Periodic Reporting for period 1 - FERROMON (Ferrotransmons and Ferrogatemonsfor Scalable Superconducting Quantum Computers)
Période du rapport: 2023-11-01 au 2024-10-31
Alternative approaches to superconducting qubit technology are the main goals of the FERROMON project. State-of-the-art architectures require flux-bias lines to tune the qubit frequency. The use of currents to control these lines is detrimental to qubit performance and presents a severe bottleneck for scalability. In this project, we propose two novel superconducting qubit designs capable of overcoming this challenge by simplifying the whole layout of the qubit circuit and its operation. This will involve the investigation of JJs which include a ferromagnetic layer in their design, and their integration into quantum processors. One innovation track will implement superconductor- insulator- thin superconductor -ferromagnet – superconductor (SIsFS) JJs in a transmon geometry— namely the ferrotransmon (Fmon). The other will hybridize gatemons and π-junction to deliver a ferrogatemon (FGmon).
The idea of the use of ferromagnetic transmons rather than the qubits based on standard Al-tunnel JJs is motivated by the possibility of tuning the relevant scaling energy, the Josephson energy EJ of the junction, by using microwaves or magnetic field pulses, instead of a static magnetic field applied during all the operations of the qubit.
The idea of the use of ferromagnetic transmons rather than the qubits based on standard Al-tunnel JJs is motivated by the possibility of tuning the relevant scaling energy, the Josephson energy EJ of the junction, by using microwaves or magnetic field pulses, instead of a static magnetic field applied during all the operations of the qubit.
The work performed in the first year has basically developed according to the original plan, and deliverables confirm main achievements and indicate the flow of current and future efforts. The two main directions focus the Fmon and the FGmon respectively.
For the Fmon, the research activities have been developed in order to: i) find out the appropriate layout and the right construction parameters of the tunnel ferromagnetic JJs (FJJs); ii) to study the magnetic response of the JJs; iii) to scale down the junctions to the submicron regime; iv) to find out doping of ferromagnetic materials towards softer F layers in SIsFS JJs; v) to integrate tunnel FJJs in 2D and 3D microwave waveguides, which includes the design of the line to apply in-plane magnetic field pulses.
The key results, mostly documented in D.1.1 are:
i) The SIsFS JJs to be integrated in the Fmon must fall in the series-like transport regime. This allows to design and fabricate a standard transmon circuit and ex-situ deposit the ferromagnetic interlayer and the superconducting top electrode of the JJ.
ii) Accurate analysis of the magnetic response of the JJ allows to define their scaling energies.
iii) Successful scaling of the F-JJs to sub-micron regime -definition of the geometry and JJ parameters and of the fabrication steps respectively.
iv) A systematic study on films and nanopatterned s/F bilayers has allowed to identify (Fe11Ni81Gd8)80 Nb20 as the best possible F solutions to lower the exchange field of the F barrier.
v) We have designed the junction geometry in 2D and 3D microwave waveguides (deliverable D1.2.) to align with the necessary transmon parameters and found an optimal solution a size of around 0.03 µm², targeting an anharmonicity of approximately 300 MHz.
The Helmotz flux coil is compatible with a full 5 qubit device based on Fmon. Quantware (QW) has identified the necessary modifications to their standard fabrication process in order to realize Fmon (D3.1). QW has implemented the other primary change to integrate the ferromagnetic layer into the standard Al SIS fabrication process. This new design incorporates the deposition of a ferromagnetic layer under a third angle, following the standard two-angle aluminum deposition process. All this work makes it possible to fully design the 5-qubit Fmon SOPRANO. For the FGmon platform, different material growth approaches have been investigated to identify the best suited configuration that allows for gate voltage control.
We have found that 2DEG-based triple-hybrid materials platform does not meet the necessary criteria for effective π junctions at this stage of development. Our parallel efforts indicate that nanowires remain the most promising platform for further research. We plan to shift our primary focus to the VLS nanowire platform, which has already demonstrated the capability to host π junctions. The successful demonstration of induced superconductivity in vapor-liquid-solid (VLS) grown InAs nanowires proximitized by EuS (ferromagnetic insulator) and Al shells is discussed in D.2.1. These wires were used to fabricated gate-tunable hybrid FJJs. QW has also identified necessary changes in the fabrication process for insulating FGmon in combination with deliverables D2.1 and D2.2. This involves combining in-house fabrication procedures for the qubit architecture with UCPH's nanowire fabrication process, which enables precise positioning of the nanowires using a micromanipulator and the realization nanowire gating and contacting structures. The flip-chip platform, with its ability to separate quantum circuitry from input/output wiring and its customizable design, offers a versatile solution for integrating various hybrid structures without compromising performance.
Significant progress in understanding key aspects of high-fidelity control and readout of Fmons and FGmons has been also achieved, as discussed in D4.1. Qblox has theoretically investigated the analog control aspects of Fmons and FGmons to understand the effects of various forms of signal imperfections. All milestones planned by the end of the first year have been achieved.
