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This research receives funding from the European Union's Horizon 2020 research and innovation program under Grant Agreement No. 820466 (CiViQ). This research is also funded by the German Federal Ministry of Education and Research within the project Hardware-based Quantum Security. The authors gratefully acknowledge the support from the Scottish Universities Physics Alliance and the Engineering and Physical Sciences Research Council. This project was supported within the framework of the International Max Planck Partnership with Scottish Universities. ; Agile cryptography allows for a resource-efficient swap of a cryptographic core in case the security of an underlying classical cryptographic algorithm becomes compromised. Conversely, versatile cryptography allows the user to switch the cryptographic task without requiring any knowledge of its inner workings. In this paper, we suggest how these related principles can be applied to the field of quantum cryptography by explicitly demonstrating two quantum cryptographic protocols, quantum digital signatures (QDS) and quantum secret sharing (QSS), on the same hardware sender and receiver platform. Crucially, the protocols differ only in their classical postprocessing. The system is also suitable for quantum key distribution (QKD) and is highly compatible with deployed telecommunication infrastructures, since it uses standard quadrature phase-shift keying encoding and heterodyne detection. For the first time, QDS protocols are modified to allow for postselection at the receiver, enhancing protocol performance. The cryptographic primitives QDS and QSS are inherently multipartite, and we prove that they are secure not only when a player internal to the task is dishonest, but also when (external) eavesdropping on the quantum channel is allowed. In our first proof-of-principle demonstration of an agile and versatile quantum communication system, the quantum states are distributed at GHz rates. A 1-bit message may be securely signed using our QDS protocols in less than 0.05 ms over a 2-km fiber link and in less than 0.2 s over a 20-km fiber link. To our knowledge, this also marks the first demonstration of a continuous-variable direct QSS protocol. © 2021 authors. ; Publisher PDF ; Peer reviewed

Quantum state tomography is both a crucial component in the field of quantum information and computation, and a formidable task that requires an incogitably large number of measurement configurations as the system dimension grows. We propose and experimentally carry out an intuitive adaptive compressive tomography scheme, inspired by the traditional compressed-sensing protocol in signal recovery, that tremendously reduces the number of configurations needed to uniquely reconstruct any given quantum state without any additional a priori assumption whatsoever (such as rank information, purity, etc) about the state, apart from its dimension. ; The research supporting the results here reported have recieved additional funding from BK21 Plus Program (21A20131111123) funded by the Ministry of education (MOE, Korea) and National Research Foundation of Korea (NRF), the NRF grant funded by the Korea government (MSIP) (Grant No. 2010-0018295Canada Research Chairs (CRC), the Spanish MINECO (Grant No. FIS2015-67963-P), the Grant Agency of the Czech Republic (Grant No. 18-04291S), and the IGA Project of the Palacký University (Grant No. IGA PrF 2018-003).