Quantum Communication Protocols with Superconducting Circuits

Abstract

In recent years superconducting qubits coupled to coplanar transmission line resonators were developed sufficiently to convincingly be considered a promising candidate for real- izing a scalable quantum computer. One of the main advantages of superconducting quantum circuits is the variability of many of its parameters and the possibility to engineer and fabricate the circuits with high accuracy. Furthermore, it is possible to make the couplings between its individual con- stituents (superconducting qubits) on the order of several hundreds of megahertz. Given typical decoherence rates of the system on the order of one hundred kilohertz, this allows for fast and high-fidelity quantum operations. It enables the realization of long coherence times of the system and high-fidelity quantum operations at the same time. In this thesis, the realization and characterization of important experimental build- ing blocks of a quantum processor are presented. Moreover, the fundamental concepts of circuit quantum electrodynamics are discussed. In particular we describe a typical exper- imental setup as well as the design and fabrication of superconducting quantum circuits. We investigate two quantum communication protocols with up to four transmon qubits and carefully characterize its quantum gates in order to achieve high fidelities. The protocol that we experimentally study first is deterministic quantum teleportation with real-time quantum feedback. We specifically concentrate on the characterization of the protocol by quantum process tomography. Furthermore, we investigated an entan- glement distillation protocol up to the single-shot readout, where two copies of weakly entangled states are used to achieve higher entanglement on one of the pairs. This proto- col is specifically interesting as it is using only local operations, classical communication and measurement. A clear increase of the entanglement by the coherent part of the pro- tocol is observed with averaged readout, where the entanglement is quantified with the entanglement measure concurrence and with quantum channel capacities. The presented techniques and results contribute towards realizing a scalable quantum computer with superconducting circuits and quantum communication in the microwave regime.