Superconducting NbTi Radiofrequency Resonator for Surface ion Traps

Trapped-ion qubits are one approach among many for achieving scalable quantum computers. An ion trap has to be operated with several dc and radio frequency (rf) signals to trap and control its qubits. Many ion trap setups are operated at cryogenic temperatures to reduce thermal influences, to reach very high vacuum, and to achieve high fidelity for quantum operations. A resonance circuit, consisting of a coil and the capacitance of the trap electrodes, is used to step-up a low power rf signal to high amplitudes in close proximity to the ion trap. These ac fields are used to confine ions
$70 \,{\rm \mu }\mathrm{m}$
above a surface ion trap chip. An increased quality factor (Q-factor) of the resonance circuit leads to a higher voltage gain, but the experiment also benefits from a better attenuation of parasitic frequency components in the confining electric field. The Q-factor is inversely proportional to the trap capacitance. Since ion traps are growing in size due to a larger number of qubits, the capacitance is increasing. Therefore, the development of a coil with low losses becomes even more important. In this work, we present the setup of a superconducting coil for a high-Q resonator, measurements of the Q-factor, and its temperature dependency. The coil is made of a niobium-titanium (NbTi) wire wound on a threaded bobbin made of ceramics and equipped with further thermalization structures. The superconducting resonator is a very promising approach to satisfy the needs for future trapped-ion quantum computing setups.