Duquennoy, Rocco (2024) Dibenzoterrylene: a molecular platform for photonic quantum technologies. [Tesi di dottorato]

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Abstract

In recent years, individual molecules have emerged as a valuable platform for the development of quantum technologies. The properties of molecules can be synthetically engineered, allowing to tailor them to the specific need or application. This thesis focuses on a specific organic molecule, dibenzoterrylene (DBT), a particular polycyclic aromatic hydrocarbon. The delocalization of the $\pi$-electron cloud over the entire surface of this molecule allows for strong interaction between the system and the electromagnetic fields and, consequently, with light. This is why a common element in all experimental works presented in this thesis is the use of light to investigate the behavior of a single DBT molecule. After an introductory chapter that sets the context for this research, in Chapter 2 I delve into the molecular physics that governs the interactions between DBT and its surrounding environment, both through light and the vibrational couplings with the anthracene crystal it is inserted in. I then describe the photonic properties of DBT, demonstrating its ability to emit stable photons with excellent purity even at room temperature. When cooled to liquid-helium temperatures, where the experiments discussed here have been conducted, the emitted spectrum narrows down to present a close-to-Fourier-limited emission line with excellent frequency stability. I pay special attention to purely quantum properties, such as single-photon emission purity and the indistinguishability of the produced quantum states, as they play a crucial role in quantum computation or quantum communication protocols. In Chapter 3, I introduce two different methods for creating a local electric field capable of shifting the emission frequency of DBT. Indeed, in order to scale up the number of photons available in quantum protocols based on light, we need to be able to tune the emission from different sources to obtain indistinguishable photons. I report on the demonstration of how, by exploiting the interplay between these techniques, frequency shifts can be achieved with minimal impact on the emission stability. In Chapter 4, I present the results from tests aimed at quantifying the degree of indistinguishability of photons emitted by distinct DBT molecules. This work achieves one of the few realizations of this type of experiment, resulting in one of the highest visibility measurements reported in pulsed excitation, reaching a maximum of 40 %. A promising aspect of this result is that the system utilized involved minimal engineering of the source, leaving significant room for future improvements. Chapter 5, the only one that is not mainly experimental, reports on a data analysis method applied to the two-photon interference coincidence profiles from the previous chapter. This method efficiently extracts information on the frequency differences between the utilized quantum emitters. Chapter 6 discusses the use of DBT emitters as nano-thermometers between 3 K and 20 K. In this temperature range, obtaining spatial information on temperature profiles with high resolution is challenging. In the reported case, we leverage the exponential dependence of the emission spectrum width of DBT, combined with the small size of the crystals in which it is embedded in order to measure the temperature profile of a two-dimensional membrane. This approach achieves one of the highest sensitivities reported for a thermometer with a minimally invasive approach. Finally, the last chapter describes the experimental methodologies that enabled the calibration of a single-photon detector by comparing it with an analog one previously traced to the international standard of radiative flux. The sensitivity error obtained with the photon flux emitted by DBT is found to be an order of magnitude lower than that obtained with an attenuated laser, demonstrating a possible immediate application for the hereby discussed single-photon source.

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