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Figure 2.5: Example Qubit Coupling Circuits – a) Capacitive coupling: Phase
qubits can be easily coupled via a capacitor that provides fixed-strength alwayson
coupling. b) Resonator coupling: Phase and charge qubits can be coupled via
a resonant bus. For the charge qubit the resonator enables long-distance coupling
despite the qubit’s high impedance, while for the phase qubit the resonator can
be used to provide a band-pass filter for the coupling.
[McDermott et al., 2005]. The low impedance allows the coupling wiring to be
quite long and thus gives great flexibility in the design of coupling geometries for
many qubits.
The flux qubit is commonly coupled by placing two qubits right next to each
other [Majer et al., 2005]. This allows their flux degree of freedom to interact
via the resulting mutual inductance between them. Long-distance coupling between
flux qubits has not yet been demonstrated, but a one-dimensional chain of
nearest-neighbor coupled qubits is sufficient to achieve universal quantum computation.
The required fidelities to successfully implement error correction for such
a geometry are much more stringent, though.
The high-impedance charge qubit is the least flexible when it comes to coupling.
It cannot be coupled with simple wires as the capacitive impedance to
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