Single organic molecules for photonic quantum technologies.

Isolating single molecules in the solid state has allowed fundamental experiments in basic and applied sciences. When cooled down to liquid helium temperature, certain molecules show transition lines that are tens of megahertz wide, limited by only the excited-state lifetime. The extreme flexibility in the synthesis of organic materials provides, at low costs, a wide palette of emission wavelengths and supporting matrices for such single chromophores. In the past few decades, their controlled coupling to photonic structures has led to an optimized interaction efficiency with light. Molecules can hence be operated as single-photon sources and as nonlinear elements with competitive performance in terms of coherence, scalability and compatibility with diverse integrated platforms. Moreover, they can be used as transducers for the optical read-out of fields and material properties, with the promise of single-quanta resolution in the sensing of charges and motion. We show that quantum emitters based on single molecules hold promise to play a key role in the development of quantum science and technologies.

Enhancing the radiative emission rate of single molecules by a plasmonic nanoantenna weakly coupled with a dielectric substrate.

Enhancing the spontaneous emission of single emitters has been an important subject in nano optics in the past decades. For this purpose, plasmonic nanoantennas have been proposed with enhancement factors typically larger than those achievable with optical cavities. However, the intrinsic ohmic losses of plasmonic structures also introduce an additional nonradiative decay channel, reducing the quantum yield. Here we report on experimental studies of a weakly coupled dielectric substrate and a plasmonic nanoantenna for enhancing the radiative decay rate of single terrylene molecules embedded in an ultrathin organic film. We systematically investigate how the refractive index of the dielectric substrate affects the lifetime and the quantum efficiency and show that the coupled structure could moderately enhance the radiative decay rate while maintaining a high quantum efficiency.

Spectroscopic detection and state preparation of a single praseodymium ion in a crystal.

The narrow optical transitions and long spin coherence times of rare earth ions in crystals make them desirable for a number of applications ranging from solid-state spectroscopy and laser physics to quantum information processing. However, investigations of these features have not been possible at the single-ion level. Here we show that the combination of cryogenic high-resolution laser spectroscopy with optical microscopy allows one to spectrally select individual praseodymium ions in yttrium orthosilicate. Furthermore, this spectral selectivity makes it possible to resolve neighbouring ions with a spatial precision of the order of 10 nm. In addition to elaborating on the essential experimental steps for achieving this long-sought goal, we demonstrate state preparation and read out of the three ground-state hyperfine levels, which are known to have lifetimes of the order of hundred seconds.

Spontaneous emission enhancement of a single molecule by a double-sphere nanoantenna across an interface.

We report on two orders of magnitude reduction in the fluorescence lifetime when a single molecule placed in a thin film is surrounded by two gold nanospheres across the film interface. By attaching one of the gold particles to the end of a glass fiber tip, we could control the modification of the molecular fluorescence at will. We find a good agreement between our experimental data and the outcome of numerical calculations.

Single-photon spectroscopy of a single molecule.

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.

Efficient coupling of single photons to single plasmons.

We demonstrate strong coupling of single photons emitted by individual molecules at cryogenic and ambient conditions to individual nanoparticles. We provide images obtained both in transmission and reflection, where an efficiency greater than 55% was achieved in converting incident narrow-band photons to plasmon-polaritons (plasmons) of a silver nanoparticle. Our work paves the way to spectroscopy and microscopy of nano-objects with sub-shot noise beams of light and to triggered generation of single plasmons and electrons in a well-controlled manner.

Near-infrared single-photons from aligned molecules in ultrathin crystalline films at room temperature.

We investigate the optical properties of Dibenzoterrylene (DBT) molecules in a spin-coated crystalline film of anthracence. By performing single molecule studies, we show that the dipole moments of the DBT molecules are oriented parallel to the plane of the film. Despite a film thickness of only 20 nm, we observe an exceptional photostability at room temperature and photon count rates around 10(6) per second from a single molecule. These properties together with an emission wavelength around 800 nm make this system attractive for applications in nanophotonics and quantum optics.

Quantum interference of tunably indistinguishable photons from remote organic molecules.

