Iordanis Kerenidis

As in modern communication networks, the security of quantum networks will rely on complex cryptographic tasks that are based on a handful of fundamental primitives. Weak coin flipping (WCF) is a significant such primitive which allows two mistrustful parties to agree on a random bit while they favor opposite outcomes. Remarkably, perfect information-theoretic security can be achieved in principle for quantum WCF, which is impossible for a classical coin flip without computational assumptions or trusting a third party. In this work, we overcome conceptual and practical issues that have prevented the experimental demonstration of this primitive to date, and demonstrate how quantum resources can provide cheat sensitivity, whereby each party can detect a cheating opponent, and an honest party is never sanctioned. Such a property is not known to be classically achievable with information-theoretic security. Our experiment implements a refined, loss-tolerant version of a recently proposed theoretical protocol and exploits heralded single photons generated by spontaneous parametric down-conversion, a carefully optimized linear optical interferometer including beam splitters with variable reflectivities and a fast optical switch for the verification step. High values of our protocol benchmarks are maintained for attenuation corresponding to several kilometers of telecom optical fiber.

The goal of demonstrating a quantum advantage with currently available experimental systems is of utmost importance in quantum information science. While this remains elusive for quantum computation, the field of communication complexity offers the possibility to already explore and showcase this advantage for useful tasks. Here, we define such a task, the Sampling Matching problem, which is inspired by the Hidden Matching problem and features an exponential gap between quantum and classical protocols in the one-way communication model. Our problem allows by its conception a proof-of-principle photonic implementation based on encoding in the phase of coherent states of light, the use of a fixed size linear optic circuit, and single-photon detection. This enables us to demonstrate experimentally an advantage in the transmitted information resource beyond a threshold input size, which would have been impossible to reach for the original Hidden Matching problem. Our demonstration has implications in various communication and cryptographic settings. Specifically we have used it to introduce a robust practical quantum money-scheme. Our scheme involves an honest Bank who prepares the note by independently and uniformly selecting multiple n-bit binary secret strings which are encoded into the single photon states. The note is then distributed among untrusted holders. To carry out the transaction, the note holder sends the note to the honest local verifiers of the Bank. The verifier runs the Sampling Matching scheme on some randomly selected copies of the note and forwards the classical measurement outcome to the Bank. The Bank then declares the validity of the note. Our private-key money scheme includes multiple features such as single round classical interaction of the local verifier with the Bank, optimal note re-usability (linear in the size of Bank note), linear verification circuit size, and an unconditional security against any adversary trying to forge the Bank note while tolerating the noise of up to 21.4%. The simplistic nature of our verification scheme using Sampling Matching allows for the ability to reach a maximal theoretical noise tolerance of 25%, as conjectured by Amiri et al [Phys Rev A 95, 062334].

Device-independent cryptographic protocols are by definition more secure than their device-dependent counterparts, since they do not rely on any assumptions regarding the internal workings of the apparatus used to implement them. Thus far, the device-independent approach has been successfully applied to problems such as quantum-key distribution and randomness generation, but it is not a priori clear whether it can be applied to protocols in the mistrustful cryptography class, where the parties do not trust one another. In this work we show that for bit-commitment and coin flipping a device-independent treatment is possible.

We give the first exponential separation between quantum and bounded-error randomized one-way communication complexity. Specifically, we define the Hidden Matching Problem and prove that its quantum one-way communication complexity is O(log n), yet any randomized one-way protocol with bounded error must use Omega(sqrt{n}) bits of communication. No asymptotic gap for one-way communication was previously known.

We present a composably secure protocol allowing n parties to test an entanglement generation resource controlled by a possibly dishonest party. The test consists only in local quantum operations and authenticated classical communication once a state is shared among them and provides composable security, namely it can be used as a secure subroutine by n honest parties within larger communication protocols to test if a source is sharing quantum states that are at least Ɛ-close to the GHZ state. This claim comes on top of previous results on multipartite entanglement verification where the security was studied in the usual game-based model. Here, we improve the protocol to make it more suitable for practical use in a quantum network and we study its security in the Abstract Cryptography framework to highlight composability issues and avoid hidden assumptions. This framework is a top-to-bottom theory that makes explicit any piece of information that each component (party or resource) gets at every time-step of the protocol. Moreover any security proof, which amounts to showing indistinguishability between an ideal resource having the desired security properties (up to local simulation) and the concrete resource representing the protocol, is composable for free in this setting. This allows us to readily compose our basic protocol in order to create a composably secure multi-round protocol enabling honest parties to obtain a state close to a GHZ state or an abort signal, even in the presence of a noisy or malicious source. Our protocol can typically be used as a subroutine in a Quantum Internet, to securely share a GHZ state among the network before performing a communication or computation protocol.

We experimentally implement a quantum coin flipping protocol that guarantees a strictly better security than classically possible against an all powerful adversary over a distance suitable for communication in metropolitan area networks. The implementation is based on a practical plug’n'play system, originally designed for quantum key distribution. Furthermore, we show that our protocol can be combined with quantum coin flipping protocols that provide almost perfect security against adversaries with limited resources and hence enhance them with a level of unconditional security. Our results offer a powerful theoretical and experimental toolbox for future secure quantum communications.