Deletrious Effects of Spectator Qubits in Multiqubit Circuits

For a number of quantum technologies, such as superconducting qubits, multiqubit algorithms are performed by concatenating a series of one- and two-qubit gates. However, in such an architecture there will be no true one- and two-qubit gates because of spectator qubits, i.e., qubits which are connected, but should be idle during the gate. For drive activated two-qubit gates, such as the cross-resonance gate, the role of these spectator qubits can be non-trivial. Here we will discuss our studies on these spectator qubits, for example, by measuring the entanglement dynamics of the spectator qubit with the active qubits. Understanding spectator dynamics will lead to improved designs of multiqubit gates for quantum computing technologies.