Schrieffer-Wolff Methods for Annealing Qubits*

A robust method for characterizing the low-energy Pauli decomposition of coupled superconducting qubits is based on a generalization of Bravyi et al’s Schrieffer-Wolff transformation but applied in infinitesimal steps [1]. This method has several desirable properties. The effective Hamiltonian remains block diagonal so that the computational subspace is completely decoupled from the non-computational one. The adiabatic connection terms generated by the time-dependence of the computational subspace are block off-diagonal and thus do not generate any terms within the computational subspace. This work is a continuation of work described last year focused on the readout of single flux qubits. Subsequent numerical advances now permit the extension of the method to interacting tunable flux qubits including novel flux qubits capable of implementing strong non-stoquastic interactions.

[1] Bravyi et al, Annals of Physics 326, 2793 (2011)