  We propose a simple scheme for generating a high-fidelity controlled-Z (CZ)
gate in a three-component qubit/bus/qubit device. The corresponding tune/detune
pulse is single-step, with a near-resonant constant undershoot between the 200
and 101 states. During the pulse, the frequency of the first qubit is kept
fixed, while the frequency of the second qubit is varied in such a way as to
bring the 200 and 101 states close to resonance. As a result, the phase of the
101 state is accumulated via the corresponding second-order anticrossing. For
experimentally realistic qubit frequencies and a 75 MHz coupling (150 MHz
splitting), a 45 ns gate time can be realized with >99.99% intrinsic fidelity,
with errors arising due to the non-adiabaticity of the ramps. The CZ pulse is
characterized by two adjustable parameters: the undershoot magnitude and
undershoot duration. The pulse does not load an excitation into the bus. This
by-passes the previously proposed need for two additional qubit-to-bus and
bus-to-qubit MOVE operations. Combined with the recently predicted
high-fidelity idling operation in the RezQu architecture [A. Galiautdinov, J.
Martinis, A. Korotkov (unpublished)], this controlled-Z scheme may prove useful
for implementations on the first generation quantum computers.
