  Noise is the central obstacle to building large-scale quantum computers.
Quantum systems with sufficiently uncorrelated and weak noise could be used to
solve computational problems that are intractable with current digital
computers. There has been substantial progress towards engineering such
systems. However, continued progress depends on the ability to characterize
quantum noise reliably and efficiently with high precision. Here we describe
such a protocol and report its experimental implementation on a 14-qubit
superconducting quantum architecture. The method returns an estimate of the
effective noise and can detect correlations within arbitrary sets of qubits. We
show how to construct a quantum noise correlation matrix allowing the easy
visualization of correlations between all pairs of qubits, enabling the
discovery of long-range two-qubit correlations in the 14 qubit device that had
not previously been detected. Our results are the first implementation of a
provably rigorous and comprehensive diagnostic protocol capable of being run on
state of the art devices and beyond. These results pave the way for noise
metrology in next-generation quantum devices, calibration in the presence of
crosstalk, bespoke quantum error-correcting codes, and customized
fault-tolerance protocols that can greatly reduce the overhead in a quantum
computation.
