# getConcurrence

returns the concurrence between two qbits, using Wooters formula (Entanglement of Formation of an Arbitrary State of Two Qubits)

## Contents

- Syntax
- Description
- Example of two qbits which are coupled via a cavity

## Syntax

getConcurrence()

## Description

getConcurrence() returns the concurrence, which is a measure for entanglement between two qbits. This command can be used in the context of an arbitrary system. However, if the system contains more than 2 qbits all other qibts have to be traced out first using the traceOut(name) command.

## Example of two qbits which are coupled via a cavity

    clear; close all;
    s = System;
    a = Nlevel(1);
    s.addEntity(a,'qbit');
    s.addEntity(Qoscillator(3,1),'cavity');
    s.addEntity(Nlevel(1),'qbit2');
    s.addCoupling('qbit','cavity',0.05);
    s.addCoupling('cavity','qbit2',0.05);
    s.addDissipation('qbit',200);
    s.addExternalField(Gausspulse(0.04,100,30,1),'qbit');
    s.setTmax(400);
    s.setTimestep(0.1);
    s.simulate;
    s.traceOut('cavity')
    h =plot(s.time,s.getConcurrence);
    xlabel('Time in au');
    ylabel('Concurrence')

The above graphs show the concurrence between the two indirectly coupled qbits. This concurrence is calculated using the reduced density matrix reduced_rho_hist.