During the first two years of the project, substantial progress has been made on all three core objectives. The main achievements include:
1.a Implementation of novel control techniques for spectral diffusion in single-photon sources, enabling enhanced indistinguishability for multi-photon quantum interference.
After identifying charge noise as a limiting factor, we exploited the intrinsic anisotropy of the molecules to finely tune their emission frequency without inducing additional instability.
[R. Duquennoy, et al., “Enhanced Control of Single-Molecule Emission Frequency and Spectral Diffusion,” ACS Nano, vol. 18, no. 47, pp. 32508–32516, 2024]
1.b Deployment of a molecule-based single photon source in quantum readiometry.
We leveraged QUINTESSEnCE technology to embed single molecules in polymeric photonic structures and demonstrated their use as quantum light standards for detector calibration. Unlike lasers or thermal sources, our emitters cannot produce two photons within the same pulse, enabling highly precise and accurate calibration.
[P. Lombardi, et al., “Advances in Quantum Metrology with Dielectrically Structured Single Photon Sources Based on Molecules”, Advanced Quantum Technologies 2400107 (2024)]
2.a Theoretical framework for hybrid molecular optomechanics.
We developed a model describing how molecules—being the smallest and lightest optomechanical systems—can access unconventional cavity optomechanical regimes. Using high-power, multi-color excitation, we demonstrated the ability to sustain macroscopic populations in vibrationally excited states despite fast relaxation.
[D. De Bernardis, et al., “Hybrid interfaces at the single quantum level in fluorescent molecules”, arXiv:2503.20872v3 23 Apr 2025]
2.b Quantum thermometry at cryogenic temperatures using single molecules.
We demonstrated a nanoscale thermometer operable in the 2–10 K range, based on the measurement of molecules' optical linewidth. The technique reaches a sensitivity of ~0.01 K at 5 K and has been applied to complex phononic crystal membranes, where standard heat-mapping techniques are ineffective.
[V. Esteso et al., “Quantum thermometry with single molecules in portable nanoprobes [V. Esteso, et al., “Quantum thermometry with single molecules in portable nanoprobes”, Phys. Rev. X Quantum 4, 040314 (2023)]
3.a New protocols for characterizing molecular spin qubits.
We established a robust characterization method based on cantilever torque magnetometry, validated on custom-designed Ni(II) complexes. This approach offers a high-throughput, cost-effective alternative for screening molecular qubit candidates.
[J.T. Janetzki, et al., “Cantilever Torque Magnetometry for Designing Molecular Qubits”, in preparation]
In addition to these scientific milestones, the project has fostered a highly interdisciplinary research environment, merging synthetic chemistry, quantum optics, and molecular spectroscopy. The second half of the project will capitalize on this foundation to advance toward full quantum control and sensing with scalable, molecule-based platforms.
