photonic bonds strengthens security
Technology Research News
The virtually perfect security afforded
by quantum cryptography has been demonstrated in laboratories. But getting
from the laboratory to practical applications is especially difficult
for a technology that relies on the stranger aspects of quantum mechanics.
is based on manipulating and measuring the quantum states of individual
photons, and keeping any particle in its quantum state means isolating
it from its environment -- a difficult challenge in the real world of
fiber-optic cables and communications devices.
A team of researchers in Austria has adapted a 5-year-old scheme for boosting
quantum communications signals so they can be transmitted with relatively
simple equipment, which should make it possible to build devices for transmitting
secure information over long distances.
"For many protocols
of quantum communication... it is necessary that two points which communicate
with each other establish entanglement over long distances," said Anton
Zeilinger, a professor of physics at the University of Vienna.
Two particles can become entangled, or linked, when they are in the quantum
mechanical state of superposition, which is a mixture of all possible
quantum states. Quantum states include particle attributes like the electron
spin states spin up and spin down.
When one of the entangled particles is measured, the measurement knocks
it out of superposition into a random state and the other particle immediately
collapses into the same state, regardless of the physical distance between
Because it is impossible to observe a particle, such as a photon, in its
quantum state without altering that state, anyone who eavesdrops on a
message encoded in quantum particles will alter the message and reveal
the security breach. If one person sends a key to another using quantum
cryptography and they see that no one intercepted it, they can safely
use the key to encrypt their communications.
"The problem is that if we create a pair of entangled particles in a source,
the quality of the entanglement degrades with... distance," said Zeilinger.
The greater the distance, the greater the amount of noise in the system.
"This noise is caused by the interaction of the particles -- in our case
photons -- with the environment, in our case a glass fiber," he said.
Like other entanglement purification schemes, the University of Vienna
proposal starts with some number of partially entangled pairs of photons
and ends up with a smaller number of more highly entangled pairs. All
of the pairs have the same level of entanglement to begin with and the
various schemes eliminate some of the photons, which leaves the remaining
ones more highly entangled.
The University of Vienna scheme sends the photons through polarization
beam splitters, which filter photons based on their polarization. Photons
are particles of electromagnetic energy and they have an electric field
and a magnetic field. The electric field of a non polarized photon vibrates
in a plane perpendicular to the direction the photon is traveling in.
A polarized photon has an electric field that vibrates in only one direction
of the perpendicular plane.
A sender transmits one photon from each of two entangled pairs of photons
and keeps the other halves of the pairs. The sender and receiver then
put their photons through a polarization beam splitter, which has two
inputs and two outputs. If the photons have the same polarization, one
will exit from each output.
If both sender and receiver have one photon come through each output,
they both measure one photon from the same output channel of their polarization
beam splitters. If the measured photons match each other, they keep the
remaining photon pair, which is more highly entangled and therefore more
suitable for quantum communications.
The key to the University of Vienna scheme is that it is simple enough
to potentially be done outside a lab. Previous schemes have performed
more complicated manipulations that require correspondingly more complicated
Combined with quantum repeaters, the method should allow for unconditionally
secure quantum cryptographic key over arbitrary distances, said Zeilinger.
Quantum repeaters don't exist yet, but are theoretically possible. Classical
repeaters copy a weakened signal and transmit the copy, effectively boosting
the original signal. "But a classical repeater is worse than worthless
for quantum states," said Daniel Gottesman, a fellow of the Clay Mathematics
Institute working at the University of California at Berkeley. "Because
it looks at the state, it actually creates more noise," he said.
A quantum repeater would need to be able to regenerate the state without
looking at it. "That's where entanglement purification comes in," said
Gottesman. "It's a way to take a number of noisy quantum states and distill
out a few accurate ones. It's much harder than building a classical repeater,
but perhaps this [University of Vienna] work will bring it within reach.
In the context of quantum cryptography, that means you're back in business.
Less noise means no foothold for an eavesdropper," he said.
Developing quantum cryptographic systems is still a difficult task; the
researchers also have not addressed the key problem of how to store photons,
said Seth Loyd, an associate professor of mechanical engineering at the
Massachusetts Institute of Technology. "I think it's an excellent idea,
but I'm not sure that it really improves the prospects for implementing
quantum cryptography as much as [the researchers] claim," he said.
Practical quantum communication systems could be developed in 10 years,
Zeilinger's research colleagues were Jian-Wei Pan, Christoph Simon and
Caslav Brukner of the University of Vienna. They published the research
in the April 26, 2001 issue of the journal Nature. The research was funded
by the Austrian Science Foundation, the Austrian academy of sciences and
the European Union.
Timeline: 10 years
TRN Categories: Quantum Computing
Story Type: News
Related Elements: Technical paper, "Entanglement purification
for quantum communication," Nature, April 26, 2001
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