On the Device-Independent Approach to Quantum Physics

Alexei Grinbaum

Valerio Scarani

Rotem Arnon-Friedman

The field of device-independent (DI) quantum information processing concerns itself with devising and analysing protocols, such as quantum key distribution, without referring to the quality of the physical devices utilised to execute the protocols. Instead, the analysis is based on the observed correlations that arise during a repeated interaction with the devices and, in particular, their ability to violate the so called Bell inequalities. Since the analysis of DI protocols holds irrespectively of the underlying physical device, it implies that any device can be used to execute the protocols: If the apparatus is of poor quality, the users of the protocol will detect it and abort; otherwise, they will accomplish their goal. This strong statement comes at a price-- the analysis of DI protocols is, a priori, extremely challenging. The thesis presents an approach that can be taken to simplify the analysis of DI information processing protocols. The idea is the following: Instead of analysing the most general device leading to the observed correlations, one should first analyse a significantly simpler device that, in each interaction with the user, behaves in an identical way, independently of all other interactions. We call such a device an independently and identically distributed (IID) device. As the next step, special techniques are used to prove that, without loss of generality, the analysis of the IID device implies similar results for the most general device. These are termed reductions to IID. We present two mathematical techniques that can be used as reductions to IID in the DI setting, each accompanied by a showcase-application that exemplifies the reduction's usage and benefits. Performing the analysis via a reduction to IID leads to simpler proofs and significant quantitive improvements, matching the tight results proven when analysing IID devices.

Valerio Scarani

Rafael Luiz da Silva

Anubhav Chaturvedi, Manik Banik

Nonlocal correlations are useful for device-independent (DI) randomness certification (P ironio S. et al. , Nature (London) , 464 (2010) 1021). The advantage of this DI protocol over the conventional quantum protocol is that randomness can be certified even when experimental apparatuses are not trusted. Quantum entanglement is the necessary physical source for the nonlocal correlation required for such DI task. However, nonlocality and entanglement are distinct concepts. There exist entangled states which produce no nonlocal correlation and hence are not useful for the DI randomness certification task. Here we introduce the measurement-device–independent randomness certification task where one has trusted quantum state preparation devices but the mesurement devices are completely unspecified. Interestingly we show that there exist entangled states, with local description, that are a useful resource in such task but are useless in the corresponding DI scenario.