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IEEE Photonics Conference

Procrustean entanglement concentration in dense wavelength-division multiplexing

Hsuan‐Hao Lu, Muneer Alshowkan, Jude Alnas, Joseph M. Lukens, Nicholas A. Peters

Published Nov 12, 2023• 0 citations
Publisher

Abstract

We employ polarization-dependent loss emulators for Procrustean filtering of a noisy quantum state affected by classical crosstalk, resulting in a substantial improvement of two-qubit entangled state fidelities from approximately 75% to over 92%.
DOI
10.1109/ipc57732.2023.10360731

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The success of a future quantum internet will rest in part on the ability of quantum and classical signals to coexist in the same optical fiber infrastructure, a challenging endeavor given the orders of magnitude differences in flux of single-photon-level quantum fields and bright classical traffic. We theoretically describe and experimentally implement Procrustean entanglement concentration for polarization-entangled states contaminated with classical light, showing significant mitigation of crosstalk noise in dense wavelength-division multiplexing. Our approach leverages a pair of polarization-dependent loss emulators to attenuate highly polarized crosstalk that results from imperfect isolation of conventional signals copropagating on shared fiber links. We demonstrate our technique both on the tabletop and over a deployed quantum local area network, finding a substantial improvement of two-qubit entangled state fidelity from approximately 75% to over 92%. This local filtering technique could be used as a preliminary step to reduce asymmetric errors, potentially improving the overall efficiency when combined with more complex error-mitigation techniques in future quantum repeater networks.
Understanding quantum noise is essential for accurately creating desired quantum states and for examining a given state's evolution in any protocol. Using spontaneous parametric downconversion, we can create a wide variety of single- and two-qubit polarization states, including nearly perfect Bell states, mixed states (i.e., "noisy" states) and maximally entangled mixed states (MEMS). To characterize these states we use several different measures, including fidelity, "tangle" and linear entropy. In the course of our experiments, we have discovered and numerically investigated an extreme imbalance in the sensitivity of these different two-qubit state measures. We have also experimentally realized a "Procrustean" filtering technique to remove noise from MEMS. For moderate amounts of filtering, the experimental procedure works as desired to increase the tangle and decrease the linear entropy. However, for large amounts of filtering, the process becomes dominated by perturbations in the starting density matrix. The final outcome is a pure (i.e., zero entropy) product state (i.e., zero entanglement).
Although spontaneous Raman scattering is often regarded as the dominant impairment in hybrid quantum– classical fiber links, practical deployment also requires clear limits on linear inter-channel crosstalk between entangled photons and neighboring dense wavelength-division multiplexing (DWDM) channels. In this work, we experimentally determine such limits using a controlled bench-top testbed in which a high-fidelity (F > 0.97) polarization-entangled photon source shares a DWDM demultiplexer with a single classical optical channel separated by 100 GHz. Residual classical leakage into the quantum receiver is controlled through spectral filtering and systematically varied by sweeping the classical power incident on the detector from −120 dBm to −40 dBm. A thin-film bandpass filter with 0.9 nm bandwidth, providing approximately 10 dB of additional suppression, is employed in the quantum path. Entanglement quality is assessed through polarization-interference visibility measured in multiple polarization bases and through the coincidence-to-accidental ratio (CAR). We observe that high visibility and CAR are maintained over a broad range of crosstalk levels, with degradation governed by the relative rate of accidental coincidences rather than by absolute optical power alone. Acceptable performance, defined here as visibility ≥ 90% and CAR ≥ 20, is preserved provided that the classical leakage power at the detector does not exceed the quantum-signal power by more than 12 dB for a continuous-wave carrier or for a 100 GBd dual-polarization QPSK signal. These experimental results establish filtering and isolation requirements for entanglement distribution in hybrid quantum–classical DWDM optical systems.
Boosting the communication rate of quantum networks is a central challenge in quantum information science. Recently, an efficient entanglement distribution scheme employing quasi-deterministic Bell-pair sources based on time-frequency multiplexing, referred to as zero-added-loss multiplexing~(ZALM), has been proposed. Its implementation, however, requires high-fidelity entanglement swapping across densely multiplexed time-frequency modes, which has remained an experimental challenge. Here we demonstrate entanglement swapping across 16 parallel frequency modes with a high average fidelity of 93.9±\pm±\SI{1.4}{\%}. Notably, polarization-entangled photon pairs in each frequency mode are spectrally single-mode using only off-the-shelf 50-GHz dense wavelength-division multiplexing~(DWDM) filters, eliminating the need for additional narrowband filtering. Furthermore, in order to fully exploit the temporal degree of freedom, the pump pulse is operated with a repetition frequency of \SI{3.0}{GHz}. By combining the frequency and time multiplexing, the total swapping rate reaches 5.38$\pm$0.17\,\si{pairs\,s^{-1}}, which corresponds to the ZALM Bell-pair rate of \SI{8.2e2}{pairs\,s^{-1}}. Our results establish the key experimental capabilities required for ZALM and demonstrate a scalable route toward practical high-rate quantum repeaters and long-haul quantum networks.