⚛️ quantum physics

Superconducting qubit based on altermagnets

This paper proposes a new class of high-performance superconducting qubits based on superconductor-altermagnet-superconductor junctions that leverage programmable Josephson potentials to achieve large anharmonicity and intrinsic protection against charge and flux noise through coherent two-Cooper-pair tunneling.

Original authors: Xue-Feng Pan, Xin-Lei Hei, Franco Nori, Peng-Bo Li

Published 2026-09-22

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Xue-Feng Pan, Xin-Lei Hei, Franco Nori, Peng-Bo Li

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build a powerful quantum computer, scientists are racing to create the perfect switch, a tiny device that can hold a piece of information in a fragile state of being both on and off at the same time. The most promising candidates for this job are superconducting qubits, circuits made from materials that conduct electricity with zero resistance. To make these circuits work as switches, they rely on a special barrier called a Josephson junction. This barrier acts like a gate that allows pairs of electrons, known as Cooper pairs, to tunnel through it, creating a non-linear behavior essential for the qubit to function. However, these delicate circuits are easily disturbed by their environment. Tiny fluctuations in electric charge or magnetic fields can cause the information to leak away, a problem known as decoherence. To solve this, researchers have spent years trying to engineer the shape of the energy landscape these circuits sit in, hoping to find a design that is both stable and easy to control.

A team of researchers has now proposed a new way to build these switches by introducing a previously overlooked type of magnetic material into the heart of the junction. This material, called an altermagnet, possesses a unique property: it has a magnetic order that splits electron spins based on their direction of motion, yet it has no overall magnetic field. This is a crucial distinction because traditional magnets usually generate stray fields that destroy superconductivity, but altermagnets avoid this problem entirely. By sandwiching a thin layer of this altermagnet between two superconductors, the researchers showed that they could precisely shape the flow of electrons through the junction. They demonstrated that by adjusting the thickness of the magnetic layer and the alignment of its internal structure, they could create a junction that behaves in ways previously impossible with standard materials.

The core of their discovery lies in how these junctions handle the tunneling of electron pairs. In a typical junction, electrons tunnel through one pair at a time. In this new design, the researchers showed that the junction can be tuned to allow two pairs of electrons to tunnel together as a single unit. This process, known as double-Cooper-pair tunneling, fundamentally changes the energy landscape of the qubit. It creates a potential well that is much steeper and more distinct than usual, which is a desirable trait because it makes the qubit's energy levels more widely spaced. This spacing, called anharmonicity, is vital for ensuring that the computer can distinguish between the zero and one states without accidentally jumping to a third, unwanted state.

Perhaps the most significant finding is how this design protects the qubit from noise. The researchers found that in a specific configuration, the quantum states of the qubit develop a property called parity, which acts as a shield against electrical charge fluctuations. In this state, the qubit becomes completely insensitive to a common type of electrical noise that usually causes errors. This protection is not just a minor improvement; the simulations suggest it can be total, effectively silencing a major source of error that plagues other types of superconducting qubits. Furthermore, the team showed that this protection can be turned on and off or adjusted by applying a small external magnetic flux, giving engineers a new dial to control the device's sensitivity and performance.

The researchers also explored how to build a complete circuit using these new junctions. They proposed a design where two of these specialized junctions are connected in a loop, similar to a standard superconducting quantum interference device, but with the added ability to tune the magnetic properties of the loop. This setup allows the frequency of the qubit and its resistance to noise to be controlled by an external magnetic field. While adding this loop introduces a new vulnerability to magnetic noise, the team showed that by choosing the right operating point, this sensitivity can be minimized. The result is a versatile platform where the magnetic material itself becomes a programmable tool, allowing scientists to engineer the exact behavior they need for a high-performance quantum bit.

This work suggests a new path forward for quantum computing hardware. By combining the stability of superconductors with the unique spin-splitting properties of altermagnets, the researchers have opened a door to qubits that are both highly stable and highly controllable. The simulations indicate that these devices could offer long coherence times, meaning they can hold information for longer periods, while also being robust against the electrical noise that typically limits current technology. The study does not claim to have built a working computer, but it provides a detailed blueprint for how such a device could be constructed. It establishes altermagnets as a powerful new ingredient for the next generation of quantum devices, offering a way to engineer the very laws of motion for electrons in a circuit to suit the needs of quantum information.

Technical Summary: Superconducting Qubit Based on Altermagnets

Problem Statement
Superconducting qubits rely on Josephson junctions to provide the essential nonlinearity required for quantum computation. However, the energy spectrum and anharmonicity of these qubits are constrained by the intrinsic current-phase relation (CPR) of conventional $s$ -wave junctions, which typically exhibit a simple sinusoidal dependence. While alternative architectures like fluxonium and 0- $\pi$ qubits have improved coherence by engineering effective potentials, they often face trade-offs between anharmonicity and noise sensitivity. Furthermore, proposals utilizing unconventional superconductors (e.g., $d$ -wave) to access higher-harmonic CPRs often suffer from quasiparticle noise and operate in regimes susceptible to charge noise. There is a need for a platform that allows for the programmable engineering of Josephson potentials to achieve high anharmonicity while providing intrinsic protection against environmental noise, particularly charge noise, without relying on exotic superconducting materials that are difficult to fabricate.

