Mixedness and Entanglement in the presence of Localized Closed Timelike Curves

TL;DR

This paper explores how different models of quantum closed timelike curves (CTCs) affect the mixedness and entanglement of quantum states, revealing that P-CTCs can increase entanglement and purity more than D-CTCs.

Contribution

It introduces a comparative analysis of D-CTC and P-CTC effects on quantum states, highlighting P-CTC's ability to enhance entanglement and purity significantly.

Findings

P-CTC can increase the mixedness of the CR system.

P-CTC can convert mixed states into pure states.

P-CTC can maximally entangle partially entangled states.

Abstract

We examine mixedness and entanglement of the chronology-respecting (CR) system with assuming that quantum mechanical closed timelike curves (CTCs) exist in nature and by introducing the qubit system and applying the general controlled operations between CR and CTC systems. We use the magnitude of Bloch vector as a measure of mixedness. While Deutschian-CTC (D-CTC) either preserves or decreases the magnitude, postselected-CTC (P-CTC) can increases it. Nonintuitively, even the completely mixed CR-qubit can be converted into a pure state after CTC-qubit travels around the P-CTC. It is also shown that while D-CTC cannot increase the entanglement of CR system, P-CTC can increase it. Surprisingly, any partially entangled state can be maximally entangled pure state if P-CTC exists. Thus, distillation of P-CTC-assisted entanglement can be easily achieved without preparing the multiple copies of the partially entangled state.