What this document is
This paper, first posted to arXiv in October 2017 by a team of researchers including several from IBM, addresses a practical problem in the early era of quantum computing: today's quantum devices have several hundred qubits at most and only limited error correction, well short of the fault-tolerant, error-free machines that full-scale quantum computing eventually requires.
Rather than waiting for that future hardware, the authors describe a hybrid variational approach that runs partly on a quantum device and partly on a classical computer, using the quantum hardware to prepare highly entangled states and a classical optimizer to adjust the parameters controlling those states until they minimize a target cost function. The approach is presented as applicable both to classical optimization problems and to simulating quantum chemistry.
The paper's central claim is that useful quantum algorithms may be achievable within the shallow circuit depth that near-term devices can reliably run, rather than requiring the deep, long gate sequences that only a fault-tolerant quantum computer could execute without accumulating errors.
Where to find it
Available from arXiv.