José Carlos Abadillo-Uriel

Recent publications and preprints

1. 

   Spin-cQED with bulk germanium spin qubits

   Ahmad Fouad Kalo, Esteban A Rodrı́guez-Mena, Jose Carlos Abadillo-Uriel, Michele Filippone, and Yann-Michel Niquet

   arxiv, 2026

   Abs arXiv Bib URL

   Unstrained bulk germanium is a particularly attractive material for circuit quantum electrodynamics with spins (spin-cQED). We show, through systematic modeling and comparison with state-of-the-art strained germanium heterostructures, that hole spins in bulk germanium double quantum dots readily reach the strong-coupling regime with superconducting microwave resonators, achieving spin-photon coupling strengths g_s/2\pi≳100 MHz. This enhancement originates from large spin-orbit interactions beyond the perturbative regime. In addition, the coupling is much less sensitive to the orientation of the applied magnetic field, which shall ease operation and limit the impact of device-to-device variability. Our results establish bulk germanium as a compelling platform for scalable spin-cQED.

   @article{kalo2026bulkge,
     title = {Spin-cQED with bulk germanium spin qubits},
     author = {Kalo, Ahmad Fouad and Rodr{\'\i}guez-Mena, Esteban A and Abadillo-Uriel, Jose Carlos and Filippone, Michele and Niquet, Yann-Michel},
     year = {2026},
     doi = {arXiv:2607.24967v1},
   }

2. 

   All-electrical dephasing-protected spin qubits in altermagnets

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

   arxiv, 2026

   Abs arXiv Bib URL

   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.

   @article{abadillo2026altermagnet,
     title = {All-electrical dephasing-protected spin qubits in altermagnets},
     author = {Abadillo-Uriel, Jose Carlos and Maiani, Andrea and Cortijo, Alberto and Aguado, Ram{\'o}n and Seoane Souto, Rub{\'e}n},
     year = {2026},
     doi = {arXiv:2606.26066v2},
   }

3. 

   Sweet-spot protection of hole spins in sparse arrays via spin-dependent magnetotunneling

   Esteban A Rodrı́guez-Mena, Biel Martı́nez, Ahmad Fouad Kalo, Yann-Michel Niquet, and Jose Carlos Abadillo-Uriel

   Physical Review B, 2026

   Abs arXiv Bib URL

   Recent advances in the scaling of spin qubits have led to the development of sparse architectures where spin qubits are distributed across multiple quantum dots. This distributed approach enables qubit manipulation through hopping and flopping modes, as well as protocols for spin shuttling to entangle spins beyond nearest neighbors. Therefore, understanding spin tunneling across quantum dots is fundamental for the improvement of sparse array encodings. Here, we develop a microscopic theory of a minimal sparse array formed by a hole in a double quantum dot. We show the existence of spin-dependent magnetic corrections to the tunnel couplings that help preserve existing sweet spots, even for quantum dots with different -factors, and introduce new ones that are not accounted for in the simplest models. Our analytical and numerical results explain observed sweet spots in state-of-the-art shuttling and cQED experiments, are relevant to hopping and flopping modes, and apply broadly to sparse array encodings of any size.

   @article{magnetotunnel,
     title = {Sweet-spot protection of hole spins in sparse arrays via spin-dependent magnetotunneling},
     author = {Rodr{\'\i}guez-Mena, Esteban A and Mart{\'\i}nez, Biel and Kalo, Ahmad Fouad and Niquet, Yann-Michel and Abadillo-Uriel, Jose Carlos},
     journal = {Physical Review B},
     volume = {113},
     pages = {205303},
     year = {2026},
     publisher = {APS},
     doi = {10.1103/qmdt-4plp},
   }

4. 

