The goal to develop quantum
computers—a long-awaited type of computer that could solve otherwise
intractable problems, such as breaking complex encryption codes—has
inspired scientists the world over to invent new devices that could
become the brains and memory
of these machines. Many of these tiny devices use particles of light,
or photons, to carry the bits of information that a quantum computer
will use.
But while each of these pieces of hardware can do some jobs well,
none are likely to accomplish all of the functions necessary to build a
quantum computer. This implies that several different types of quantum
devices will need to work together for the computer or network to
function. The trouble is that these tiny devices frequently create
photons of such different character that they cannot transfer the
quantum bits of information between one another. Transmuting two vastly
different photons into two similar ones would be a first step toward
permitting quantum information components to communicate with one
another over large distances, but until now this goal has remained
elusive.
[1] A single photon is produced by a quantum dot (QD). Simultaneously, a
pair of photons is produced by a parametric down-conversion crystal
(PDC). [2] One of the PDC photons—which has different characteristics
than the QD photon—is routed into a cavity and filter, [3] rendering
this PDC photon and the QD photon nearly identical. Credit: Suplee, NIST
However, the team has demonstrated that it is possible to take
photons from two disparate sources and render these particles partially
indistinguishable. That photons can be made to "coalesce" and become
indistinguishable without losing their essential quantum properties
suggests in principle that they can connect various types of hardware
devices into a single quantum information network. The team's
achievement also demonstrates for the first time that a "hybrid" quantum
computer might be assembled from different hardware types.
The team connected single photons from a "quantum dot," which could be useful in logic circuits,
with a second single-photon source that uses "parametric down
conversion," which might be used to connect different parts of the
computer. These two sources typically produce photons that differ so
dramatically in spectrum that they would be unusable in a quantum
network. But with a deft choice of filters and other devices that alter
the photons' spectral shapes and other properties, the team was able to
make the photons virtually identical.
"We manipulate the photons to be as indistinguishable as possible in
terms of spectra, location and polarization—the details you need to
describe a photon. We attribute the remaining distinguishability to
properties of the quantum dot," says Glenn Solomon, of NIST's Quantum
Measurement Division. "No conceivable measurement can tell
indistinguishable photons
apart. The results prove in principle that a hybrid quantum network is
possible and can be scaled up for use in a quantum network."
From physorg
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