Browsing by Author "Zurel, M."
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Item Open Access On the extremal points of the Λ-polytopes and classical simulation of quantum computation with magic states(Rinton Press, Inc., 2021-11) Okay, Cihan; Zurel, M.; Raussendorf, R.We investigate the Λ-polytopes, a convex-linear structure recently defined and applied to the classical simulation of quantum computation with magic states by sampling. There is one such polytope, Λn, for every number n of qubits. We establish two properties of the family {Λn,n∈N}, namely (i) Any extremal point (vertex) Aα∈Λm can be used to construct vertices in Λn, for all n>m. (ii) For vertices obtained through this mapping, the classical simulation of quantum computation with magic states can be efficiently reduced to the classical simulation based on the preimage Aα. In addition, we describe a new class of vertices in Λ2 which is outside the known classification. While the hardness of classical simulation remains an open problem for most extremal points of Λn, the above results extend efficient classical simulation of quantum computations beyond the presently known range.Item Open Access The role of cohomology in quantum computation with magic states(Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften, 2023-04-13) Raussendorf, R.; Okay, Cihan; Zurel, M.; Feldmann, P.A web of cohomological facts relates quantum error correction, measurement-based quantum computation, symmetry protected topological order and contextuality. Here we extend this web to quantum computation with magic states. In this computational scheme, the negativity of certain quasiprobability functions is an indicator for quantumness. However, when constructing quasiprobability functions to which this statement applies, a marked difference arises between the cases of even and odd local Hilbert space dimension. At a technical level, establishing negativity as an indicator of quantumness in quantum computation with magic states relies on two properties of the Wigner function: their covariance with respect to the Clifford group and positive representation of Pauli measurements. In odd dimension, Gross' Wigner function-an adaptation of the original Wigner function to odd-finite-dimensional Hilbert spaces-possesses these properties. In even dimension, Gross' Wigner function doesn't exist. Here we discuss the broader class of Wigner functions that, like Gross', are obtained from operator bases. We find that such Clifford-covariant Wigner functions do not exist in any even dimension, and furthermore, Pauli measurements cannot be positively represented by them in any even dimension whenever the number of qudits is n ≥ 2. We establish that the obstructions to the existence of such Wigner functions are cohomological.