Singles become pairs: New insights into the light scattering of atoms
Researchers at the Humboldt University of Berlin, partners of the DAALI project, have demonstrated a surprising effect present in the fluorescent light of a single atom
Researchers headed by Jürgen Volz and Arno Rauschenbeutel from the Department of Physics at the Humboldt University of Berlin, partners of the DAALI project, have gained new insights into the scattering of light by a fluorescent atom, which could also be useful for quantum communication. The research team has now published their results in the scientific journal Nature Photonics.
In 1900, Max Planck formulated the hypothesis that light cannot exchange arbitrary amounts of energy with matter, such as an atom, but only certain discrete “energy packets” called quanta. Five years later, Albert Einstein then proposed that these quanta were not a mere computational quantity, but that light itself consisted of quanta, which we now call photons. In fact, nowadays there are photodiodes which are sensitive enough to register a single photon. With continuous illumination, these do not produce a steady electrical signal, but rather a series of short current pulses. Each current pulse then indicates the detection of a single photon.
Under the magnifying glass: scattering of laser light
When the fluorescent light of a laser-excited atom hits a highly sensitive photodiode, we will never detect two photons simultaneously. In this respect, the fluorescent light from a single atom differs from the laser light used for excitation, as laser light does contain simultaneous photons. However, if two laser photons hit a single atom at the same time, the atom will only absorb one and let the second pass through. The atom then radiates the absorbed laser photon in a random direction and is only then ready to absorb another.
In other words, a single atom can only scatter one photon at a time. The photons in the fluorescent light of a single atom strike the detector in a sequential manner, like pearls on a string. The DAALI project and other research on quantum technologies exploit this property. For example, in quantum communication, natural or artificial atoms emit single photons for tap-proof communication.
Through the filter: single photons become pairs
A research team at Humboldt University recently demonstrated a surprising effect using the fluorescent light of a single atom. By removing a specific color component from the light with a filter, the team transformed the single photon stream into simultaneously detected photon pairs. Removing certain photons from a single photon stream makes the remaining photons appear as pairs. This effect defies our everyday understanding; if you remove all green cars from a street, the remaining cars won’t suddenly drive in pairs. The previous certainty that a single atom can only scatter one photon at a time also seems disproven: when viewed through the correct filter, the atom can scatter two photons simultaneously. Jean Dalibard and Serge Reynaud at ENS Paris predicted this effect in their theoretical work about 40 years ago. However, a team led by quantum physicists Jürgen Volz and Arno Rauschenbeutel only recently demonstrated it experimentally.
“This is a wonderful example of the extent to which our intuition fails us when we try to get an idea of how processes occur at the microscopic level,” says Jürgen Volz. “However, this is much more than just a curiosity,” adds Arno Rauschenbeutel. “Indeed, the photon pairs generated are quantum mechanically entangled. So there is the spooky action at a distance between the two photons that Einstein didn’t want to believe in and thanks to which one can teleport quantum states, for example.” “That a single atom is ideally suited as a source for such entangled photon pairs,” Volz and Rauschenbeutel agree, “is something hardly anyone would have believed until recently.”
Quantum Revolution: Brighter Entangled Photon Pairs for Advanced Quantum Communication
In fact, the demonstrated effect lends itself to realizing sources of entangled photon pairs whose brightness reaches the theoretically possible maximum and thus surpasses existing sources. Furthermore, the photon pairs inherently match the atoms from which they were emitted. This allows one to directly interface the photons with quantum repeaters or quantum gates which use the same atoms and are required for long-distance quantum communication.
A single atom is excited by laser light and scatters one photon after another. An optical filter removes certain colour components from this stream of single photons. This causes the remaining photons to become pairs that leave the filter simultaneously. (Image: Department of Physics, Humboldt-Universität zu Berlin)
Publication: On the simultaneous scattering of two photons by a single two-level atom, Luke Masters, Xin-Xin Hu, Martin Cordier, Gabriele Maron, Lucas Pache, Arno Rauschenbeutel, Max Schemmer, Jürgen Volz, Nature Photonics (2023), https://www.nature.com/articles/s41566-023-01260-7
About DAALI
DAALI is a European Project that brings together a team theoretical and experimental experts from ICFO, Humboldt University, Sorbonne University, the Center for Nanoscience and Nanotechnology (C2N), Weizmann Institute of Science and IXBlue. The realisation of efficient interfaces between photons and atoms is the basis for a wide range of applications such as quantum memories for light and nonlinear optics at the single-photon level. For this purpose, the DAALI team has the goal of developing new physical paradigms that facilitate novel schemes to understand and control light-matter interactions, with the use of a wide range of different platforms based on ordered atom arrays, nanocavities, waveguide QED, and solid-state emitters, among others.
Contact
Jürgen Volz
Department of Physics
Humboldt-Universität zu Berlin
E-Mail: juergen.volz@hu-berlin.de
Telefon: +49 30 2093 82155
Arno Rauschenbeutel
Department of Physics
Humboldt-Universität zu Berlin
E-Mail: arno.rauschenbeutel@hu-berlin.de
