Quantum 5, 522 (2021). https://doi.org/10.22331/q-2021-08-09-522 Although quantum channels underlie the dynamics of quantum states, maps which are not physical channels — that is, not completely positive — can often be encountered in settings such as […]
Quantum 5, 521 (2021). https://doi.org/10.22331/q-2021-08-09-521 Covariant codes are quantum codes such that a symmetry transformation on the logical system could be realized by a symmetry transformation on the physical system, usually with limited capability of […]
Quantum 5, 520 (2021). https://doi.org/10.22331/q-2021-08-09-520 The operational formulations of quantum theory are drastically time oriented. However, to the best of our knowledge, microscopic physics is time-symmetric. We address this tension by showing that the asymmetry […]
Quantum 5, 519 (2021). https://doi.org/10.22331/q-2021-08-09-519 We analyze bipartite matrices and linear maps between matrix algebras, which are respectively, invariant and covariant, under the diagonal unitary and orthogonal groups’ actions. By presenting an expansive list of […]
Quantum 5, 518 (2021). https://doi.org/10.22331/q-2021-08-05-518 In a recent work, it was shown by one of us (EGC) that Bell-Kochen-Specker inequality violations in phenomena satisfying the no-disturbance condition (a generalisation of the no-signalling condition) cannot in […]
Quantum 5, 517 (2021). https://doi.org/10.22331/q-2021-08-05-517 The surface code is a leading candidate quantum error correcting code, owing to its high threshold, and compatibility with existing experimental architectures. Bravyi et al. (2006) showed that encoding a […]
Quantum 5, 516 (2021). https://doi.org/10.22331/q-2021-08-03-516 Quantitative characterization of the spatial structure of single photons is essential for free-space quantum communication and quantum imaging. We introduce an interferometric technique that enables the complete characterization of a […]
Quantum 5, 515 (2021). https://doi.org/10.22331/q-2021-08-03-515 We study the relation of causal influence between input systems of a reversible evolution and its output systems, in the context of operational probabilistic theories. We analyse two different definitions […]
Quantum 5, 514 (2021). https://doi.org/10.22331/q-2021-08-03-514 Given a closed quantum system, the states that can be reached with a cyclic process are those with the same spectrum as the initial state. Here we prove that, under […]
Quantum 5, 513 (2021). https://doi.org/10.22331/q-2021-07-28-513 This paper proposes a highly efficient quantum algorithm for portfolio optimisation targeted at near-term noisy intermediate-scale quantum computers. Recent work by Hodson et al. (2019) explored potential application of hybrid […]
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