Work

Few-Electron Spin Qubits in Quantum Dots Probed Through Nuclear Spin Magnetism

Year: 2026

Type: article

Abstract: The knowledge of the energy spectrum completely defines the dynamics of a quantum system for a given initial state. This makes spectroscopy a key characterization technique when studying or designing qubits and complex quantum systems. In semiconductor quantum dots, the electronic quantum states can be probed through charge transport or charge sensing, but the technique is practically limited to gate-defined quantum dots. Epitaxial quantum dots benefit from excellent optical properties, but are usually incompatible with charge transport, while alternative spectroscopy techniques provide only limited information. Here we demonstrate a spectroscopy technique which utilizes nuclear spins as a non-invasive probe. By using the cotunneling-assisted transfer of spin angular momentum instead of the charge currents we achieve near-equilibrium probing. Experiments are conducted on low-strain GaAs/AlGaAs epitaxial dots, revealing energy spectra for charge configurations with up to seven electrons and with resolution limited only by the thermal broadening. Beyond the few-electron energy spectroscopy, we apply non-invasive probing via nuclear spins to reveal the subtle many-body physics of the quantized electronic states. We map out the magnetic field dependence of the electron spin lifetimes, revealing an unusual inverse quadratic dependence. The rich variety of further observations includes the long-lived spin-qubit states in the p shell, electron ground-state phase transitions, strong spin-orbit coupling regimes, and an anomalously fast nuclear spin diffusion process. Experiments are backed up by good agreement with the first-principles configuration interaction numerical modeling. Our work uncovers few-electron states as a new operating regime for optically active quantum dots. Accurate control and probing of many-body states offers a test-bed system for fundamental physics studies, while prospective technological applications include electron spin qubits with extended coherence and scalable electrical control.

Source: PRX Quantum

DOI: 10.1103/j8xw-w72c