Direct observations of π-leaps of superconducting phase differences in π-junction-based SQUIDs

Abstract

We directly observed π-leaps of superconducting phase differences in π-junction-based superconducting quantum interference devices (SQUIDs). The SQUIDs studied here are formed by introducing a π-junction to a conventional-junction (0-junction)-based direct current (DC)-SQUID, which is referred to as the 0-0-π SQUID. Either clockwise- or counter-clockwise-circulating current flows in the 0-0-π SQUID spontaneously because of a π-phase shift of the π-junction. In other words, the 0-0-π SQUID has a bistable state corresponding to the directions of circulating currents. π-leaps are generated in transiting between the two states of the bistable state. π-leaps are an ultra-fast phenomenon and are difficult to be observed as are. We prepared a half-flux quantum (HFQ)-SQUID that comprised two 0-0-π SQUIDs. π-leaps are reflected in a static characteristic, that is, a modulation pattern of the critical current in the HFQ-SQUID. We formed π-junctions with the PdNi layer on 0-junction-based circuits supplied by the National Institute of Advanced Industrial Science and Technology. The modulation pattern of the HFQ-SQUID had a period corresponding to π-leaps as expected, although some microstructures were observed. We demonstrated that the microstructures originated from the asymmetry inside each 0-0-π SQUID by analysing the relationship between the phase change of 0-0-π SQUIDs and the modulation patterns.

Energy efficient half-flux-quantum circuit aiming at milli-kelvin stage operation

“…Additionally, 0-π SQUIDs with extremely small LI c products are employed as inductors with enhanced self-inductance [36]. Measurements of HFQ interference between 0-0-π SQUIDs have indicated the propagation of an HFQ in HFQ circuits [37]. Furthermore, an HFQ JTL connected to DC/SFQ and SFQ/DC converters has also been evaluated at 4.2 K, suggesting the possibility of integrating SFQ and HFQ circuits [38].…”

Flux transfer circuits breaking conventional limit in transfer coefficient based on a negative inductance of a π-junction

“…π-junctions are also made with a magnetic Josephson junction such as a superconductor-ferromagnet-superconductor (SFS) or a superconductor-ferromagnet-insulator-superconductor sandwich structure in which the ferromagnetic layer is carefully controlled [5][6][7][8][9][10][11][12][13][14][15]. π-junctions based on a magnetic Josephson junction have been applied to various superconductor circuits in recent years [16][17][18][19][20][21][22]. In particular, magnetic junctions using an alloy of Pd 89 Ni 11 as a ferromagnetic material showed relatively small spread I critical currents [9] and have become applicable to the integrated circuits [20][21][22].…”

“…π-junctions based on a magnetic Josephson junction have been applied to various superconductor circuits in recent years [16][17][18][19][20][21][22]. In particular, magnetic junctions using an alloy of Pd 89 Ni 11 as a ferromagnetic material showed relatively small spread I critical currents [9] and have become applicable to the integrated circuits [20][21][22]. If single-π-junction SQUIDs (π-SQUIDs) are formed under the condition 2πLI c /Φ 0 < 1, the internal flux exceeds the external flux, in other words, the slope exceeds the unity.…”

NbN-based tunnel-type π-junctions for low-power half-flux-quantum circuits

“…Furthermore, HFQ logic circuits based on all-π-junctions utilizing a superconductor/insulator/ferromagnet/superconductor (SIFS) structure have the potential to lower static power consumption and simplifying the fabrication process by eliminating the shunt resistors [9]. In terms of material choice, niobium nitride (NbN) holds advantages over conventional niobium (Nb) electrodes for monolithic HFQ circuits integration in qubit control applications [10][11][12][13][14]. The high kinetic inductance of NbN provides design benefits for compact HFQ circuits, reducing their footprint on the chip [15].…”

“…We developed a fabrication process for HFQ circuits utilizing all SIFS π-junctions. Unlike the conventional fabrication process for Nb-based circuits where the ground plane is located at the bottom of the chip [13,18], our fabrication process involved grounding the circuit using a ground plane on the chip's surface, allowing the epitaxial growth of NbN-based junctions from the substrate. This HFQ circuit layout can be compatible with NbN-based flux qubits for a monolithic fabrication process [19][20][21].…”