Chirality and spin selectivity in electron transfer processes: from quantum detection to quantum enabled technologies

Project Information

CASTLE

Grant agreement ID: 101071533

DOI

10.3030/101071533

EC signature date 22 November 2022

Start date 1 January 2023

End date 31 December 2028

Funded under

• European Research Council (ERC)

Total cost

€ 8 976 957,00

EU contribution

€ 8 976 957,00

Coordinated by

UNIVERSITA DEGLI STUDI DI FIRENZE
Italy

Periodic Reporting for period 2 - CASTLE (Chirality and spin selectivity in electron transfer processes: from quantum detection to quantum enabled technologies)

Reporting period: 2024-07-01 to 2025-12-31

Summary of the context and overall objectives of the project

Molecular spins are promising building blocks of future quantum technologies thanks to the flexibility of chemistry, which enables the design of complex structures for specific applications. However, their weak interaction with external stimuli hampers access to their state at the single-molecule level, essential for quantum computing and sensing. We propose an innovative solution exploiting the interplay between chirality and magnetism via the chirality-induced spin selectivity (CISS) effect in electron transfer processes.
The phenomenon is still not fully understood and only partially explored.

CASTLe aims to contribute to the development of a universal quantum computer by demonstrating an efficient method for spin qubit initialization, manipulation, and readout at high temperatures, connecting a spin qubit to a dyad where an electron donor and acceptor are linked by a chiral bridge. This goal will be pursued through four objectives:

• O1: Establish the CISS effect at the intramolecular level
• O2: Investigate its quantum properties through coherent detection
• O3: Model CISS at the quantum mechanical level and design applications
• O4: Demonstrate CISS-based initialization and readout in a device

CASTLe leverages the complementary expertise of four PIs:

• UNIFI: magnetic molecular systems and novel magnetism concepts
• NWU: photo-induced ET and advanced transient laser and EPR spectroscopies
• FUB: pulsed and time-resolved magnetic resonance, including EDMR
• UNIPR: realistic models and quantum computing schemes for molecular spin systems, plus unconventional NMR
• Partners: the Weizmann Institute and the Italian National Interuniversity Consortium of Material Science and Technology (INSTM)

The impact extends beyond quantum technologies, as electron transfer is ubiquitous in chemistry and biology and linked to spin. Chirality, central in natural evolution, remains largely unexplored for ET control in materials science. Achieving CASTLe’s objectives will expand control over fundamental chemical processes. CISS and spin polarization transfer may also enhance NMR sensitivity and chirality detection.

Visit https://www.castle.unifi.it/
for more information.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

• A roadmap was defined to transform CISS into an enabling technology for quantum information science (Adv. Mater., 2023, 35, 2300472), including simulations of initialization, one- and two-qubit gates, and readout of a molecular spin linked to a D--A unit, exploiting CISS to control individual spins.
• Intramolecular CISS was demonstrated via time-resolved EPR on chiral D--A molecules aligned with liquid crystals. A clear spectral feature absent in achiral references provided unambiguous evidence (Science, 2023, 382, 197). Additional systems studied include randomly oriented dyads (J. Am. Chem. Soc. 2024, 146, 24125 ), chromophore-linked DNA hairpins (Proc. Natl. Acad. Sci. USA 2025, 122, e2515120122), and a hetero-helicene dyad (submitted).
• Transport experiments on thia-bridged hetero-helicenes showed high CISS efficiency and low-voltage conductivity in neutral (ACS Nano, 2023, 17, 15189) and radical cationic forms (J. Mater. Chem. C, 2024, 12, 10029).
• Electron-transfer dynamics were described via the Haberkorn formalism including CISS-induced polarization, enabling simulations of initialization, readout, and gates.
• A model including bridge degrees of freedom showed sizable acceptor polarization arising from coherent and incoherent dynamics and strong correlations (Nano Letters 2024, 24, 12133).
• Molecular spin qubits based on vanadium(IV) and copper(II) were investigated. Porphyrin-based qubits coupled to photo-generated spins showed high electronic and nuclear polarization (J. Am. Chem. Soc. 2025, 147, 331) and enabled entangling gates between two vanadyl qubits (J. Am. Chem. Soc. 2026, 10.1021/jacs.5c17205).
• An optically pumped NMR setup using a high-repetition-rate 3rd harmonic Nd-YAG pumped OPO laser is under development.
• A eutectic Ga-In electrode setup for micrometric CISS measurements has been developed (manuscript submitted).

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The most relevant result is the demonstration that CISS operates in electron transfer at the molecular level, pushing the field beyond the state of the art and adding a new degree of freedom to ET reactions. As noted in Science by J. E. Subotnik: "These results suggest that the standard ET theory should be modified to include both energy and total (orbital plus spin) angular momentum conservation, thereby opening the door to new CISS applications, including “green” hydrogen generation." The polarization of nuclear spins via CISS will also be explored.

Schematic view of how qubits could be controlled and readout using CISS