World’s First Superconducting Flux Qubit Operating without Magnetic field

 The National Institute of Information and Communications Technology (NICT, President: TOKUDA Hideyuki, Ph.D.), NTT Corporation (NTT, President: Mr. SHIMADA Akira), Tohoku University (President: Dr. TOMINAGA Teiji) and the Tokai National Higher Education and Research System Nagoya University (President: Dr. SUGIYAMA Naoshi) succeeded in developing a new type of superconducting flux qubit that operates in zero magnetic field.

Conventional superconducting flux qubits require an external magnetic field generated by bulky coils or an additional local flux bias line near the qubit. The newly developed superconducting flux qubit using a ferromagnetic Josephson junction (named π-junction) demonstrates the realization of a flux qubit operating at zero magnetic field. The qubit lifetime is in the microsecond range, which is the longest among the superconducting qubits with ferromagnetic π-junction. By further improving the ferromagnetic junction material in the future, this π-junction and flux qubit that can operate in zero magnetic field may become essential elements of high-performance quantum devices that are indispensable for quantum computers.

This result was published in the British scientific journal “Communications Materials” on October 11, 2024.

In the information technology of the future, quantum computers are expected to play a crucial role in various fields, from material and pharmaceutical development to information security. Among the promising technologies, superconducting qubits stand out for their relatively easy control of quantum states. Josephson junctions, which are essential components of superconducting qubits, provide anharmonicity to the circuit, enabling qubit operation. The characteristics of a few representative superconducting qubits are shown in Table 1.

Transmon qubits [1], widely used today for superconducting quantum computers, have low anharmonicity, which may cause problems such as frequency crowding when integrating many qubits. 

On the other hand, flux qubits [2, 3], which use three Josephson junctions, have higher anharmonicity and can mitigate frequency crowding problems (see Figure 1(a)). However, flux qubits require external coils to apply half a magnetic flux quantum (Φ₀ = 2.07×10^-15 Wb) to the superconducting loop for optimal operation, leading to challenges like low-frequency noise from the coils or the need for additional control lines for each qubit, hindering large-scale integration.

To address this, researchers at Tohoku University, led by Prof. Taro Yamashita (then at Nagoya University), proposed incorporating a π-junction into the flux qubit (see Figure 1(b)). This π-junction, which uses a ferromagnetic Josephson junction, generates a 180-degree (π) phase difference without external magnetic fields, allowing the qubit to reach its optimal operating point spontaneously.  This is expected to reduce external noise, simplify the circuit, and make qubit integration easier.
 

Previous studies include the work of Prof. Alexey Ustinov’s research team at the  Karlsruhe Institute of Technology (Germany) [4], who reported a coherence time of 4 nanoseconds in a phase qubit composed of Nb/AlO_x/Nb Josephson junctions and Nb/CuNi/Nb π-junctions. Additionally, they attempted to introduce a π-junction into a flux qubit, but did not observe quantum coherent qubit operation [5].

In our research, we utilized PdNi, which maintains a more stable π-state than CuNi, to form a π-junction on an NbN electrode. Furthermore, we combined NICT’s NbN/AlN/NbN Josephson junction technology with NTT’s optimal design for flux qubits in 3D cavity resonators to develop a new type of flux qubit that operates optimally in zero magnetic fields (see Figure 2). Measurements using NTT’s long-coherence-time qubit measurement system confirmed that the optimal operating point is at zero magnetic field, with a coherence time of 1.45 microsecond (see Figure 3). This marks a 360-fold improvement in coherence time compared to phase qubits with π-junctions.

However, conventional flux qubits fabricated without a π-junction exhibit an energy relaxation time of 16 microseconds (as reported by NICT on September 20, 2021), indicating that the π-junction of the current NbN/PdNi/NbN structure negatively impacts quantum coherence.

While further improvement in quantum coherence is still needed, this achievement represents the world's first realization of a flux qubit that does not require an external magnetic field and exhibits a coherence time on the order of microseconds. This technology is a crucial step toward miniaturizing and integrating various quantum circuits, including qubits. By eliminating the need for an external magnetic field, it will contribute to simplifying circuits, saving energy, and reducing costs.

An element (Josephson element) with a structure in which two superconducting electrodes (S) are separated by an extremely thin insulator (I) or a thin film of normal metal. A current (Josephson current I_S) with zero electrical resistance (zero voltage) can flow in a Josephson junction because of the tunnel effect between the superconducting electrodes. The magnitude of this Josephson current is determined by the difference (φ) in the macroscopic phase between the two superconducting electrodes (DC Josephson effect), so conversely, the macroscopic phase between the superconducting electrodes can be controlled by how much current is passed through the Josephson element. Many superconducting devices, including superconducting quantum bits, use macroscopic phase control by this Josephson element as their basic operating principle.

The property that the energy difference between the ground state and first excited state of a superconducting quantum bit is different from the energy difference between the first and second excited states. It is quantified by the difference (E₁₂-E₀₁), where E01 is the energy difference between the ground state |0> and the first excited state |1>, and E₁₂ is the energy difference between the first excited state |1> and the next energy level |2>.

A type of superconducting quantum bit. It has a circuit structure in which a quantum bit consisting of one or two Josephson junctions is coupled with a capacitor with large capacitance (also called a shunt capacitor). By reducing the charging energy, this quantum bit is more resistant to charge noise and has a better coherence time.

A type of superconducting quantum bit. It is composed of a relatively large Josephson junction embedded in a superconducting circuit and is characterized by a large ratio of Josephson energy (Ej) to charging energy (Ec), typically on the order of 10⁶. It uses two different quantum oscillation states of the current across the Josephson junction to represent the two states of the quantum bit.