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Applied Physics Letters

High-temperature superconductor quantum flux parametron for energy efficient logic

Han Cai, Jay C. LeFebvre, Hao Li, Ethan Y. Cho, Nobuyuki Yoshikawa, Shane A. Cybart

Published May 20, 2024• 7 citations
Publisher

Abstract

We report the fabrication and measurement of quantum flux parametron logic from high-transition-temperature Josephson junctions operating at 25 K, above the temperature of liquid helium. The circuits are written directly into the plane of a single-layer thin film of YBa2Cu3O7 using a focused helium ion beam. A single-cell quantum flux parametron was constructed and correct logic operation was verified by using an on-chip superconducting quantum interference device for readout. A three-cell shift register was also fabricated and tested using a three-phase clock cycle. We estimate the average bit energy to be about 1×10−20 J at 1 GHz.

DOI
10.1063/5.0206445

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We report the fabrication and measurement of quantum flux parametron logic from high-transition-temperature Josephson junctions operating at 25 K, above the temperature of liquid helium. The circuits are written directly into the plane of a single-layer thin film of YBa2Cu3O7 using a focused helium ion beam. A single-cell quantum flux parametron was constructed and correct logic operation was verified by using an on-chip superconducting quantum interference device for readout. A three-cell shift register was also fabricated and tested using a three-phase clock cycle. We estimate the average bit energy to be about 1×10−20 J at 1 GHz.
As we rapidly advance through the information age, the power consumed by computers, data centers, and networks grows exponentially. This has inspired a race to develop alternative low-power computational technologies. A new adiabatic configuration of a decades-old superconducting digital logic device has darted into the lead called quantum flux parametrons (QFP). QFP operate with dissipation so low that they seemingly violate the laws of thermodynamics. In just a short span of time, they have gone from simple single NOT gates to complex processors containing thousands of gates. They are fabricated from elemental niobium superconductors cooled to just a few degrees above absolute zero. However, their efficiency is so great that for large high-performance computers with several gates, the energy savings are immense. For smaller computational platforms QFPs from high-temperature superconductors (high-Tc) are highly desirable. In this work, we take the first steps towards this goal with the demonstration of a high-T C QFP shift register. Our device is fabricated using focused helium ion beam lithography where the material is modified with an ion beam at the nanoscale to directly pattern these circuits into a high-T C thin film. We validate the correct logical operation at 25 K, over 6 times higher than niobium devices with an estimated bit energy of 0.1 attoJoule at 10 GHz.
YBa2_{2}2​Cu3_{3}3​O7−δ_{7-\delta }7−δ​Single Flux Quantum Flip Flop Directly Written With a Focused Helium Ion Beam
Superconducting logic circuits based on low transition temperature (Tc) are attractive due to their intrinsic low power consumption and high operating frequency. They have a wide variety of applications (particularly in future computing), but are difficult to use because they require cooling with liquid helium to operate. The current high Tclogic circuit based on YBa2Cu3O7-δ(YBCO) already has the ability to operate at a temperature above 4 K. However, this material is difficult to integrate into large-scale circuits by the traditional YBCO junction fabrication process. The limits of fabrication methods are the bottleneck stifling this material's potential for integration into logic circuits. As such, in this paper we aim to advance the state of the art in junction fabrication technology. Here we report the demonstration of a high Tcsuperconducting single flux quantum (SFQ) circuit made by a focused helium ion beam that is easily fabricated and can readily be incorporated into a large-scale integration with nano-scale junctions for future applications. An inductively coupled readout superconducting quantum interference device (SQUID) is employed to reduce the circuit complexity. The circuit parameters and performance are discussed.
We have implemented a simple circuit of the rapid single-flux-quantum (RSFQ) logic family using a single-layer YBa2Cu3O7−x thin-film structure with 14 in-plane Josephson junctions formed by direct electron beam writing. The circuit includes two dc/SFQ converters, two Josephson transmission lines, a complete RS SFQ flip-flop, and an SFQ/dc converter (readout SQUID). Low-frequency testing has shown that the dc-current-biased circuit operates correctly and reliably at T∼30 K, a few degrees below the effective critical temperature of the junctions. Prospects for a further increase of the operation temperature and implementation of more complex RSFQ circuits are discussed in brief.
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.
Abstruct-Quantum Flux Parametron (QFP) logic gates were designed and fabricated from a single layer of YJ3a2Cu307 thin film based on step-edge Josephson junctions. Self-inductance (L) of the QFP loop, an important circuit parameter, as is critical current (I,) 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.
A 3 bit single flux quantum (SFQ) shift register based on high-Tc bicrystal Josephson junctions has been designed, fabricated, and experimentally tested. The circuit consists of 26 bicrystal Josephson junctions and includes the shift register itself, two dc-SFQ converters, one readout superconducting quantum interference device, serving as a SFQ-dc converter, and three Josephson transmission lines. The correct operation of all circuit components has been demonstrated by low frequency testing at a temperature of 50 K.