Los Alamos Gets Closer To Quantum Computing

"Based on these recent experiments and theoretical work, it appears the barriers to constructing a working quantum computer will be technical, rather than fundamental to the laws of physics," said Richard Hughes of Los Alamos' Neutron Science and Technology Group.

Hughes also said that a quantum computer like the one Los Alamos is building, in which single ionized atoms act like a computer memory, could be capable of performing small computations within three years.

A quantum computer would be much faster at certain computations than traditional computers because it uses the quantum properties of particles such as photons or ions to represent the zeroes and ones that traditional computers employ as binary digits, or bits. These ions, trapped by electromagnetic fields, are termed qubits, short for quantum bits.

A quantum computer won't replace desktop personal computers or even supercomputers, but scientists predict that it will be much better than present-day computers at specialized tasks, such as factoring huge numbers or searching for specific bits of information in massive data bases.

"Quantum computing could fundamentally change the way we protect sensitive information," Hughes said.

Public key cryptography, in which sender and receiver share a "key" for breaking a large number into its factors, is now widely used to provide security for government and industrial information. This cryptographic method has been used since the late 1970s to protect secure electronic messages and telephone communications, but its security depends on the length of time it takes to factor the large numbers used as cryptographic keys.

A quantum computer theoretically could factor such a key number in a few seconds, instead of the many years required by traditional computers. In addition, spies using a quantum computer could reach back into the past and crack any secrets that had been encrypted by the public key method. In conjunction with its research on quantum computing, Los Alamos is developing a new encryption technique known as quantum cryptography in anticipation of the day that public key encryption becomes obsolete.

Los Alamos' quantum computer at this early stage consists of a string of up to eight trapped calcium ions, or qubits, and prototype optical switches that allow researchers to direct a pulsed laser beam onto individual ions and switch the beam from ion to ion.

The Los Alamos team uses lasers to cool the calcium ions to a state of rest in a so-called ion trap. A titanium-sapphire laser will be used to apply light pulses to the ions to perform logic gate operations. These logic gates are the elementary operations at the basis of all computer calculations.

Traditional computers combine individual digital bits in operations such as "and" or "or." Quantum computers can perform other operations not possible with traditional computers and perform reversible versions of ordinary operations. Theoretically, this gives quantum computers the capability of simultaneously performing exponentially larger numbers of operations than traditional computers, Hughes said.

Using the laser, the Los Alamos experiment successfully demonstrated optical addressing, an essential step that a quantum computer will have to execute in actually performing a calculation, Hughes said.

"What we've done is show that we can apply a single laser pulse to a single ion in a trap," Hughes said. "We've done this with very low cross-talk to the locations of the neighboring ions, which can be as close as 20 microns away. This is an essential requirement to perform logic gates on the qubits."

For example, to set up a logic gate involving two qubits will require a sequence of five laser pulses.

The team has been working on its prototype quantum computer for more than two years while other Los Alamos researchers have been trying to solve some of the fundamental theoretical questions that are obstacles to an operating quantum computer. At the APS meeting, Hughes discussed their preliminary answers.

"One question that Los Alamos physicists have solved is crucial to development of a quantum computer. Ions will only remain in a coherent quantum state for a short time. Some scientists have said trapped ions would lose their coherence too quickly to be used practically for computing. We have analyzed how many computational operations can be performed, or in other words, how many of these quantum logic gates can you apply to a calculation?" Hughes said.

The answer is that theoretically as many as 100,000 logic operations could be applied to registers that consist of up to 50 qubits.

Another key question is how long it would take a quantum gate to complete a single operation. The answer, Hughes said, is a few microseconds.

"These experimental and theoretical results clear the path to let us start exploring quantum computing," Hughes said. "We don't expect to start practical computing for years, but we think we're on the right track."

Los Alamos National Laboratory is operated by the University of California for the U.S. Department of Energy.