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Superconducting Microwave Cavities for the Optoelectronic Oscillator and the Study of Magnetic Levitation

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Superconducting radio frequency cavities have applications in fields including particle accelerators, force sensing, narrow filtering, and more. Here, we look at a progression of work with SRF cavities starting from the design and fabrication stage, then moving to frequency and quality factor measurements. A specific type of microwave cavity, the coaxial stub cavity, is considered. We look at the finite element simulations that calculate the TEM-like coaxial mode of the cavity and discuss the configuration of the electric and magnetic field components.

Once the prototypes are fabricated and tested, we turn to applications of this specific type of cavity. The high Q nature of this particular configuration allows for a highly selective filter element and is used in a hybrid optical-electrical device called the optoelectronic oscillator. We characterize the room temperature implementation of this device, which consists of a laser, a modulator, an optical delay line, a photodiode, an amplifier, and the cavity. This device produces an optical pulsetrain with a repetition rate determined by the filter element in the loop along with a high spectral purity, low phase noise microwave output whose frequency is also determined by the filter element. The phase noise, microwave, and optical spectra are characterized for room temperature and initial results from cryogenic experiments are shown for the microwave output.

Additionally, the coupling of a levitated magnet's mechanical oscillations to a radio frequency (RF) mode inside of a superconducting cavity may lay the groundwork for coupling to quantum objects whose states can be probed and controlled, such as magnons or transmons. We have previously reported levitation of a strong, mm-scale neodymium magnet within a cm-scale coaxial microwave resonator. In trying to better understand the behavior of the system, we have developed a finite element model that allows us to calculate the potential energy landscape of the cavity-magnet system. This can be done for a wide array of cavity and magnet specifications as well as any magnet orientation. By identifying the stable points, we can design the cavity such that levitation can be forced in regions of interest, such as regions of maximum electric and magnetic fields. We can also control the system's sensitivity to the magnet's motion and approximate the mechanical frequency at which the magnet vibrates about equilibrium. The calculated potential energy landscapes are used, in combination with other geometry-based FEM simulations, in comparison with experimental measurements of the shift in resonance frequency with the motion of the magnet when there is no optical access inside of the cavity at cryogenic temperatures.

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This item is under embargo until June 24, 2024.