Zenith Grant Awardee

Caslav Brukner

Institute of Quantum Optics and Quantum Information

Project Title

Quantum information without time and without causal order

Project Summary

One of the most deeply rooted concepts in science and in our everyday life is causality: the idea that events in the present are caused by events in the past and, in turn, act as causes for what happens in the future. If an event A is a cause of an effect B, then B cannot be a cause of A. According to quantum mechanics, however, objects can lose their well-defined classical properties, such as a particle that is in a \'superposition\' of two different locations at the same time. If we believe that quantum mechanics governs all phenomena, it is natural to expect that the order of events could also be indefinite, similarly to the location of a particle. The existence of such \'superpositions of causal orders\' may have far-reaching implications for the foundations of quantum mechanics, quantum gravity and quantum computing. In our project we will address the following research questions. Can we derive no-go theorems that will provable exclude any \'hidden\' causal explanation of superpositions of causal orders? Can these superpositions be harnessed to outperform the conventional, causal, quantum computers, similarly as the latter are shown to outperform their classical counterparts in solving hard computational problems?

Technical Abstract

Both quantum theory and general relativity appear to be fundamental, but the difference in their mathematical formulations and foundational concepts is a major hindrance to find a unified framework for the two theories. In quantum physics it is standardly assumed that the background time or definite causal structure pre-exists such that every operation is either in the future, in the past or space-like separated from any other operation. Our group has recently proposed a quantum framework that allows for correlations in which two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operation is before or after the other. In our project we will explore the bounds on quantum correlations with indefinite causal order, develop theory of such multiparty correlations and analyse communication tasks (\'causal games\') where using them as a resource have advantages over conventional, causal, quantum correlations. We will also derive Bell\'s type no-go theorem to exclude any \'hidden\' (local) causal explanation of quantum correlations with indefinite causal order. Our project might not only contribute to the development of new paradigm for quantum communication, but also sparks new ideas and methodological tools in quantum theories of gravity.