Llorenç Escolà-Farràs
Current Organizations / Institutes
The organizations/institutes from the scholar's latest article
Previous Organizations / Institutes
The organizations / institutes from the scholar's other articles in the past ten years
Research Subjects
Llorenç Escolà-Farràs has a total of 13 co-authors. Here are the top co-authors by number of shared publications:
- Florian Speelman
QuSoft
3 shared publications - Rene Allerstorfer
CWI Amsterdam QuSoft
2 shared publications Andreas Bluhm
Univ. Grenoble Alpes
1 shared publication- Andreas Winter
Departament de Física Grup d'Informació Quàntica Universitat Autònoma de Barcelona 08193 Bellaterra (Barcelona Spain
1 shared publication - Arpan Akash Ray
Eindhoven University of Technology
1 shared publication - Boris Škorić
Eindhoven University of Technology
1 shared publication
Latest Publications
- Open Access
- 22 December 2025
- Published by American Physical Society (APS) in Physical Review Letters
- Vol. 135 (26) , 260801
Making Existing Quantum Position Verification Protocols Secure Against Arbitrary Transmission Loss
Signal loss threatens the security of quantum cryptography, especially in quantum position verification (QPV) protocols, where even small losses can compromise security. This Letter modifies traditional QPV to make high transmission loss between verifiers and the prover irrelevant for a class of protocols, including a practically interesting one based on BB84 states ( QPV BB 84 f ). Using photon presence detection and a small time delay, as well as a commitment before proceeding, the protocol’s relevant loss rate is reduced to only that of the prover’s lab, and the modified protocol has essentially the same security guarantees as the original one. The adapted protocol c − QPV BB 84 f thus offers strong security guarantees and feasibility over longer distances. We also discuss the practical implementation of the protocol and parameter estimates.
- 22 December 2025
- Published by American Physical Society (APS) in Physical Review Letters
- Vol. 135 (26) , 260801
Open AccessMaking Existing Quantum Position Verification Protocols Secure Against Arbitrary Transmission Loss
Signal loss threatens the security of quantum cryptography, especially in quantum position verification (QPV) protocols, where even small losses can compromise security. This Letter modifies traditional QPV to make high transmission loss between verifiers and the prover irrelevant for a class of protocols, including a practically interesting one based on BB84 states ( QPV BB 84 f ). Using photon presence detection and a small time delay, as well as a commitment before proceeding, the protocol’s relevant loss rate is reduced to only that of the prover’s lab, and the modified protocol has essentially the same security guarantees as the original one. The adapted protocol c − QPV BB 84 f thus offers strong security guarantees and feasibility over longer distances. We also discuss the practical implementation of the protocol and parameter estimates.
- 11 December 2024
- Published by American Physical Society (APS) in Physical Review A
- Vol. 110 (6)
Continuous-variable quantum position verification secure against entangled attackers
- 11 December 2024
- Published by American Physical Society (APS) in Physical Review A
- Vol. 110 (6)
Continuous-variable quantum position verification secure against entangled attackers
- Open Access
A quantum cloning game with applications to quantum position verification
We introduce a quantum cloning game in which $k$ separate collaborative parties receive a classical input, determining which of them has to share a maximally entangled state with an additional party (referee). We provide the optimal winning probability of such a game for every number of parties $k$, and show that it decays exponentially when the game is played $n$ times in parallel. These results have applications to quantum cryptography, in particular in the topic of quantum position verification, where we show security of the routing protocol (played in parallel), and a variant of it, in the random oracle model.Open AccessA quantum cloning game with applications to quantum position verification
We introduce a quantum cloning game in which $k$ separate collaborative parties receive a classical input, determining which of them has to share a maximally entangled state with an additional party (referee). We provide the optimal winning probability of such a game for every number of parties $k$, and show that it decays exponentially when the game is played $n$ times in parallel. These results have applications to quantum cryptography, in particular in the topic of quantum position verification, where we show security of the routing protocol (played in parallel), and a variant of it, in the random oracle model. - 6 October 2023
- Published by American Physical Society (APS) in Physical Review Letters
- Vol. 131 (14) , 140802
Single-Qubit Loss-Tolerant Quantum Position Verification Protocol Secure against Entangled Attackers
We give a tight characterization of the relation between loss tolerance and error rate of the most popular protocol for quantum position verification, which is based on BB84 states. Combining it with classical information, we show, using semidefinite programming, for the first time a fault-tolerant protocol that is secure against attackers who preshare a linear amount of entanglement (in the classical information), arbitrarily slow quantum information and that tolerates a certain amount of photon loss. We also extend this analysis to the case of more than two bases, showing even stronger loss tolerance for that case. Finally, we show that our techniques can be applied to improve the analysis of one-sided device-independent quantum key distribution protocols.
