Pentacene Dimers Boost Quantum Sensing Towards Single-Proton Detection

Researchers at the Institute of Translational Medicine, led by Maria Grazia Concilio, demonstrate that pentacene dimers, leveraging triplet pair states generated via singlet fission, represent a significant advancement over conventional pentacene monomers for nanoscale detection of nuclear magnetic resonance (NMR) signals and alternating current (AC) magnetic fields. Their computational modelling, employing a Lindblad master equation, indicates comparable sensitivity for single-spin detection but a demonstrably superior interaction cross-section when probing small ensembles of nuclear spins. This work establishes a theoretical framework for utilising these high-spin states as versatile, high-sensitivity quantum probes, exhibiting optimised performance in low-magnetic field environments and with a sensitivity that scales proportionally to the number of pulses employed within the sensing protocol.

Pentacene dimers significantly enhance nanoscale magnetic resonance sensitivity and quantum sensing

A 30% improvement in the interaction cross-section for detecting small ensembles of nuclear spins is observed with a pentacene dimer, in comparison to traditional pentacene monomers. This enhancement addresses a persistent challenge in nanoscale magnetic resonance detection, which has historically limited the reliable sensing of these smaller nuclear spin groups. The interaction cross-section, a measure of the probability of interaction between the sensor and the target spins, is crucial for signal strength and detection efficiency. Modelling, utilising a Lindblad master equation, a standard tool in quantum optics for describing the time evolution of open quantum systems, reveals comparable sensitivity for detecting isolated single spins, offering a flexible platform for diverse quantum sensing applications. The Lindblad master equation accounts for both coherent evolution of the quantum system and incoherent processes like decay and dephasing, providing a realistic simulation environment. This suggests that dimers do not sacrifice single-spin sensitivity while gaining substantial benefits for ensemble detection.

High-spin multi-excitonic states now provide a foundation for adaptable, high-sensitivity quantum probes, proving particularly effective in low-magnetic field regimes and with performance scaling alongside the number of applied sensing pulses. The principle behind this scaling lies in the cumulative effect of repeated measurements, enhancing the signal-to-noise ratio. The superior performance of the pentacene dimer extends to active decoupling sequences, including spin echo, XY4, and XY8, all of which demonstrated effective signal modulation. These decoupling sequences are employed to suppress unwanted interactions and improve the clarity of the NMR signal. Simulations reveal optimised sensitivity in low-magnetic field regimes, which are important for many practical sensing applications, including biological imaging and materials science, where strong magnetic fields can be detrimental. Furthermore, analytical modelling of fluorescence modulation establishes a direct correlation between signal strength and the number of sensing pulses applied, offering a pathway to amplify detection. This modulation arises from the interaction between the triplet states and the external magnetic field, altering the fluorescence properties of the pentacene dimer. Detailed kinetic modelling, incorporating parameters such as fluorescence and phosphorescence rates, accurately reproduced transient absorption and triple electron-paramagnetic resonance (triple-EPR) data, with the dimer’s singlet fission process simulated using parameters mirroring experimental observations. Singlet fission is the process where a high-energy singlet excited state splits into two lower-energy triplet excited states, and accurately modelling this process is vital for predicting the dimer’s behaviour.

Idealised simulations establish performance benchmarks for future sensor development

Pentacene dimers offer a promising route to enhanced nanoscale sensing of nuclear spins, although the simulations assume ideal conditions, neglecting real-world environmental noise and decoherence. The simulations were conducted in a vacuum, without considering the influence of temperature, solvent effects, or other molecules that could interact with the pentacene dimers. Maintaining quantum coherence, the delicate state enabling precise measurements, is notoriously difficult outside of carefully controlled laboratory settings. Quantum coherence refers to the preservation of the phase relationship between quantum states, and its loss leads to signal degradation. The impact of imperfections on sensitivity remains an open question, but establishing this baseline performance is essential for accurately assessing these real-world factors and guiding future experimental designs focused on mitigating decoherence, the loss of quantum information. Decoherence arises from interactions with the surrounding environment, causing the quantum states to lose their superposition and entanglement. Pentacene dimers, formed through singlet fission where one excited molecule splits into two, offer a distinct advantage for detecting groups of atomic nuclei. While single pentacene molecules remain effective at detecting individual spins, the dimer’s architecture enhances sensitivity when multiple nuclei are present, allowing for more effective nanoscale magnetic resonance. This is because the two triplet states within the dimer can collectively interact with the ensemble of nuclear spins, leading to a stronger overall signal. The potential applications of this technology extend to diverse fields, including high-resolution biomolecular imaging, where detecting the magnetic moments of individual proteins or nucleic acids could provide unprecedented insights into their structure and function. Furthermore, the ability to detect weak magnetic fields at the nanoscale could revolutionise materials science, enabling the characterisation of magnetic materials with atomic precision and the development of novel spintronic devices. The 19 MHz resonant frequency used in the simulations is relevant to many biologically important nuclei, such as ¹H and ¹³C, further highlighting the potential for biomedical applications.

The research demonstrated that pentacene dimers exhibit a stronger interaction when detecting small groups of nuclear spins compared to single pentacene molecules. This finding matters because it suggests a pathway to improve the sensitivity of nanoscale magnetic resonance, potentially enabling the detection of weaker signals. Simulations using dynamical decoupling sequences, including spin echo, XY4, and XY8, established a theoretical baseline for utilising high-spin states as quantum probes. The study indicates sensitivity is optimised at low magnetic fields and scales with the number of pulses used in the sensing protocol, with a resonant frequency of 19MHz relevant to biologically important nuclei.