For the Fmon, the research activities have been developed in order to: i) find out the appropriate layout and the right construction parameters of the tunnel ferromagnetic JJs (FJJs); ii) to study the magnetic response of the JJs; iii) to scale down the junctions to the submicron regime; iv) to find out doping of ferromagnetic materials towards softer F layers in SIsFS JJs; v) to integrate tunnel FJJs in 2D and 3D microwave waveguides, which includes the design of the line to apply in-plane magnetic field pulses.
The key results, mostly documented in D.1.1 are:
i) The SIsFS JJs to be integrated in the Fmon must fall in the series-like transport regime. This allows to design and fabricate a standard transmon circuit and ex-situ deposit the ferromagnetic interlayer and the superconducting top electrode of the JJ.
ii) Accurate analysis of the magnetic response of the JJ allows to define their scaling energies.
iii) Successful scaling of the F-JJs to sub-micron regime -definition of the geometry and JJ parameters and of the fabrication steps respectively.
iv) A systematic study on films and nanopatterned s/F bilayers has allowed to identify (Fe11Ni81Gd8)80 Nb20 as the best possible F solutions to lower the exchange field of the F barrier.
v) We have designed the junction geometry in 2D and 3D microwave waveguides (deliverable D1.2.) to align with the necessary transmon parameters and found an optimal solution a size of around 0.03 µm², targeting an anharmonicity of approximately 300 MHz.
The Helmotz flux coil is compatible with a full 5 qubit device based on Fmon. Quantware (QW) has identified the necessary modifications to their standard fabrication process in order to realize Fmon (D3.1). QW has implemented the other primary change to integrate the ferromagnetic layer into the standard Al SIS fabrication process. This new design incorporates the deposition of a ferromagnetic layer under a third angle, following the standard two-angle aluminum deposition process. All this work makes it possible to fully design the 5-qubit Fmon SOPRANO. For the FGmon platform, different material growth approaches have been investigated to identify the best suited configuration that allows for gate voltage control.
We have found that 2DEG-based triple-hybrid materials platform does not meet the necessary criteria for effective π junctions at this stage of development. Our parallel efforts indicate that nanowires remain the most promising platform for further research. We plan to shift our primary focus to the VLS nanowire platform, which has already demonstrated the capability to host π junctions. The successful demonstration of induced superconductivity in vapor-liquid-solid (VLS) grown InAs nanowires proximitized by EuS (ferromagnetic insulator) and Al shells is discussed in D.2.1. These wires were used to fabricated gate-tunable hybrid FJJs. QW has also identified necessary changes in the fabrication process for insulating FGmon in combination with deliverables D2.1 and D2.2. This involves combining in-house fabrication procedures for the qubit architecture with UCPH's nanowire fabrication process, which enables precise positioning of the nanowires using a micromanipulator and the realization nanowire gating and contacting structures. The flip-chip platform, with its ability to separate quantum circuitry from input/output wiring and its customizable design, offers a versatile solution for integrating various hybrid structures without compromising performance.
Significant progress in understanding key aspects of high-fidelity control and readout of Fmons and FGmons has been also achieved, as discussed in D4.1. Qblox has theoretically investigated the analog control aspects of Fmons and FGmons to understand the effects of various forms of signal imperfections. All milestones planned by the end of the first year have been achieved.
Most of the accomplishments are obviously beyond the state of the art because both the Fmon and the FGmon are two novel types of superconducting qubits. All performed work converges into two main results: the complete design of the 5-qubit Fmon SOPRANO and of the 5-qubit FGmon SOPRANO.
In the final layout of the devices all experiments and realized test junctions have provided information on how to design the 5-qubit quantum processor. For the Fmon we have found an appropriate ferromagnet (Fe11Ni81Gd8)80 Nb20 as barrier material. These fields have been already achieved through a Helmholtz flux coil realized through Airbridges. Size of the junction of around 0.03 µm², targeting an anharmonicity of approximately 300 MHz, aligns the junction properties with the necessary transmon parameters and the microwave waveguides.
The feasibility of hybrid structures employing VLS-grown InAs nanowires with fully-overlapping EuS/Al hybrid shells for hosting gate-tunable JJs has been established in view of FGmon realization. The precise positioning and gating of nanowires using micromanipulation techniques have allowed for the fabrication of gate-tunable hybrid JJs with reliable functionality. These advancements represent a significant step toward realizing the full potential of FGmons for scalable quantum processors.
In the final layout of the devices all experiments and realized test junctions have provided information on how to design the 5-qubit quantum processor. For the Fmon we have found an appropriate ferromagnet (Fe11Ni81Gd8)80 Nb20 as barrier material. These fields have been already achieved through a Helmholtz flux coil realized through Airbridges. Size of the junction of around 0.03 µm², targeting an anharmonicity of approximately 300 MHz, aligns the junction properties with the necessary transmon parameters and the microwave waveguides.
The feasibility of hybrid structures employing VLS-grown InAs nanowires with fully-overlapping EuS/Al hybrid shells for hosting gate-tunable JJs has been established in view of FGmon realization. The precise positioning and gating of nanowires using micromanipulation techniques have allowed for the fabrication of gate-tunable hybrid JJs with reliable functionality. These advancements represent a significant step toward realizing the full potential of FGmons for scalable quantum processors.