  On today's noisy imperfect quantum devices, execution fidelity tends to
collapse dramatically for most applications beyond a handful of qubits. It is
therefore imperative to employ novel techniques that can boost quantum fidelity
in new ways.
  This paper aims to boost quantum fidelity with Clifford canary circuits by
proposing Quancorde: Quantum Canary Ordered Diverse Ensembles, a fundamentally
new approach to identifying the correct outcomes of extremely low-fidelity
quantum applications. It is based on the key idea of diversity in quantum
devices - variations in noise sources, make each (portion of a) device unique,
and therefore, their impact on an application's fidelity, also unique.
  Quancorde utilizes Clifford canary circuits (which are classically simulable,
but also resemble the target application structure and thus suffer similar
structural noise impact) to order a diverse ensemble of devices or
qubits/mappings approximately along the direction of increasing fidelity of the
target application. Quancorde then estimates the correlation of the
ensemble-wide probabilities of each output string of the application, with the
canary ensemble ordering, and uses this correlation to weight the application's
noisy probability distribution. The correct application outcomes are expected
to have higher correlation with the canary ensemble order, and thus their
probabilities are boosted in this process.
  Doing so, Quancorde improves the fidelity of evaluated quantum applications
by a mean of 8.9x/4.2x (wrt. different baselines) and up to a maximum of 34x.
