  There is great interest in using near-term quantum computers to simulate and
study foundational problems in quantum mechanics and quantum information
science, such as the scrambling measured by an out-of-time-ordered correlator
(OTOC). Here we use an IBM Q processor, quantum error mitigation, and weaved
Trotter simulation to study high-resolution operator spreading in a 4-spin
Ising model as a function of space, time, and integrability. Reaching 4 spins
while retaining high circuit fidelity is made possible by the use of a
physically motivated fixed-node variant of the OTOC, allowing scrambling to be
estimated without overhead. We find clear signatures of ballistic operator
spreading in a chaotic regime, as well as operator localization in an
integrable regime. The techniques developed and demonstrated here open up the
possibility of using cloud-based quantum computers to study and visualize
scrambling phenomena, as well as quantum information dynamics more generally.
