Operator-Schmidt decompositions and the Fourier transform, with applications to the operator-Schmidt numbers of unitaries

The operator-Schmidt decomposition is useful in quantum information theory for quantifying the nonlocality of bipartite unitary operations. We construct a family of unitary operators on C ⊗ C whose operatorSchmidt decompositions are computed using the discrete Fourier transform. As a corollary, we produce unitaries on C ⊗ C with operatorSchmidt number S for every S ∈ {1, ..., 9}. This corollary was unexpected, since it contradicted reasonable conjectures of Nielsen et al [Phys. Rev. A 67 (2003) 052301] based on intuition from a striking result in the two-qubit case. By the results of Dür, Vidal, and Cirac [Phys. Rev. Lett. 89 (2002) 057901], who also considered the two-qubit case, our result implies that there are nine equivalence classes of unitaries on C3⊗C3 which are probabilistically interconvertible by (stochastic) local operations and classical communication. As another corollary, a prescription is produced for constructing maximally-entangled unitaries from biunimodular functions. Reversing tact, we state a generalized operator-Schmidt decomposition of the quantum Fourier transform considered as an operator C1 ⊗ C2 → C1 ⊗ C2 , with M1M2 = N1N2. This decomposition shows (by Nielsen’s bound) that the communication cost of the QFT remains maximal when a net transfer of qudits is permitted. In an appendix, a canonical procedure is given for removing basis-dependence for results and proofs depending on the “magic basis” introduced in [S. Hill and W. Wootters, “Entanglement of a pair of quantum bits,” Phys Rev. Lett 78 (1997) 5022-5025]. PACS numbers: 03.67.Hk, 03.65.Ud ∗[email protected]