We demonstrate two-photon interference using two remote single molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we tune and explore the effect of photon distinguishability. We discuss future improvements on the brightness of single-photon beams, their integration by large numbers on chips, and the extension of our experimental scheme to coupling and entanglement of distant molecules.

A single-molecule optical transistor.

The transistor is one of the most influential inventions of modern times and is ubiquitous in present-day technologies. In the continuing development of increasingly powerful computers as well as alternative technologies based on the prospects of quantum information processing, switching and amplification functionalities are being sought in ultrasmall objects, such as nanotubes, molecules or atoms. Among the possible choices of signal carriers, photons are particularly attractive because of their robustness against decoherence, but their control at the nanometre scale poses a significant challenge as conventional nonlinear materials become ineffective. To remedy this shortcoming, resonances in optical emitters can be exploited, and atomic ensembles have been successfully used to mediate weak light beams. However, single-emitter manipulation of photonic signals has remained elusive and has only been studied in high-finesse microcavities or waveguides. Here we demonstrate that a single dye molecule can operate as an optical transistor and coherently attenuate or amplify a tightly focused laser beam, depending on the power of a second 'gating' beam that controls the degree of population inversion. Such a quantum optical transistor has also the potential for manipulating non-classical light fields down to the single-photon level. We discuss some of the hurdles along the road towards practical implementations, and their possible solutions.

Controlled coupling of counterpropagating whispering-gallery modes by a single Rayleigh scatterer: a classical problem in a quantum optical light.

We present experiments where a single subwavelength scatterer is used to examine and control the backscattering induced coupling between counterpropagating high-Q modes of a microsphere resonator. Our measurements reveal the standing wave character of the resulting symmetric and antisymmetric eigenmodes, their unbalanced intensity distributions, and the coherent nature of their coupling. We discuss our findings and the underlying classical physics in the framework common to quantum optics and provide a particularly intuitive explanation of the central processes.

Influence of a single quantum dot state on the characteristics of a microdisk laser.

We report a quantum dot microcavity laser with a cw sub-microW lasing threshold, where a significant reduction of the lasing threshold is observed when a single quantum dot (QD) state is aligned with a cavity mode. The quality factor exceeds 15,000 before the system lases. When no QD states are resonant, below threshold the cavity mode initially degrades with increasing pump power, after which saturation occurs and then the cavity mode recovers. We associate the initial cavity mode spoiling with QD state broadening that occurs with increasing pump power.

Realization of two Fourier-limited solid-state single-photon sources.

We demonstrate two solid-state sources of indistinguishable single photons. High resolution laser spectroscopy and optical microscopy were combined at T = 1.4 K to identify individual molecules in two independent microscopes. The Stark effect was exploited to shift the transition frequency of a given molecule and thus obtain single photon sources with perfect spectral overlap. Our experimental arrangement sets the ground for the realization of various quantum interference and information processing experiments.

Controlled photon transfer between two individual nanoemitters via shared high-Q modes of a microsphere resonator.

We realize controlled cavity-mediated photon transfer between two single nanoparticles over a distance of several tens of micrometers. First, we show how a single nanoscopic emitter attached to a near-field probe can be coupled to high-Q whispering-gallery modes of a silica microsphere at will. Then we demonstrate transfer of energy between this and a second nanoparticle deposited on the sphere surface. We estimate the photon transfer efficiency to be about 6 orders of magnitude higher than that via free-space propagation at comparable separations.

Influence of a sharp fiber tip on high-Q modes of a microsphere resonator.

We investigate the degradation of the Q factor of a fundamental whispering-gallery mode of a microsphere resonator when a fiber tip is placed in the evanescent field of the mode. With a tip diameter of 80 nm it is possible to maintain a Q factor exceeding 10(8), even when the tip is as close as 10 nm to the sphere surface. This result demonstrates the possibility of using such a tip as a "nanotool" to actively place a single nanoparticle in a single high- Q mode with great precision to achieve well-controlled coupling.

Mapping and manipulating whispering gallery modes of a microsphere resonator with a near-field probe.

We report high spatial resolution mapping of high-Q whispering gallery modes in microsphere resonators with a near-field probe. We present experimental results on the effect of Q-factor degradation when the probe interacts with the evanescent field and discuss future applications of our experimental set-up for realization of novel nanolasers and nano light-emitting-diodes.