Strong Interaction between Light and Matter

A microscopic cavity of two highly reflective mirrors is used to allow a quantum dot to interact with a single photon. A photon is emitted and reabsorbed up to 10 times by the quantum dot before it is lost. The quantum dot is electrically controlled within a semiconductor chip. (Source: U. Basel)

Researchers have succeeded in creating an efficient quantum-mechanical light-matter interface using a micro­scopic cavity. Within this cavity, a single photon is emitted and absorbed up to 10 times by an arti­ficial atom. This opens up new prospects for quantum tech­nology, report physicists at the University of Basel and Ruhr-University Bochum.

Achieving an inter­action between a single photon and a single atom is a huge challenge due to the tiny size of the atom. However, sending the photon past the atom several times by means of mirrors significantly increases the probability of an inter­action. In order to generate photons, the researchers use artificial atoms, known as quantum dots. These semi­conductor structures consist of an accu­mulation of tens of thousands of atoms, but behave much like a single atom: when they are optically excited, their energy state changes and they emit a photon. “However, they have the techno­logical advantage that they can be embedded in a semi­conductor chip,” says Daniel Najer, who conducted the experiment at the Department of Physics at the University of Basel.

Normally, these light particles fly off in all directions like a light bulb. For their experiment, however, the researchers posi­tioned the quantum dot in a cavity with reflective walls. The curved mirrors reflect the emitted photon back and forth up to 10,000 times, causing an inter­action between light and matter. Measure­ments show that a single photon is emitted and absorbed up to 10 times by the quantum dot. At the quantum level, the photon is transformed into a higher energy state of the arti­ficial atom, at which point a new photon is created. And this happens very quickly, which is very desirable in terms of quantum techno­logical appli­cations: one cycle lasts just 200 picoseconds.

The conversion of an energy quantum from a quantum dot to a photon and back again is theoretically well supported, but “nobody has ever observed these oscil­lations so clearly before,” says Richard J. Warburton from the Depart­ment of Physics at the University of Basel. The success­ful experiment is parti­cularly signi­ficant because there are no direct photon-photon interactions in nature. However, a controlled inter­action is required for use in quantum infor­mation processing.

By trans­forming light into matter according to the laws of quantum physics, an interaction between indi­vidual photons becomes indirectly possible – namely, via the detour of an entanglement between a photon and a single electron spin trapped in the quantum dot. If several such photons are involved, quantum gates can be created through entangled photons. This is a vital step in the generation of photonic qubits, which can store infor­mation by means of the quantum state of light particles and transmit them over long distances.

The experiment takes place in the optical frequency range and places high technical demands on the size of the cavity, which must be adapted to the wave­length, and the reflec­tivity of the mirrors, so that the photon remains in the cavity for as long as possible. (Source: U. Basel)

Reference: D. Najer et al.: A gated quantum dot strongly coupled to an optical microcavity, Nature, online 21 October 2019; DOI: 10.1038/s41586-019-1709-y

Link: Nano-Photonics, Dept. of Physics, University of Basel, Basel, Switzerland

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