Periodic Reporting for period 1 - QUINTESSEnCE (Quantum interfaces with single molecules)
Reporting period: 2023-06-01 to 2025-11-30
QUINTESSEnCE addresses a central challenge in quantum technologies: how to scale up complex quantum systems while maintaining control at the single-quantum level. The project builds on the unique properties of molecules, which combine the coherence of atoms with the flexibility and integrability of solid-state systems. By exploiting molecular complexity, the project aims to develop a new class of scalable, hybrid quantum devices where single molecules act as coherent interfaces between photons (optical qubits), spins (magnetic qubits), and phonons (mechanical modes).
At the heart of QUINTESSEnCE is the ambition to leverage the intrinsic reproducibility and tunability of molecules. Unlike many solid-state platforms that suffer from inhomogeneity and fabrication-induced decoherence, molecules can be synthesized with atomic precision and embedded in custom nanostructures. This allows for the creation of nominally identical quantum units, crucial for scalability.
The project pursues three groundbreaking objectives:
1. Molecular nanophotonics for the generation and manipulation of complex quantum states of light on-chip.
2. Single-molecule cavity optomechanics enabling strong coupling between individual photons and phonons.
3. Optical control of molecular spins, establishing a scalable and coherent spin-photon interface.
These objectives tackle critical bottlenecks in quantum communication, sensing, and information processing. If successful, QUINTESSEnCE will enable multi-photon quantum interference experiments, single-photon optomechanical coupling, and the optical read-out of molecular spin qubits — all within a unified molecular platform.
The expected impact is broad and transformative. Scientifically, it will open unexplored quantum regimes by combining photonic, magnetic, and mechanical degrees of freedom in a single coherent system. Technologically, it will provide scalable, chip-integrated components for hybrid quantum devices. By advancing this new "lab-in-a-molecule" concept, QUINTESSEnCE is poised to influence diverse fields from molecular engineering and materials science to quantum optics and computing.
At the heart of QUINTESSEnCE is the ambition to leverage the intrinsic reproducibility and tunability of molecules. Unlike many solid-state platforms that suffer from inhomogeneity and fabrication-induced decoherence, molecules can be synthesized with atomic precision and embedded in custom nanostructures. This allows for the creation of nominally identical quantum units, crucial for scalability.
The project pursues three groundbreaking objectives:
1. Molecular nanophotonics for the generation and manipulation of complex quantum states of light on-chip.
2. Single-molecule cavity optomechanics enabling strong coupling between individual photons and phonons.
3. Optical control of molecular spins, establishing a scalable and coherent spin-photon interface.
These objectives tackle critical bottlenecks in quantum communication, sensing, and information processing. If successful, QUINTESSEnCE will enable multi-photon quantum interference experiments, single-photon optomechanical coupling, and the optical read-out of molecular spin qubits — all within a unified molecular platform.
The expected impact is broad and transformative. Scientifically, it will open unexplored quantum regimes by combining photonic, magnetic, and mechanical degrees of freedom in a single coherent system. Technologically, it will provide scalable, chip-integrated components for hybrid quantum devices. By advancing this new "lab-in-a-molecule" concept, QUINTESSEnCE is poised to influence diverse fields from molecular engineering and materials science to quantum optics and computing.
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.
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.
The impact of QUINTESSEnCE results spans over different aspects.
The possibility of increasing multiphoton indistinguishability opens the opportunity of deploying molecular emitters for quantum optical simulation such as Boson sampling experiments or for linear optical quantum computing. Further research will focus on actively depleting charge noise, building on our demonstrated strategy of minimizing molecular sensitivity (Result 1.a). These developments could pave the way for deployable, high-performance quantum photonic sources.
The integration of single molecules in photonic structures as demonstrated in Result 1.b opens a new direction in quantum metrology, particularly for the precise calibration of single-photon detectors. The next generation of sources will incorporate resonant enhancement to boost emission rates and collection efficiency. These advancements hold clear potential for industrial uptake, particularly in sectors such as quantum sensing and quantum communications, pending the success of upcoming proof-of-concept demonstrations.
The theoretical framework developed in Result 2.a provides a roadmap for molecular cavity optomechanics, with the potential to unlock hybrid quantum interfaces between electronic and mechanical degrees of freedom. A particularly impactful application is the frequency conversion of single THz photons to optical photons, which could enable novel detection schemes across quantum technologies, spectroscopy, and telecommunications.
The nanoscale thermometer developed in Result 2.b is now being applied to probe phonon-mediated heat transport in engineered nanostructures, such as phononic crystals. These experiments may provide insight into the long-standing question of coherent heat transport at the nanoscale, and contribute to fundamental understanding in thermal physics and phononics.
The characterization protocol in Result 3.a provides a high-throughput strategy to screen transition metal complexes for potential use as molecular spin qubits. This approach, combined with optical characterization, will enable the development of optically addressable spin systems — a critical step toward scalable molecular platforms for quantum information processing.
The possibility of increasing multiphoton indistinguishability opens the opportunity of deploying molecular emitters for quantum optical simulation such as Boson sampling experiments or for linear optical quantum computing. Further research will focus on actively depleting charge noise, building on our demonstrated strategy of minimizing molecular sensitivity (Result 1.a). These developments could pave the way for deployable, high-performance quantum photonic sources.
The integration of single molecules in photonic structures as demonstrated in Result 1.b opens a new direction in quantum metrology, particularly for the precise calibration of single-photon detectors. The next generation of sources will incorporate resonant enhancement to boost emission rates and collection efficiency. These advancements hold clear potential for industrial uptake, particularly in sectors such as quantum sensing and quantum communications, pending the success of upcoming proof-of-concept demonstrations.
The theoretical framework developed in Result 2.a provides a roadmap for molecular cavity optomechanics, with the potential to unlock hybrid quantum interfaces between electronic and mechanical degrees of freedom. A particularly impactful application is the frequency conversion of single THz photons to optical photons, which could enable novel detection schemes across quantum technologies, spectroscopy, and telecommunications.
The nanoscale thermometer developed in Result 2.b is now being applied to probe phonon-mediated heat transport in engineered nanostructures, such as phononic crystals. These experiments may provide insight into the long-standing question of coherent heat transport at the nanoscale, and contribute to fundamental understanding in thermal physics and phononics.
The characterization protocol in Result 3.a provides a high-throughput strategy to screen transition metal complexes for potential use as molecular spin qubits. This approach, combined with optical characterization, will enable the development of optically addressable spin systems — a critical step toward scalable molecular platforms for quantum information processing.