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arXiv:2508.08835 · Physical Review Research

Phosphorescence detection as the first step toward all-optical spin manipulation in single perylene molecules

Robert Smit, Β. Kozankiewicz, Michel Orrit

Published Dec 12, 2025• 0 citations
arXivHTMLPDF

Abstract

The on-demand manipulation of triplet states in closed-shell single molecules is still out of reach due to a lack of information about the energy of those triplet states. Yet, the access to triplet states would open up a route toward an all-optical single-molecule photonic switch/transistor and, moreover, would provide a way of performing coherent spin operations from the spinless ground state. In this work, we take an important step toward those aims by measuring the triplet energy from the weak phosphorescence signal of perdeuterated perylene, embedded as a guest molecule in a dibenzothiophene host matrix, which well preserves the coherence properties of the perylene guest. We find that perylene’s phosphorescence can be enhanced in this host matrix, when acting as an intermediary for the generation of triplet excitons. The triplet energy that we find can be used to significantly narrow down the search for the ultraweak spin-forbidden transitions from the ground singlet to the triplet states of a single molecule.

DOI
10.1103/vlfy-134r

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The on-demand manipulation of triplet states in closed-shell single molecules is still out of reach due to a lack of information about the energy of those triplet states. Yet, the access to triplet states would open up a route toward an all-optical single-molecule photonic switch/transistor and, moreover, would provide a way of performing coherent spin operations from the spinless ground state. In this work, we take an important step toward those aims by measuring the triplet energy from the weak phosphorescence signal of perdeuterated perylene, embedded as a guest molecule in a dibenzothiophene host matrix, which well preserves the coherence properties of the perylene guest. We find that perylene’s phosphorescence can be enhanced in this host matrix, when acting as an intermediary for the generation of triplet excitons. The triplet energy that we find can be used to significantly narrow down the search for the ultraweak spin-forbidden transitions from the ground singlet to the triplet states of a single molecule.
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.
Experiments with single quantum systems are at the frontier of modern physics. Probing single molecules allows for very precise control of light-matter interaction and for quantum optical studies. Moreover, a single guest molecule in a solid host matrix is a very sensitive probe of local dynamics in its nano-environment. Fluorescence excitation spectroscopy on single molecules is a powerful tool for spectroscopic characterizations at highest resolution, as information can be revealed that is not accessible in experiments on ensembles of molecules. The detection of the fluorescence signal of a single absorber is achieved by exploiting the high spectral selectivity given by the ratio (10 10) of homogeneous to inhomogeneous linewidths of optical transitions at cryogenic temperatures (zero-phonon transitions). Two-photon fluorescence excitation, a fundamental nonlinear optical process, was observed on a single quantum system [1]. Single diphenyl-octatetraene molecules trapped in an n-tetradecane matrix were excited from the 1Ag ground to the lowest excited singlet state by simultaneous absorption of two photons using a single-mode cw laser. Excited molecules were observed through weak one-photon emission resulting from a symmetry breaking induced by the crystal field. The fundamental principles and the experimental technique of single molecule spectroscopy will be introduced briefly and some particular single molecule experiments presented. This work was done in collaboration with T. Plakhotnik, A. Renn and U.P. Wild
Optically readable molecular spins are attractive as quantum sensors due to their nanoscale modularity, synthetic tunability, and scope for sensitive readout. In particular, photoexcited spin-triplet states in organic molecules are appealing in supporting high optical-spin contrast at room temperature. Their utility is underpinned by their coherence, which warrants a detailed understanding of it. Here, from first principles, we systematically explore the Hahn-echo decoherence of the benchmark molecular system for room-temperature optically detected spin coherence---pentacene guest molecules coupled to a para-terphenyl host. Using generalized cluster-correlation expansion methods, we investigate the mechanisms of nuclear-spin-induced decoherence from zero to high magnetic field, exploring the role of guest vs host molecules, specific nuclei, zero-field splitting interactions, and hyperfine parameters. We describe how zero-field decoherence is driven by ~6 nuclei on the guest, while high-field decoherence is driven by ~600 nuclei in the host; how the longitudinal zero-field splitting parameter, DDD, can prolong T2T_2T2​; and the magnetic-field-dependent processes which drive decoherence. These results advance our understanding of decoherence in optically readable molecular spins, providing insight for their synthetic enhancement and deployment as quantum probes.
Molecular materials that enable coherent control over an electron's spin state at room temperature are promising candidates for quantum technologies, including quantum sensors and ultra-low noise microwave amplifiers, known as masers. Host-guest molecular crystals enable independent control of spin-active guests and their local environments to enhance molecular spin properties and so improve device performance. Using electron paramagnetic resonance and optically-detected magnetic resonance, we demonstrate the ability to tune triplet population, depopulation, and spin-lattice relaxation by modulating host-dependent lattice rigidity and vibrational coupling to significantly reduce the operating requirements for building useful masers. Importantly, the most rigid host, picene, reveals the ability to slow spin-lattice relaxation without lengthening triplet lifetime, though at the cost of strain-induced line width broadening and reduced triplet spin polarisation. We also find that deuteration reduces the triplet resonance line width and vibrationally-mediated triplet depopulation. Consequently, we find that perdeuterated pentacene in perdeuterated p-terphenyl is the most viable candidate for building a continuous wave maser. This work demonstrates host-guest engineering as an important and practical method for tuning the spin-dependent performance of room-temperature molecular quantum technologies.
We show that spin-orbit coupling in a quantum dot molecule allows for coherent manipulation of two-electron spin states using Raman transitions. Such two-electron spin states defined by the singlet and triplet states of two exchange-coupled quantum dots can have favorable coherence properties. In addition, two of the four metastable ground states in this system can be used as auxiliary states that could facilitate implementation of tasks such as mapping of spin states to that of a single propagating photon. We find that even weak spin-orbit effects---manifesting themselves as slightly different ggg factors for the electron and the hole---would allow for the coherent Raman coupling of the singlet-triplet states. We also discuss the possibilities for implementing quantum optical techniques for spin preparation and manipulation.
The spin-selective photokinetics of a single matrix-isolated impurity molecule with a triplet-triplet optical transition, T 0–T 1, is considered and the manifestations of the photokinetics in the fluorescence excitation spectra and intensity autocorrelation functions g (2)(τ) of the molecule undergoing narrow-band optical excitation is studied to resolve the fine structure of the transition. The rates of intersystem crossings (ISCs) T 1→S→T 0 to and from a nonradiating singlet state S of the molecule and the rate of population relaxation among the ground (T 0) state sublevels can be obtained from the spectra and g (2)(τ) using the analytical expressions obtained. New experiments on an individual NV defect center in nanocrystals of diamond, where, for the first time, the fine structure of its triplet-triplet 3 A-3 E zero-phonon optical transition (~637 nm) at 1.4 K was resolved, are interpreted. It is concluded that the rate of the ISC transition from the m S =0 sublevel of the excited 3 E state to the singlet 1 A state (~1 kHz) is much slower than the rates from the m S =±1 substates, while the rates of ISC transitions to different m S substates of the ground 3 A state are close to each other (~1 Hz). As a result, only the optical transition between m S =0 sublevels in the 3 A-3 E manifold contributes strongly to the fluorescence. The experimentally observed double-exponential decay of the g (2)(τ) function is explained by the two pathways available to the center for it to leave the S state: (i) the S→ T 0(m S )=0) transition and (ii) the S→T 0(m S =±1) transitions followed by the slow spin-lattice relaxation T 0(m S =±1)→T 0(m S =0) (rate ~0.1 Hz). The work is important for studies where the NV center is used as a single photon source or for quantum information processing.