Quantum code splits secrets

By Eric Smalley, 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 cryptographic protocols provide 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 of entanglement.

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 a zero.

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 said.

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.

Timeline:   Unknown
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


October 10, 2001

Page One

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Mobile radios make intranet

Quantum code splits secrets

Computer tells convincing story

Virtual beings boost evolutionary theory


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