Fabrication Process for Monolithic Integration of a Nitride Superconductor-Based Superconducting Qubit with a Single Flux Quantum Control Circuit

Miyajima, Shigeyuki; Yamashita, Taro; Tanaka, Masamitsu; Takeuchi, Naoki; Inomata, K.; Terai, Hirotaka

doi:10.1109/tasc.2023.3241270

Cited by 4 publications

(4 citation statements)

References 14 publications

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“…Finally, a 500 nm-thick NbN ground plane was deposited covering the chip. Since the ground plane was placed on the top layer of the chip, this fabrication process allows the epitaxial growth of NbN/AlN/NbN junctions, which are used for NbN-based flux qubits with an improved coherence time [19][20][21]. Therefore, it is compatible with the fabrication process of NbN-based flux qubit for monolithic integration.…”

Section: Methodsmentioning

confidence: 99%

“…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]. Subsequently, we designed and fabricated an HFQ-TFF circuit consisting of SIFS π-junctions.…”

Section: Introductionmentioning

confidence: 99%

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

Pham,

Li,

Oba

et al. 2024

Supercond. Sci. Technol.

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We have developed half-flux-quantum (HFQ) circuits using all-π-junctions formed from an NbN/AlN/PdNi/NbN (SIFS) structure. The circuits were fabricated using a novel process that incorporated a ground plane on top of the chip, enabling the epitaxial growth of NbN-based junctions from the substrate. The π-state of the junctions was demonstrated through a half-flux-quantum shift. Notably, these π-junctions exhibited self-overdamped current-voltage characteristics, enabling them to function as switching components without the need for shunt resistors. The elimination of shunt resistors and the high sheet inductance of NbN are expected to enhance the density of HFQ circuits. To evaluate the performance and power consumption of the all-π-junctions HFQ circuits, we designed and fabricated an HFQ toggle flip-flop (HFQ-TFF) circuit utilizing π-π-π SQUIDs as the fundamental components. Our findings reveal that the NbN-based HFQ-TFF circuit correctly operates as a frequency divider while consuming only around 30% of the power compared to single-flux-quantum TFF (SFQ-TFF) circuits. These results suggest that the HFQ circuit using SIFS-π-junctions has promising potential for integrated circuits requiring low-power consumption at cryogenic temperatures, such as qubit control.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

Pham,

Li,

Oba

et al. 2024

Supercond. Sci. Technol.

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“…Due to our experimental conditions, a 50 nm thick PdNi film is patterned as the bias resistors, despite it may cause flux trapping in the adjacent superconducting loop and affect the circuit operation. PdAu or Pd resistors are desired in much more compacted circuits for milli-kelvin stage demonstrations [22,25]. The microphotograph of the HFQ TFF in figure 5 shows multiple π-π-π SQUIDs connected in serial or parallel, where the middle JJ acts as the π phase shifter while the upper and bottom JJs serve as the switching JJs.…”

Section: Hfq Tffmentioning

confidence: 99%

“…However, the well-established SFQ circuits were designed to operate at 4 K, while qubits are typically located at the millikelvin stage of a dilution refrigerator. Recently, SFQ researchers are trying to demonstrate SFQ circuits with lowered critical current to support the operation at milli-kelvin stage [21][22][23]. Since information is stored as a magnetic flux in SFQ circuit, the small critical current (I c ) inevitably leads to a larger inductance (L) according to LI c = 0.4 ∼ 1.2Φ 0 , where Φ 0 is the magnetic flux quantum.…”

Section: Introductionmentioning

confidence: 99%

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

Li,

Pham,

Takeshita

et al. 2023

Supercond. Sci. Technol.

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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 Icn 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 RSFQ 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.

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