Proof-of-Principle Implementation of Quantum Key Distribution in an Indefinite Causal Order

In quantum physics, the order in which different operations occur can be placed in superposition. The resulting processes have an indefinite causal order which is of fundamental interest and can be viewed as a novel quantum resource enabling new protocols. Here, we report an experimental proof-of-principle 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 (0.02) per shared qubit, with eavesdropper detection performed through measurements of the control qubit rather than comparing the key. Unlike 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. Although the present implementation does not yet constitute a secure quantum key distribution protocol, owing to 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.