  The engineering of quantum devices has reached the stage where we now have
small scale quantum processors containing multiple interacting qubits within
them. Simple quantum circuits have been demonstrated and scaling up to larger
numbers is underway. However as the number of qubits in these processors
increases, it becomes challenging to implement switchable or tunable coherent
coupling among them. The typical approach has been to detune each qubit from
others or the quantum bus it connected to, but as the number of qubits
increases this becomes problematic to achieve in practice due to frequency
crowding issues. Here, we demonstrate that by applying a fast longitudinal
control field to the target qubit, we can turn off its couplings to other
qubits or buses (in principle on/off ratio higher than 100 dB). This has
important implementations in superconducting circuits as it means we can keep
the qubits at their optimal points, where the coherence properties are
greatest, during coupling/decoupling processing. Our approach suggests a new
way to control coupling among qubits and data buses that can be naturally
scaled up to large quantum processors without the need for auxiliary circuits
and yet be free of the frequency crowding problems.
