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Optics Express

Photophysics of a single quantum emitter based on vanadium phthalocyanine molecules

Richard Escalante, Mohan Chandra Mathpal, Catalina Ruiz-Tagle, Vicente H. Alvarado, Felipe A Pinto, Luis Javier Martínez, Loïk Gence, Griselda García, Iván A. González, Jeronimo R. Maze

Published Jul 16, 2024• 4 citations
Publisher

Abstract

Single quantum emitters play a fundamental role in the development of quantum technologies such as quantum repeaters, and quantum information processing. Isolating individual molecules with stable optical emission is an essential step for these applications, especially for those molecules that present large coherence times at room temperature. Among them, vanadium-oxide phthalocyanine (VOPc) molecules stand out as promising candidates due to the large coherence times of their ground state electronic spin, which are on the order of microseconds when measured in the ensemble. However, the optical properties of such VOPc molecules at the single emitter level have not yet been reported. Here we show that single VOPc molecules with stable optical properties at room temperature can be isolated. We find that the optical response of the molecule under laser illumination of different polarizations agrees well with a system having pyramidal C 4 v symmetry. Furthermore, we provide theoretical calculations that support our experimental findings and provide insight into the role of phonons and the internal electronic structure of the molecule. These results demonstrate that this single paramagnetic molecule can function as a single quantum emitter while displaying optical stability under ambient conditions to have their intrinsic properties investigated.

