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Development of ferromagnetic Josephson junctions for phase-shift flux quantum bits

Taro Yamashita, Akira Kawakami, Hirotaka Terai

The Japan Society of Applied PhysicsFeb 8, 20180 citations

Flux-bias-free flux quantum bit with ferromagnetic π junction

Taro Yamashita, Kun Zuo, Yoshiro Urade +4 more

The Japan Society of Applied PhysicsJan 28, 20200 citations

Superconducting π Qubit with a Ferromagnetic Josephson Junction

禎通前川

Physical Review LettersJan 1, 20050 citations

Superconducting flux qubit with ferromagnetic Josephson π-junction operating at zero magnetic field

Sunmi Kim, Leonid Victorovich Abdurakhimov, Duong Pham +6 more

Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a π-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as π-junction, it is possible to realize a flux qubit operating at zero magnetic flux. Here, we report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic π-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity. Conventional superconducting flux qubits require a finely tuned magnetic field to operate, hindering their on-chip integration. Here, ferromagnetic Josephson junctions with a π-phase shift in the superconducting order parameter allow the realization of a flux qubit operating at zero magnetic field.Conventional superconducting flux qubits require the application of a precisely tuned magnetic field to set the operation point at half a flux quantum through the qubit loop, which complicates the on-chip integration of this type of device. It has been proposed that by inducing a π-phase shift in the superconducting order parameter using a precisely controlled nanoscale-thickness superconductor/ferromagnet/superconductor Josephson junction, commonly referred to as π-junction, it is possible to realize a flux qubit operating at zero magnetic flux. Here, we report the realization of a zero-flux-biased flux qubit based on three NbN/AlN/NbN Josephson junctions and a NbN/PdNi/NbN ferromagnetic π-junction. The qubit lifetime is in the microsecond range, which we argue is limited by quasiparticle excitations in the metallic ferromagnet layer. Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity. Conventional superconducting flux qubits require a finely tuned magnetic field to operate, hindering their on-chip integration. Here, ferromagnetic Josephson junctions with a π-phase shift in the superconducting order parameter allow the realization of a flux qubit operating at zero magnetic field.

arXiv:2401.14597Communications MaterialsOct 11, 202420 citations

Fabrication and measurements of hybrid Nb/Al Josephson junctions and flux qubits withπ-shifters

A V Shcherbakova, Kirill G. Fedorov, K V Shulga +8 more

We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π -phase shifters fabricated using well-established Nb-based technology of superconductor–ferromagnet–superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in situ Ar etching using e-beam gun. Shadow-evaporated Al/AlO x /Al Josephson junctions with Nb bias pads show the expected current–voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π -shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without π -junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.We describe fabrication and testing of composite flux qubits combining Nb- and Al-based superconducting circuit technology. This hybrid approach to making qubits allows for employing π -phase shifters fabricated using well-established Nb-based technology of superconductor–ferromagnet–superconductor Josephson junctions. The important feature here is to obtain high interface transparency between Nb and Al layers without degrading sub-micron shadow mask. We achieve this by in situ Ar etching using e-beam gun. Shadow-evaporated Al/AlO x /Al Josephson junctions with Nb bias pads show the expected current–voltage characteristics with reproducible critical currents. Using this technique, we fabricated composite Nb/Al flux qubits with Nb/CuNi/Nb π -shifters and measured their magnetic field response. The observed offset between the field responses of the qubits with and without π -junction is attributed to the π phase shift. The reported approach can be used for implementing a variety of hybrid Nb/Al superconducting quantum circuits.

arXiv:1405.0373Superconductor Science and TechnologyJan 13, 201540 citations

SUPERCONDUCTING QUANTUM BIT WITH FERROMAGNETIC π-JUNCTION

T. YAMASHITA, Satoshi Takahashi, Sadamichi Maekawa

Foundations of Quantum Mechanics in the Light of New TechnologyJun 1, 20060 citations

Phase-shift flux qubit with a ferromagnetic π junction

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