Methodology
The authors propose a superconducting qubit architecture utilizing an Altermagnetic Josephson Junction (AMJJ) as the nonlinear element. The AMJJ consists of a trilayer structure with two $s$ -wave superconducting leads separated by an altermagnetic interlayer (specifically modeled as a $d_{xy}$ -wave altermagnet).

Microscopic Modeling: The authors formulate a microscopic Hamiltonian for the S-AM-S junction using the Bogoliubov-de Gennes (BdG) formalism. They analyze the Andreev bound states in the short-junction limit, where the Josephson current is dominated by discrete bound states. The model incorporates momentum-dependent spin splitting characteristic of altermagnets, which induces phase shifts in the Andreev levels dependent on the transverse momentum and the altermagnetic parameters (exchange energy $J$ , layer thickness $L$ , and interface barrier $Z$ ).
Potential Engineering: Using the Furusaki-Tsukada formula, the authors numerically calculate the total Josephson current and the resulting macroscopic Josephson potential. They demonstrate that by tuning microscopic parameters (such as $L$ , $J$ , and chemical potential mismatch $\delta\mu$ ), the CPR can be engineered to include significant higher-harmonic terms, specifically the second harmonic ( $\sin(2\phi)$ ).
Inverse Design: To achieve specific target CPRs (e.g., $\phi$ -junctions or $2\phi$ -junctions), the authors employ an inverse-design workflow combining Bayesian optimization for global parameter search and quasi-Newton methods with automatic differentiation for local refinement.
Qubit Modeling: The AMJJ is shunted by a large capacitor to form a transmon-like qubit. The system Hamiltonian includes terms for single-Cooper-pair tunneling ( $I_1 \sin \phi$ ) and double-Cooper-pair tunneling ( $I_2 \sin 2\phi$ ). The authors analyze the eigenenergy spectrum, anharmonicity, and noise coupling strengths (charge and flux) across different regimes defined by the ratio $\eta = I_2/I_1$ .
Flux Tunability: A flux-tunable implementation is proposed using a symmetric SQUID composed of two identical AMJJs. This allows external magnetic flux to modulate the effective Josephson potential, enabling control over the relative weights of the first and second harmonics.

Key Contributions and Results

Programmable Josephson Potentials: The study establishes that altermagnets provide a versatile platform for engineering non-sinusoidal Josephson potentials. By adjusting the altermagnetic layer thickness and interface parameters, the system can be tuned into $\phi$ -junction, $0$-junction, or $2\phi$ -junction regimes.
Enhanced Anharmonicity: The inclusion of double-Cooper-pair tunneling (the second harmonic) significantly enhances the qubit's anharmonicity compared to standard transmons, even when operating in the transmon regime ( $E_J/E_C \gg 1$ ).
Intrinsic Charge Noise Protection:
- In the $2\phi$ -junction regime (where $\eta > 0$ and exceeds a critical value $\eta_c$ ), the ground and first excited states of the qubit acquire the same parity.
- Because the charge operator is odd-parity, the matrix element between these states vanishes ( $\langle \psi_0 | \hat{N} | \psi_1 \rangle = 0$ ).
- This parity protection leads to the complete suppression of transverse charge-noise-induced energy relaxation.
- Similarly, low-frequency charge noise (pure dephasing) is suppressed because the diagonal matrix elements of the charge operator also vanish due to parity symmetry.
Quasiparticle Noise Suppression: The use of $s$ -wave superconductors ensures an isotropic energy gap, which exponentially suppresses quasiparticle noise. The authors show that the introduction of double-Cooper-pair tunneling further suppresses this noise, particularly in the $2\phi$ regime where parity selection rules can completely eliminate quasiparticle-induced relaxation.
Flux Control and Trade-offs: The flux-tunable SQUID implementation allows for precise control of the qubit frequency and anharmonicity. While higher-order harmonics generally increase sensitivity to flux noise, the authors demonstrate that operating at specific bias points (e.g., $\phi_{ext} = \pm 2\pi$ ) can suppress this sensitivity to negligible levels.

Significance
The paper claims to establish altermagnets as a programmable platform for Josephson-potential engineering. The proposed architecture offers a new route toward high-performance superconducting qubits that simultaneously combine:

Large Anharmonicity: Facilitated by the engineered multi-harmonic CPR.
Intrinsic Noise Protection: Specifically against charge noise via parity protection in the $2\phi$ regime, and against quasiparticle noise via the $s$ -wave gap and parity selection.
Broad Tunability: Achieved through device design parameters and external magnetic flux.

The work suggests that this approach overcomes the limitations of previous $d$ -wave based qubits (which suffer from quasiparticle noise) and conventional transmons (which suffer from reduced anharmonicity in the noise-insensitive regime), all while remaining compatible with established $s$ -wave superconducting fabrication techniques. The authors note that independent studies on similar topics have recently emerged, reinforcing the relevance of this research direction.

More like this