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arXiv:2606.26066

All-electrical dephasing-protected spin qubits in altermagnets

José Carlos Abadillo-Uriel, Andrea Maiani, Alberto Cortijo, Ramón Aguado, Rubén Seoane Souto

cond-mat.mes-hallquant-phSubmitted Jun 24, 2026• Announced Jun 25, 2026

v1 · Jun 24, 2026 v2 · Jul 20, 2026

Abstract

We introduce altermagnetic semiconductors as a materials platform for scalable, all-electrical controllable spin qubits that operate without magnetic fields in gate-defined quantum dots. The altermagnet intrinsic finite-momentum spin polarization provides a field-free qubit splitting that is fully and continuously tunable through the dot ellipticity with electrostatic gates, delivering individually addressable qubit frequencies. Furthermore, the altermagnetic spin splitting is first-order insensitive to electric field fluctuations by construction and quantization-axis fluctuations produce only transverse coupling, yielding relaxation without pure dephasing, a distinct dephasing-protection yielding an advantage over conventional spin qubits. The net-zero magnetization eliminates stray-fields and renders the platform compatible with superconducting resonators thus enabling high-fidelity, non-demolition circuit-QED readout by dispersive coupling of the qubit spin-dependent electric dipole couples to a resonator. Starting from an effective quantum-dot description, supported by a microscopic lattice-model, we derive the qubit Hamiltonian and establish that it naturally supports a complete gate set: electric-dipole spin resonance controls single qubits, while tunable exchange combined with per-qubit frequency addressing directly implements the fermionic simulation (fSim) family of two-qubit gates. Crucially, within the same platform it is possible to implement a singlet-triplet qubit with fully electrical control over both exchange and splitting differences, removing the need for micromagnets or nuclear polarization gradients, while enabling baseband operation. Our results establish altermagnetic quantum dots as a novel, all-electrical route to spin qubits with enhanced protection.

Comments: 13 pages, 6 figures + appendices

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We introduce altermagnetic semiconductors as a materials platform for scalable, all-electrical controllable spin qubits that operate without magnetic fields in gate-defined quantum dots. The altermagnet intrinsic finite-momentum spin polarization provides a field-free qubit splitting that is fully and continuously tunable through the dot ellipticity with electrostatic gates, delivering individually addressable qubit frequencies. Furthermore, the altermagnetic spin splitting is first-order insensitive to electric field fluctuations by construction and quantization-axis fluctuations produce only transverse coupling, yielding relaxation without pure dephasing, a distinct dephasing-protection yielding an advantage over conventional spin qubits. The net-zero magnetization eliminates stray-fields and renders the platform compatible with superconducting resonators thus enabling high-fidelity, non-demolition circuit-QED readout by dispersive coupling of the qubit spin-dependent electric dipole couples to a resonator. Starting from an effective quantum-dot description, supported by a microscopic lattice-model, we derive the qubit Hamiltonian and establish that it naturally supports a complete gate set: electric-dipole spin resonance controls single qubits, while tunable exchange combined with per-qubit frequency addressing directly implements the fermionic simulation (fSim) family of two-qubit gates. Crucially, within the same platform it is possible to implement a singlet-triplet qubit with fully electrical control over both exchange and splitting differences, removing the need for micromagnets or nuclear polarization gradients, while enabling baseband operation. Our results establish altermagnetic quantum dots as a novel, all-electrical route to spin qubits with enhanced protection.

Singlet-triplet qubits offer an attractive encoding for semiconductor quantum computing, combining ancilla-free readout, reduced sensitivity to common-mode noise, and baseband voltage control. However, the Zeeman energy difference

ΔE_\mathrm{Z}

is typically fixed by local magnetic field gradients or

g

-factor inhomogeneities, leaving the exchange interaction

J

as the only dynamically tunable parameter. This always-on

ΔE_\mathrm{Z}

precludes orthogonal control of the qubit's rotation axes and introduces unwanted state rotations during idling. Here we demonstrate all-electrical orthogonal control of a degenerate singlet-triplet (DST) qubit formed by two hole spins in a germanium double quantum dot. Exploiting the electrically tunable anisotropic

g

-factors of the two spins, we identify a regime where both

ΔE_\mathrm{Z}

and

J

vanish, making the

S

and

T_0

states degenerate at the idle point. By applying only baseband voltage pulses, we independently control both

J

and

ΔE_\mathrm{Z}

, enabling fully orthogonal

Z

- and

X

-axis rotations. Randomized benchmarking yields an average physical single-qubit gate fidelity of 99.53\% for a gate duration of approximately 100 ns. Finally, we electrically tune the degenerate point across a wide range of magnetic field orientations, enabling operation in a regime of enhanced coherence time and offering a route towards multi-qubit scaling under a shared global magnetic field.

