Experimental Measurement of Lower and Upper Bounds of Concurrence for Mixed Quantum States

TL;DR

This paper experimentally measures the bounds of quantum entanglement (concurrence) in two-qubit mixed states using photonic systems, introducing a simple method for obtaining two copies of the state for entanglement estimation.

Contribution

It presents a novel, straightforward technique to generate two identical copies of a mixed quantum state for concurrence measurement in photonic systems.

Findings

Measured concurrence bounds agree with density matrix reconstructions.

Proposed a simple method for obtaining two copies of a mixed state.

Method can be extended to multi-photon entanglement estimation.

Abstract

We experimentally measure the lower and upper bounds of concurrence for a set of two-qubit mixed quantum states using photonic systems. The measured concurrence bounds are in agreement with the results evaluated from the density matrices reconstructed through quantum state tomography. In our experiment, we propose and demonstrate a simple method to provide two faithful copies of a two-photon mixed state required for parity measurements: Two photon pairs generated by two neighboring pump laser pulses through optical parametric down conversion processes represent two identical copies. This method can be conveniently generalized for entanglement estimation of multi-photon mixed states.