Passive Polarization Stabilization for Practical and Robust Entanglement Distribution

TL;DR

The paper tackles the problem of robust entanglement distribution over lossy and noisy fiber channels by introducing a passive stabilization scheme based on cross-aligned polarization-maintaining fibers (PMFs). Through Jones-calculus modeling and fidelity analysis, it predicts high entanglement fidelity even with realistic misalignments and small length mismatches. Experimentally, the cross-aligned PMF configuration preserves polarization and phase without active compensation, achieving average fringe visibility of $V rightarrow 0.867$ under instability ( versus SMF’s $0.444$) and maintaining stable fringes with low error propagation, thereby enabling practical entanglement distribution for field deployments. This approach promises simplified, robust quantum-state distribution suitable for QKD, distributed quantum computing, and large-scale quantum networks.

Abstract

Quantum entanglement is a key resource in quantum information science, playing an essential role in quantum key distribution (QKD), quantum networks, and distributed quantum computing. However, practical applications require techniques capable of reliably distributing entanglement under lossy and noisy conditions. In this work, we demonstrate that a simple configuration using two polarization-maintaining fibers (PMFs) arranged in a cross-axis alignment enables stable distribution of entangled photon pairs without the need for real-time polarization compensation. To support this, we performed quantum information modeling and fidelity simulations for the cross-aligned PMF pair, and experimentally compared the entanglement preservation and interference fringe stability in setups based on standard single-mode fibers (SMFs) and PMFs. The experimental results show that the PMF-based configuration achieves an average visibility of 0.867 and an error propagation of 0.023 under unstable conditions, whereas the SMF setup exhibits a significantly lower stability with an average visibility of 0.444 and an error propagation of 0.167 under the same conditions. These results indicate that the cross-aligned PMF pair allows robust entanglement transmission without complex active compensation devices, suggesting its suitability for reliable quantum state distribution in long-distance quantum communication, drone-based QKD, and multi-user quantum networks.

Figures (4)

• Figure 1: A schematic of the system for transmitting bipartite quantum states in a self-compensating manner is shown. Both Alice and Bob employ cross-aligned PMF pairs. Without loss of generality, the axis of Alice’s first PMF ($A_1$) is assumed to align with the horizontal polarization of the reference frame. Alice’s second PMF ($A_2$) is rotated by $90^\circ + \theta$ relative to $A_1$, while Bob’s first PMF ($B_1$) is rotated by an angle $\phi$ with respect to $A_1$, representing a realistic scenario.
• Figure 2: This figure shows the simulated fidelity of entangled photon pairs in the state $\ket{\psi^-}$ with a central wavelength of $\lambda = 810~\text{nm}$ transmitted through the cross-aligned polarization-maintaining fiber (PMF) pairs of Alice and Bob. It is assumed that the fiber length mismatch $\Delta L$ and the fast axis angle error $\theta$ are the same for each PMF pair of Alice and Bob, and only the reference frame difference $\phi$ is present. The final output state after phase compensation is calculated under these conditions. Each subplot represents a different fiber length mismatch and is labeled from (a) to (i).
• Figure 3: Overview of the experiment for generating and distributing entangled photon pairs. Entangled photon pairs are generated via a Sagnac interferometer, and the measurement station selects the measurement basis using HWP and PBS. Alice and Bob each measure the quantum states using two SPADs, and single count and coincidence count data for four channels are acquired using TCSPC. (a) The entanglement source and measurement station are connected via SMF, with one HWP and two QWPs inserted for polarization and phase compensation. (b) The entanglement source and measurement station are connected via a cross-aligned PMF pair, and a tilted QWP is used as a fixed phase retarder for simple phase compensation.
• Figure 4: Interference fringes of entangled photon pairs measured using SMF and cross-aligned PMF under stable and unstable environmental conditions. (a) and (b) correspond to the cases using SMF, while (c) and (d) correspond to the cases using cross-aligned PMF pair. Each graph was obtained by setting Alice's HWP angle to $2\theta_A = 0^\circ$ and $45^\circ$, then varying Bob's HWP angle $2\theta_B$ and measuring the coincidence counts between detectors $D_{A1}$ or $D_{A2}$ and $D_{B1}$. In the case of the PMF, high visibility and fringe stability were maintained even under external vibrations and bending conditions.