  As progress is made towards the first generation of error-corrected quantum
computers, robust characterization and validation protocols are required to
assess the noise environments of physical quantum processors. While standard
coherence metrics and characterization protocols such as T1 and T2, process
tomography, and randomized benchmarking are now ubiquitous, these techniques
provide only partial information about the dynamic multi-qubit loss channels
responsible for processor errors, which can be described more fully by a
Lindblad operator in the master equation formalism. Here, we introduce and
experimentally demonstrate Lindblad tomography, a hardware-agnostic
characterization protocol for tomographically reconstructing the Hamiltonian
and Lindblad operators of a quantum noise environment from an ensemble of
time-domain measurements. Performing Lindblad tomography on a small
superconducting quantum processor, we show that this technique characterizes
and accounts for state-preparation and measurement (SPAM) errors and allows one
to place bounds on the fit to a Markovian model. Comparing the results of
single- and two-qubit measurements on a superconducting quantum processor, we
demonstrate that Lindblad tomography can also be used to identify and quantify
sources of crosstalk on quantum processors, such as the presence of always-on
qubit-qubit interactions.
