David Poulin, Université de Sherbrooke

Practical characterization of quantum devices without tomography

co-authors: Marcus Da Silva, Olivier Landon-Cardinal

Université de Sherbrooke

Quantum tomography is the main method used to assess the quality of quantum information processing devices, but its complexity presents a major obstacle for the characterization of even moderately large systems. However, tomography generates much more information than is usually sought.

Taking a more targeted approach, we develop schemes that enable (i) estimating the fidelity of an experiment to a theoretical ideal description [1], (ii) learning which description within a reduced subset best matches the experimental data [2-3]. Both these approaches yield a significant reduction in resources compared to tomography. In particular, we show how to estimate the fidelity between a predicted pure state and an arbitrary experimental state using only a constant number of Pauli expectation values selected at random according to an importance-weighting rule. In addition, we propose methods for identifying a state inside interesting variational classes such as matrix product states (MPS) [2], which include GHZ, W and cluster states, and multi-scale entanglement renormalisation ansatz (MERA) [3], which describes critical quantum systems.

[1] da Silva, Landon-Cardinal and Poulin, PRL 107, 210404 (2011).
[2] Cramer, Plenio, Flammia, Somma, Gross, Bartlett, Landon-Cardinal, Poulin, Liu, Nature Commun. 1, 149 (2010).
[3] Landon-Cardinal, Poulin, arXiv:1204.0792