Quantum dice debut

By Eric Smalley, Technology Research News

Researchers have overcome a major obstacle to generating random numbers on quantum computers by limiting the possibilities in the otherwise unlimited randomness of a set of quantum particles.

Random numbers play a key role in classical computing by providing an element of chance in games and simulations, a reliable method for encrypting messages, and a means of accurately sampling huge amounts of data.

Researchers from the Massachusetts Institute of Technology and the National Atomic Energy Commission in Argentina have shown that short sequences of random operations -- randomly shifting laser pulses or magnetic fields -- acting on a string of quantum bits can, in effect, generate random configurations of qubits.

Being able to generate random numbers in quantum computing could make quantum computers easier to build by countering the noise that eventually destroys qubits, which represent the 1s and 0s of computer information. Quantum computers promise to be fantastically fast at solving certain types of large problems, including the mathematics that underpins today's security codes.

Quantum random numbers could also be useful for increasing the efficiency of quantum secret-sharing schemes, quantum encryption and various forms of quantum communications.

Qubits can represent not only 1 and 0 but any number in between; a string of 100 qubits can represent every possible 100-digit binary number, and a single set of operations can search every possible answer to a problem at once. This gives quantum computers their power, but also poses a problem for generating random numbers. The nearly infinite number of possible qubit configurations theoretically requires an impossibly large number of calculations.

In the quantum world, no outcome is certain, and in most aspects of quantum computing, the goal is to reduce the uncertainty in order to get a definite answer to a problem. The researchers' scheme, however, aims for uncertainty. It limits the possible outcomes without making them predictable.

The scheme generates quantum states in such a way that the probabilities of the limited set of outcomes are as evenly distributed over the nearly infinite range of possible outcomes as quantum theory allows, said Joseph Emerson, one of the MIT researchers who is now a fellow at the Perimeter Institute for Theoretical Physics in Canada. "These pseudo-random transformations are a practical substitute for truly... random transformations," he said.

The number of operations required to represent a truly random configuration increases exponentially with the number of qubits in the configuration. For example, if the quantum equivalent of generating random numbers takes 22, or four, operations for two qubits, 15 qubits would require 215, or 32,768, operations.

The researchers' pseudo-random number method could be used to help build quantum computers by providing a practical way to estimate imperfections or errors in quantum processors, said Emerson. "This is addressing a very big problem -- imperfections such as decoherence and inadequate control of the coherence between the qubits are the main limiting factors in the creation of large-scale quantum computers," he said.

A quantum particle decoheres, or is knocked out of its quantum state, when it interacts with energy from the environment in the form of light, heat, electricity or magnetism. Researchers are looking for ways to fend off decoherence for as long as possible in order to make qubits last long enough to be useful.

A way to estimate decoherence would allow researchers to assess the strength and type of environmental noise limiting the precision of a given quantum device, said Emerson. Random quantum operations can be used as control operations that, when subjected to the noise affecting a prototype quantum computer, will generate a response that depends only on the noise, he said. This way the noise can be characterized with many fewer measurements than existing methods, which are dependent on the interactions of the qubits and so require a number of measurements that increases exponentially with the number of qubits, he said.

In addition to helping build quantum computers, random operators would be useful for quantum communications tasks like encryption, said Emerson. "The idea is to randomize a specific configuration of qubits containing the message, and then transmit this randomized state," he said.

In this case, if each bit that makes up the message is encrypted, or changed randomly, it is not possible for an eavesdropper to find any type of pattern that may lead to cracking the message.

The researchers tested the method on a three-qubit prototype liquid nuclear magnetic resonance (NMR) quantum computer. The computer consists of a liquid sample containing the amino acid alanine, which is a molecule made of three carbon-13 atoms. The qubits are the atoms' spins, which are analogous to a top spinning clockwise or counterclockwise. The two directions, spin up and spin down, can be used to represent 1 and 0. The qubits are controlled by magnetic fields generated by the nuclear magnetic resonance device.

Being able to diagnose faulty quantum computer components in a way that is independent of the number of qubits is very important, said Daniel Lidar, an assistant professor of theoretical chemical physics at the University of Toronto. "For this reason alone I suspect random [operators] will find widespread applications as quantum computer benchmarking becomes an experimental reality," he said.

It is also likely that future quantum algorithms will make increasing use of pseudo-random operators, said Lidar.

The researchers are working on making the random-number-generation system more precise, said Emerson. "Right now one can only estimate very coarse properties of the noise, such as [its] overall strength," he said. "I would like to devise methods to get a much more detailed analysis of the noise operators."

Complete noise-estimation experiments could be implemented in rudimentary quantum computers within the next few years, said Emerson. Researchers generally agree that practical quantum computers are a decade or two away.

Emerson's research colleagues were Yaakov S. Weinstein, Marcos Saraceno, Seth Lloyd, and David G. Corey. The work appeared in the December 19, 2003 issue of Science. The research was funded by the National Science Foundation (NSF), the Defense Advanced Research Projects Agency (DARPA) and the Cambridge-MIT Institute.

Timeline:   2 years, 10-20 years
Funding:   Government; University
TRN Categories:  Quantum Computing and Communications; Physics
Story Type:   News
Related Elements:  Technical paper, "Pseudo-Random Unitary Operators for Quantum Information Processing," Science, December 19, 2003


January 14/21, 2004

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