Connecting processes with indefinite causal order and multi-time quantum states. (20th October 2017)

• Record Type: Journal Article
• Authors: Silva, Ralph; Guryanova, Yelena; Short, Anthony J; Skrzypczyk, Paul; Brunner, Nicolas; Popescu, Sandu
• Is Part Of: New journal of physics. Volume 19:Number 10(2017:Oct.)
• Publication Date: 2017-10-20
• Subjects: process matrices -- multi-time states -- post-selection -- indefinite causal order -- two-time states; Physics -- Periodicals; Physics; Periodicals; 530.05
• DOI: 10.1088/1367-2630/aa84fe
• Languages: English
• ISSNs: 1367-2630

Abstract

Abstract: Recently, the possible existence of quantum processes with indefinite causal order has been extensively discussed, in particular using the formalism of process matrices. Here we give a new perspective on this question, by establishing a direct connection to the theory of multi-time quantum states. Specifically, we show that process matrices are equivalent to a particular class of pre- and post-selected quantum states. This offers a new conceptual point of view to the nature of process matrices. Our results also provide an explicit recipe to experimentally implement any process matrix in a probabilistic way, and allow us to generalize some of the previously known properties of process matrices. Finally we raise the issue of the difference between the notions of indefinite temporal order and indefinite causal order, and show that one can have indefinite causal order even with definite temporal order.