Experimental Simulation of Closed Timelike Curves

TL;DR

This paper experimentally simulates quantum systems with closed timelike curves, revealing effects like perfect state discrimination and state preparation differences, providing insights into quantum causality and non-linearities.

Contribution

It presents the first experimental simulation of a qubit interacting with its past self, demonstrating key quantum effects in closed timelike curve scenarios.

Findings

Perfect discrimination of non-orthogonal states

Ability to distinguish different state preparations

Effects depend on initial states and decoherence

Abstract

Closed timelike curves are among the most controversial features of modern physics. As legitimate solutions to Einstein's field equations, they allow for time travel, which instinctively seems paradoxical. However, in the quantum regime these paradoxes can be resolved leaving closed timelike curves consistent with relativity. The study of these systems therefore provides valuable insight into non-linearities and the emergence of causal structures in quantum mechanics-essential for any formulation of a quantum theory of gravity. Here we experimentally simulate the non-linear behaviour of a qubit interacting unitarily with an older version of itself, addressing some of the fascinating effects that arise in systems traversing a closed timelike curve. These include perfect discrimination of non-orthogonal states and, most intriguingly, the ability to distinguish nominally equivalent ways of preparing pure quantum states. Finally, we examine the dependence of these effects on the initial qubit state, the form of the unitary interaction, and the influence of decoherence.