On the way to the quantum computer
The scientists already know theoretically how to deal with them. For many researchers, the only question is whether and how the idea can be realized at all. Meanwhile, Los Alamos physicists are making experimental advances in the development of quantum computers, and in three years they aim to have the first humble quantum computer ready. Richard Hughes of Los Alamos National Laboratory's Neutron Science and Technology Group stated at the American Physical Society annual meeting in March 1998: "Judging from the most recent experimental and theoretical work, it appears that the obstacles to building a working quantum computer are more technical in nature and not so much based on fundamental physical problems." According to him, within the next three years a quantum computer, such as the one currently being built at Los Alamos, could perform small calculations.
The strengths and weaknesses of quantum computers
A quantum computer could perform certain arithmetic operations much faster than a conventional computer. It uses the quantum properties of particles such as photons or ions - the so-called qubits (from quantum bits) - to store information. In each qubit there is a superposition of particle states, so that arithmetic operations can be carried out simultaneously with a large number of starting values. However, this method limits the possible uses of quantum computers, which is why they will neither replace the usual desktop PCs nor conventional mainframes. However, they would be extremely well suited for special tasks such as factoring large numbers or searching for specific information in large databases.
A worthwhile application for quantum computers would be the encryption and decoding of messages. "Working with quantum computers could fundamentally change the way we protect confidential or classified information," said Hughes.
Public key cryptography has been in widespread use since the 1970s. A message encoded using this method remains secret as long as it is not possible to break down a perhaps 100-digit, publicly known number - the key - into its factors. While conventional computers need many years to do this, a quantum computer would theoretically do this work in a few seconds. The encryption method would then be worthless. To compensate, the team of scientists at Los Alamos are also developing a new encryption technique known as quantum cryptography. It is intended to be used as soon as the traditional encryption method is outdated.
The "quantum computer" in Los Alamos
At this early stage, Los Alamos quantum computers consist of a chain of up to eight "trapped" calcium ions -- known as qubits -- and prototype optical switches, which the researchers use to direct a laser beam at a single ion and switch the beam from Ion to Ion.
The team uses lasers to cool the calcium ions into a quiescent state in what is known as an ion trap. Flashes of light from a titanium-sapphire laser are directed at the ions to start logical operations that are the basis for computer-aided calculations of all kinds. The researchers have so far succeeded in experimentally establishing optical addressing. 'We have shown that you can hit exactly a single ion in an ion trap with a single laser pulse,' explained Hughes. "We were able to do this with only a slight effect on the neighboring ions, which only have to be 20 micrometers apart. This is an essential requirement to realize logic circuits with qubits."
Theoretical Questions of Time
While one team worked on its prototype quantum computer for over two years, other researchers have been trying to solve some of the fundamental theoretical questions that could prove obstacles to a working quantum computer. At the American Physical Society meeting, Hughes presented the working group's preliminary answers:
"One question Los Alamos physicists have solved is essential for the development of a quantum computer: ions remain in a coherent quantum state for only a short time. Some scientists suspect that trapped ions lose their coherence too quickly to be useful for practical calculations. We analyzed how many arithmetic operations can be carried out theoretically." The answer was that a quantum computer with a register of 50 qubits should be able to perform at least 100,000 logical operations without any problems. Each individual operation would only take a few microseconds.
"These experimental and theoretical results pave the way for research into quantum computing," said Hughes. "We don't expect any practical use in the coming years, but we think we're on the right track."
