# Quantum measurements in Majorana circuit quantum electrodynamics

- Quantenmessungen in der Schaltkreis-Quantenelektrodynamik der Majorana Quasiteilchen

Ohm, Christoph; Hassler, Fabian (Thesis advisor); Schmidt, Thomas (Thesis advisor); DiVincenzo, David P. (Thesis advisor)

*Aachen (2016, 2017)*

Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2016

Abstract

Quantum computation is a multifaceted field of research aiming for the physical realization of quantum systems and their manipulation. This thesis discusses the combination of two notable approaches in the pursuit of a fully operational quantum computer---circuit quantum electrodynamics and topological quantum computation based on Majorana quasiparticles. In circuit quantum electrodynamics quantum information is stored into small superconducting circuit elements whose interaction with electromagnetic radiation in the range of microwaves allows to process quantum information very efficiently. This approach has proven extremely useful for control and readout of superconducting qubits, i.e., small circuit elements that carry quantum information. Because of remarkably strong light-matter couplings that can be achieved for superconducting qubits in microwave resonators, the circuit quantum electrodynamics architecture is particularly useful to perform highly sensitive quantum measurements.Superconductivity by itself is an intriguing state of matter that shows a great variety of different phenomena. In particular, the discovery of topological phases in superconductors opened new horizons for quantum computation. One notable system that admits topological superconductivity is a semiconductor-superconductor nanowire with special zero modes occurring at its ends. These so-called Majorana zero modes are remarkably robust against decoherence and therefore well-suited for fault-tolerant quantum computation.The first part of this thesis examines the coupling of Majorana zero modes to electromagnetic radiation with microwave frequencies. The light-matter coupling mechanism that is considered here arises for Majorana zero modes located at a voltage-biased superconducting tunneling junction. The emission of microwave radiation in presence of Majorana zero modes gives rise to coherent radiation which is emitted at half of the usual Josephson frequency. On the basis of this fractional Josephson radiation, we propose a microwave readout scheme for Majorana qubits. As usual for typical measurements in circuit quantum electrodynamics, the proposed readout implements a quantum non-demolition measurement for Majorana qubit.In the last part of the thesis we propose a novel scheme for the implementation of measurement-induced entanglement between remote superconducting qubits as required for quantum communication. By detecting a single photon, that passes a Mach-Zehnder interferometric setup, deterministic entanglement with single-shot efficiency is achieved. This scheme essentially relies on the strong coupling between the qubits and the photon.

Institutions

- Junior Professorship of Theoretical Physics (Condensed Matter) [137230]
- Department of Physics [130000]

### Identifier

- URN: urn:nbn:de:hbz:82-rwth-2016-120797
- DOI: 10.18154/RWTH-2016-12079
- RWTH PUBLICATIONS: RWTH-2016-12079