Entanglement gain in measurements with unknown results

TL;DR

This paper investigates how non-selective global projective measurements can increase entanglement between two particles, showing they can enhance pure non-maximally entangled states but not mixed states, with implications for open system dynamics.

Contribution

It characterizes the entanglement gain possible through non-selective global measurements and analyzes their limitations and approximations in open quantum systems.

Findings

Non-selective global measurements can increase entanglement of pure non-maximally entangled states.

Such measurements cannot increase entanglement of mixed states.

Markovian open system dynamics approximate these measurements exponentially fast.

Abstract

We characterise non-selective global projective measurements capable of increasing quantum entanglement between two particles. We show that non-selective global projective measurements are capable of increasing entanglement between two particles, in particular, entanglement of any pure non-maximally entangled state can be improved in this way (but not of any mixed state) and we provide detailed analysis for two qubits. It is then shown that Markovian open system dynamics can only approximate such measurements, but this approximation converges exponentially fast as illustrated using Araki-Zurek model. We conclude with numerical evidence that macroscopic bodies in a random pure state do not gain entanglement in a random non-selective global measurement.