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arXiv:2509.14813 · Physical review. B./Physical review. B

Spin-photon coupling using circular double quantum dots

Ferdinand Omlor, Florinda Viñas Boström, Martin Leijnse

Published Feb 2, 2026• 0 citations
arXivHTMLPDF

Abstract

We propose and analyze a microwave spin-photon interface based on a circular double quantum dot, inspired by recent experimental observations of anisotropic g -factors and orbital ring states in InAs nanowires. We develop an effective theoretical model, capturing the interplay of spin-orbit coupling and the magnetic flux, and show how ring states form at crossings of odd and even geometric parity orbital states. Similar to bonding and antibonding states of conventional double quantum dots, the ring states can be changed into single dot states by detuning the dots, which enables a high degree of control over the system's properties. Applying a tilted magnetic field induces spin-charge hybridization which enables spin-photon coupling. For low disorder, the photons couple states of simultaneously (almost) opposite spin and angular momentum. With increasing disorder, the spin-photon coupling becomes analogous to the flopping mode mechanism of conventional double quantum dots with intrinsic spin-orbit coupling where the spin is hybridized with the bonding and antibonding orbital states. We show that the system exhibits a third-order charge-noise sweet spot at a specific magnetic field angle, which lowers the system's sensitivity to dephasing while retaining a substantial spin-photon coupling strength. Moreover, the photon coupling mechanism can be switched off either electrically, by detuning to the single-dot regime, or magnetically, by rotating the field to disable the spin-charge hybridization.

DOI
10.1103/qtfz-3rh9

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The interaction of qubits via microwave frequency photons enables long-distance qubit-qubit coupling and facilitates the realization of a large-scale quantum processor. However, qubits based on electron spins in semiconductor quantum dots have proven challenging to couple to microwave photons. In this theoretical work we show that a sizable coupling for a single electron spin is possible via spin-charge hybridization using a magnetic field gradient in a silicon double quantum dot. Based on parameters already shown in recent experiments, we predict optimal working points to achieve a coherent spin-photon coupling, an essential ingredient for the generation of long-range entanglement. Furthermore, we employ input-output theory to identify observable signatures of spin-photon coupling in the cavity output field, which may provide guidance to the experimental search for strong coupling in such spin-photon systems and opens the way to cavity-based readout of the spin qubit.
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We theoretically investigate quantum correlations and energy storage in a single-electron silicon double quantum dot (eDQD) coupled to a single-mode microwave cavity. The charge and spin degrees of freedom are hybridized by Rashba spin-orbit coupling (SOC), while the cavity interacts with the eDQD through spin-photon and charge-photon couplings. Using the reduced thermal density matrix, we characterize spin-charge entanglement by concurrence and quantum coherence by the l1l_1l1​ norm. At low temperature, the cavity strongly modifies the spin-charge correlations, producing distinct regimes of enhanced and suppressed entanglement. We identify a nonlinear crossover between an eDQD-dominated regime and a photon-dressed regime. This crossover is revealed independently by two signatures: a rapid change in the concurrence and the onset of finite photon occupation in the cavity. Its boundary exhibits a dominant Gc∝ΩG_{\rm c}\propto\sqrtΩGc​∝Ω​ dependence and is robust against cavity Hilbert-space truncation. Beyond the Rashba coupling studied in [Ferreira et al., Phys. Rev. A 107, 052408 (2023)], we show that the cavity provides an additional tunable means of controlling quantum correlations and energy storage. We further characterize the eDQD as a quantum battery under coherent charging and evaluate its stored energy and ergotropy. The ergotropy varies markedly across a cavity-dressing crossover resembling that identified from entanglement, connecting cavity-induced modification of the dressed states to extractable work. Optimization over the charge-photon coupling reveals parameter-dependent optimal charging regimes and enhanced maximum ergotropy with increasing Rashba coupling. Our results establish cavity coupling and SOC as complementary controls of quantum correlations and extractable energy in semiconductor eDQD-cavity systems.