Unexpected Quantum Phenomenon: Single Atoms Yielding Entangled Pairs of Photons

Researchers from the Humboldt University of Berlin, collaborating with the DAALI (Disruptive Approaches to Atom-Light Interfaces) project, have uncovered an unexpected phenomenon related to the emission of light by a solitary atom.

The study, led by scientists Jürgen Volz and Arno Rauschenbeutel from the Department of Physics at Humboldt University, provides novel insights into how a single fluorescent atom scatters light. These findings could have significant applications in the realm of quantum communications. The researchers have published their work in the prestigious journal Nature Photonics.

At the turn of the 20th century, physicist Max Planck posited that the exchange of energy between light and matter, such as an atom, happens in discrete units, termed “quanta.” Albert Einstein further expanded on this concept five years later, suggesting that light itself was composed of these quanta, now commonly referred to as photons. In the modern era, photodiodes have been developed that are sufficiently sensitive to detect individual photons. When subjected to a constant light source, these photodiodes do not emit a continuous electrical signal but generate a sequence of brief electrical pulses, each corresponding to the capture of a single photon.

Detailed Examination of Scattering of Laser Light

When the light emitted by a lone atom, excited to fluorescence by a laser, reaches a highly sensitive photodiode, the device never registers two photons at the same time. This characteristic sets the emitted fluorescent light apart from the laser light used to excite it, where photons do indeed appear simultaneously. When a single atom is exposed to two simultaneous laser photons, it absorbs only one and allows the other to pass. The atom then emits the absorbed photon in a random direction and is only then ready to absorb another.

This property has been capitalized upon in the DAALI project and other quantum technology research initiatives. For instance, it is employed in secure quantum communication systems that utilize individual photons emitted either naturally or artificially.

Unanticipated Findings Regarding Paired Photons

However, the researchers have now showcased a particularly startling effect in the light emission from a solitary atom. By using a filter to eliminate a specific color component from the emitted light, they transformed the stream of individual photons into simultaneously appearing photon pairs.

This result defies conventional wisdom and everyday experience; for instance, removing all green cars from a street doesn’t cause the remaining cars to suddenly pair up and drive alongside each other.

It also appears to refute the previously held belief that a single atom can only scatter one photon at a time. With the right color filter in place, the atom can, in fact, scatter two photons simultaneously.

This phenomenon was theoretically predicted four decades ago by Jean Dalibard and Serge Reynaud at ENS Paris but has only now been experimentally verified by the team led by Volz and Rauschenbeutel.

Jürgen Volz noted the limitations of human intuition when trying to comprehend processes at the microscopic scale. Arno Rauschenbeutel added that the generated photon pairs are entangled at the quantum level, exhibiting the “spooky action at a distance” that Einstein once refuted. This has significant implications for applications like quantum state teleportation.

Both scientists concur that a single atom being an optimal source for such entangled photon pairs was an idea that few would have considered plausible until recently. This discovery could pave the way for the development of entangled photon pair sources that outperform current ones, while inherently matching the atoms that emitted them. This compatibility could be vital for long-distance quantum communication technologies involving quantum repeaters or quantum gates.

Reference: “On the simultaneous scattering of two photons by a single two-level atom” by Luke Masters, Xin-Xin Hu, Martin Cordier, Gabriele Maron, Lucas Pache, Arno Rauschenbeutel, Max Schemmer, and Jürgen Volz, published on July 27, 2023, in Nature Photonics. DOI: 10.1038/s41566-023-01260-7

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More about Quantum Phenomenon

• Nature Photonics Journal
• Humboldt University Department of Physics
• Introduction to Quantum Communication
• Max Planck and Quantum Theory
• Albert Einstein and the Photoelectric Effect
• DAALI Project Overview
• Quantum Entanglement Explained
• Jean Dalibard and Serge Reynaud’s Theoretical Work