Quantum light would let us peer into atoms like never before, paving a way to solve longstanding mysteries in materials physics.
IMAGE: DAVID WALL VIA GETTY IMAGES |
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 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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