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 secretsharing 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 100digit 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 pseudorandom 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' pseudorandom 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 largescale 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 threequbit 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 carbon13 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 pseudorandom operators, said Lidar.
The researchers are working on making the randomnumbergeneration
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 noiseestimation 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 CambridgeMIT Institute.
Timeline: 2 years, 1020 years
Funding: Government; University
TRN Categories: Quantum Computing and Communications; Physics
Story Type: News
Related Elements: Technical paper, "PseudoRandom Unitary
Operators for Quantum Information Processing," Science, December 19, 2003
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January 14/21, 2004
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