DOI
10.1364/oe.527703

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Single quantum emitters play a fundamental role in the development of quantum technologies such as quantum repeaters, and quantum information processing. Isolating individual molecules with stable optical emission is an essential step for these applications, especially for those molecules that present large coherence times at room temperature. Among them, vanadium-oxide phthalocyanine (VOPc) molecules stand out as promising candidates due to the large coherence times of their ground state electronic spin, which are on the order of microseconds when measured in the ensemble. However, the optical properties of such VOPc molecules at the single emitter level have not yet been reported. Here we show that single VOPc molecules with stable optical properties at room temperature can be isolated. We find that the optical response of the molecule under laser illumination of different polarizations agrees well with a system having pyramidal C 4 v symmetry. Furthermore, we provide theoretical calculations that support our experimental findings and provide insight into the role of phonons and the internal electronic structure of the molecule. These results demonstrate that this single paramagnetic molecule can function as a single quantum emitter while displaying optical stability under ambient conditions to have their intrinsic properties investigated.
Single quantum emitters play a fundamental role in the development of quantum technologies such as quantum repeaters, and quantum information processing. Isolating individual molecules with stable optical emission is an essential step for these applications, specially for those molecules that present large coherence times at room temperature. Among them, vanadium-oxide phthalocyanine (VOPc) molecules stand out as promising candidates due to their large coherence times measured in ensemble. However, the optical properties of individual molecules have not yet been reported. Here we show that single VOPc molecules with stable optical properties at room temperature can be isolated. We find that the optical response of the molecule under laser illumination of different polarization agrees well with a system having pyramidal C4v_{4v}4v​ symmetry. Furthermore, the molecule reveals a non-radiative transition rate that depends on the excitation wavelength when its lifetime is interrogated. We provide theoretical calculations that support our experimental findings and provide insight to the role of phonons and internal electronic structure of the molecule. These results demonstrate that this single paramagnetic molecule can function as a single quantum emitter while displaying optical stability under ambient conditions to have their intrinsic properties investigated.
ingle luminescent molecules provide a unique approach in the development of quantum technologies utilizing single photon sources. This includes quantum metrology by using a molecule’s sensitivity of its emission and magnetic properties to the local environment. In this thesis, we present our investigation of the optical properties of several different classes of luminescent molecules. We begin by providing some theoretical background of single quantum emitters as well as a brief description of the experimental methodologies and equipment. Next, we present an optical investigation of an ensemble of iron phthalocyanines molecules. This molecule possesses a ground state triplet, which is a desirable property for optically active spin qubits, but has a very weak optical emission. Diffraction limited spots displayed photo-instability in the form of blinking and irreversible bleaching. In ensemble form however, their optical stability allowed us to identify a possible Raman peak where we calculated the associated phonon frequency. Next, we present our single molecule study of vanadium phthalocyanine. This molecule has been documented as displaying very long spin coherence times even at room temperatures. We confirmed the presence of a single molecule by measuring the second order correlation function. Additionally, we looked at the intensity and spectral response as a function of the excitation laser polarization. The spectrum was fitted to a two Gaussian function, which may correspond to the two dipole transitions as suggested by theoretical calculations. Lastly, we looked at the optical properties of rare earth europium complexes known for having very sharp optical transitions in the emission spectrum, with each having varying levels of sensitivity to the local environment. Motivated by techniques to investigate non-radiative decay channels, we looked at the optical response of four different europium complexes under two 1 µs pulses of 515 nm laser separated by 1 µs. Each displayed a very different results and allowed us to identify the best candidates for single molecule studies. Finally, we looked at the emission spectrum as well as the optical response under a 6 µs long pulse using time-correlation single photon spectroscopy.
Single photons are highly desirable for encoding, sending and processing quantum information. Suitable engineering of single quantum emitters and their environment can lead to a reliable single photon source and to mediated photon-photon interactions, essential assets for quantum communication and computation purposes. Dye molecules, our chosen single quantum emitter, have attracted much attention in recent years due to their high photostability and narrow emission lines. In this thesis I present a detailed analysis of the optical properties of single dibenzoterrylene molecules in an anthracene matrix. Single photon emission is demonstrated at room and cryogenic temperatures. In the latter case, an analysis of the second-order correlation function of the emission of a single molecule is studied at various temperatures. The data agree with a full solution of the optical Bloch equations over a wide range of temperature, and provide some insight into the dephasing process of the molecules. I then present a technique for integrating single dye molecules with nano-photonic devices. These are hybrid plasmonic waveguides, designed to guide light and to enhance the interaction with a single emitter by a tight confinement of the waveguide mode. Single molecules are deposited over the waveguides and a first evidence of coupling is presented. I conclude with proposed improvements to the optical setup and to the deposition methods. Finally, I present another type of integrated device, where the light matter interaction is enhanced by inserting single quantum emitters into coupled optical resonators. The theoretical description for the case of resonant two-photon absorption from single atoms in coupled photonic crystal cavities is developed. I conclude by proposing a possible realisation of this system, and describing the first steps towards the fabrication of such a device.
Optical interfaces that connect long-lived spin qubits to photons are a central requirement for quantum networking and distributed quantum information processing. Currently, solid-state atomic defects are leading candidates due to their inherent spin and optical coherence. Building on these advancements, synthetically tailored molecular systems represent a fundamental change in the field, utilizing precise atomic control and consistent bottom-up assembly. However, the lack of a robust spin-photon interface combining bright fluorescence, high spectral stability, and the persistent spin lifetimes inherent to ground-state systems has prohibited the detection of individual molecular qubits. Here we show that a triplet ground state carbene molecule, embedded within a structurally matched host crystal, functions as a robust spin-photon interface with single-molecule addressability. The system exhibits narrow zero-phonon lines, spectral stability over more than an hour, spin-selective optical transitions and single-molecule optically detected magnetic resonance. Coherent control yields millisecond-scale dynamical-decoupling coherence and tens-of-milliseconds spin relaxation at a temperature of 4.5 K. These results establish molecular qubits as a viable platform for single-emitter quantum optics while preserving the advantages of bottom-up chemical design and processable materials.
Abstract Electron spins are ideal two-level systems that may couple with microwave photons so that, under specific conditions, coherent spin-photon states can be realized. This represents a fundamental step for the transfer and the manipulation of quantum information. Along with spin impurities in solids, molecular spins in concentrated phases have recently shown coherent dynamics under microwave stimuli. Here we show that it is possible to obtain high cooperativity regime between a molecular Vanadyl Phthalocyanine (VOPc) spin ensemble and a high quality factor superconducting YBa2Cu3O7 (YBCO) coplanar resonator at 0.5 K. This demonstrates that molecular spin centers can be successfully integrated in hybrid quantum devices.
Organic chemistry offers the potential to synthesize individual molecules for specific tasks in quantum information processing. One of their striking properties is the emission of single photons with nearly ideal coherence. Here, we implement Ramsey spectroscopy to measure the electronic state coherence of a single molecule. Conventionally, the emitter’s coherence is characterized by saturation or intensity auto-correlation measurements. However, both methods are under the steady influence of continuous interaction with the excitation laser. This influence can be eliminated by using a pump–probe sequence of two optical pulses to observe the decoherence. We have measured a near-transform-limited decoherence time of 17.6 ns, while the radiative lifetime is 10.1 ns. We also perform frequency-detuned excitation, gaining richer insights into the dephasing behavior of the molecule. The experiments exhibit that optical Ramsey spectroscopy is a promising tool for measuring the emitter’s coherence properties.