<i>In situ</i> control of the resonant frequency of kinetic inductance detectors with multiplexed readout

Abstract

Large multiplexing factors are a primary advantage of kinetic inductance detectors (KIDs), but the implementation of high-density arrays still presents significant challenges. Deviations between designed and achieved resonant frequencies are common, and differential loading and responsivity variation across an array may lead to dynamic inter-resonator interactions. It is, therefore, valuable to be able to both set and maintain the resonant frequency of a KID in situ, using the readout system. We show that it is possible to alter the resonant frequency of the devices by more than one resonator linewidth through the application of readout current and establish a new stable operational bias point at the driven frequency by making use of the hysteretic bistability commonly seen as bifurcation in frequency-domain measurements. We examine this interaction using a readout tone at fixed frequency positioned near or within the unbiased resonant bandwidth. Development of a control methodology based on this principle remains in an early stage, but a foundational step is understanding the interaction of the readout current with the resonator, in particular its influence on the resonant frequency. In this work, we study conventional KIDs with no physical isolation from the substrate, so we posit that the readout current primarily interacts with the resonator via non-thermal mechanisms, resulting in a predominantly reactive response. This behavior is reproduced by a simple lumped-element circuit model of the resonance and readout system, providing a straightforward framework for analysis and interpretation. This demonstration is an important early step in the development of techniques that seek to dynamically alter the resonant frequencies of conventional KID arrays and sets the stage for fast active resonant frequency control under operational conditions.