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Journal of Physics Conference Series

Design and operation of distributed double-SQUID amplifier for RSFQ circuits

Komei Higuchi, Hiroshi Shimada, Yoshinao Mizugaki

Published Sep 1, 2019• 3 citations
Publisher

Abstract

Recently, many groups have been developing on-chip Rapid Single-Flux-Quantum (RSFQ) output amplifiers which are designed to connect RSFQ circuitry to room temperature electronics. One of such amplifiers is a distributed SQUID amplifier comprising SQUIDs embedded in cascaded RSFQ RS flip-flops. In this paper, we propose a distributed double-SQUID amplifier, where a SQUID is replaced with a double-SQUID. Test circuit was fabricated using a niobium integration technology. Measurements were performed in a liquid helium bath. We confirmed that the maximum output voltage swing of a 4-stage distributed double- SQUID amplifier was 2.93 mV, which was comparable with the corresponding numerical value of 3.54 mV.

DOI
10.1088/1742-6596/1293/1/012060

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Recently, many groups have been developing on-chip Rapid Single-Flux-Quantum (RSFQ) output amplifiers which are designed to connect RSFQ circuitry to room temperature electronics. One of such amplifiers is a distributed SQUID amplifier comprising SQUIDs embedded in cascaded RSFQ RS flip-flops. In this paper, we propose a distributed double-SQUID amplifier, where a SQUID is replaced with a double-SQUID. Test circuit was fabricated using a niobium integration technology. Measurements were performed in a liquid helium bath. We confirmed that the maximum output voltage swing of a 4-stage distributed double- SQUID amplifier was 2.93 mV, which was comparable with the corresponding numerical value of 3.54 mV.
We report the enhanced voltage swing of a rapid-single-flux-quantum (RSFQ) distributed amplifier by replacing a single superconducting quantum interference device (SQUID) with a double-stack-SQUID (DSS). A DSS is composed of two stacked 2-junction SQUIDs sharing one sensing inductor. Thanks to its stack structure, a DSS is expected to generate twofold output voltage. We have designed a 12-stage RSFQ distributed amplifier equipped with DSSs. The maximum output voltage swing reached 10.2 mV in simulation. Test chips were fabricated using a 25-µA/µm2 Nb integration process. In measurements, a test chip was cooled in a liquid helium bath. The experimental output voltage swing was up to 8.34 mV.
The novel element of the rapid single flux quantum (RSFQ) logic family-the quasi digital voltage amplifier (QDVA)-which was recently proposed theoretically, has now been designed and experimentally investigated. The QDVA can be employed both as a logic element in digital circuits and as analogue voltage amplifier. The correct operation of the SFQ doubler has been demonstrated up to the output voltage 0.5 mV and with very large margins (up to 60%). The proposed QDVA can be integrated on the same chip with a SQUID sensor; it significantly simplifies the matching circuits and seems to be very attractive for the multichannel and digital SQUID systems.
We report on the development and test of an integrated system composed of a flux qubit and a rapid single-flux quantum (RSFQ) circuit that allows qubit manipulation. The goal is to demonstrate the feasibility of control electronics integrated on the same chip as the qubit, in view of the application in quantum computation with superconducting devices. RSFQ logic relies on the storage and transmission of magnetic flux quanta and can be profitably used with superconducting qubits because of the speed, scalability, compatibility with the qubit fabrication process and low temperature environment. While standard RSFQ circuitry is well assessed, the application to quantum computing requires a complete rescaling of parameter values, in order to preserve the qubit coherence and reduce the power dissipation. In the system presented in this paper, the qubit role is played by a superconducting loop interrupted by a small dc SQUID, usually called a double SQUID, which behaves as a tunable rf-SQUID. Its energy potential has the shape of a double well, with the barrier between the wells controlled by magnetic flux applied to the inner dc SQUID. Here for the first time we report measurements at a base temperature of 370 mK in which flux control pulses with desired characteristics were supplied by a RSFQ circuit fabricated using non-standard parameters in the same chip as the qubit.
In addition to their fast switching with low power dissipation, precise voltage generation is a unique feature of single-flux-quantum (SFQ) circuits. This feature is attractive for realizing ac voltage standards consisting of SFQ-based digital-to-analog converters (DACs). Among several key components in SFQ-based DACs, a double-flux-quantum amplifier (DFQA) is a potential candidate to amplify the output voltage. In this paper, we present a redesigned DFQA without flux bias lines. We found a parameter set that provided wide bias margins and zero-flux-bias operation. Improved bias regions were numerically demonstrated, whereas a ±20-fold DFQA excluding flux biases was fabricated using Nb integration technology and tested in a liquid helium bath.
A low-noise single-chip two-stage superconducting quantum interference device (SQUID) system with a double relaxation oscillation SQUID as the second stage has been realized. The system was operated in a direct voltage readout mode, with a closed loop bandwidth up to 1 MHz. Operated at 4.2 K, the white flux noise measured in flux locked loop was 1.3 μΦ0/√Hz, corresponding to an energy sensitivity of ε≈27h. Owing to the large flux-to-voltage transfer of up to 3.6 mV/Φ0, the room-temperature preamplifier noise did not dominate the overall flux noise.
Some basic cells in the rapid single-flux quantum (RSFQ) family of superconductive logic circuits were studied. An input stage, a buffer, and an OR gate were simulated and laid out for a standard niobium-based fabrication process. The resulting circuits perform properly for clock speeds up to 1 GHz. For the simpler circuits tested, the measured margins are wide, consistent with simulations, and not very dependent on clock speed. However, margin decreases with increased circuit complexity. The input stage converts a rising edge into an SFQ pulse, which has a small amplitude and narrow width in time. On the present circuits, these pulses are about 200 mu V in amplitude and 10 ps in width. To facilitate measurement, the authors have chosen as the output an asymmetrical superconducting quantum interference device (SQUID) consisting of two Josephson junctions. It converts SFQ pulses into 2.5 mV latching output levels on chip. By inductively coupling SFQ pulses into the SQUID loop, DC current isolation between the RSFQ circuit-under-test and the output stage is provided. The SQUID was optimized for 2 GHz operations.>