- 6 October 2023
- Published by American Physical Society (APS) in Physical Review Letters
- Vol. 131 (14) , 140802
Single-Qubit Loss-Tolerant Quantum Position Verification Protocol Secure against Entangled Attackers
We give a tight characterization of the relation between loss tolerance and error rate of the most popular protocol for quantum position verification, which is based on BB84 states. Combining it with classical information, we show, using semidefinite programming, for the first time a fault-tolerant protocol that is secure against attackers who preshare a linear amount of entanglement (in the classical information), arbitrarily slow quantum information and that tolerates a certain amount of photon loss. We also extend this analysis to the case of more than two bases, showing even stronger loss tolerance for that case. Finally, we show that our techniques can be applied to improve the analysis of one-sided device-independent quantum key distribution protocols.
- 14 December 2022
- Published by American Physical Society (APS) in Physical Review A
- Vol. 106 (6) , 062415
Quantifying causal influence in quantum mechanics
We extend Pearl's definition of causal influence to the quantum domain, where two quantum systems with finite-dimensional Hilbert space are embedded in a common environment and propagated with a joint unitary . For a finite-dimensional Hilbert space , we find the necessary and sufficient condition on for a causal influence of on and vice versa. We introduce an easily computable measure of the causal influence and use it to study the causal influence of different quantum gates, its mutuality, and quantum superpositions of different causal orders. For two two-level atoms dipole-interacting with a thermal bath of electromagnetic waves, the space-time dependence of causal influence almost perfectly reproduces the one of reservoir-induced entanglement.- 14 December 2022
- Published by American Physical Society (APS) in Physical Review A
- Vol. 106 (6) , 062415
Quantifying causal influence in quantum mechanics
We extend Pearl's definition of causal influence to the quantum domain, where two quantum systems with finite-dimensional Hilbert space are embedded in a common environment and propagated with a joint unitary . For a finite-dimensional Hilbert space , we find the necessary and sufficient condition on for a causal influence of on and vice versa. We introduce an easily computable measure of the causal influence and use it to study the causal influence of different quantum gates, its mutuality, and quantum superpositions of different causal orders. For two two-level atoms dipole-interacting with a thermal bath of electromagnetic waves, the space-time dependence of causal influence almost perfectly reproduces the one of reservoir-induced entanglement.- Open Access
- 24 February 2020
- Published by American Physical Society (APS) in Physical Review Research
- Vol. 2 (1) , 012044
All tight correlation Bell inequalities have quantum violations
It is by now well established that there exist nonlocal games for which the best entanglement-assisted performance is not better than the best classical performance. Here we show in contrast that any two-player XOR game, for which the corresponding Bell inequality is tight, has a quantum advantage. In geometric terms, this means that any correlation Bell inequality for which the classical and quantum maximum values coincide, does not define a facet, i.e., a face of maximum dimension, of the local (Bell) polytope. Indeed, using semidefinite programming duality, we prove upper bounds on the dimension of these faces, bounding it far away from the maximum. In the special case of nonlocal computation games, it had been shown before that they are not facet defining; our result generalizes and improves this. As a by-product of our analysis, we find a similar upper bound on the dimension of the faces of the convex body of quantum correlation matrices, showing that (except for the trivial ones expressing the non-negativity of probability) it does not have facets.
- 24 February 2020
- Published by American Physical Society (APS) in Physical Review Research
- Vol. 2 (1) , 012044
Open AccessAll tight correlation Bell inequalities have quantum violations
It is by now well established that there exist nonlocal games for which the best entanglement-assisted performance is not better than the best classical performance. Here we show in contrast that any two-player XOR game, for which the corresponding Bell inequality is tight, has a quantum advantage. In geometric terms, this means that any correlation Bell inequality for which the classical and quantum maximum values coincide, does not define a facet, i.e., a face of maximum dimension, of the local (Bell) polytope. Indeed, using semidefinite programming duality, we prove upper bounds on the dimension of these faces, bounding it far away from the maximum. In the special case of nonlocal computation games, it had been shown before that they are not facet defining; our result generalizes and improves this. As a by-product of our analysis, we find a similar upper bound on the dimension of the faces of the convex body of quantum correlation matrices, showing that (except for the trivial ones expressing the non-negativity of probability) it does not have facets.