Experimental quantum cryptography with classical users Francesco Massa ∗1 , Preeti Yadav † 2,3 , Amir Moqanaki 1 , Walter O. Krawec 4 , Paulo Mateus 2,3 , Nikola Paunkovi´ c 2,3 , Andr´ e Souto 2,5 , and Philip Walther ‡ 1 1 Vienna Center for Quantum Science and Technology (VCQ), Faculty of Physics, University of Vienna, Boltzmanngasse 5, Vienna A-1090, Austria 2 Instituto de Telecomunica¸ c˜oes, 1049-001 Lisbon, Portugal 3 Departamento de Matem´atica, Instituto Superior T´ ecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal 4 Computer Science and Engineering Department, University of Connecticut, Storrs, CT 06269, USA 5 LASIGE, Departamento de Inform´atica, Faculdade de Ciˆ encias, Universidade de Lisboa, 1749-016 Lisboa, Portugal Abstract The exploit of quantum systems allows for insights that promise to revolutionise information processing, although a main challenge for practical implementations is technological complexity. Due to its feasibility, quantum cryptography, which allows for perfectly secure communication, has become the most prominent application of quantum technology. Nevertheless, this task still requires the users to be capable of performing quantum operations, such as state preparation or measurements in multiple bases. A natural question is whether the users’ technological requirements can be further reduced. Here we demonstrate a novel quantum cryptography scheme, where users are fully classical. In our protocol, the quantum operations are performed by an untrusted third party acting as a server, which gives the users access to a superimposed single photon, and the key exchange is achieved via interaction-free measurements on the shared state. Our approach opens up new interesting possibilities for quantum cryptography networks. Quantum key distribution (QKD) is a technique that allows two users, traditionally called Alice and Bob, to exchange a cryptographic key in an information-theoretic secure way. This means that the security of the key relies on information theory and cannot be broken even by an eavesdropper with unlimited resources. Since the first QKD proposal, the BB84 protocol [1], much progress, both theoretical and experimental, has been made in the field. The practicality of this technology is underlined by numerous experimental and even commercial endeavors, supporting its development [2, 3, 4, 5]. Most QKD protocols require Alice or Bob to share a quantum state, or a direct quantum channel, and to perform quantum operations, i.e. operations on quantum bits (qubits) that do not have any counterpart in classical communication, such as generation or measurement in multiple ∗ francesco.massa@univie.ac.at † pri8.phy@gmail.com ‡ philip.walther@univie.ac.at 1 arXiv:1908.01780v1 [quant-ph] 5 Aug 2019