Shining Light on Single Molecule Dynamics: One Photon at a Time

Karl Otto Greulich

Throughout the development and confirmation of the photon concept, "Gedankenexperiments" have been invoked where a light source is attenuated until "only one photon is in the apparatus." Such a statement requires, however, knowledge on details of the light generation process, which were not known until the advent of true single-photon light sources such as isolated single atoms or molecules. Here, starting from a two atom light source, it is shown that probably multi-atom or multi-molecule light sources do not generate, even not in small fractions, antibunching photons and that attenuation down to one photon per time unit is not reliably possible. Thus, one of the strongest arguments in favour of the existence of single "photons" in free space, the accumulation time argument, which discusses the absorption of a single photon by a single atom or molecule, becomes questionable.

Vahid Sandoghdar

Ars Technica: Orbital angular momentum is one of the quantum properties of photons and has a theoretically infinite number of possible values. This makes it a subject of interest for researchers trying to increase the amount of data a single photon can represent. A team led by Xinlun Cai of the University of Bristol in the UK may have taken the first real steps toward that goal. The researchers shined IR laser light through ring-shaped silicon chambers scored with grooves, which diffracted the light, causing helical interference patterns. The patterns were dependent on the photons' original polarization and the diameter and grooving of the ring chambers. The team also created additional patterns by combining light that passed through different rings. Data transfer is only one possible use for orbital angular momentum manipulation, but researchers must overcome the loss of pattern coherence due to interactions with molecules in air or fiber-optic cables before it is useful in quantum devices.

Rezus, Walt, Zumofen +3 more

Karl Otto Greulich

Single molecule techniques improve our understanding of the photon and light. If the single photon double slit experiment is performed at the "single photon limit" of a multi-atom light source, faint light pulses with more than one photon hamper the interpretation. Single molecules, quantum dots or defect centres in crystals should be used as light source. "Single photon detectors" do not meet their promise-only "photon number resolving single photon detectors" do so. Particularly, the accumulation time argument, the only safe basis for the postulate of a strictly particle like photon, has so far not yet been verified.