"Quantum control has been a dream for many years."
What happens when you flick on a light switch? The room is flooded with bright light. It is one of those everyday things we take for granted, but what’s taking place at the atomic level is really quite fascinating.
The process is most often described in terms of the particle nature of light. An atom or molecule in a fluorescent tube enters an excited state when it’s heated, and spontaneously decays to a lower energy state, releasing a photon — the fundamental particle of light. When the photon enters your eye, something similar happens, but in reverse. The photon is absorbed by a molecule in the retina, and its energy boosts the molecule into an excited state.
But light is both a particle and a wave, and this duality is fundamental to the physics that rules the world of atoms and molecules. Despite this significance, the wave nature of light was often ignored, until recently.
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Kater Murch, assistant professor of physics in Arts and Sciences at Washington University in St. Louis, is one of the first in the world to look at the spontaneous emission of light with an instrument sensitive to its wave, rather than its particle nature.
The instrument consists of an artificial atom — a microscopic superconducting circuit with two energy levels cooled to just above absolute zero — and an interferometer, which works by merging two or more sources of light. In this experiment, the electromagnetic wave of the emitted light interferes with a reference wave of the same frequency, and this interference is measured.
According to Murch, all that a photon detector can tell you about spontaneous emission is whether an atom is in its excited state or its ground state. On the other hand, interferometers can catch the atom diffusing through a quantum “state space” made up of all the possible combinations of its two energy states.
However, this is really tough to accomplish because the electromagnetic field associated with a single photon produces a very faint signal that is often disguised in quantum noise. Luckily, the noise holds information about the state of the artificial atom so the researchers could still trace its evolution.
What they discovered was really strange. When viewing light in its wave nature, the researchers observed that the artificial atom could move from a lower energy state to a higher one even as it decayed.
"You'd never see that if you were detecting photons," Murch said in a press release.
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But it gets even more bizarre. Given that an atom’s average excitation can increase even when it decays is a sign that how we look at light might give us some control over the atoms that are emitted, explained Murch.
How? By one of the weirdest of all quantum effects — entanglement. According to quantum physics, when an atom emits light, the light and the atoms must become connected, or entangled, so that measuring a property of one instantly reveals the value of that property for the other, no matter how far away it is.
In other words, every measurement of an entangled object influences its entangled partner. And it is this effect that could allow a light detector to control the light emitter.
"Quantum control has been a dream for many years," Murch said. "One day, we may use it to enhance fluorescence imaging."
"That's very long term, but that's the idea," he concluded.
The work will be described in the May 20th issue of the journal Nature Communications, and is described further in the video below.
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