A mind-boggling new type of “quantum light” might allow scientists to peer inside atoms and control some of the bizarre powers of light particles, or photons, at these tiny quantum scales, reports a new study.
Though this form of quantum light remains theoretical at this point, scientists believe it may soon be experimentally demonstrated with existing equipment, potentially leading to novel technologies in microscopy and quantum computation. The technique could also inspire discoveries in fields like “attoscience,” which deals with interactions that occur on the extremely short timescale of the attosecond (a billionth of a billionth of a second), and could solve longstanding mysteries in the physics of materials.
Light is so ubiquitous in our lives that it is easy to forget it’s there at all. Every day, we are bathed in rays from the Sun, our home fixtures, our urban environments, and countless other sources. At these larger scales, photons are governed by the familiar laws of classical physics, but if you shrink down to the scale of atoms, the rules of quantum mechanics take over. At this size, the universe is filled with all manner of trippy nonsense, such as quantum entanglement, whereby particles become synced together even across vast distances.
Now, scientists have envisioned an entirely new state of light that puts a quantum spin on a phenomenon called high-harmonic generation, which involves energizing photons from lasers into much higher frequencies.
While high-harmonic generation has been well-studied from a classical physics standpoint, harnessing its quantum properties could “[pave] the way towards the engineering of novel states of light over a broadband spectrum” and contribute to “the ambitious goal of bringing together quantum optics and attoscience,” according to a study published on Thursday in Nature Physics.
“The quantum mechanical laws of light lead to many new behaviors of light that cannot be explained by classical theory,” said Nicholas Rivera, a junior fellow at Harvard University who co-authored the study, in an email to Motherboard. “We call light exhibiting those properties ‘quantum states of light.’”
“Especially attractive are ‘many-photon quantum states of light’ which are simply put, states of light with uniquely quantum properties that also are comprised of many photons which are believed to enable advances for ultra-precise measurements, for communication systems, and for quantum computation,” he added.
In classical physics experiments, high harmonics can be induced by stripping electrons from the atoms in a material, such as a solid or gas, by bathing the material with extremely strong laser blasts. The electrons then recombine with the atoms, releasing photons that have much higher frequencies than the original laser light. For instance, high harmonics could be produced by converting laser photons at infrared wavelengths into the more energetic ultraviolet or X-ray bands of the light spectrum.
“High-harmonic generation is a process of extreme ‘upconversion’ that converts low-frequency photons into high-frequency photons,” Rivera explained. “Moreover, the upconverted photons can exist as very short pulses, with durations of roughly 100 attoseconds. The extremely short duration of these pulses is quite attractive because of the promise of enabling visualization of physical and chemical processes happening at these same ultrashort time scales.”
“For example, the motion of electrons in atoms and molecules occurs on these extremely small timescales,” he noted. “In general, high harmonic generation promises to give a new window into the properties of electrons, atoms, and molecules with tons of applications throughout all of the sciences.”
Rivera and his colleagues first started exploring whether high-harmonic generation could produce many-photon quantum states of light a few years ago, and outlined their initial findings in a 2020 study. That project revealed that the quantum dimensions of high-harmonic generation were virtually unexplored, so the researchers spent the past few years working out the mathematical foundations of this phenomenon on atomic scales.
In the new research, the team suggests that a quantum version of high-harmonic generation could be produced if the atoms in the target material were entangled, meaning that their properties would become correlated in that weird way that is only seen at very small scales.
What does all this mean? This novel form of quantum light could usher in sophisticated new techniques for imaging materials, such as biological samples, with unprecedented clarity, and it could also expose the hidden details about the ultra-fast interactions and properties of entities at atomic scales.
“The vision is that the quantum properties of light produced by a many-body system of correlated quantum matter should reflect the intrinsic correlations between the constituents of the matter,” Rivera said. “The study of correlations is foundational to the study of modern materials—and so, by having an experimental technique that could extract correlations with high fidelity, one could then use it to clarify the physics of materials that have evaded a complete understanding (of which there are many in modern materials physics).”
“This said, the work here is just the beginning of this vision, and it will ultimately be a while before we can know the full potential of using correlations of light to usefully infer correlations of matter,” he noted.
To that end, the researchers pinpoint two existing experimental setups that could potentially produce this novel form of quantum light, and note that their new theory “can be adopted or generalized in several different research directions,” according to the study.
“The question of ‘what is the best’ implementation is ultimately still an open question for us, and one we are now working towards understanding,” Rivera said. “That said, one exciting area, where we believe that some of these ideas could be tested, is in the field of ‘high-harmonic generation in solids,’ where the target which produces high-frequency light is not a gas, but a solid material.”
“Solids can have strong quantum correlations between the constituent electrons, and so it is interesting to explore how those correlations manifest in the emitted light,” he concluded. “[T]his is a question that will require extension of our theory to describe quantum light emission by solids accurately—and is an area we are excited to develop.”
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