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Molecular Single Photons for Atomic Experiments

Wilhelm Kiefer, Petr Siyushev, K. Kafenda, Jörg Wrachtrup, Ilja Gerhardt

Published Jan 1, 2014• 0 citations
Publisher

Abstract

Single organic molecules allow for the generation of high-flux single photons. At cryogenic conditions, these photons can be as narrow as a few MHz. It is possible to find suitable molecules matching to a variety of atomic transitions, allowing for quantum hybrid systems of atoms and molecules.
DOI
10.1364/cleo_qels.2014.ff2a.6

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The generation of non-classical light, especially of single photon states, is mostly limited to non-linear optics in crystals and single emitters. Here we perform research on single organic dye molecules under cryogenic conditions. The molecules are well-behaved and do not blink or bleach. Also they exhibit an enormous brightness (>106 detected counts per second) and have narrow-band emission (<15-20MHz). By atomic sodium vapor we are able to suppress unwanted background emission from residual impurities in the sample. This light is now analyzed for the use in all-optical gates and further quantum optical schemes. A first step is the characterization in a Hong-Ou-Mandel type experiment. Different variations of this are possible – reaching from indistinguishable to distinguishable photons impinging on a beam-splitter.
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.
Single organic molecules make excellent single photon sources, emitting photons with high efficiency and at favourable wavelengths for coupling to other quantum systems, including alkali atoms. I will discuss techniques to create organic crystals containing single photon emitting molecules, and our recent results applying strain to these crystals to tune their emission wavelength. I will show that subsequent photons emitted by a single molecule can undergo quantum interference at a beam splitter, a vital tool in optical quantum computing and communication, and will discuss how discuss how to assess photon indistinguishability using both continuous and pulsed lasers. While molecular excitation and radiative emission is efficient, generated photons can be difficult to collect without the use of photonic structures. I will discuss our recent efforts in coupling single molecules to waveguides and nanophotonic cavities.
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A single molecule under cryogenic conditions allows one to realize an extremely bright and simultaneously narrow-band single-photon source. We present a review on the different excitation schemes of a single molecule and present the corresponding single-photon nature of the emitted light. Single-molecule spectroscopy has been recently interlinked with atomic spectroscopy. This optical interconnect among the different quantum systems might be enhanced by a so-called Faraday anomalous dispersion optical filter—an ideal tool for many experiments in quantum optics. We introduce our theoretical and experimental approach on these filters which are based on hot atomic sodium vapor. The electrical tunability together with the brightness of a single molecule allows us to record a full atomic spectrum of this filter with the single photons originating from a single-molecular emitter.
Efficient interaction of light and matter at the ultimate limit of single photons and single emitters is of great interest from a fundamental point of view and for emerging applications in quantum engineering. However, the difficulty of generating single-photon streams with specific wavelengths, bandwidths, and power as well as the weak interaction probability of a single photon with an optical emitter pose a formidable challenge toward this goal. Here, we demonstrate a general approach based on the creation of single photons from a single emitter and their use for performing spectroscopy on a second emitter situated at a distance. While this first proof of principle realization uses organic molecules as emitters, the scheme is readily extendable to quantum dots and color centers. Our work ushers in a new line of experiments that provide access to the coherent and nonlinear couplings of few emitters and few propagating photons.