17
percent
Memory efficiency
record for vapor-based
quantum systems
69
percent
Memory efficiency
of new crystal-based
quantum system
Solidifying memories made of light
Crystal offers more efficient quantum information storage
By Alexandra Witze
Sun-drenched summer vacations may
yield pleasant memories, but physicists
have now harnessed light to remember
something else: quantum information.
Researchers have coaxed laboratory
crystals to capture and release information carried within a light pulse at the
highest efficiency yet. The work, reported
in the June 24 Nature, could lead to new
secure communications that exploit
weird properties of the quantum world.
“It’s quite an important step towards
our dream of extending the distances over
which we can do quantum communication,” says Wolfgang Tittel, a physicist at
the University of Calgary in Canada who
was not involved in the work.
Until now, researchers have tried to
fashion “quantum memories” for light
primarily by sending laser beams into a
vapor of atoms. The atoms preserve infor-
mation in the light that can then be read
out again, like playing back the data on a
DVD. But quantum memories based on
vapors are inefficient: The best system so
far has 17 percent efficiency (of 100 light
particles put in, only 17 make it out). Physi-
cists don’t need perfect efficiency, but a
system needs at least 50 percent recall to
be useful in quantum applications.
A crystal containing praseodymium
can store information encoded in light.
of the crystal, so that one end absorbs
strongly at the blue part of the spectrum
and the other end toward the red. Quantum information from the light is stored
in the oscillations of the crystal’s atoms;
reversing the electric field gets the atoms
to reemit light containing the same information as the original pulse.
The crystal that Hedges and colleagues
created is made of praseodymium, a rare
earth element, combined with yttrium,
silicon and oxygen. The scientists are now
beginning to study quantum memories
using other rare earth elements, such as
europium. Their next goal: to coax the
crystal to retain quantum memory for
longer than a few microseconds.
Supercold atoms in free fall
may offer clues about gravity
tower in Bremen, Germany. Since freely
falling objects are essentially weightless,
the successful drop demonstrates the
ability to monitor quantum objects in
near-zero gravity. That ability may lead
to a deeper understanding of heavy topics
such as general relativity, an interna-
tional team reports in the June 18 Science.
The new study is “an impressive technological advance,” says MIT physicist
Wolfgang Ketterle, who shared a Nobel
Prize in 2001 for creating Bose-Einstein
condensates, or BECs, in the lab.
Quantum physics
takes flying leap
One challenge was miniaturizing the
jungle of complex equipment usually
needed to create a BEC. “These guys fit
the equivalent equipment into a 60-by-
60 centimeter by 2-meter capsule and
sent it down 120 meters and it smashed
By Laura Sanders
In an experiment that puts the good old-fashioned egg drop to shame, European
physicists dropped a small blob of ultracold atoms 120 meters down a shaft. The
result: no yolk on their faces.
The researchers created a cloud of
about 10,000 ultracold rubidium atoms
that fused into a quirky quantum object
called a Bose-Einstein condensate, then
dropped the stuff off a needle-shaped
at the bottom,” says physicist Paulo
Nussenzveig of the University of São
Paulo. “It’s really, really amazing.”
A camera caught the BEC expand-
ing before the capsule crashed into an
8-meter-deep pit of plastic balls. The
atoms’ behavior in near-weightlessness
largely agreed with theoretical predic-
tions, although tiny stray magnetic per-
turbations caused the BEC to expand a
little less than predicted, the team found.
