Longitudinal Coupling Structures
A large burden of small but not negligible stray couplings is troublesome for many multi-qubit layout concepts, particularly those involving superconducting qubits.
Our investigations have recently uncovered [DD2] the possibility of altering the basic form of the qubit-bus coupling; the Jaynes-Cummings, or “transverse” form of coupling has clear disadvantages compared with a “longitudinal” form.
Longitudinal coupling avoids many potential long-distance effective couplings that are seen with the systematic application of perturbation (Schrieffer-Wolff) theory to the existing transverse-coupling model. The longitudinal-coupling approach has the potential of substantially removing crosstalk between neighboring qubits, allowing them to be fabricated exactly at the same frequency.