When Light Interacts With Atoms, Shape Matters

Scientists in Singapore have shown that a photon’s shape affects how it is absorbed by a single atom.

AsianScientist (Dec. 7, 2016) – Scientists in Singapore have shown that a photon’s shape affects how it is absorbed by a single atom. Their work was published in Nature Communications.

Vision is all about photons of light, which are packets of energy, interacting with the atoms or molecules in what you’re looking at. Some photons reflect off, reaching your eyes, while others get absorbed. The photon’s energy decides which will happen. Now, scientists at the Center for Quantum Technologies at the National University of Singapore have mapped the interaction of a single atom with a single photon, which may inform the design of quantum devices.

We don’t often think of photons as being spread out in time and space and thus having a shape, but the ones in this experiment were some four meters long. Dr. Christian Kurtsiefer and his team have learned to shape these photons with extreme precision. For the present study, the team worked with rubidium atoms and infrared photons. They shone the photons one at a time onto a single atom. A four-meter photon takes about 13 nanoseconds to pass the atom. Every time a photon was sent towards the atom, the team watched to see if and when the atom got excited.

The team tested two different photon shapes: one rising in brightness, the other decaying. Hundreds of millions of measurements made over 1,500 hours showed that the overall probability that a single rubidium atom would absorb a single photon of either type was just over four percent.

However, when the team looked at the process on nanoscale timeframes, they saw that the probability of absorption at each moment depends on the photon’s shape. The researchers found that if the photon arrived dimly, from the atom’s point of view, then ended brightly, the peak probability of excitation was just over 50 percent higher than when the photon arrived bright and had a long, fading tail.

The work also builds understanding for technologies that rely on light-matter interactions. Some proposals for quantum technologies, such as communication networks, sensors and computers, require that a photon writes information into an atom by being absorbed. To build reliable devices, scientists will need to control this interaction.


The article can be found at: Leong et al. (2016) Time-resolved Scattering of a Single Photon by a Single Atom.

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Source: National University of Singapore; Photo: Shutterstock.
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