Many applications of quantum communication require a high amount of entanglement between the states of two separated parties, for example quantum teleportation or superdense coding. However, when states are locally entangled and then separated, errors arise during transportation of the qubits. This results in a lower amount of entanglement between the states. Thus, we need protocols that apply local operations on the qubits of the two separate parties to achieve higher entanglement. This is called entanglement concentration. We analyze two protocols that classically can be used for the extraction of randomness and apply it to the concentration of entanglement: Von Neumann’s protocol and Elias’s protocol. These protocols are well-described in the literature. We analyze the performance of the protocols on quantum computers with varying error rates. We find that Von Neumann’s protocol extracts entanglement similar to the theory on the best 5-qubit processor (Athens): there is an average percentage change in the concurrence when the protocol succeeds of 49.0%, which is close to the theoretical value of 52.5%. Elias’s protocol requires many more qubits than Von Neumann’s protocol. The only quantum computer large enough to run the protocol (Melbourne) has high error rates. Also, Elias’s protocol requires more operations. This results in higher errors when we execute the protocol on Melbourne. We execute the protocol with 2 initial states and find an average percentage decrease of the concurrence when the protocol succeeds of 72.6%. Thus, we find that Elias’s protocol is not suited to use in practice and the Von Neumann protocol can be used to extract entanglement with the best quantum processors used in this work.

For many quantum applications we require high-fidelity entanglement between multiple pairs of solid state qubits at a distance. To achieve a high fidelity, we have to minimize the time during which the generated qubits need to stay coherent. Entanglement protocols often used in practice only generate one qubit at the same time. To generate multiple entangled pairs, the protocol is repeated. However during the time it takes for all pairs to be generated, the memory qubits will dephase. The required coherence time increases with the inverse transmission probability of the photons, which decreases exponentially with distance. This thesis is concerned with entanglement generation protocols that herald multiple entangled pairs simultaneously and in general herald N-dimensional entangled bipartite states. The main advantage of using more than 2 dimensions is that the qudits only dephase during the time in which the protocol executes. With simulations we show that the fidelity of the entangled pairs created with our protocols is higher than the fidelity of pairs created by protocols that heralds one entangled pair for distances L > 10 km. We also show a polynomial relation between the total success probability of the tailored protocol with dimension, which is an exponential improvement with respect to previous works.