, 2025•6 citations

Coexistent quantum channel characterization using quantum process tomography with spectrally resolved detection

Joseph C. Chapman, Joseph M. Lukens, Muneer Alshowkan +3 more

The coexistence of classical and quantum signals over the same optical fiber is critical for quantum networks operating within the existing communications infrastructure. Here, we characterize the quantum channel that results from distributing approximate single-photon polarization-encoded qubits simultaneously with classical light of varying intensities through a 25 km fiber-optic channel. We use spectrally resolved quantum process tomography with a newly developed Bayesian reconstruction method to estimate the quantum channel from experimental data, both with and without classical noise. Furthermore, we show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel if the noise is dominated by Raman scattering. These results aid future development of quantum repeater designs and quantum error-correcting codes which benefit from realistic channel error models.

Quantum Computing, Communication, and Simulation•Mar 8, 2023•0 citations

Toward Ancilla-Assisted Process Tomography for Dynamic In-Situ Metrology of Metropolitan Quantum Network Links

Cory M. Nunn, W. McKenzie, Evan Dowling +7 more

We characterize a subset of the Washington DC metropolitan quantum network (DC-QNet) using Bayesian ancilla-assisted process tomography with polarization qubits, demonstrating quantum-classical coexistence with White Rabbit synchronization over deployed fiber in a reconfigurable network topology.

•Jan 1, 2026•0 citations

Dynamic quantum process tomography on the Washington DC metropolitan quantum network (DC-QNet)

Cory M. Nunn, Wayne McKenzie, Evan Dowling +6 more

In a large-scale quantum network, quantum process tomography is a powerful tool for quantifying and verifying the quality of quantum channels. Within this framework, ancilla-assisted process tomography (AAPT) provides a significant advantage for in situ network metrology, as the same states used for entanglement distribution can also be used for tomography. Here, we perform AAPT within a reconfigurable three-node topology on the Washington DC metropolitan quantum network (DC-QNet), consisting of deployed fiber links up to 11.5 km in length. Using a source of polarization-entangled photon pairs at one node, single photons are routed to either of the two receiver nodes via optical switch, and correlations to the local (ancilla) qubits are leveraged to characterize all relevant links in the network. Additionally, AAPT performance is characterized in the presence of noise from coexistent classical synchronization signals, and the advantages of dynamic routing are shown theoretically for larger network topologies.

Quantum Computing, Communication, and Simulation•Mar 5, 2026•0 citations

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.

The coexistence of quantum and classical signals over the same optical fiber with minimal degradation of the transmitted quantum information is critical for operating large-scale quantum networks over the existing communications infrastructure. Here, we systematically characterize the quantum channel that results from simultaneously distributing approximate single-photon polarization-encoded qubits and classical light of varying intensities through fiber-optic channels of up to 15 km. Using spectrally resolved quantum process tomography with a Bayesian reconstruction method that we develop, we estimate the full quantum channel from experimental photon counting data, both with and without classical background. Furthermore, although we find the exact channel description to be a weak function of the pump polarization, we nevertheless show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel when the noise is dominated by Raman scattering. These results provide a basis for the future development of quantum repeater designs and quantum error-correcting codes for real-world channels and inform models used in the analysis and simulation of quantum networks.

The development of large-scale quantum networks requires reliable quantum channels, the quality of which can be quantified by the framework of quantum process tomography. In this work, we leverage ancilla-assisted process tomography (AAPT) and Bayesian inference to probe a 1.6 km deployed fiber-optic link. We send one of the two polarization-entangled photons at Alice in one building to Bob in another, exploiting the local qubit as an ancilla system to characterize the corresponding quantum channel. Monitoring over a 24 h period returns a steady process fidelity of 97.6(1)%, while controllable spectral filtering with passbands from 0.025 to 4.38 THz finds fidelities that first increase, and then level off with bandwidth, suggesting both stable operation with time and minimal polarization mode dispersion. To our knowledge, these results represent the first AAPT of a deployed quantum link, revealing a valuable tool for in situ analysis of entanglement-based quantum networks.

The coexistence of classical and quantum signals over the same optical fiber is critical for quantum networks operating within the existing communications infrastructure. Here, we characterize the quantum channel that results from distributing approximate single-photon polarization-encoded qubits simultaneously with classical light of varying intensities through a 25 km fiber-optic channel. We use spectrally resolved quantum process tomography with a newly developed Bayesian reconstruction method to estimate the quantum channel from experimental data, both with and without classical noise. Furthermore, we show that the coexistent fiber-based quantum channel has high process fidelity with an ideal depolarizing channel if the noise is dominated by Raman scattering. These results aid future development of quantum repeater designs and quantum error-correcting codes which benefit from realistic channel error models.

In a large-scale quantum network, quantum process tomography is a powerful tool for quantifying and verifying the quality of quantum channels. Within this framework, ancilla-assisted process tomography (AAPT) provides a significant advantage for in situ network metrology, as the same states used for entanglement distribution can also be used for tomography. Here, we perform AAPT within a reconfigurable three-node topology on the Washington DC metropolitan quantum network (DC-QNet), consisting of deployed fiber links up to 11.5 km in length. Using a source of polarization-entangled photon pairs at one node, single photons are routed to either of the two receiver nodes via optical switch, and correlations to the local (ancilla) qubits are leveraged to characterize all relevant links in the network. Additionally, AAPT performance is characterized in the presence of noise from coexistent classical synchronization signals, and the advantages of dynamic routing are shown theoretically for larger network topologies.