Quantum-flux-parametron using rf-SQUIDs with Josephson π-junction as phase shfters

Stefano Poletto, CHIARELLO F., CASTELLANO M. G. +4 more

Kota Arai, Naoki Takeuchi, Taro Yamashita +1 more

Superconductor/ferromagnet/superconductor Josephson junctions across which the phases of superconducting order parameters are shifted by π in the ground state are called π Josephson junctions (π-JJs). Recently, several groups have established the fabrication technology of π-JJs so that superconductor logic families exploit π-JJs to improve circuit performance. In this paper, we investigate the advantages of π-JJs in adiabatic quantum-flux-parametron (AQFP) logic, which is an adiabatic superconductor logic based on the quantum-flux-parametron. First, we show that AQFP logic with π-JJs (π-AQFP) can operate using only π-JJs and can be powered and clocked in the same way as the conventional AQFP logic. Then, we show that π-AQFP logic can realize inverters without using signal transformers, which generally occupy a large area in AQFP gates and are difficult to miniaturize. We demonstrate π-AQFP inverters using numerical simulation to establish that π-AQFP logic operates adiabatically and robustly. Our results show that π-JJs can be easily incorporated into AQFP logic and enhance the circuit density of AQFP logic.

H. Hasegawa, Y. Tarutani, Uki Kabasawa +3 more

Quantum Flux Parametron (QFP) logic gates were designed and fabricated from a single layer of YBa/sub 2/Cu/sub 3/O/sub 7/ thin film based on step-edge Josephson junctions. Self-inductance (L) of the QFP loop, an important circuit parameter, as is critical current (J/sub c/) of the Josephson junction, was determined by using directly coupled dc superconducting quantum interference devices (SQUIDs). Magnetically coupled dc SQUIDs were used to detect the output signal from the QFP. Simulated waveforms were used to identify the operation modes of the QFP as a function of the circuit parameters. From examination of the output waveforms of the QFP, operation in agreement with simulations was confirmed.

B. Ruggiero, C. Granata, Antonio Vettoliere +5 more

Feng Li, Yuto Takeshita, Masamitsu Tanaka +1 more

We adopt superconductor/ferromagnet/insulator/superconductor (SFIS) Josephson junctions (JJs) as both switching JJs and intrinsic π phase shifters in superconductor digital circuits. The critical current density (Jc) and characteristic voltage (Vc) of the SFIS junctions are about 22 A/cm2 and 22 μV, respectively. The intrinsic π phase shift is confirmed by measuring the suppressed nominal critical current Icn and half-period-shifted modulation pattern of a π–π–π superconducting quantum interference device (SQUID) that contains three π-JJs in a superconducting loop. A single-flux-quantum (SFQ) circuit composed of a DC/SFQ, Josephson transmission line (JTL), and SFQ/DC converter based on SFIS JJs alone is demonstrated at 4.2 K. The energy dissipation of the SFQ/DC converter decreases by 80% because some JJs are self-biased by the π phase shifter. The intrinsic circulating currents induced by the π phase shifters lead to a narrow bias margin (±5%) and even error function, which can be solved by parameters optimization or circuit initialization in the future. The half-modulation period (Φ0/2) of a half-flux-quantum (HFQ) SQUID (a partial HFQ JTL) exhibits propagation of HFQ between π–π–π SQUIDs, indicating that more complex HFQ circuits can be developed with π-JJs alone in the future.

Yoshinao Mizugaki