Entangling measurements for the cautious: paper in Physical Review Letters

Researchers thinking ahead to an era of commercial quantum technologies want to safeguard against unscrupulous suppliers. Their concern is to ensure that quantum procedures can be carried out properly even if the devices that do them are purchased from a possibly untrustworthy source.

In a paper published 25 July in Physical Review Letters, CQT's Valerio Scarani and his colleagues tackle this problem for a device needed to do quantum teleportation, proposing a design that could be certified to function prior to use. "The ultimate goal is, I don't trust the box, I want to check if it behaves as it should," says Valerio. The paper is highlighted by the journal as an "Editors' Suggestion".

In order to teleport the state of system A in Alice's hands to system B in Bob's hands, B must be initially entangled with another system in Alice's hands (A1). Teleportation happens when Alice performs a measurement that entangles A and A1. Valerio and other CQT researchers Rafael Rabelo, Melvyn Ho and Daniel Cavalcanti, together with Nicolas Brunner of the University of Bristol, UK, show how to test that an uncharacterized device creates such entanglement.

In their protocol, the device must be able to perform three measurements: two are used to check that two systems aren't entangled to start with, and the third does the desired entangling. Entanglement between the systems is confirmed by looking for a violation of Bell's Inequality — a quantity that only exceeds 2 when quantum correlations are present, reaching a maximum of 2√2 for maximally entangled states.

As long as the device passes the test, the cautious experimenter can be reassured that the device's third setting works as it should. The experimenter can then use with confidence the 'entangling' measurement in other procedures, such as teleportation. "It's the first time a device-independent proof has been made for a measurement box," says Valerio. Previous protocols were devised for sources.

For further details, see "Device-Independent Certification of Entangled Measurements", Physical Review Letters 107, 050502 (2011); arXiv:1105.3138.

Valerio is a Principal Investigator at CQT and an Associate Professor at NUS. His other CQT collaborators are members of his group: Rafael is a PhD student, Melvyn is a Master student and Daniel is a Research Fellow.