Photon shape influences light-matter interactions

08 Dec 2016

Singapore team shows how photons with rising or falling energy are absorbed differently; could inform design of quantum devices.

Scientists at the Centre for Quantum Technologies (CQT) at the National University of Singapore have shown that a photon's shape affects how it is absorbed by a single atom. They say that this discovery is significant because it means that a photon’s energy, based on its wavelength, is no longer the only factor determining its behavior.

Christian Kurtsiefer, Principal Investigator at CQT, and his team have learned to shape photons with extreme precision. He commented, “We don't often think of photons as being spread out in time and space and thus having a shape, but the ones in our experiment were some four meters long.”

For the research, published 29 November in Nature Communications, the team worked with rubidium atoms and infrared photons. The photons were directed one at a time onto a single atom of the alkali metal. "Our experiments investigate the most fundamental interaction between matter and light," commented Victor Leong, a PhD student involved with the research.

A four-metre long photon takes about 13 nanoseconds to pass such an atom. Every time a photon was directed towards the atom, the team watched to see if and when the atom got excited. By noting the excitation times and collecting them together, the researchers could map the probability of the atom absorbing the photon as a function of time.

Two different photon shapes were investigated; one rising in brightness, the other decaying. Hundreds of millions of measurements made over 1500 hours showed that the overall probability that a single rubidium atom would absorb a single photon of either type was just over 4%. 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 CQT team 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% higher than when the photon arrived bright and had a long, fading tail.

As expected

The researchers had expected that the atoms might prefer to soak up the rising photons. Coauthor Matthias Steiner explained, “That is because of what happens naturally when an excited atom decays. Then, the atom spits out a decaying photon. Imagine running the process backwards – the equations say it should look the same – and the atom would arrive with rising brightness. Our choice of photon shape was inspired by the time symmetry of quantum mechanics."

The team also commented that the work 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. The photon knocks the atom into an excited state. To build reliable devices, scientists will need to control the interaction. You can only engineer what you can understand," said coauthor Alessandro Cerè.

Paper conclusions

The key conclusions described in the Nature Communications paper were as follow: “We have shown that the scattering dynamics depends on the envelope of the photon, in particular that an atom is indeed more efficiently excited by a photon with an exponentially rising temporal envelope compared with an exponentially decaying one. The advantage of exciting single atoms with exponentially rising photons is a larger peak excitation probability within a narrower time interval. Such a synchronization can be beneficial to quantum networks.”