   Coherence of a hole spin flopping-mode qubit in a circuit quantum electrodynamics environment

   Léo Noirot, Cécile X Yu, Jose Carlos Abadillo-Uriel, Étienne Dumur, Heimanu Niebojewski, Benoit Bertrand, and 2 more authors

   Nature Physics, 2026

   Abs arXiv Bib URL

   The entanglement of microwave photons and spin qubits in silicon represents a pivotal step forward for quantum information processing utilizing semiconductor quantum dots. Such hybrid spin circuit quantum electrodynamics (cQED) has been achieved by granting a substantial electric dipole moment to a spin by de-localizing it in a double quantum dot under spin-orbit interaction, thereby forming a flopping-mode (FM) spin qubit. Despite its promise, the coherence properties demonstrated to date remain insufficient to envision FM spin qubits as practical single qubits. Here, we present a FM hole spin qubit in a silicon nanowire coupled to a high-impedance niobium nitride microwave resonator for readout. We report Rabi frequencies exceeding 100 MHz and coherence times in the microsecond range, resulting in a high single gate quality factor of 380. This establishes FM spin qubits as fast and reliable qubits. Moreover, using the large frequency tunability of the FM qubit, we reveal for the first time that photonic effects predominantly limit coherence, with radiative decay being the main relaxation channel and photon shot-noise inducing dephasing. These results highlight that optimized microwave engineering can unlock the potential of FM spin qubits in hybrid cQED architectures, offering a scalable and robust platform for fast and coherent spin qubits with strong coupling to microwave photons.

   @article{noirot2025coherence,
     title = {Coherence of a hole spin flopping-mode qubit in a circuit quantum electrodynamics environment},
     author = {Noirot, L{\'e}o and Yu, C{\'e}cile X and Abadillo-Uriel, Jose Carlos and Dumur, {\'E}tienne and Niebojewski, Heimanu and Bertrand, Benoit and Maurand, Romain and Zihlmann, Simon},
     journal = {Nature Physics},
     year = {2026},
     url = {https://www.nature.com/articles/s41567-026-03262-y},
     doi = {10.1038/s41567-026-03262-y},
   }

5. 

   Granular aluminum induced superconductivity in germanium for hole spin-based hybrid devices

   Giorgio Fabris, Paul Falthansl-Scheinecker, Devashish Shah, Daniel Michel Pino, Maksim Borovkov, Anton Bubis, and 16 more authors

   arxiv, 2026

   Abs arXiv Bib URL

   In superconductor-semiconductor hybrid structures, superconductivity and spin polarization are competing effects as magnetic fields break Cooper pairs. They can be combined using thin films and in-plane magnetic fields, an approach that enabled the pursuit of Majorana zero modes, Kitaev chains, and Andreev spin qubits (ASQs), but remains challenging for materials with small in-plane g-factors or when out-of-plane fields are required. We demonstrate that granular aluminium (grAl), composed of nanometre-scale aluminium grains embedded in an amorphous oxide matrix, can overcome this limitation. By depositing grAl on Ge/SiGe heterostructures, we induce a hard superconducting gap with BCS peaks at 305 \mueV and magnetic-field resilience for both the in-plane and out-of-plane directions, allowing Zeeman splitting of Yu-Shiba-Rusinov (YSR) states beyond 50 \mueV (12 GHz). Leveraging this robustness, we reveal signatures of hole physics and discuss a driving mechanism for ASQs regardless of the strength of the Rashba spin-orbit coupling in planar germanium.

   @article{fabris2026granular,
     title = {Granular aluminum induced superconductivity in germanium for hole spin-based hybrid devices},
     author = {Fabris, Giorgio and Falthansl-Scheinecker, Paul and Shah, Devashish and Pino, Daniel Michel and Borovkov, Maksim and Bubis, Anton and Roux, Kevin and Sokolova, Dina and Juanes, Alejandro Andres and Costanzo, Tommaso and Taha, Inas and Gen{\c c}, Aziz and Arbiol, Jordi and Calcaterra, Stefano and De Cerdeira Oliveira, Afonso and Chrastina, Daniel and Isella, Giovanni and Seoane Souto, Rub{\'e}n and Calder{\'o}n, Maria Jos{\'e} and Aguado, Ram{\'o}n and Abadillo-Uriel, Jose Carlos and Katsaros, Georgios},
     year = {2026},
     doi = {arXiv:2602.21364v4},
   }

6. 

