World's first zero-magnetic-field superconducting flux qubit achieved

World's first zero-magnetic-field superconducting flux qubit achieved
by Riko Seibo
Tokyo, Japan (SPX) Oct 16, 2024

A research collaboration between the National Institute of Information and Communications Technology (NICT), NTT Corporation, Tohoku University, and the Tokai National Higher Education and Research System at Nagoya University has successfully developed a superconducting flux qubit that operates without requiring an external magnetic field. This breakthrough was accomplished using a ferromagnetic Josephson junction, known as a p-junction, enabling the qubit to function optimally in a zero magnetic field environment.

Traditional superconducting flux qubits typically depend on external magnetic fields, which are generated using cumbersome coils or local flux bias lines. The innovative approach by the research team utilizes a p-junction that introduces a 180-degree phase shift naturally, removing the need for external magnetic fields. This design simplifies the circuit and could pave the way for easier integration and scaling of quantum devices. The qubit has demonstrated a microsecond-range coherence time, marking the longest coherence observed in superconducting qubits with ferromagnetic p-junctions. Future advancements in the ferromagnetic junction materials could enhance the performance, making these qubits a key component of next-generation quantum computing hardware.

The study was published in the journal *Communications Materials* on October 11, 2024.

Quantum computers are anticipated to significantly impact various fields, including materials science, pharmaceuticals, and cybersecurity. Among several approaches, superconducting qubits are promising due to their straightforward state control, with Josephson junctions playing a central role in enabling qubit operations by introducing anharmonicity. While transmon qubits are widely used, they face challenges with frequency crowding due to low anharmonicity. Conversely, flux qubits offer higher anharmonicity, which can mitigate such issues but traditionally require an external magnetic field to reach their optimal operating point.

The new qubit design addresses these limitations by incorporating a ferromagnetic p-junction, which allows the qubit to operate at its optimal point without an external field, potentially reducing noise and energy requirements. The integration of NICT's niobium nitride-based qubit technology with ferromagnetic Josephson junctions has made this innovation possible. Researchers used a palladium-nickel alloy, which exhibits greater p-state stability than other materials, to form the p-junction. This was coupled with NTT's advanced qubit design and measurement capabilities to achieve a 1.45-microsecond coherence time, a substantial improvement over previous efforts involving phase qubits with p-junctions.

Earlier attempts to use p-junctions in flux qubits, such as those conducted by researchers at Karlsruhe Institute of Technology, did not yield successful results in maintaining quantum coherence. In contrast, the current study demonstrates that a flux qubit with a p-junction can indeed sustain coherent operation at zero magnetic field, achieving a 360-fold increase in coherence time compared to prior phase qubits utilizing p-junctions.

Despite this achievement, there is still a gap between the coherence times of traditional flux qubits (16 microseconds) and those with the new p-junction structure. The current results suggest that further refinements in the p-junction materials and qubit design are necessary to enhance coherence times.

The development of this zero-magnetic-field qubit marks an important milestone in quantum circuit miniaturization and integration. The elimination of bulky magnetic field-generating components could lead to simpler, more efficient, and cost-effective quantum circuits. Ongoing efforts aim to refine the device's structure and manufacturing process to improve uniformity and performance, with the long-term objective of surpassing the capabilities of current aluminum-based superconducting qubits.

Researchers envision that future advancements in p-junction technology will enable longer coherence times and establish the zero-magnetic-field flux qubit as a crucial element in quantum computing applications.

Research Report:Superconducting flux qubit with ferromagnetic Josephson p-junction operating at zero magnetic field