Proof-of-Principle Implementation of Quantum Key Distribution in an Indefinite Causal Order
Abstract
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 \AB's measurement-and-preparation operations in a photonic quantum SWITCH. By embedding \AB\ 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.
Versions
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BibTeX
@misc{valibouse2026proofofprinciple,
title = {{Proof-of-Principle Implementation of Quantum Key Distribution in an Indefinite Causal Order}},
author = {Yann Valibouse and Martí Cladera-Rosselló and Michael Antesberger and Hector Spencer-Wood and Kyrylo Simonov and Patrik Sund and Mathieu Bozzio and Philip Walther and Lee A. Rozema},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2608.13561},
eprint = {2608.13561},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.13561},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - Proof-of-Principle Implementation of Quantum Key Distribution in an Indefinite Causal Order
AU - Yann Valibouse
AU - Martí Cladera-Rosselló
AU - Michael Antesberger
AU - Hector Spencer-Wood
AU - Kyrylo Simonov
AU - Patrik Sund
AU - Mathieu Bozzio
AU - Philip Walther
AU - Lee A. Rozema
PY - 2026
DA - 2026/10/20
PB - arXiv
DO - 10.48550/arxiv.2608.13561
UR - https://arxiv.org/abs/2608.13561
AB - 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 \AB's measurement-and-preparation operations in a photonic quantum SWITCH. By embedding \AB\ 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.
ER -