Abstract
Superconducting qubits are currently one of the leading qubit designs, showing great success in some of the world's most powerful quantum computers. Superconducting qubits, however, communicate at microwave frequencies, which is problematic when it comes to the task of long distance communication as unless the communication channel is cryogenically cooled the microwave photons are swamped by thermal noise. Optical photons, on the other hand, are ideal for long distance communication, particularly at frequencies that lie within the telecommunications band. One solution to the superconducting qubits communication problem is to create a device that can convert an input microwave photon into an optical photon with unit quantum efficiency. A device of such could then be used alongside existing optical fibre networks to connect distant superconducting processors. Furthermore, an upconversion device would allow superconducting qubit systems to take advantage of quantum technologies in the optical region, such as optical quantum memories.
Several different approaches have been taken in attempts to experimentally implement such a device. Recently a theoretical proposal showed that the strong non-linearities that occur near the collective resonances in fully concentrated rare-earth crystals can be used to efficiently upconvert microwave photons. In this thesis we begin experimental investigations of two fully concentrated rare-earth crystals, GdVO4 and DyPO4, for their application to microwave upconversion.
Absorption spectra of the two crystals in the optical region are measured using a Fourier transform infrared spectrometer, in order to characterise and explore the possible optical states to be used in the upconversion process. Experiments are then carried out at microwave frequencies to explore the coupling between the collective spin resonances (magnons) that occur within the crystals and a microwave cavity. It is found that not only can the ultrastrong coupling regime be reached in these systems but that the linewidth of the magnon mode is narrow, measuring 35MHz at T = 25mK. This is the first experimental evidence of ultrastrong coupling between a microwave cavity and antiferromagnetic magnon modes in a rare-earth crystal. Furthermore, narrow transition linewidths are a crucial requirement in the proposed upconversion method, therefore the narrow linewidth experimentally measured here is an important validation of the proposals feasibility.
The results presented in this thesis show that microwave upconversion mediated by fully concentrated rare-earth crystals has an exciting future. Perhaps one day rare-earth crystals will be used to overcome the major communication problem superconducting qubits currently face.