   Disorder-independent hole spin manipulation by hopping

   Biel Martı́nez, Ana Sempere-Sanchis, Jose Carlos Abadillo-Uriel, and Yann-Michel Niquet

   arxiv, 2026

   Abs arXiv Bib URL

   Spin manipulation by hopping has recently emerged as a promising strategy to control hole spins in quantum dots using exclusively baseband control, thereby mitigating power dissipation and high-frequency management constraints in large-scale architectures. Unlike conventional approaches such as electron dipole spin resonance (EDSR), this mechanism exploits dot-to-dot variations of the spin precession axes to enable spin rotations. However, it is intrinsically disorder-dependent: in the absence of sufficient variability, the precession axes remain aligned and spin manipulation becomes ineffective. This fundamental reliance on disorder raises concerns regarding its compatibility with the long-term evolution of spin-qubit platforms toward improved material quality, cleaner interfaces, and enhanced device reproducibility. Here, we numerically assess the viability of spin manipulation by hopping as a function of disorder strength and demonstrate that its implementation is indeed increasingly constrained as disorder is reduced. To overcome this limitation, we propose an alternative strategy based on hopping between intentionally squeezed quantum dots. This approach retains the advantages of baseband control while being independent of disorder and robust against moderate variability, thereby offering improved prospects for scalable hole-spin quantum computing architectures.

   @article{martinez2026disorder,
     title = {Disorder-independent hole spin manipulation by hopping},
     author = {Mart{\'\i}nez, Biel and Sempere-Sanchis, Ana and Abadillo-Uriel, Jose Carlos and Niquet, Yann-Michel},
     year = {2026},
     doi = {arXiv:2602.20740v1},
   }

7. 

   Switchable spin-photon coupling with hole spins in single-quantum dots

   Carlos Sagaseta, Marı́a José Calderón, and Jose Carlos Abadillo-Uriel

   Materials for Quantum Technology, 2026

   Abs arXiv Bib URL VIDEO Code

   Spin qubits in semiconductor quantum dots offer a gate-tunable platform for quantum information processing. While two-qubit interactions are typically realized through exchange coupling between neighboring spins, coupling spin qubits to photons via hybrid spin-cQED devices enables long-range interactions and integration with other cQED platforms. Here, we investigate hole spin-photon coupling in compact single quantum dot setups. By incorporating ubiquitous strain inhomogeneities to our theory, we identify three main spin-photon coupling channels: a vector-potential-spin-orbit geometric mechanism–dominant for vertical magnetic fields–, an inhomogeneous Rashba term generalizing previous spin-orbit field models, and strain-induced -tensor terms–most relevant for in-plane fields. Comparing Si, unstrained (relaxed) Ge, and biaxially strained Ge wells, we find that Si and unstrained Ge provide optimal coupling strengths (tens of MHz) thanks to their reduced heavy-hole, light-hole splitting. We demonstrate efficient switching of the spin-photon coupling while preserving sweet spot operation. Finally, we evaluate quantum state transfer and two-qubit gate protocols, achieving >99% fidelity for state transfer and >90%for two-qubit gates with realistic coherence times, establishing single-dot hole spins as a viable platform for compact spin-cQED architectures and highlighting unstrained Ge as a promising candidate for spin-photon interactions.

   @article{sagaseta2025switchable,
     title = {Switchable spin-photon coupling with hole spins in single-quantum dots},
     author = {Sagaseta, Carlos and Calder{\'o}n, Mar{\'\i}a Jos{\'e} and Abadillo-Uriel, Jose Carlos},
     journal = {Materials for Quantum Technology},
     volume = {6},
     pages = {016302},
     year = {2026},
     publisher = {IOP},
     url = {https://iopscience.iop.org/article/10.1088/2633-4356/ae4087},
     doi = {10.1088/2633-4356/ae4087},
   }