Driving late at night, you come upon a red light and stop the car. You lift your hand wearily to block the red glow streaming through your windshield. Suddenly, both the green and yellow lights come on, hitting your eyeballs at the same time. Confused, you take your hand away, and again only the red color appears.
This surreal scenario is what would actually happen if the traffic light was a single atom illuminated by a laser beam, as recently shown experimentally by researchers in Berlin. They looked at the light scattered by an atom and saw that photons—the tiniest particles of light—arrived at the detector one at a time. The scientists blocked the brightest color they saw, and suddenly pairs of photons of two slightly different colors started arriving at their detector simultaneously. They reported their findings in Nature Photonics in July.
The reason for this counterintuitive effect is that single atoms are skilled little multitaskers. Through different underlying processes, they can scatter a variety of colors at the same time like a dangerous traffic light that shines all three colors at once. Yet because of quantum interference between these processes, an observer only sees one of the metaphorical traffic light’s colors at a time, preserving peace on the road.
This experiment also paves the way for novel quantum information applications. When the brightest color is blocked, the photons that pop up simultaneously are entangled with each other, behaving in sync even when they are separated over large distances. This provides a new tool for quantum communication and information processing in which entangled photon pairs can serve as distributed keys in quantum cryptography or store information in a quantum memory device.
Atoms can be surprisingly picky about their couplings with light. Based on the varying arrangements of their constituent electrons, atoms of different elements each display clear preferences for which colors of light they strongly scatter. Proving as much is as simple as shining a laser at an atom, with the laser tuned to a particular color that closely matches that atom’s scattering preference. As expected, your detector will show the atom scattering photons of that predominant color. But strangely, the scattered photons will stream into the detector one at a time, as if in a single-file line. Up through the early 1980s physicists generally accepted a naive explanation for this strange effect: the photons arrive as if in a queue because the atom can only scatter one photon at a time.
In 1984, however, two researchers dug into the math governing this phenomenon and found that the reality is much more complicated—and much more inherently quantum. They theorized that the atom is actually doing many things simultaneously: scattering not only single photons but also, through an entirely different process, photonic pairs, triplets and quadruplets. Nevertheless, only one photon at a time arrives at the detector because of quantum interference among these processes.
Regular interference occurs between two waves like ripples on a pond, overlapping in a pattern of crests and troughs. A distinctive feature of the quantum world is that interference occurs not only between actual waves but also between probabilities: a photon sent through two slits has some probability of going through the left slit and some probability of going through the right one. The two possible paths interfere with each other, forming a pattern of crests and troughs. Block either slit, and the pattern disappears. “I like to tell my students, ‘Imagine that you want to prevent a burglar from entering your house and going into the living room. Just leave two doors open, and then you will have destructive interference, and the thieves cannot go into the living room,’” jokes physicist Jean Dalibard, who co-authored the 1984 paper.
In Dalibard’s model, however, this interference is not a joke at all. It actually happens between the two underlying processes, the single-photon and multiphoton scattering. And it happens not in space but in time such that a probability trough appears for two photons arriving at the same time. So the atom multitasks, yet it does so in a way that looks suspiciously like doing just one thing.
Caught in the Acts
Dalibard’s complex description of the multitasking atom languished in relative obscurity until recently. “I was very happy that the group from Berlin found this paper. I don’t know how they did,” he says. From their end, the researchers in Berlin were fascinated by the counterintuitive theory introduced by Dalibard and his co-author, physicist Serge Reynaud. “When we started to dig into the old literature from the 1980s, we really got intrigued,” says Max Schemmer, a former postdoctoral researcher at Humboldt University of Berlin and a co-author of the recent work.
Schemmer and his colleagues saw the potential of recently developed technology to experimentally test this theory. First, they cooled a cloud of rubidium atoms to just shy of absolute zero. Then they used optical tweezers—a tightly focused laser beam strong enough to grab extremely tiny objects—to isolate and hold one atom. Next they illuminated that atom with another laser tuned to rubidium’s scattering preference and placed a lens off to the side to collect the scattered light and channel it into an optical fiber.
To block the brightest color, the researchers guided the light into a finely tuned filter created by a ring of optical fiber. The length of the ring was chosen and adjusted precisely to create destructive interference for only one color of light. When this filter was included in the light’s path, they saw the brightest color disappear. And as Dalibard and Reynaud had predicted, photons of two slightly different colors suddenly started arriving at the detector in simultaneous pairs.
By blocking the brightest color, thus taking the atom’s single-photon-generating process offline, Schemmer and his colleagues were able to see the other process in action without the destructive interference created by the dominant single atom—much like a traffic light that shines both green and yellow when red is blocked.
A Practical Promise
The atom’s “second task” of scattering photons in pairs could come in handy for quantum computing and communication. Once the brightest color is blocked, the pairs of photons that arrive simultaneously are entangled with each other—entanglement being the not-so-secret ingredient that gives quantum approaches advantages over classical ones.
Entangled photon pairs could be used to share quantum information across vast distances or to transmit it between different mediums. Conveniently, the photon pairs produced with this technique come in a very precise color rather than being spread across larger chunks of the rainbow like photon pairs produced by conventional methods. This makes them particularly useful for efficiently storing quantum information in a quantum memory device, Schemmer says, which could in turn lead to more robust quantum communication networks.
Additionally, these photon pairs possess a unique kind of entanglement that is not offered by other sources: a syncing in time. “There is one existing technique of producing entangled pairs of photons,” says Magdalena Stobinska, a quantum optics expert, who did not participate in the work. “But this is a different degree of freedom and therefore can be used for different types of applications. So it broadens the palette of efficiently produced entangled pairs of photons. And I think that’s cool.”
And theory predicts that photon pairs are not the end of the story. The atom is also simultaneously scattering entangled photons in threes, fours, and so on. Blocking the red on this “traffic light” makes not only yellow and green shine through but also blue, orange, and much more. Clusters of entangled photons created this way could potentially serve as resources for photon-based quantum computing. “This system is like a treasure trove of quantum correlations,” says Fabrice P. Laussy, a professor of light-matter interactions at the University of Wolverhampton in England, who reviewed the recent study but did not participate in the research. “Everything is in there.”