  Currently available superconducting quantum processors with interconnected
transmon qubits are noisy and prone to various errors. The errors can be
attributed to sources such as open quantum system effects and spurious
inter-qubit couplings (crosstalk). The ZZ-coupling between qubits in fixed
frequency transmon architectures is always present and contributes to both
coherent and incoherent crosstalk errors. Its suppression is therefore a key
step towards enhancing the fidelity of quantum computation using transmons.
Here we propose the use of dynamical decoupling to suppress the crosstalk, and
demonstrate the success of this scheme through experiments performed on several
IBM quantum cloud processors. In particular, we demonstrate improvements in
quantum memory as well as the performance of single-qubit and two-qubit gate
operations. We perform open quantum system simulations of the multi-qubit
processors and find good agreement with the experimental results. We analyze
the performance of the protocol based on a simple analytical model and
elucidate the importance of the qubit drive frequency in interpreting the
results. In particular, we demonstrate that the XY4 dynamical decoupling
sequence loses its universality if the drive frequency is not much larger than
the system-bath coupling strength. Our work demonstrates that dynamical
decoupling is an effective and practical way to suppress crosstalk and open
system effects, thus paving the way towards higher-fidelity logic gates in
transmon-based quantum computers.
