Federico Grasselli

The security of prepare-and-measure satellite-based quantum key distribution (QKD), under restricted eavesdropping scenarios, is addressed. We particularly consider cases where the eavesdropper, Eve, has limited access to the transmitted signal by Alice, and/or Bob’s receiver station. For instance, Eve can only receive an attenuated version of the transmitted signals. This results in settings where an uncharacterized bypass channel, inaccessible to Eve, can also carry signals to Bob. We obtain generic bounds on the key rate in the presence of bypass channels and apply them to continuous-variable QKD protocols with Gaussian encoding as well as to the family of BB84 protocols. We find regimes of operation in which the above restrictions on Eve can considerably improve system performance. Our work opens up new security frameworks for spaceborne quantum communications systems.

Quantum conference key agreement (CKA) is a cryptographic task for sharing a secret common key between multiple users. CKA has been established as a network protocol that can leverage multipartite entanglement (NQKD) to gain an advantage over contemporary two-party communication primitives (2QKD). Specifically, when performing QCKA in constrained quantum networks, e.g., with limited channel capacities, NQKD schemes can produce the conference key between N users with up to an N-1 rate advantage over 2QKD. QCKA has previously been implemented by direct transmission of a 4-photon GHZ state, however did not show the advantage comparison. Here we show this advantage using a universal network resource represented by a 6-qubit photonic graph state.

Paradigmatic QKD protocols establish secure keys between pairs of users, however when more than two parties want to communicate, recently introduced quantum conference quantum key agreement (CKA) protocols can outperform 2-party primitives in terms of resource cost. In this contribution we report an implementation of a four-user quantum CKA protocol using polarisation-encoded multi-partite GHZ states at telecom wavelength. We distribute these states over up to 50km of optical fibre and implement custom multiparty error correction and privacy amplification on the resulting raw keys. From a finite-key analysis, we establish an information-theoretic secure key of up to 1.15 × 10^6 bits, which is used to encrypt and securely share an image between the four users. Surpassing the previous maximum distance for GHZ state transmission by more than an order of magnitude, these results demonstrate the viability of network protocols relying on multi-partite-entanglement. Future applications beyond quantum CKA include entanglement-assisted remote clock-synchronization, quantum secret sharing, and GHZ-based repeater protocols.

We investigate three promising Quantum Digital Signature (QDS) protocols that can sign arbitrarily long documents with information-theoretic security. We re-derive the security proof of each scheme obtaining, where possible, tighter security bounds while closing security loopholes caused by using non-authenticated communication. In one case, we upgrade the QDS protocol to information-theoretic security by replacing NIST-recommended hash functions with universal hashing. We systematically compare the performance of the three protocols by optimizing over their parameters and deduce the protocol requiring the shortest signatures and the minimal number of preshared secret bits. Finally, we propose a multi-partite QDS protocol that retains the best features of the analyzed protocols while allowing for an arbitrary number of receivers.

Here we report on the experimental results implementing robust anonymous quantum conference key agreement using GHZ states. Results confirm the advantage when allowing for the use of multipartite entanglement along with bipartite entanglement.

When the outcomes of a set of parties measuring their local quantum systems exhibit non-local correlations by violating a Bell inequality, one can infer that such outcomes are secret to some extent. This is at the core of the security of many device-independent (DI) protocols, such as DI randomness expansion and DI conference key agreement. We quantify the amount of secret randomness in the parties’ outcomes by analytically computing their conditional von Neumann entropies as a function of the Bell violation, for different Bell inequalities.

We consider a device-independent scenario where N parties test the Mermin-Ardehali-Belinskii-Klyshko (MABK) inequality. By exploiting the inequality’s symmetries, we drastically simplify the general form of the quantum state that can be considered, without loss of generality. We then derive an upper bound on the maximal violation of the MABK inequality attained by an arbitrary N-qubit state, as a function of the state’s parameters. The two results enable us to derive analytical bounds on the von Neumann entropy of the parties’ outcomes, conditioned on the eavesdropper’s information. These quantities are crucial for the security of many cryptographic protocols and better bounds lead to more robust protocols. In particular, we bound the conditional entropy of a single party’s outcome and the joint conditional entropy of two parties’ outcomes, as a function of the MABK violation observed by three parties. We extend the former bound to N parties and prove its tightness, while we observe that the latter significantly improves previous results.