量子开关中通过因果序干涉术实现的纠缠认证
Entanglement certification via causal-order interferometry in a quantum switch
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中文总结 AI 辅助
该研究利用量子开关的因果序干涉术，通过插入局域幺正变换，在多种噪声模型下扩大了可认证纠缠的噪声参数区域，实现了更优的纠缠认证效果。
中文摘要 AI 辅助
纠缠认证通常针对已经历过噪声传输或处理的量子态开展。噪声会减少留存的纠缠，甚至在纠缠仍存在时导致给定判据失效。在这一背景下，量子开关作为不定因果序（ICO）的典型实现，可相干控制两个通道作用的顺序，已被证明在一系列量子信息处理任务中具备优势。本文探究这种相干控制是否能扩大纠缠仍可被认证的噪声参数区域。我们将两个序分支视为因果序干涉仪的臂，并在通道使用之间插入一个局域幺正变换以调节它们的干涉。对于随机泡利噪声，后选的ICO输出可表现出比两个确定序的任何经典混合都更大的纠缠负性；特别地，我们确定了这样的参数区域：其负性仍非零，而所有经典混合的负性均消失。合适的局域泡利幺正变换可大幅扩大这一仅ICO的区域，而依赖输入的路径差指标定性地将算子非对易性与后选负性增益关联起来。数值例子将该优势扩展到局域振幅阻尼噪声和三量子位外尔噪声。在针对3×3部分转置正定（PPT）Tile有界纠缠态的一个代表性外尔噪声点处，一个非可分解的见证算子可检测到后选ICO输出，而一个解析界排除了局域旋转见证算子族检测确定序输出及其混合的可能。这些结果表明，因果序干涉术是在不同噪声模型和维度下增强纠缠认证的一种策略。
英文摘要
Entanglement certification is often performed on states that have already undergone noisy transmission or processing. Noise can reduce the surviving entanglement and can also cause a given criterion to fail even when entanglement remains. In this context, the quantum switch, a paradigmatic realization of indefinite causal order (ICO), coherently controls the orders in which two channels act and has been shown to offer advantages across a range of quantum information-processing tasks. Here we ask whether this coherent control enlarges the noise-parameter region in which entanglement remains certifiable. We regard the two order branches as the arms of a causal-order interferometer and insert a local unitary between the channel uses to tune their interference. For stochastic Pauli noise, a postselected ICO output can exhibit greater entanglement negativity than any classical mixture of the two definite orders; in particular, we identify regimes where its negativity remains nonzero while that of every classical mixture vanishes. A suitable local Pauli unitary substantially enlarges this ICO-only region, while an input-dependent path-difference indicator qualitatively links operator noncommutativity to the postselected negativity gain. Numerical examples extend the advantage to local amplitude-damping noise and two-qutrit Weyl noise. At a representative Weyl-noise point for the $3\times 3$ positive-partial-transpose (PPT) Tiles bound-entangled state, a nondecomposable witness detects the postselected ICO output, whereas an analytic bound excludes detection of the definite-order outputs and their mixtures by the entire locally rotated witness family. These results identify causal-order interferometry as a strategy for enhancing entanglement certification across distinct noise models and dimensions.
