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Faraday Discussions

Towards witnessing quantum effects in complex molecules

Tristan Farrow, Robert Anthony Taylor, Vlatko Vedral

Published Jan 1, 2015• 2 citations
Publisher

Abstract

Whether many-body objects like organic molecules can exhibit full quantum behaviour, including entanglement, is an open fundamental question. We present a generic theoretical protocol for entangling two organic molecules, such as dibenzoterrylene in anthracene. The availability of organic dye molecules with two-level energy structures characterised by sharp and intense emission lines are characteristics that position them favourably as candidates for quantum information processing technologies involving single-photons. Quantum entanglement can in principle be generated between several organic molecules by carefully interfering their photoluminescence spectra. Major milestones have been achieved in the last 10 years showcasing entanglement in diverse systems including ions, cold atoms, superconductors, photons, quantum dots and NV-centres in diamond, but not yet in molecules.

DOI
10.1039/c5fd00101c

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Whether many-body objects like organic molecules can exhibit full quantum behaviour, including entanglement, is an open fundamental question. We present a generic theoretical protocol for entangling two organic molecules, such as dibenzoterrylene in anthracene. The availability of organic dye molecules with two-level energy structures characterised by sharp and intense emission lines are characteristics that position them favourably as candidates for quantum information processing technologies involving single-photons. Quantum entanglement can in principle be generated between several organic molecules by carefully interfering their photoluminescence spectra. Major milestones have been achieved in the last 10 years showcasing entanglement in diverse systems including ions, cold atoms, superconductors, photons, quantum dots and NV-centres in diamond, but not yet in molecules.
Quantum light-matter interactions form the basis of many quantum technologies and much fundamental research. Organic molecules are an impressive platform for the generation of quantum light, both as single photon emitters and as collective systems. In this work, I investigate quantum light-matter interactions with molecules of dibenzoterrylene in crystals of anthracene. I outline methods to grow dibenzoterrylene-doped anthracene crystals, and I characterize the optical properties of cryogenically cooled molecules. I prepare collective systems composed of pairs of molecules interacting through their free-space electromagnetic fields. I demonstrate a method to tune interacting molecules into resonance to form superradiant and subradiant collective states. Finally, I present a platform for solid-state cavity QED based on molecules of dibenzoterrylene coupled to a nanophotonic cavity. This demonstration establishes a route toward scalable deterministic photon generation with molecules. I also tune pairs of cavity-coupled molecules into resonance to form distributed collective states, opening the door to cavity-mediated many-body interactions.
Single organic molecules hold great promise for generating, manipulating, and storing single photons. Such processes form the basis for new quantum-enhanced technologies such as sensing, communications, and computation. In this talk I will focus on a particularly promising molecule – dibenzoterrylene (DBT). When DBT is introduced into a solid crystal of anthracene it is photostable and emits light between 780 and 795 nm. When cooled to cryogenic temperature, DBT is isolated from phonon-induced dephasing meaning that the photons have a lifetime-limited. I will present recent results in growing DBT-doped anthracene crystals, introducing these to nanophotonic interfaces for enhancing the collection of photons, tuning the DBT emission wavelength to coincide with rubidium atomic absorption, and finally pump-probe spectroscopy experiments to find long-lived triplet states in DBT which will be useful for building a single-molecule quantum memory.
Single molecules in solid-state matrices have been proposed as sources of single-photon Fock states back 20 years ago. Their success in quantum optics and in many other research fields stems from the simple recipes used in the preparation of samples, with hundreds of nominally identical and isolated molecules. Main challenges as of today for their application in photonic quantum technologies are the optimization of light extraction and the on-demand emission of indistinguishable photons. We here present Hong-Ou-Mandel experiments with photons emitted by a single molecule of dibenzoterrylene in an anthracene nanocrystal at 3 K, under continuous wave and also pulsed excitation. A detailed theoretical model is applied, which relies on independent measurements for most experimental parameters, hence allowing for an analysis of the different contributions to the two-photon interference visibility, from residual dephasing to spectral filtering.
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.
Single organic dye molecules have been studied for almost three decades. A majority of experiments are now conducted under ambient conditions and aim towards material science and micro-biology. The experiments under cryogenic conditions are often based on highly rigid polycyclic hydrocarbons, which have excellent fluorescing properties when they are operated at temperatures below 2 K. Their photonic advantages integrate their high flux, their narrow-band nature and their tunability over the entire visible spectrum. Therefore the use of organic molecules as efficient and narrow-band single photon sources facilitated a number of quantum optical advances in the past years. The results mostly cover quantum sensing, the formation of quantum hybrid systems and the optical combination of multiple photons in all-optical experiments. Here we present all three fields and outline our own experiments in the combination of single molecule studies, atomic spectroscopy and entanglement generation. The brightness and the narrow-band nature of the molecules are outlined and a delayed-choice quantum eraser is presented. The mode-mixing of two photons on a beam-splitter allows for the generation of a degenerate photon pair, which is resonant to the sodium D2-line. Furthermore, its spectral width matches roughly to the natural linewidth of an isolated sodium atom, such that further experiments are feasible. The entanglement of the post-selected photon pair is testified by a violation of Bell's inequality. Even the raw collected clicks violate Bell's inequality by more than two standard deviations.