提出量子态认证的近似实例最优方法,支持纠缠测量。
Instance-Optimal Quantum State Certification with Entangled Measurements
- 基于纠缠测量设计新算法,实现实例最优性能。
- 证明复制次数为最大复杂度乘以保真度,逼近最优。
- 方法可简化混合态检测下界证明,适合量子验证研究者。
我们研究量子态认证任务:给定一个假设态σ和多个未知态ρ的副本,测试器需判断两者在迹距离上是否相等或至少ε远离。已知若允许对所有副本进行纠缠测量,则所需副本数为Θ(d/ε²),但这是针对最坏情况σ的界限。目前尚不清楚该问题在每个具体实例下的最优副本复杂度。尽管此前已有针对无纠缠测量的实例最优结果,但对允许任意测量(包括纠缠)的情形仍属开放。本文首次解决此问题,证明了在允许完全纠缠测量时,量子态认证的副本复杂度近乎实例最优。类似于无纠缠情形,最优复杂度等于最坏情况复杂度乘以σ与最大混合态之间的保真度。我们通过引入新的量子版Ingster-Suslina方法构建下界,该方法本身可能具有独立价值,并借此以极简方式重新推导出混合态检测的Ω(d/ε²)下界 [OW15]。
原文摘要 · Abstract (English)
We consider the task of quantum state certification: given a description of a hypothesis state $σ$ and multiple copies of an unknown state $ρ$, a tester aims to determine whether the two states are equal or $ε$-far in trace distance. It is known that $Θ(d/ε^2)$ copies of $ρ$ are necessary and sufficient for this task, assuming the tester can make entangled measurements over all copies [CHW07,OW15,BOW19]. However, these bounds are for a worst-case $σ$, and it is not known what the optimal copy complexity is for this problem on an instance-by-instance basis. While such instance-optimal bounds have previously been shown for quantum state certification when the tester is limited to measurements unentangled across copies [CLO22,CLHL22], they remained open when testers are unrestricted in the kind of measurements they can perform. We address this open question by proving nearly instance-optimal bounds for quantum state certification when the tester can perform fully entangled measurements. Analogously to the unentangled setting, we show that the optimal copy complexity for certifying $σ$ is given by the worst-case complexity times the fidelity between $σ$ and the maximally mixed state. We prove our lower bounds using a novel quantum analogue of the Ingster-Suslina method, which is likely to be of independent interest. This method also allows us to recover the $Ω(d/ε^2)$ lower bound for mixedness testing [OW15], i.e., certification of the maximally mixed state, with a surprisingly simple proof.
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