AI 中文总结
该研究实验实现了基于不定因果序的类BB84量子密码协议，借助量子SWITCH完成窃听检测，无需披露密钥材料，为利用不定因果序保障量子密钥安全提供了原理验证。
AI 中文摘要
在量子物理学中，不同操作发生的顺序可以处于叠加态，由此产生的过程具有不定因果序，这既具有基础研究意义，也可被视为一种新型量子资源，能支持多种新协议。本文报道了此类协议之一的实验实现：我们将Alice和Bob的测量制备操作置于光子量子SWITCH中，执行类BB84量子密码术。通过将Alice和Bob嵌入量子SWITCH，该协议在每个共享量子比特上实现了0.15±0.02的平均窃听者检测概率，且窃听者检测是通过对控制量子比特进行测量完成，而非通过比对密钥。与标准BB84及相关方案需公开披露并丢弃部分原始密钥以检测窃听不同，我们的方法无需披露密钥材料：原则上，每个保留的量子比特都可在检测窃听的同时，保持用于密钥生成的可用性。该实验依赖一种新型测量技术，可在不破坏路径相干性的前提下，测量光子在量子SWITCH内部的偏振。尽管由于量子SWITCH内部测量所需的后选择，当前实现尚未构成安全的量子密钥分发协议，但它提供了原理验证，证明可利用不定因果序检测窃听，且不会损失密钥比特。
英文摘要
In quantum physics the order in which different operations occur can be placed in superposition. The resulting processes have an indefinite causal order and are both of fundamental interest and can be viewed as a novel quantum resource that enables a variety of new protocols. Here we report an experimental implementation of one such protocol, where we perform BB84-like quantum cryptography by placing Alice and Bob's measurement-and-preparation operations in a photonic quantum SWITCH. By embedding Alice and Bob within the quantum SWITCH, the protocol achieves an average eavesdropper detection probability of $0.15 \pm 0.02$ per shared qubit, with eavesdropper detection performed through measurements of the control qubit rather than by comparing the key. Unlike the standard BB84 and related schemes, which detect eavesdropping by publicly revealing and discarding a fraction of the raw key, our approach requires no disclosure of key material: every retained qubit can, in principle, be tested for eavesdropping while remaining available for key generation. The experiment relies on a new measurement technique that allows the polarization of a photon to be measured inside the quantum SWITCH without destroying path coherence. Although the present implementation does not yet constitute a secure quantum key distribution protocol, owing to the post-selection required for measurements within the quantum SWITCH, it provides a proof of principle that indefinite causal order can be exploited to detect eavesdropping without sacrificing key bits.
Comments21 pages, 7 figures
