code splits secrets
Technology Research News
IBM researchers have shown that tapping
the weird quantum properties of particles like atoms and photons would
improve on a classic technique that allows a group of people to hold pieces
of a secret that can only be revealed by combining the pieces.
When a secret is too important for any one person to know, secret-sharing
a way to break up the secret into parts held by several or even many people.
The protocols keep the secret until all or most of the parts are assembled.
Adding a quantum component to this scheme would make it harder for the
people holding the pieces to cheat or be coerced into revealing the secret.
The IBM scheme is a step in that direction. "We haven't done anything
so sophisticated in the quantum version" as splitting a secret into many
parts, said David P. DiVincenzo, a physicist at IBM Research. "We've just
been investigating the simple case of splitting a secret into two."
The quantum secret-sharing scheme is similar to quantum cryptography and
quantum computing because it relies on the quantum mechanical condition
Particles like atoms are usually either spin up or spin down, meaning
that the axes they spin around point either up or down relative to the
magnetic field around the atoms. But when atoms or other particles are
isolated from the environment and cannot be observed, they enter the quantum
mechanical state of superposition, which means they are in some mixture
of both spin up and spin down.
Two or more particles in superposition can be entangled so that even if
they are separated, when one of them is measured and becomes either spin
up or spin down the other particle immediately leaves superposition and
assumes the same spin regardless of the distance between them.
There are four possible combinations of spins for a pair of entangled
particles. One combination, called a singlet, stands out from the other
three, which are called triplets.
The quantum secret-sharing scheme represents a bit of information by creating
a string of entangled pairs of particles. An odd number of singlets in
the string represents a one, and an even number of singlets represents
Because the two particles have to be together in order to tell whether
they form a singlet or a triplet, two people sharing a secret this way
couldn't simply measure their halves of the string and compare notes to
tell whether the bit is a one or a zero. This makes quantum versions of
secret-sharing protocols more secure than classical versions.
"If the parts of the secret are actually pieces of a quantum state, then
even communication -- at least communication of the ordinary, classical
sort -- can be insufficient for them to reconstruct the secret," said
DiVincenzo. "They need to do something stronger. They need some kind of
additional quantum technology in order to unlock the secret," he said.
The needed quantum technology could be a quantum communications channel.
If the polarization of photons were used rather than the spin of atoms,
the photons could be transmitted while preserving their quantum states.
In order to carry out the scheme, however, there must be a way to store
the quantum states of particles for long periods of time.
"This scheme is not something that can be realized in the immediate future,
except as a demonstration," said Daniel Gottesman, a fellow at the Clay
Mathematics Institute and a visiting scholar at the University of California
at Berkeley. "You need to store the quantum states until it comes time
to open the secret, and it will be a while until we can do that reliably."
Quantum secret sharing "would require a good quantum memory and the ability
to measure qubits. Some of the rudiments of what are needed in this scheme
are available today," said DiVincenzo.
Practical quantum secret sharing will also require the development of
quantum repeaters in order to send quantum information over distances
greater than the roughly 10 kilometers possible today. Repeaters boost
signals traveling along communications lines.
Quantum repeaters could be developed in about six years but quantum memory
will probably take longer, said DiVincenzo. "That gets into the cloudy
future," he said.
It also remains to be seen whether the added property of requiring quantum
communications makes for a more useful form of secret sharing, Gottesman
It should be possible to make a practical form of the quantum secret-sharing
scheme before large-scale quantum computers can be built, said DiVincenzo.
Large-scale quantum computers are probably more than 20 years away, according
to many researchers.
DiVincenzo's research colleagues were Barbara M. Terhal and Debbie W.
Leung of IBM Research. They published the research in the June 18, 2001
issue of the journal Physical Review Letters. The research was funded
by the National Security Agency (NSA), the Army Research Office and IBM.
Funding: Government; Corporate
TRN Categories: Cryptography and Security; Quantum Computing
Story Type: News
Related Elements: Technical paper, "Hiding Bits in Bell
States," Physical Review Letters, June 18, 2001
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