A Quantum Paradox of Choice: More Freedom Makes Summoning a Quantum State Harder

Adlam, Emily ; Kent, Adrian Patrick (2016)


The properties of quantum information in space-time can be investigated by studying operational tasks, such as ‘summoning’, in which an unknown quantum state is supplied at one point, and a call is made at another for it to be returned at a third. Hayden-May recently proved necessary and sufficient conditions for guaranteeing successful return of a summoned state for finite sets of call and return points when there is a guarantee of at most one summons. We prove necessary and sufficient conditions when there may be several possible summonses and complying with any one constitutes success, and demonstrate the existence of an apparent paradox: the extra freedom makes it strictly harder to complete the summoning task. This result has practical applications for distributed quantum computing and cryptography and also implications for our understanding of relativistic quantum information and its localization in space-time.