One of the things that sets the quantum world apart from our everyday classical one is the capacity for entanglement—when two or more objects share an invisible connection that entwines their fates. Entanglement is the most extreme version of a quantum connection, where measuring one particle can tell you everything you need to know about another. Short of that, particles can still sync up in decidedly quantum ways, where measuring one particle will give you some incomplete information about another. Such quantum correlations can be used to make more precise measurements than classical ones. For example, they can help us detect gravitational waves.
Photons of light don’t often naturally connect in this way. But when they do, quantum-correlated photons could potentially be useful to study materials’ quantum features. Generating this quantum light is tricky business, however, and has so far been largely confined to just a few photons.
Electrons, atoms and molecules, on the other hand, participate in en masse quantum correlations inside of materials all the time. Electrons syncing up inside a metal give rise to superconductivity at low temperatures, for instance, and—physicists speculate—high-temperature superconductivity, exotic fractional electron materials, and more. Now a team of physicists in Israel, Austria, England and the U.S. has found a way to imprint the complex pattern of quantum correlations from such materials onto light. This method can produce bright quantum light at a broad range of frequencies, the team explained recently in Nature Physics.
“Imagine having quantum light that you could see with your eyes,” says Ido Kaminer, an electrical and computer engineer at Technion–Israel Institute of Technology and senior author of the study. “That would be amazing, and it would also have many advantages for applications of quantum science that you wouldn’t consider otherwise.”
The researchers’ idea builds on an existing process for creating bursts of bright light. This process, known as high harmonic generation, involves shining a bright laser beam onto a gas of atoms, or more recently a solid crystal, such as zinc oxide, the active ingredient in many mineral sunscreens. The gaseous atoms or solids absorb the laser light and in turn emit light at higher harmonics: if the input light is like a middle C on a piano, the emitted light is comparable to many C notes hundreds of octaves up.
The emissions combine to produce light pulses that pass by in the tiniest fraction of a second—a billionth of a billionth. If directed at electrons, atoms or molecules, these short bursts can be used to capture high-frame-rate videos.
For their new study, the researchers aimed to understand how quantum correlations inside a source material, be it a gas or a mineral, would impact the quantum properties of the light bursts coming out, if at all. “High harmonic generation is a very important area. And still, until recently, it was described by a classical picture of light,” Kaminer says.
In quantum mechanics, figuring out what’s going on with more than a few particles at the same time is notoriously difficult. Kaminer and Alexey Gorlach, a graduate student in his lab, used their COVID-imposed isolation to try to make progress on a fully quantum description of light emitted in high harmonics. “It’s really crazy; Alexey built a super complex mathematical description on a scale that we’ve never had before,” Kaminer says.
Next, to fully incorporate the quantum properties of the material used to generate this light, Kaminer and Gorlach teamed up with Andrea Pizzi, then a graduate student at the University of Cambridge and now a postdoctoral fellow at Harvard University.
“This is a very beautiful mathematical framework to attack the very tricky mesoscopic world,” says Elena del Valle, a light-matter interaction expert and a physicist at the Autonomous University of Madrid, who was not involved with the work. “Mesoscopic” refers to anything that combines a medium number of particles: more than a few but not so many that individual behavior is completely irrelevant. Here, that means the many photons and their quantum correlations.
The researchers’ results spell out precisely how the quantum correlations of the source will translate into quantum correlations of emitted light.
If such quantum light is successfully generated in experiments, there are two main ways it could be of practical use. First, it can give insight into the material that generated it. “Quantum properties are at the core of a lot of things, like high-temperature superconductors,” Kaminer says. “And this would teach you something you couldn’t see otherwise.”
Second, quantum light can be used as a source, especially in the case of x-ray imaging. In this realm, correlated light could pick up on extra quantum information that would be inaccessible otherwise. “Once you get to the x-ray regime,” Kaminer says, “then you can use it for imaging materials, going through samples.”
Atoms and materials used for high harmonic generation today don’t have any interesting quantum properties to speak of, Kaminer says, and so do not produce quantum light. To choose a material to work with and create this light in the lab, the scientists are aiming to team up with an experimental group. They warn that a real implementation might not be straightforward.
“From here to the experiment there will still be some hard work, innovative engineering and theoretical developments,” Pizzi says. But researchers have several promising experimental ideas, and Pizzi and his collaborators, as well as others in the field, are optimistic. “Putting all this together for a few atoms, under a strong pulsed excitation, is not science fiction at the moment,” del Valle says. If realized, this technique could allow scientists to glimpse matter’s full quantum complexity like never before.