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arXiv:2608.27625

Turning Zeeman splitting into switchable charge polarization in a double quantum dot

Chi Sun, Jacob Linder

cond-mat.mes-hallquant-phSubmitted Aug 27, 2026• Announced Aug 31, 2026
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Abstract

A magnetic field that acts identically on two quantum dots is not expected to move charge between them. Nevertheless, we show that a uniform Zeeman field can strongly reconfigure and even reverse the single-electron charge polarization of an asymmetric open double quantum dot. Using a symmetry-preserving Green's-function equation-of-motion approach, we identify regimes where the preferred dot occupation reverses while the system remains in the single-electron charge sector. Two distinct mechanisms produce this behavior. Unequal gate levels produce different occupation responses because the Zeeman-shifted resonances lie at different positions relative to the reservoir chemical potential, whereas unequal onsite interactions distinguish the dots through their many-body addition spectra. Coulomb blockade stabilizes the single-electron sector, causing the reservoir-mediated response to appear as spatial charge redistribution rather than a change in total occupation. Our results establish a mechanism for magnetic control of charge polarization at fixed electrostatic detuning.

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A magnetic field that acts identically on two quantum dots is not expected to move charge between them. Nevertheless, we show that a uniform Zeeman field can strongly reconfigure and even reverse the single-electron charge polarization of an asymmetric open double quantum dot. Using a symmetry-preserving Green's-function equation-of-motion approach, we identify regimes where the preferred dot occupation reverses while the system remains in the single-electron charge sector. Two distinct mechanisms produce this behavior. Unequal gate levels produce different occupation responses because the Zeeman-shifted resonances lie at different positions relative to the reservoir chemical potential, whereas unequal onsite interactions distinguish the dots through their many-body addition spectra. Coulomb blockade stabilizes the single-electron sector, causing the reservoir-mediated response to appear as spatial charge redistribution rather than a change in total occupation. Our results establish a mechanism for magnetic control of charge polarization at fixed electrostatic detuning.
A lateral gate-controlled double dot structure in Si has been fabricated for studying coupled two qubits. Nonequilibrium single-electron tunneling measurements at 1.4K show that the second Coulomb peak, associated with a two-electron occupation, splits into two side peaks, and that their separation displays a strong magnetic-field dependence for various interdot coupling constants. Moreover, for some fixed magnetic fields, the separation of the side peaks decays exponentially as a function of the interdot coupling. We attribute this behavior to electron spin exchange and spin swapping between singlet and triplet states in the coupled double dot in the presence of a magnetic field.
We analyze the charge and spin dynamics in a DC biased double quantum dot driven by crossed DC and AC magnetic fields. In this configuration, spatial delocalization due to inter-dot tunnel competes with intra-dot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunnelling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace (dark subspace) performing coherent spin rotations and the current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the first case, our results show that measuring the current will allow to get information on the spin relaxation time. In the last case, we will show that applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating AC magnetic fields, electrons are driven to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.
We study the spin-spin coupling between two single-electron quantum dots due to the Coulomb and spin-orbit interactions, in the absence of tunneling between the dots. We find an anisotropic XYXYXY spin-spin interaction that is proportional to the Zeeman splitting produced by the external magnetic field. This interaction is studied both in the limit of weak and strong Coulomb repulsion with respect to the level spacing of the dot. The interaction is found to be a nonmonotonic function of interdot distance a0{a}_{0}a0​ and external magnetic field and, moreover, vanishes for some special values of a0{a}_{0}a0​ and/or magnetic field orientation. This mechanism thus provides a new way to generate and tune spin interaction between quantum dots. We propose a scheme to measure this spin-spin interaction based on the spin-relaxation-measurement technique.
We have analyzed theoretically quenched dynamics of correlated double quantum dot (DQD) due to the switching "on" and "off" coupling to reservoirs. The possibility for controllable manipulation of charge and spin states in the double quantum dot was revealed and discussed. The proposed experimental scheme allows to prepare in DQD maximally entangled pure triplet state and to drive it to another entangled singlet state by tuning both applied bias and gate voltage. It was also demonstrated that the symmetry properties of the total system (double quantum dot coupled to electron reservoirs) allow to resolve the initially prepared two-electron states by detecting non-stationary spin-polarized currents flowing in both reservoirs and controlling the residual charge.
We analyze coherent spin rotations in a dc biased double quantum dot driven by crossed dc and ac magnetic fields. In this configuration, spatial delocalization due to interdot tunneling competes with intradot spin rotations induced by the time dependent magnetic field, giving rise to a complicated time dependent behavior of the tunneling current. When the Zeeman splitting has the same value in both dots and spin flip is negligible, the electrons remain in the triplet subspace performing coherent spin rotations and current does not flow. This electronic trapping is removed either by finite spin relaxation or when the Zeeman splitting is different in each quantum dot. In the last case, we will show that by applying a resonant bichromatic magnetic field, the electrons become trapped in a coherent superposition of states and electronic transport is blocked. Then, manipulating ac magnetic fields allows one to drive electrons to perform coherent spin rotations which can be unambiguously detected by direct measurement of the tunneling current.
We propose altermagnetic semiconductors as a platform for field-free, all-electrically controlled spin qubits in gate-defined quantum dots. The momentum-dependent spin splitting of an altermagnet produces a Zeeman-like qubit splitting whose magnitude and sign are set by the dot ellipticity, enabling local frequency tunability without external magnetic fields or micromagnets. Because the splitting is tied to a fixed altermagnetic quantization axis, electric-field noise is longitudinally suppressed at leading order, while quantization-axis fluctuations couple transversely and therefore cause relaxation rather than pure dephasing. The compensated magnetic order also avoids stray fields, making the platform naturally compatible with superconducting resonators and dispersive circuit-QED readout through the qubit's spin-dependent electric dipole. Starting from an effective quantum-dot model, supported by a microscopic lattice model, we derive the single- and two-dot Hamiltonian models. We show that electric-dipole spin resonance enables single-qubit control, while tunable exchange and electrically addressable qubit frequencies realize fSim two-qubit gates. The same double-dot architecture also supports singlet-triplet qubits with electrical control of both exchange and splitting gradients, removing the need for micromagnets or nuclear-polarization gradients. These results establish altermagnetic quantum dots as a route to field-free spin qubits with intrinsic electrical tunability and enhanced dephasing protection.