Tuning high-Q superconducting resonators by magnetic field reorientation

Abstract

Superconducting resonators interfaced with paramagnetic spin ensembles are used to increase the sensitivity of electron spin resonance experiments and are key elements of microwave quantum memories. Certain spin systems that are promising for such quantum memories possess 'sweet spots' at particular combinations of magnetic fields and frequencies, where spin coherence times or linewidths become particularly favorable. In order to be able to couple high-Q superconducting resonators to such specific spin transitions, it is necessary to be able to tune the resonator frequency under a constant magnetic field amplitude. Here, we demonstrate a high quality, magnetic field resilient superconducting resonator, using a 3D vector magnet to continuously tune its resonance frequency by adjusting the orientation of the magnetic field. The resonator maintains a quality factor of > 10 5 up to magnetic fields of 2.6 T, applied predominantly in the plane of the superconductor. We achieve a continuous tuning of up to 30 MHz by rotating the magnetic field vector, introducing a component of 5 mT perpendicular to the superconductor.

Magnetic field robust high quality factor NbTiN superconducting microwave resonators

“…The large value of n c ph extracted from this fit is a consequence of the continuous rise in Q int with n ph as also previously observed for NbTiN in references [34,50], but also for other materials i.e. elementary Nb [54,55], NbN [47,56] and TiN [53]. One potential explanation is that NbTiN is considered an extreme 'dirty superconductor' with a large defect density.…”

“…For higher μ 0 H ext , the fitting of the S 21 (f)-curves becomes unreliable. The observed reduction in f r is fitted with the common quadratic relation [47,48]. For the internal quality factor Q int in figure 3(d), we observe a nearly constant Q int until approximately μ 0 H ext = 130 mT and a strong decrease for the internal quality factor for higher μ 0 H ext .…”

Spin-Resonance Linewidths of Bismuth Donors in Silicon Coupled to Planar Microresonators

“…In this work, the internal Q factor is approximately 100 000, with a coupled Q factor of approximately 30 000, meaning that this device is suboptimally coupled for a high efficiency quantum memory. In future devices, higher internal quality-factor resonators-such as those already demonstrated at fields compatible with bismuth clock transitions [14,45]-could give high-efficiency (0.97) memories without impacting the cooperativity measured here. The cooperativity in the narrow-κ limit is discussed in the Supplemental Material [22] and in Refs.…”

“…Of particular interest is the limiting case in which ∂f/∂B 0 ∼ 0, for example around the 6.76 GHz S z "clock transition," where Fig. 3(b) shows the echo-detected magnetic field sweep as resonator C is tuned downward in frequency by tilting the applied field with respect to the resonator plane [14]. EDFSs are collected at each B 0 as the resonator is tuned and are also shown in Fig.…”

“…In addition to the ensemble coupling strength, which can be modified by the choice of resonator design and spin concentration, two further critical parameters are the resonator and electron-spin-resonance (ESR) linewidths. Superconducting resonators with quality factors above 10 6 (corresponding to linewidths less than 10 kHz) have been demonstrated [14,15]. In contrast, ESR linewidths of bismuth donors in bulk-doped silicon have been measured at the X band (approximately 9.7 GHz) to be 4.5 MHz in silicon with natural isotopic abundance ( nat Si) and approximately 0.1 MHz in ion-implanted isotopically enriched 28 Si [2,16].…”