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(AFP/File,
Paul J. Richards)
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Paris —
Quantum computing is getting the headlines these days, with buzz among
scientists of giga-powered number-crunching and unbreakable encryption.
The US
National Security Agency (NSA) is reportedly advancing towards a quantum
computer that could crack almost any conventional algorithm.
The NSA
plans were leaked by contractor Edward Snowden and reported by The Washington
Post on Thursday.
Details of
its work remain sketchy, though. And the agency is only one of many players,
both public and corporate, in a field that must overcome many hurdles before it
can dethrone standard computing.
Conventional
computers work by processing binary code -- an information currency that exists
in one of two states, either zero or 1.
Quantum computers,
though, break free of the two-state constraints.
They
harness the principle of quantum mechanics, when strange things occur through
the state of an atom's spin, something called angular momentum.
In a
quantum state, the atom goes into a condition called superposition. It can hold
the value of zero or 1 or both values at the same time.
This
juggling trick holds out the possibility of parallel processing on a massive
scale.
An
algorithm that a conventional supercomputer might take years to break could be
cracked by so-called qubits, or quantum bits, in a fraction of the time.
"The
special properties of qubits will allow quantum computers to work on millions
of computations at once," says IBM. "For example, a single 250-qubit
state contains more bits of information than there are atoms in the
Universe."
Daunting
engineering obstacles have to be overcome, though. In order to achieve the
fragile quantum state, a cloud of atoms has to be cooled to near-absolute zero
and controlled by pulses of laser.
Changes in
temperature, electromagnetic waves and minute defects in material can all wreck
the sought-after superposition that fuels the qubit.
Scaling up
these computers from hugely expensive, highly protective labs represents
"an enormous practical challenge," the Nobel jury said in 2012, when
it awarded that year's physics prize for fundamental work on the quantum state.
Quantum's
other big plus is a phenomenon called entanglement.
Particles
created in a quantum state behave like psychic twins.
Even if
they are far apart, a disturbance to one particle affects the other, a
phenomenon that Einstein once called "spooky action at a distance."
Thus if a
message sent in a quantum state is intercepted en route, the entanglement is
destroyed -- and alarm bells ring that someone is eavesdropping.
Achieving
quantum cryptography
Entanglement
is the big goal of quantum cryptography.
It holds
out the possibility of creating a unique, one-time code shared only by sender
and recipient that would be almost impossible to decrypt by an outsider. Better
still, the message could not even be touched during transmission.
Even
without entanglement, though, the quantum state can be useful in cryptography,
said Philippe Grangier, a specialist in quantum optics at France's National
Centre for Scientific Research (CNRS).
His team
has done tests that sends a standard-encrypted message, along with a
quantum-encrypted key, in squirts of light down a fibre-optic cable.
Once
received, the key is then used to decode the message.
The
technique uses the quantum signature in the key as a burglar alarm, Grangier
said in a phone interview.
"Just
the slightest interception of the data will reduce the size of the quantum key
when it gets to the recipient, and the spy gets detected," said Grangier.
"The
more the spy perseveres, the smaller the key becomes. Eventually, the connection
is cut."
Their
greatest length for transmission has been through a cable 80 kilometres (50
miles) long -- a distance that is useful for local communications but still way
too short for transcontinental use or more.
Going
beyond this distance lies the conundrum of how to amplify a weakening light
signal down a cable so that the data is repeated but does not lose its quantum
state through interference.
Other
techniques aim at overcoming the "repeater" problem by line of sight
laser transmission, in theory to satellites in near-Earth orbit.
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