This is a gradient echo memory setup. The image to be stored, the
letter N encoded by a signal laser beam and a mask, enters from the left
(pink light) and enters the storage cell filled with Rb atoms. The
components of this image will be absorbed by the atoms when, at
locations all over the body of the cell, a part of the signal beam and
parts of a separate “control” laser beam -- entering from the side
(shaded green) via a polarizing beam splitter (PBS) -- and (last but not
least) the strength of a magnetic field (delivered by the brown coil
around the cell) are just right. The stored image can later be read out
and observed with a CCD camera. Credit: NIST
The storage of light-encoded messages on film and compact disks
and as holograms is ubiquitous---grocery scanners, Netflix disks,
credit-card images are just a few examples. And now light signals can be
stored as patterns in a room-temperature vapor of atoms. Scientists at
the Joint Quantum Institute have stored not one but two letters of the
alphabet in a tiny cell filled with rubidium (Rb) atoms which are
tailored to absorb and later re-emit messages on demand. This is the
first time two images have simultaneously been reliably stored in a
non-solid medium and then played back
.
In effect, this is the first stored and replayed atomic movie.
Because the JQI researchers are able to store and replay two separate
images, or "frames," a few micro-seconds apart, the whole sequence can
qualify as a feat of cinematography. The new storage process was
developed by Paul Lett and his colleagues, who publish their results in
the latest issue of the journal Optics Express.
One young man was inspired by the lingo of the JQI paper, especially
the storage of images in the atomic memory, and contrived a song which
he performs on a video clip:
We don't yet need to store grocery barcodes in tiny vials of rubidium. The atomic method, however, will come into its own for storing and processing quantum information, where subtle issues of coherence and isolation from the outside world need to be addressed.
The atomic storage medium is a narrow cell some 20 centimeters long, which seems pretty large for a quantum device.
That's how much room is needed to accommodate a quantum process called
gradient echo memory (GEM). This useful protocol for storage was
pioneered at the Australian National University just in the past few
years. While many storage media
try to cram as much information into as small a place as
possible---whether on a magnetized strip or on a compact disk---in GEM
an image is stored over the whole range of that 20-cm-long cell.
The image is stored in this extended way, by being absorbed in atoms
at any one particular place in the cell, depending on whether those
atoms are exposed to three carefully tailored fields: the electric field
of the signal light, the electric field of another "control" laser
pulse, and a magnetic field (adjusted to be different along the length
of the cell) which makes the Rb atoms (each behaving like a magnet
itself) precess about. When the image is absorbed into the atoms in the
cell, the control beam is turned off. Because this process requires the
simultaneous action of two particular photons---one putting the atom in
an excited state, the other sending it back down to a slightly different
ground state---it cannot easily be undone by atoms subsequently
randomly emitting light and returning to the original ground state.
That's how the image is stored. Image readout occurs in a sort of
reverse process. The magnetic field is flipped to a contrary
orientation, the control beam turned back on, and the atoms start to
precess in the opposite direction. Eventually those atoms reemit light,
thus reconstituting the image pulse, which proceeds on its way out of
the cell.
Having stored one image (the letter N), the JQI physicists then
stored a second image, the letter T, before reading both letters back in
quick succession. The two "frames" of this movie, about a microsecond
apart, were played back successfully every time, although typically only
about 8 percent of the original light was redeemed, a percentage that
will improve with practice. According to Paul Lett, one of the great
challenges in storing images this way is to keep the atoms embodying the
image from diffusing away. The longer the storage time (measured so far
to be about 20 microseconds) the more diffusion occurs. The result is a
fuzzy image.
Paul Lett plans to link up these new developments in storing images
with his previous work on squeezed light. "Squeezing" light is one way
to partially circumvent the Heisenberg uncertainty principle governing
the ultimate measurement limitations. By allowing a poorer knowledge of a
stream of light---say the timing of the light, its phase---one gain a
sharper knowledge of a separate variable---in this case the light's
amplitude. This increased capability, at le ast for the one variable,
allows higher precision in certain quantum measurements.
"The big thing here," said Lett, "is that this allows us to do images
and do pulses (instead of individual photons) and it can be matched
(hopefully) to our squeezed light source, so that we can soon try to
store "quantum images" and make essentially a random access memory for
continuous variable quantum information. The thing that really attracted
us to this method---aside from its being pretty well-matched to our
source of squeezed light---is that the ANU group was able to get 87%
recovery efficiency from it - which is, I think, the best anyone has
seen in any optical system, so it holds great promise for a quantum
memory."
The lead author of the new Optics Express
article, Quentin Glorieux, feels that the JQI image storage method
represents a potentially important addition to the establishment of
quantum networks, equipment which exploits quantum effects for
computing, communications, and metrology. "It is very exciting because
images and movies are familiar to everyone. We want to go to the quantum
level. If we manage to store quantum information embedded in an image or maybe in multiple images, that could really hasten the advent of a quantum network/internet."
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