Abstract
The quantum bits (qubits) on which superconducting quantum computers are based have energy scales corresponding to photons with GHz frequencies. The energy of photons in the gigahertz domain is too low to allow transmission through the noisy room-temperature environment, where the signal would be lost in thermal noise. Optical photons, on the other hand, have much higher energies, and signals can be detected using highly efficient single-photon detectors. Transduction from microwave to optical frequencies is therefore a potential enabling technology for quantum devices. High conversion efficiency and high fidelity (the similarity of input microwave state to the output optical state.) are important characteristics of these devices. In such devices, the efficiency of the up-conversion process is typically improved by increasing the power in the pump. In addition to this, the optical pump is dissipated as thermal photons via absorption-induced heating into the system. Since the frequency of thermal photons and qubits are in the same range, this can affect the quality of the transferred quantum state which is characterized by fidelity. Thus, the optical pump can be a source of thermal noise and degrade the fidelity. Realizing an optimum pump power level while investigating its effect on fidelity via temperature are important tasks in the evolution of these upconversion devices. In this work, we explore two kinds of hybrid quantum devices that can be used as frequency upconverts; the first one is a resonant electro-optic modulator consisting of a microwave cavity and a WGM resonator (made of LiNbO3) and the next one is a magneto-optic modulator consisting of a microwave cavity and the rare earth crystal gadolinium vanadate (GdVO4) in the cuboid shape. We model the sub-Kelvin thermal behavior of an electro-optic transducer based on a lithium niobate whispering gallery mode resonator. We compute thermal photon numbers and find that there is an optimum power level for a continuous pump, whilst pulsed operation of the pump increases the fidelity of the conversion. In the magneto-optic modulator via photon-magnon coupling, we investigate the effect of GdVO4 temperature on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator. In the next step, we investigate the effect of absorption-induced heating of GdVO4 on the linewidth of magnons and the strength of photon-magnon coupling inside a dilution refrigerator, and the temperature of GdVO4 is estimated via comparison of two methods. The effect of heating of the GdVO4 via the mentioned methods on the transmission spectrum and the resonance frequency of the microwave cavity is studied. The transmission spectrum is used to comprehend the time evolution of the thermal equilibrium of the sample and estimate its temperature.