Abstract
Recent advances in quantum technologies have seen quantum computers using superconducting qubits approach the ability of classical computers.
These superconducting qubits are naturally microwave devices, but microwave photons are all too easily swamped by thermal noise at room temperatures, limiting the ability to connect different quantum devices. Optical photons, on the other hand, have much higher energies and can be readily used at room temperature without being lost in thermal noise. Optical photons are also compatible with photonic memories, whereas microwave photons have few successful methods of long-term storage.
Therefore, it seems likely that quantum computers will be hybrid devices, with (microwave) superconducting qubits using (optical) photonic memories and interconnects to the outside world, and so there needs to be a way to coherently convert between the two frequency regimes.
This thesis investigates the use of rare earth ions for this quantum frequency conversion between microwave photons and optical photons. In the past, spin transitions in erbium-doped yttrium orthosilicate have been seen to convert microwave photons to optical photons, but with conversion efficiencies limited by thermal excitation and the presence of unwanted erbium isotopes. Using isotopically enhanced Er-170 at dilution fridge temperatures, these limitations were addressed.
Two measurements were made of this isotopically enhanced system, one using resonant enhancement of only the microwave fields in the erbium containing material, and one with resonant enhancement of both the microwave and optical fields. A maximum conversion efficiency of 10⁻⁶ was seem for conversion between microwave photons of around 5 GHz to optical photons in the 1550 nm telecommunications band, with the limitations being identified as not fundamental properties of the system, but as control over the effective spin temperatures and detector noise.
One previously identified trade off in using doped samples is between the density of ions and their inhomogeneous linewidth. Fully concentrated materials have the potential to greatly increase the spectral density of ions while maintaining narrow spectral lines with long-range order, and there have been theoretical predictions that using collective spin excitations (magnons) could improve conversion efficiency. These are a new class of materials, and to investigate them, measurements were made of anti-ferromagnetic resonances in gadolinium vanadate. The behaviour of the spin resonances is observed, and strong coupling between the spins and the microwave field is seen, which is one requirement for high efficiency frequency conversion.