Can Quantum dots spit out clone-like photons ?
Photons could help
pave the way for quantum information processors or communications.
Researchers
of Massachusetts Institute of Technology have produced coherent
single photon emitters, a key component for future quantum computers and
communications systems.
In the global quest
to develop practical computing and communications devices based on the
principles of quantum physics, one potentially useful component has proved
elusive: a source of individual particles of light with perfectly constant,
predictable, and steady characteristics. Now, researchers at MIT and in
Switzerland say they have made major steps toward such a single photon source.
Scanning
Transmission Electron Microscope image (STEM) of single perovskite quantum
dots. New study shows that single perovskite quantum dots could be a
fundamental building block for quantum-photonic technologies for computing or
communications.
The study, which involves using a
family of materials known as perovskites to make light-emitting particles
called quantum dots, appears today in the journal Science. The
paper is by MIT graduate student in chemistry Hendrik Utzat, professor of
chemistry Moungi Bawendi, and nine others at MIT and at ETH in Zurich,
Switzerland.
The ability to produce individual
photons with precisely known and persistent properties, including a wavelength,
or color, that does not fluctuate at all, could be useful for many kinds of
proposed quantum devices. Because each photon would be indistinguishable from
the others in terms of its quantum-mechanical properties, it could be possible,
for example, to delay one of them and then get the pair to interact with each
other, in a phenomenon called interference.
"This quantum interference
between different indistinguishable single photons is the basis of many optical
quantum information technologies using single photons as information
carriers," Utzat explains. "But it only works if the photons are
coherent, meaning they preserve their quantum states for a sufficiently long
time."
Many researchers have tried to
produce sources that could emit such coherent single photons, but all have had
limitations. Random fluctuations in the materials surrounding these emitters
tend to change the properties of the photons in unpredictable ways, destroying
their coherence. Finding emitter materials that maintain coherence and are also
bright and stable is "fundamentally challenging," Utzat says. That's
because not only the surroundings but even the materials themselves "essentially
provide a fluctuating bath that randomly interacts with the electronically
excited quantum state and washes out the coherence," he says.
"Without having a source of
coherent single photons, you can't use any of these quantum effects that are
the foundation of optical quantum information manipulation," says Bawendi,
who is the Lester Wolfe Professor of Chemistry. Another important quantum
effect that can be harnessed by having coherent photons, he says, is
entanglement, in which two photons essentially behave as if they were one,
sharing all their properties.
Previous chemically-made colloidal
quantum dot materials had impractically short coherence times, but this team
found that making the quantum dots from perovskites, a family of materials
defined by their crystal structure, produced coherence levels that were more
than a thousand times better than previous versions. The coherence properties
of these colloidal perovskite quantum dots are now approaching the levels of
established emitters, such as atom-like defects in diamond or quantum dots
grown by physicists using gas-phase beam epitaxy.
One of the big advantages of
perovskites, they found, was that they emit photons very quickly after being
stimulated by a laser beam. This high speed could be a crucial characteristic
for potential quantum computing applications. They also have very little
interaction with their surroundings, greatly improving their coherence
properties and stability.
Such coherent photons could also be
used for quantum-encrypted communications applications, Bawendi says. A
particular kind of entanglement, called polarization entanglement, can be the
basis for secure quantum communications that defies attempts at interception.
Now that the team has found these
promising properties, the next step is to work on optimizing and improving
their performance in order to make them scalable and practical. For one thing,
they need to achieve 100 percent indistinguishability in the photons produced.
So far, they have reached 20 percent, "which is already very
remarkable," Utzat says, already comparable to the coherences reached by
other materials, such as atom-like fluorescent defects in diamond, that are
already established systems and have been worked on much longer.
"Perovskite quantum dots still
have a long way to go until they become applicable in real applications,"
he says, "but this is a new materials system available for quantum
photonics that can now be optimized and potentially integrated with
devices."
It's a new phenomenon and will
require much work to develop to a practical level, the researchers say.
"Our study is very fundamental," Bawendi notes. "However, it's a
big step toward developing a new material platform that is promising."
The work was supported by the U.S.
Department of Energy, the National Science Foundation, and the Swiss Federal
Commission for Technology and Innovation
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