quantum crypto demoed
Technology Research News
Researchers at Northwestern University
are tapping the laws that govern subatomic matter and energy in order
to securely encrypt data and transmit it at 250 megabits per second over
Sending a secret message involves two steps: encrypting, or scrambling,
the message so that only the right person can read it, and making sure
that the key to unscramble the message gets to the intended recipient
and no one else.
Most quantum cryptography research has focused on the second step: finding
ways to securely exchange encryption keys, which are random strings of
numbers used to encrypt and decrypt messages.
The approach uses individual photons to represent each bit of a string
of random bits. Because individual photons cannot be observed without
altering their quantum states, an eavesdropper cannot look at a message
sent using single photons without tipping off the sender and receiver
to his presence. If the sender and receiver are sure that no one spied
on their bit string, they can use it as an encryption key.
The Northwestern researchers have addressed the first step with a way
to use quantum physics in the encryption process itself, with the premise
that the parties communicating have already exchanged an encryption key.
"No one has been addressing actual data encryption," said Prem Kumar,
a professor of electrical and computer engineering and physics at Northwestern
The researchers' technique uses the quantum noise present in ordinary
lasers. There is always a certain amount of randomness at the level of
atoms and subatomic particles because the quantum world is ruled by probabilities.
This randomness is known as quantum noise.
Quantum noise in lasers is caused by random fluctuations in the number
of photons generated over a given amount of time, said Kumar. For example,
if a laser produces 10,000 photons a nanosecond, or billionth of a second,
it would have a fluctuation proportional to the square root of 10,000,
which is 100, he said. "There will be a spread around the mean value of
10,000 and that spread would have a standard deviation of 100 photons,"
he said. "So sometimes I will have 10,100 photons, sometimes 10,090 photons."
The researchers' scheme mixes the message and the encryption key with
this random quantum noise to produce a pattern that appears to be random
to anyone without the encryption key.
The researchers demonstrated the technique by sending an encrypted message
at a speed of 250 megabits per second over 4 kilometers of optical fiber
in the laboratory and across a 2-kilometer fiber-optic line between two
Several research teams have used the single-photon quantum key distribution
method to demonstrate communications systems that are impervious to eavesdroppers.
But this method has major drawbacks: it is exceedingly slow and is limited
to short distances, said Kumar.
Because each bit of the key is used to encrypt one bit of the message,
and each message requires a new key, messages can be transmitted only
as fast as keys can be sent.
The light sources that generate single photons can only produce data at
about one kilobit, or thousand bits, per second, said Kumar. This translates
to about 40 words a second, or about five single-spaced pages of text
per minute. It would take about 17 minutes to transmit a print-quality
image that takes up about one megabit, or million bits, and about 70 hours
to transmit 250 megabits. "In communications networks we talk about tens
of gigabits per second," Kumar said. A gigabit is one billion bits.
And because photons can't be copied without changing their quantum states
and thus destroying the information they represent, quantum cryptographic
transmissions can't be sent through the repeaters used to boost signals
over longer distances in ordinary communications lines.
By sidestepping the issue of secure key exchange, the researchers were
able to send secure data using the faster, standard lasers used in optical
communications rather than devices that emit just one photon at a time,
The signals can also pass through amplifiers unharmed, and thus traverse
long distances, Kumar said. "The encrypted communications can theoretically
span thousands of kilometers, he said.
There are many efforts under way to study implementations of quantum cryptography
using common, practical equipment, rather than single-photon generators,
said Nicolas Gisin, a professor of applied physics at the University of
Geneva in Switzerland. Several research teams have been developing quantum
key distribution systems based on ordinary laser beams, he said. "We are
also working on this."
It is too early, however, to judge the impact of the work, Gisin said.
"It is not even clear whether these new schemes are really secure," he
added. Researchers have not been able to analyze how the new schemes would
handle the most powerful eavesdropping attacks, he said.
The Northwestern University researchers' claim for security against eavesdropping
has to be tested, said Anton Zeilinger, a professor of physics at the
University of Vienna in Austria. "If it is valid then this is a very significant
[development]," he said. "We are analyzing that claim in my group," he
The researchers next plan to boost the data rate of their system to 2.5
gigabits per second, and increase the distance it covers, said Kumar.
They also plan in the next year or two to tackle the problem of using
ordinary laser beams to distribute perfectly secure encryption keys, he
The researchers expect to demonstrate the feasibility of their data encryption
scheme in the next couple of years, and practical applications could be
possible in five years, said Kumar.
Kumar's research colleagues were Horace Yuen, Geraldo Barbosa, Eric Corndorf
and Chuang Liang. The research was funded by the Defense Advanced Research
Projects Agency (DARPA).
Timeline: 5 years
TRN Categories: Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Secure communications
using coherent states," scheduled to appear in the Proceedings of QCMC'02:
Quantum Communication, Measurement, and Computing; "Continuous Variable
Quantum Cryptography Using Coherent States," Grosshans and Grangier, Physical
Review Letters, February 4, 2002
27/December 4, 2002
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