arXiv:2605.09019quant-phcs.LG2026-05

提出无扰动量子态重构算法,适用于任意维纯态。

Learning Pure Quantum States in Any Dimension (Almost) Without Regret

  • 分阶段局部测量,用切向投影差获取误差线性观测
  • 累计遗憾上界为 $\mathcal{O}(d^3\log^2 T)$,在线保真度 $\mathcal{O}(d^3\log T/t)$
  • 适合高维量子系统精密表征,尤其关注低干扰测量场景

我们将最小累积扰动的量子态层析方法从比特推广到任意有限维纯态。学习者依次接收未知纯态的新副本,根据先前结果选择一维投影算子进行两结果投影测量。目标是在学习状态的同时使所选投影算子尽量贴近未知态以减少扰动。由于量子比特依赖布洛赫球几何,不直接适用于高维情形(纯态构成弯曲流形),我们通过在纯态流形上局部操作克服此障碍。算法按阶段进行:每阶段固定当前估计,沿相反切向方向移动获得一对邻近投影算子,取其测量结果差,从而得到误差切向分量的精确线性观测。将局部线性模型与鲁棒方差自适应估计器及热启动正则化结合,实现跨阶段精度传递。对任意维度 $d$ 的未知纯态,在 $T$ 次测量后,协议达到累计遗憾 $\mathcal{O}(d^3\log^2 T)$,且在任意中间时间 $t \leq T$,当前估计的在线保真度为 $\mathcal{O}(d^3\log T/t)$。因此,近乎无扰动的纯态层析并非仅限于量子比特,而是普遍存在的几何现象。

原文摘要 · Abstract (English)

We extend quantum state tomography with minimal cumulative disturbance, first investigated in [arXiv:2406.18370], to arbitrary finite-dimensional pure states. A learner sequentially receives fresh copies of an unknown pure state, chooses a rank-one projector for each copy using the previous outcomes, and performs the corresponding two-outcome projective measurement. The goal is to learn the state while keeping the chosen projectors close to the unknown state in order to minimize disturbance. The qubit solution relies on the special geometry of the Bloch sphere and does not extend directly to qudits, where pure states form a curved manifold. We show that this obstruction can be overcome by working locally on the pure-state manifold. The algorithm proceeds in epochs. In each epoch, it fixes a current estimate, measures pairs of nearby rank-one projectors obtained by moving in opposite tangent directions, and takes differences of the corresponding outcomes. This gives an exact linear observation of the tangent component of the error. The resulting local linear models are combined with a robust variance-adaptive estimator and a hot-start regularization that transfers precision across epochs. For every unknown pure state in dimension \(d\), after \(T\) measured copies, our protocol achieves cumulative regret \(\mathcal{O}(d^3\log^2 T)\), and at each intermediate time \(t\leq T\) its current estimate has online infidelity \(\mathcal{O}(d^3\log(T)/t)\). Hence, pure-state tomography with essentially no cumulative disturbance is not a peculiarity of qubits but a geometric phenomenon that persists for qudits.

量子层析纯态学习无扰测量高维系统

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