Ferrite-based non-reciprocal cryogenic isolators are one of the critical components in the superconducting circuits that limit the realization of the large-scale system for radio astronomy and superconducting quantum computer due to their size. In a receiver frontend of a radio telescope, isolators are used between a superconductor-insulator-superconductor (SIS) mixer and cryogenic low-noise amplifier to mitigate the multiple reflection and provides the ideal condition in order for the cryogenic active components to be independently operated in the system. Performance of the microwave ferrite-based isolators reported in recent has greatly improved, showing acceptable low insertion loss over multi-octave bandwidths [1]. However, these conventional isolators are in principle physically large compared to scale of the superconducting millimeter/microwave integrated circuits. This makes it difficult to minimize physical size of the receiver frontend, and therefore, provides limitation of the number of pixels in multibeam heterodyne receivers.
To break through this limitation, we propose a novel isolator that could be fully integrated on a superconducting planar circuit. This isolator consists of two frequency converters (mixers), two phase-delay components that have phase shifts of π/4+nπ/2 (where n is an arbitrary integer), and a local oscillator (LO) source. The up-converted signals in the upper and lower sidebands at the first mixer are combined at the subsequent mixer in certain phase differences. Because each sideband is synthesized in phase for one direction and out of phase for the other direction, the circuit principally functions as an isolator with broadband from near DC to LO frequency. An isolator using commercially available microwave components successfully demonstrated more than 17 dB of isolation across 0.01–1.50 GHz at an LO frequency of 2.30 GHz [2]. Additionally, we conducted a proof-of-concept experiment using waveguide SIS mixers that have a positive gain [3]. This result showed a gain of 0–3 dB and an isolation of 20 dB across 0.1–5.0 GHz. Those results open up a new possibility of an on-chip superconducting isolator without using the ferrite material. At this conference, we will introduce the principle of the isolator, the results of the experiments using commercially available microwave components, and those of the experiments using W-band waveguide SIS mixers.
[1] L. Zeng, C. E. Tong, R. Blundell, P. K. Grimes and S. N. Paine, "A low-loss edge-mode isolator with improved bandwidth for cryogenic operation", IEEE Trans. Microw. Theory Techn., vol. 66, no. 5, pp. 2154-2160, May 2018.
[2] S. Masui, T. Kojima, Y. Uzawa and T. Onishi, "A Novel Microwave Nonreciprocal Isolator Based on Frequency Mixers," in IEEE Microwave and Wireless Technology Letters, vol. 33, no. 7, pp. 1051-1054, July 2023.
[3] T. Kojima, S. Masui, W. Shan and Y. Uzawa, "Characterization of a low-noise superconductor–insulator–superconductor-based microwave amplifier with local oscillator phase-adjusting architecture", Appl. Phys. Lett., vol. 122, no. 7, Feb. 2023.
This work was supported in part by JST [Moonshot R&D] Grant Number [JPMJMS2067] and JSPS KAKENHI Grant Numbers JP18H03881, JP19H02205, JP22H04955, and JP23K19203.
Keywords: Superconducting non-reciprocal device, SIS mixer, Isolator, Gyrator