Controlled initialization of superconductingπ-phaseshifters and possible applications

Abstract

The rapid single-flux quantum electronics (RSFQ) is a superconducting, naturally digitalcircuit family which is currently close to being commercially applied. RSFQ is outstandingbecause of its very low switching energy resulting in very low power consumption. Thisadvantage causes, however, a significant influence of thermal noise. For industrialapplications, a certain noise immunity is required which is still a challenge, especially forcircuits of higher complexity.Integrating phase-shifting elements is a new concept for further improvements concerningstability against the influence of thermal noise. We have already shown that theimplementation of phase-shifting elements significantly reduces the influence ofthermal noise on circuit behavior by experimentally analyzing the bit-error rate(Mielke et al 2009 IEEE Trans. Appl. Supercond. 19 621–5). Concepts which areeasily implementable in standard niobium technology are especially promising.The π-phaseshifter consists of a superconducting loop which is able to store a single flux quantum. Theloop current related to the stored flux creates a well-defined phase shift. To achieve the correctfunctionality of complex circuits it is essential to store exactly one flux quantum in eachπ-phaseshifter during the cooling down of the chip. Thus, for studyingthe feasibility of this new approach, the initialization reliability of theπ-phaseshifter needs to be verified.We present an experimental investigation of this reliability to obtain a general assessment for the applicationof the π-phaseshifter in niobium technology. Furthermore, we compare the configuration shieldedby a solid ground plane with a configuration with a ground-plane hole below theπ-phaseshifter.Justified by the experimental results we suggest programmable RSFQ circuits based onπ-phaseshifters. The characteristics of these devices can be influenced by a controlled initialization ofthe π-phaseshifter.The fabrication was performed by FLUXONICS Foundry.