Quantitative verification of entanglement and fidelities from incomplete measurement data

TL;DR

This paper develops methods to estimate the entanglement and fidelities of multi-qubit quantum states using incomplete measurement data, enabling efficient verification without full state tomography.

Contribution

It introduces a semi-definite programming approach to bound entanglement from partial data, specifically for graph states with stabilizer measurements, reducing experimental complexity.

Findings

Provides analytical solutions for entanglement bounds

Demonstrates numerical methods applicable to experiments

Shows bounds are comparable to full tomography results

Abstract

Many experiments in quantum information aim at creating multi-partite entangled states. Quantifying the amount of entanglement that was actually generated can, in principle, be accomplished using full-state tomography. This method requires the determination of a parameter set that is growing exponentially with the number of qubits and becomes infeasible even for moderate numbers of particles. Non-trivial bounds on experimentally prepared entanglement can however be obtained from partial information on the density matrix. The fundamental question that needs to be addressed in this context is then formulated as: What is the entanglement content of the least entangled quantum state that is compatible with the available measurement data? We formulate the problem mathematically employing methods from the theory of semi-definite programming and then address this problem for the case, where the goal of the experiment is the creation of graph states. The observables that we consider are the generators of the stabilizer group, thus the number of measurement settings grows only linearly in the number of qubits. We provide analytical solutions as well as numerical methods that may be applied directly to experiments, and compare the obtained bounds with results from full-state tomography for simulated data.