Parity-protected superconducting qubits offer intrinsically long coherence, but many current implementations require magnetic-flux biasing, which introduces flux noise, control overhead, and limited scalability. Here we propose a parity-protected qubit based on a gate-tunable superconductor-altermagnet-superconductor Josephson junction. Altermagnets are compensated magnets with momentum-dependent spin splitting and zero net magnetization, providing spin-dependent functionality without external magnetic fields. In the proposed junction, the two spin sectors acquire opposite phase shifts, generating two Josephson channels whose interference is controlled electrically by the chemical potential. At the tuned

0

-

π

transition, the first Josephson harmonic is strongly suppressed while the second harmonic dominates, yielding a double-well potential with two nearly degenerate states of opposite Cooper-pair parity. For realistic gatemon-compatible parameters, we estimate coherence times of up to tens of milliseconds while maintaining fully gate-controlled qubit operations. These results establish altermagnetic Josephson junctions as a promising route toward protected superconducting qubits with local, scalable, and all-electrical control.

Micromagnet-free operation of electron spin qubits in Si/Si

_{1-x}

Ge

_x

vertical double quantum dots

We study a vertical double quantum dot (DQD) in a Si/Si

_{1-x}

Ge

_x

/Si double-well heterostructure for full electrical control of electron Loss-DiVincenzo (LD) spin qubits, using realistic device modeling and numerical simulations. Due to the emerging spin-orbit interaction in the DQD, as well as strain from the gate electrodes, small (percentage range) but finite

g

tensor variations emerge. In addition, we find a large valley splitting, on the order of

E_v{\sim}250\,μ

eV. As a result, multiple avenues for fast electrical single qubit rotations emerge. An ac electric field gives rise to electric dipole spin resonance (EDSR), while electron spin resonance (ESR) in the presence of an ac magnetic field can be electrically controlled by local gates due to varying

g

factors in DQDs. We also show that shuttling between neighboring dots, in vertical and horizontal direction, results in ultrafast single qubit gates of less than a nanosecond. Remarkably, this DQD architecture completely eliminates the need for micromagnets, significantly facilitating the scalability of LD spin qubits in semiconductor foundries.

We propose gate-defined spin qubits in electrostatically confined altermagnetic quantum dots. Elliptical confinement of the

d

-wave altermagnetic structure produces a low-energy doublet with opposite spin polarization. For the range of parameters used here, the qubit states energy gap lies in the microwave range while the leakage gap remains in the meV range. Even without spin-orbit coupling, time-dependent simulations show that a phase-controlled quadrupolar gate drive about a fixed bias point implements

X_{π/2}

and

X_π

rotations by resonantly modulating the confinement anisotropy. We extend the study to two-qubits using a double quantum dot. We show that the double quantum dot spectrum can be cleanly projected onto isolated quantum dot product states with a nonzero nonlocal Pauli block in the effective logical two-qubit Hamiltonian. Resonant central-barrier modulation then drives the logical two-qubit component close to a maximally entangled state. These calculations show anisotropic altermagnetic quantum dots as a route to locally gate-controlled spin qubits without requiring spin-orbit coupling.

A pair of self-assembled InGaAs quantum dots filled with two electrons can act as a singlet-triplet spin qubit that is robust against nuclear spin fluctuations as well as charge noise. This results in a T2* coherence time two orders of magnitude longer than that of a single electron, provided the qubit is operated at a particular "sweet spot" in gate voltage. However, at this fixed operating point the ground-state splitting can no longer be tuned into resonance with e.g. another qubit, limiting the options for coupling multiple qubits. Here, we propose using a four-electron coupled quantum dot to implement a singlet-triplet qubit that features a magnetically tunable level splitting. As a first step towards full experimental realization of this qubit design, we use optical spectroscopy to demonstrate the tunability of the four-electron singlet-triplet splitting in a moderate magnetic field.

Altermagnets, characterized by vanishing net magnetization and momentum-dependent spin splitting, provide a promising platform for next-generation Josephson devices. Here, we exploit the Josephson effect in superconductor-altermagnet-superconductor junctions and show how to engineer prescribed current-phase relations by device design. Based on these programmable Josephson potentials utilizing altermagnetism, we propose a new class of superconducting qubits that combine large anharmonicity with enhanced robustness against decoherence via coherent two-Cooper-pair tunneling. We show that in the

2φ

-junction regime, this kind of qubit is intrinsically protected against both charge and flux noise due to parity protection. Magnetic flux can be used to precisely control the qubit and, under appropriate bias, this architecture further suppresses charge and flux noise. Our results establish altermagnets as a versatile platform for Josephson-potential engineering and open a new route toward high-performance superconducting qubits combining high coherence, large anharmonicity, and broad tunability.