Superconducting qubit ditches magnetic field for speed in a Quantum breakthrough

Japanese researchers have developed the world’s first superconducting flux qubit that functions in a zero magnetic field.

Conventional superconducting flux qubits need external magnetic fields from large coils or flux lines. The new qubit, using a ferromagnetic Josephson junction (π-junction), works without a magnetic field.

A team led by researchers from Japan’s National Institute of Information and Communications Technology (NICT) and Tohoku University claims the new π-junction qubit has the longest lifetime among its type, and future improvements could make it key to high-performance quantum computers.

“Our results pave the way for developing quantum coherent devices, including qubits and sensors, that utilize the interplay between ferromagnetism and superconductivity,” said the team, in the study abstract .

Magnetic-free qubit

Quantum computers are expected to play a key role in future technology, benefiting fields like material science, pharmaceuticals, and information security. Superconducting qubits stand out due to their easier control of quantum states. A crucial part of these qubits is the Josephson junction, which helps control qubit operations.

Due to their low anharmonicity, transmon qubits—which are frequently seen in modern quantum computers—can result in problems like frequency crowding when numerous qubits are added.

Flux qubits, which employ three Josephson junctions and have higher anharmonicity, can lessen this problem. However, flux qubits are difficult to scale up because they require external coils to deliver a magnetic flux to the circuit, which can result in issues like noise and the requirement for additional control lines.

To address this, Tohoku University researchers used a ferromagnetic Josephson junction to create a π-junction. Without requiring external magnetic forces, this π-junction produces a 180-degree phase shift, enabling the qubit to function at its best independently.

According to the team, the innovation reduces noise, simplifies the circuit, and makes qubit integration easier. fields, reducing noise and enabling easier large-scale integration.

Enhanced qubit coherence

In the study, researchers combined NICT’s nitride superconducting qubit technology, based on niobium nitride (NbN) grown on silicon , with ferromagnetic Josephson devices to create a flux qubit with a π-junction.

The team claims the qubit operates optimally without an external magnetic field, and its coherence properties were successfully demonstrated. Unlike earlier studies, which did not achieve quantum coherent operation with a flux qubit, this new qubit showed a significant improvement.

The scientists improved on earlier experiments using CuNi by using a stable material, PdNi, to generate the π-junction. A device that verified an ideal working point at zero magnetic fields was used to measure the qubit, which was created using sophisticated design procedures. Its coherence time was 1.45 microseconds, 360 times better than that of earlier phase qubits.

According to researchers, while the coherence time is still shorter than conventional flux qubits without a π-junction, this breakthrough is a critical step toward simplifying and integrating quantum circuits. It could lead to more energy-efficient, cost-effective quantum technologies by removing the need for external magnetic fields.

With an eye toward large-scale integration, the team now intends to improve device uniformity and prolong coherence time by optimizing the circuit topology and fabrication method. Their goal is to create a new quantum hardware platform that performs better than traditional aluminum qubits.

Researchers hope to develop a π-junction flux qubit with a longer coherence time that can function in zero magnetic fields by enhancing materials and the ferromagnetic junction structure. This qubit could be a crucial part of future quantum technologies, such as quantum computer chips.

The details of the team’s research were published in the journal Communications Materials .