Researchers from various Japanese institutions have successfully operated a superconducting flux qubit at zero magnetic field. The team achieved this by inducing a π phase shift in the superconducting order parameter using a precisely controlled nanoscale thickness superconductor-ferromagnet-superconductor Josephson junction. The qubit’s lifetime is in the microsecond range, limited by quasiparticle excitations in the metallic ferromagnet layer. With further improvements in the materials of the ferromagnetic junction, the zero flux-biased flux qubits could become a promising platform for quantum computing.
Superconducting Flux Qubit Operating at Zero Magnetic Field
A team of researchers from the Advanced ICT Research Institute, National Institute of Information and Communications Technology (NICT), NTT Basic Research Laboratories, NTT Corporation, Graduate School of Engineering, Nagoya University, and the Institute for Photon Science and Technology, The University of Tokyo, have successfully operated a superconducting flux qubit at zero magnetic field. The team includes Sunmi Kim, Leonid V Abdurakhimov, Duong Pham, Wei Qiu, Hirotaka Terai, Sahel Ashhab, Shiro Saito, Taro Yamashita, and Kouichi Semba.
The Conventional Superconducting Flux Qubit
The operation of a conventional superconducting flux qubit requires the application of a precisely tuned magnetic field. This makes the scaling of quantum circuits based on this type of qubits difficult. However, 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, it is possible to realize a flux qubit operating at zero magnetic flux.
The Zero Flux-Biased Flux Qubit
The researchers report the realization of a zero flux-biased flux qubit based on three NbNAlNNbN Josephson junctions and a NbNPdNiNbN ferromagnetic π-junction. The qubit’s lifetime is in the microsecond range, which the researchers argue is limited by quasiparticle excitations in the metallic ferromagnet layer. With further improvements in the materials of the ferromagnetic junction, the zero flux-biased flux qubits can become a promising platform for quantum computing.
The Essential Component in Superconducting Quantum Bits
The essential component in superconducting quantum bits (qubits) is the Josephson junction (JJ), composed of a nanoscale tunnel barrier sandwiched between two superconducting layers. These junctions introduce circuit nonlinearity while preserving its quantum nature, enabling the circuit to behave as a macroscopic artificial atom. The conventional choice for these JJs is the AlAlOxAl JJ, which is preferred due to its simplicity of fabrication and its ability to provide a reliable sample quality for achieving long coherence times.
Challenges and Alternative Approaches
Despite significant progress in improving the coherence times of Al-based qubits, there remain challenges in terms of material improvements to deal with two-level fluctuators originating from uncontrollable defects in the amorphous AlOx in Al-based JJs. Consequently, there is a growing need for materials-oriented research to enhance the performance of superconducting qubits and design innovations that reduce noise or the sensitivity of qubits to noise. Alternative approaches have been explored to enhance device coherence, such as using titanium nitride (TiN) for capacitors and/or microwave resonators to mitigate microwave dielectric loss caused by uncontrolled defects in oxides at their surfaces and interfaces.
The π-Junction Qubit
The researchers present the successful operation of a flux qubit that contains a π-junction, referred to as π-junction qubit. This qubit has its optimal operation point at zero applied flux and exhibits coherence times in the microsecond range. For the π-junction, the researchers utilized a ferromagnetic junction with a diluted ferromagnetic Pd89Ni11 layer between NbN superconductors. The longer decay length of the PdNi junction resulting from weaker spin flip scattering makes it attractive for device applications, allowing for better control of critical currents and ensuring functional π-junctions even in the presence of spatial fluctuations in the PdNi layer thickness.
NbN-Based Flux Qubit with/without π-Junction
Conventional flux qubits and π-junction qubits were fabricated using NbN-based fabrication techniques employing a TiN buffer layer on a Si substrate. Both types of qubits had the same basic design of capacitively shunted flux qubits with NbNAlNNbN JJs. The only difference between the qubits was the inclusion of an additional and relatively large π-phase shifter made of a NbNPdNiNbN junction.
“Superconducting flux qubit operating at zero magnetic field” was published on January 25, 2024, by authors Sunmi Kim, L. V. Abdurakhimov, Duong Pham, Wei Qiu, Hirotaka Terai, Sahel Ashhab, Shiro Saito, Taro Yamashita, and Kouichi Semba. The article was sourced from arXiv, a repository managed by Cornell University.
