Robust selftesting of twoqubit states
Abstract
It is well known that observing nonlocal correlations allows us to draw conclusions about the quantum systems under consideration. In some cases this yields a characterisation which is essentially complete, a phenomenon known as selftesting. Selftesting becomes particularly interesting if we can make the statement robust, so that it can be applied to a real experimental setup. For the simplest selftesting scenarios the most robust bounds come from the method based on operator inequalities. In this work we elaborate on this idea and apply it to the family of tilted ClauserHorneShimonyHolt (CHSH) inequalities. These inequalities are maximally violated by partially entangled twoqubit states and our goal is to estimate the quality of the state based only on the observed violation. For these inequalities we have reached a candidate bound and while we have not been able to prove it analytically, we have gathered convincing numerical evidence that it holds. Our final contribution is a proof that in the usual formulation, the CHSH inequality only becomes a selftest when the violation exceeds a certain threshold. This shows that selftesting scenarios fall into two distinct classes depending on whether they exhibit such a threshold or not.
 Publication:

Physical Review A
 Pub Date:
 May 2019
 DOI:
 10.1103/PhysRevA.99.052123
 arXiv:
 arXiv:1902.00870
 Bibcode:
 2019PhRvA..99e2123C
 Keywords:

 Quantum Physics
 EPrint:
 8 pages, 1 figure. Comments are welcome