Gert Zumofen

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.

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.

Efficient coupling of photons to a single molecule and the observation of its resonance fluorescence

Single dye molecules at cryogenic temperatures exhibit many spectroscopic phenomena known from the study of free atoms and are thus promising candidates for experiments in fundamental quantum optics. However, the existing techniques for their detection have either sacrificed information on the coherence of the excited state or have been inefficient. Here, we show that these problems can be addressed by focusing the excitation light near to the extinction cross-section of a molecule. Our detection scheme enables us to explore resonance fluorescence over nine orders of magnitude of excitation intensity and to separate its coherent and incoherent parts. In the strong excitation regime, we demonstrate the first direct observation of the Mollow fluorescence triplet from a single solid-state emitter. Under weak excitation, we report the detection of a single molecule with an incident power as faint as 600 aW, paving the way for studying nonlinear effects with only a few photons.

Perfect reflection of light by an oscillating dipole.

We show theoretically that a directional dipole wave can be perfectly reflected by a single pointlike oscillating dipole. Furthermore, we find that, in the case of a strongly focused plane wave, up to 85% of the incident light can be reflected by the dipole. Our results hold for the full spectrum of the electromagnetic interactions and have immediate implications for achieving strong coupling between a single propagating photon and a single quantum emitter.

Coupled rotation within single F0F1 enzyme complexes during ATP synthesis or hydrolysis

F0F1 ATP synthases are the smallest rotary motors in nature and work as ATP factories in bacteria, plants and animals. Here we report on the first observation of intersubunit rotation in fully coupled single F0F1 molecules during ATP synthesis or hydrolysis. We investigate the Na+‐translocating ATP synthase of Propionigenium modestum specifically labeled by a single fluorophore at one c subunit using polarization‐resolved confocal microscopy. Rotation during ATP synthesis was observed with the immobilized enzyme reconstituted into proteoliposomes after applying a diffusion potential, but not with a Na+ concentration gradient alone. During ATP hydrolysis, stepwise rotation of the labeled c subunit was found in the presence of 2 mM NaCl, but not without the addition of Na+ ions. Moreover, upon the incubation with the F0‐specific inhibitor dicyclohexylcarbodiimide the rotation was severely inhibited.

Quantum interference of tunably indistinguishable photons from remote organic molecules

We demonstrate two-photon interference using two remote organic molecules as bright solid-state sources of indistinguishable photons. By varying the transition frequency and spectral width of one molecule, we explore the effect of photon distinguishability.

Strong extinction of a laser beam by a single molecule

We present an experiment where a single molecule strongly affects the amplitude and phase of a laser field emerging from a subwavelength aperture. We achieve a visibility of -6% in direct and +10% in cross-polarized detection schemes. Our analysis shows that a close to full extinction should be possible using near-field excitation.

Energy transfer as a random walk. II. Two‐dimensional regular lattices

In this paper, we study the incoherent energy transfer in two‐dimensional regular systems where the intermolecular interactions are due to Coulombic and exchange forces. This work follows our treatment for three‐dimensional lattices [Blumen and Zumofen, J. Chem. Phys. 75, 892 (1981)], where long‐range transfer steps were also taken into account. We analyze the random walk of the excitation for the triangular, square, and hexagonal lattices both analytically and numerically, using series and matrix inversions as well as simulation techniques. We show that in a first passage model the survival probability of a random walker on a lattice with randomly distributed traps is known if the distribution of the number Rn of distinct visited sites is given. The decay law is expressed as a cumulant expansion which involves moments of Rn. The first few moments of Rn are obtained from the numerical procedures; here the simulation is particularly helpful. The determined mean Sn and variance σn2 agree well with analytica...

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.

Controlling the phase of a light beam with a single molecule.

We employ heterodyne interferometry to investigate the effect of a single organic molecule on the phase of a propagating laser beam. We report on the first phase-contrast images of individual molecules and demonstrate a single-molecule electro-optical phase switch by applying a voltage to the microelectrodes embedded in the sample. Our results may find applications in single-molecule holography, fast optical coherent signal processing, and single-emitter quantum operations.