would boost quantum messages
Technology Research News
Quantum physics makes it possible to send
perfectly secure messages, and researchers have already achieved quantum
cryptography in the laboratory.
The main stumbling block to using quantum cryptography
in practical systems, however, is figuring out how to send the fragile
quantum states of light used in the schemes over long distances. "At the
moment, quantum cryptography is restricted to several tens of kilometers,"
said Ignacio Cirac, a professor of physics at the University of Innsbruck
Cirac and several colleagues have found a way to boost quantum signals
that could help make quantum cryptography practical within a decade.
Signals, whether optical or electrical, fade as they travel down communications
lines. Messages wouldn't get very far if it weren't for repeaters, which
are simple devices that receive a weakening optical or electrical pulse
and send out a stronger pulse.
Ordinary repeaters, however, don't work with quantum communications. This
is because quantum signals contain photons that are in the weird quantum
mechanical condition of superposition. This means the photons are in some
unknown mix of all possible states. For example, a photon is both vertically
and horizontally polarized when it is in superposition, and so could come
out of superposition horizontally or vertically polarized.
When a photon is observed or otherwise comes into contact with its environment,
it is knocked out of superposition and can no longer be used for quantum
communications. The trouble with ordinary optical repeaters is they have
to observe photons in order to copy them.
To get around this problem, the researchers have proposed a way of storing
quantum information in small clouds of atoms and forwarding the information
from one atom cloud to another using photons. The device would transfer
the weakened quantum information carried by inbound photons to the atoms,
correct any errors in it, and then transfer it to outbound photons to
produce a stronger signal. This would take place without disturbing the
quantum state of the information.
"We have found a way of building quantum repeaters using [sets of atoms],"
said Cirac. "A set of several thousands or millions of atoms are used
to store quantum information in a given location, correct it, and send
it to the next set of atoms."
Other proposals for building quantum repeaters call for transferring quantum
information between individual atoms and photons, which is difficult to
do, said Cirac. The researchers' scheme has several advantages over these
proposals because "we do not have to isolate atoms, no low temperature
is required, and quantum gates are not required either," he said. Quantum
logic gates take the quantum states of particles through a series of changes
in order to perform simple mathematical calculations. This is difficult
to do even in carefully controlled laboratory environments.
The researchers' proposal quantum-mechanically links, or entangles, two
distant containers of gas atoms. When two or more photons are entangled,
one or more of their properties stay in lockstep while the particles are
in superposition. For example, researchers can entangle two photons so
that when one of the photons is knocked out of superposition and becomes,
for instance, horizontally polarized, the other photon also leaves superposition
and becomes horizontally polarized at the same instant, regardless of
the physical distance between them.
The work is an improvement over other schemes because it uses large numbers
of atoms to store the information light carries in quantum communications,
said Emanuel Knill, a mathematician at Los Alamos National Laboratory.
Other researchers are beginning to conduct experiments that demonstrate
the advantages of using these groups of atoms in quantum information processing,
One advantage of the researchers' proposal is that most of the errors
this scheme is likely to generate yield no photons, said Knill. In quantum
communications, there are two types of errors: photons appearing when
none are called for and an absence of photons when they are expected.
"Some of their suggested applications intrinsically reject errors, which
only results in a relatively mild -- though not negligible -- loss in
efficiency over distance," he said.
The experimental setup needed to implement the proposal is similar to
the one recently used by researchers at the University of Aarhus in Denmark
to demonstrate entanglement between two samples of gas atoms, said Cirac.
"As soon as quantum cryptography is used in practical applications --
this may happen in five to ten years -- quantum repeaters will be needed
to extend the distances," said Cirac. "Our proposal can then play a...
Cirac's research colleagues were Lu-Ming Duan of the University of Innsbruck
and the University of Science and Technology of China, Mikhail D. Lukin
of Harvard University and Peter Zoller of the University of Innsbruck.
They published the research in the November 22, 2001 issue of the journal
Nature. The research was funded by the Austrian Science Foundation, the
European Union (EU), the European Science Foundation, the National Science
Foundation (NSF) and the Chinese Science Foundation.
Timeline: 5-10 years
TRN Categories: Quantum Computing; Cryptography and Security
Story Type: News
Related Elements: Technical paper, "Long-distance quantum
communication with atomic ensembles and linear optics," Nature, November
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