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Applied Physics Letters

Superconducting MgB2 rapid single flux quantum toggle flip flop circuit

Daniel Cunnane, Ke Chen, X. X. Xi

Published Jun 3, 2013• 13 citations
Publisher

Abstract

Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.

DOI
10.1063/1.4809587

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Using self-shunted MgB2/MgO/MgB2 Josephson junctions, we have designed, fabricated, and tested a rapid single flux quantum toggle flip flop (TFF) circuit. The junctions used MgB2 films grown by hybrid physical-chemical vapor deposition with MgO barrier layer and insulating layer deposited by RF magnetron sputtering. The result showed the frequency-division function of the TFF circuit, evidenced by the output voltage being half the input voltage, up to 63 GHz at 20 K and 180 GHz at 3.2 K, which demonstrates the potential of high operating speeds and high working temperatures in MgB2 superconducting integrated circuits.
Improved All-MgB2\hbox{All-MgB}_{2}All-MgB2​ Toggle Flip-Flop Circuit With Increased Operating Speed and Temperature Range
We have fabricated and tested rapid single flux quantum toggle flip-flop circuits using self-shunted MgB2/ MgO/MgB2Josephson junctions with a single ground layer. The MgB2films were deposited using HPCVD, and the MgO barrier was deposited using DC reactive sputtering. We highlight the circuit's voltage divider operation dependence on current bias and temperature which show operation from 7 to 30 K. The highest attained operating speed is 335 GHz at 7 K. These results demonstrate the versatile temperature range and speed of MgB2digital circuits.
We designed and fabricated a rapid-single-flux-quantum T flip-flop (TFF) with high- T c interface-engineered Josephson junctions. Y 1 Ba 2 Cu 3 O 7− d and Sr 2 AlTaO 6 were deposited for the superconducting layer and the insulating layer, respectively. The Josephson junction was formed through an interface treatment process using Ar ion milling and vacuum annealing. We simulated a TFF circuit and designed a physical layout using WRspice and Xic. The fabricated TFF has a minimum junction width of 3 μm. Through the measurement of the voltage divider operation, the maximum operation frequency was estimated to be 53 GHz at 22 K and 106 GHz at 12 K.
We report a developed single-flux-quantum (SFQ) circuit fabrication process for vertically-stacked interface-treated Josephson junctions by using high-temperature superconductor (HTS) and also report the 500 GHz operation of a toggle flip-flop (TFF). As the first trial of the circuit operation, we fabricated TFF circuits, because TFF is used as a benchmark circuit to indicate potential of the circuit operating speed. To develop the circuit fabrication process, we utilized the counter YBCO layer as a wiring layer. We obtained resistively-shunted-junction behavior of single junction fabricated by new process. In addition, we observed current-voltage characteristics at the input port and the output port of the TFF, and confirmed that the input voltage coincided with twice the output voltage up to 1.1 mV at 4.2 K, which corresponds to the correct operation up to 500 GHz. This frequency is the highest value in HTS circuits ever observed, which suggests the high potential of the vertically-stacked JJs for the circuit operation.
Rapid Single Flux Quantum (RSFQ) T-flip flops (TFFs) operating up to 770 GHz have been demonstrated at 4.2 K. The devices, consisting of either resistively shunted or unshunted Josephson junctions, are fabricated using a planarized Nb/AlO/sub x//Nb trilayer process. Electron beam lithography is used to pattern all levels with a minimum junction area less than 0.1 /spl mu/m/sup 2/. Critical current densities of 0.5 mA//spl mu/m/sup 2/ and 2.5 mA//spl mu/m/sup 2/ are used for the shunted (tested at 1.8 K) and unshunted devices (tested at 4.2 K) respectively. The input and output frequencies of the TFFs are obtained from the input and output voltages by the Josephson relation. The output voltage is exactly half of the input voltage when the divide-by-two operation is correct.
Abstract Half-flux-quantum (HFQ) circuits are based on 0–π superconducting quantum interference devices (SQUIDs) and is one of the energy-efficient superconductor digital circuits. The bit energy is determined by the critical current I cn of 0–π SQUID, which can be easily tuned with the loop inductance and junction critical current. In this work, an alternative π–π–π SQUID is adopted to demonstrate HFQ circuits to simplify the fabrication process and enhance circuit energy efficiency. The properties of superconductor/ferromagnet/insulator/superconductor Josephson junctions (π-JJs) are measured with temperature dependence from 4.2 K down to 10 mK. HFQ toggle flip-flops (TFFs) are successfully demonstrated at frequencies of up to 6.7 GHz and 44.5 GHz at temperatures of 4.2 K and 10 mK, respectively. Comparing the HFQ TFF with its rapid single-flux quantum counterpart under the same fabrication process, it is anticipated that the HFQ TFF will exhibit approximately 70% reduction in both static and dynamic energy dissipation. This research establishes the foundation for developing cryogenic interface control and readout circuits for large-scale quantum computing in the future.
We report on the functionality of a Nb-based superconducting single flux quantum (SFQ) toggle flip-flop (TFF) circuit, comprising a complementary superconductor–ferromagnet–superconductor (SFS) Josephson π-junction. The SFS junction was used as a phase shifting element inserted in the storage loop of the TFF. The fabricated circuits demonstrated correct functionality with the operation parameter ranges of ± 20%. The application of SFS π-junctions makes the SFQ circuits very compact, may substantially improve their stability, and may also be suitable for integration with Josephson quantum circuits (qubits).