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Digital signatures are widely used in modern communication to guarantee authenticity and transferability of messages. The security of currently used classical schemes relies on computational assumptions. We present a quantum signature scheme that does not require trusted quantum channels. We prove that it is unconditionally secure against the most general coherent attacks, and show that it requires the transmission of significantly fewer quantum states than previous schemes. We also show that the quantum channel noise threshold for our scheme is less strict than for distilling a secure key using quantum key distribution. This shows that "direct" quantum signature schemes can be preferable to signature schemes relying on secret shared keys generated using quantum key distribution.

Recent advances in theoretical and experimental quantum computing bring us closer to scalable quantum computing devices. This makes the need for protocols that verify the correct functionality of quantum operations timely and has led to the field of quantum verification. In this paper we address key challenges to make quantum verification protocols applicable to experimental implementations. We prove the robustness of the single server verifiable universal blind quantum computing protocol of Kashefi (2012 arXiv:1203.5217) in the most general scenario. This includes the case where the purification of the deviated input state is in the hands of an adversarial server. The proved robustness property allows the composition of this protocol with a device-independent state tomography protocol that we give, which is based on the rigidity of CHSH games as proposed by Reichardt et al (2013 Nature 496 456-60). The resulting composite protocol has lower round complexity for the verification of entangled quantum servers with a classical verifier and, as we show, can be made fault tolerant.The approaches that have been so far successful are those based on interactive proof systems [6,7], where a trusted, computationally limited verifier (also known as client, in a cryptographic setting) exchanges messages with an untrusted, powerful quantum prover, or multiple provers (also known as servers). The verifier attempts to certify that, with high probability, the provers are performing the correct quantum operations. Because we are dealing with a new form of computation, the verification protocols, while based on established techniques are fundamentally different from their classical counterparts. A number of quantum verification protocols have been developed, for different functionalities of devices and using a variety of different strategies to achieve verification [1,2,[8][9][10][11][12][13][14][15][16][17]. The assumptions made depend on the specific target and desired properties of the protocol. For example, if the emphasis is on creating an immediate practical implementation, then this should be reflected in the technological requirements leading to a testable application with current technology [17]. Alternatively, if the motivation is to prove a theoretical result, we may relax some requirements such as efficient scaling [2]. An important open problem in the field of quantum verification, is whether a scheme with a fully classical verifier is possible [18,19]. We know, however, that verification is possible in the following two scenarios.(1)A verifier with minimal quantum capacity (ability to prepare random single qubits) and a single quantum prover [1]. This is the Fitzsimons and Kashefi (FK) protocol.(2)A fully classical verifier and two non-communicating quantum provers that share entanglement [20]. This is the Reichardt, Unger and Vazirani (RUV) protocol.One of our objectives is to obtain a device-independent (allowing untrusted quantum devices) version of the FK protocol, by composing it with the RUV protocol.Here we ...

Quantum digital signatures (QDSs) allow the sending of messages from one sender to multiple recipients, with the guarantee that messages cannot be forged or tampered with. Additionally, messages cannot be repudiated--if one recipient accepts a message, she is guaranteed that others will accept the same message as well. While messaging with these types of security guarantees are routinely performed in the modern digital world, current technologies only offer security under computational assumptions. QDSs, on the other hand, offer security guaranteed by quantum mechanics. All thus far proposed variants of QDSs require long-term, high quality quantum memory, making them unfeasible in the foreseeable future. Here, we present a QDS scheme where no quantum memory is required, which also needs just linear optics. This makes QDSs feasible with current technology.

Digital signatures guarantee the authenticity and transferability of messages and are widely used in modern communication. The security of currently used classical digital signature schemes, however, relies on computational assumptions. In contrast, quantum digital signature (QDS) schemes offer information-theoretic security guaranteed by the laws of quantum mechanics. We present two QDS protocols which have the same experimental requirements as quantum key distribution, which is already commercially available. We also give a security proof for the presented QDS schemes against coherent forging attacks.