Safe havens offer practical quantum processing

By Eric Smalley, Technology Research News

One of the biggest obstacles to practical quantum computers is that the strange quantum states that make the computers possible are quite fragile. The slightest influence from the environment can cause quantum processors to break down.

Researchers at Los Alamos National Laboratory have taken a step toward making quantum computers more robust by identifying sections of quantum processors that are immune to environmental noise. Decoherence-Free Subspaces (DFSs), which were predicted by theory, can serve as safe zones where quantum computing can take place.

DFSs may allow future quantum computer designers to avoid difficult-to-implement Quantum Error Correction Codes that would otherwise be required to handle the effects of environmental noise.

The researchers' DFS experiment used a pair of entangled photons. When two or more atoms or subatomic particles are entangled, a change to one is immediately reflected by the same change in the other regardless of the physical distance between them.

The entangled photons form a rudimentary quantum bit, or qubit, which is the basic unit of quantum computing. Qubits have an exponential number of possible states. For example, four qubits have 24 or 16 states, and eight qubits have 28 or 256 states. This property means that a relatively small number of qubits can represent very large numbers, giving quantum computers the potential to be much more powerful than conventional computers.

The researchers subjected the entangled photons to environmental noise by sending them through a piece of quartz. Ironically, environmental noise, or decoherence, is necessary to create DFSs.

The key to DFSs is that the qubits in a quantum computer are all subjected to the same noise. In some cases when that happens, a subset of those possible states is protected from the noise. In theory, researchers could use that subset of states as a quantum processor.

"The concept of Decoherence-Free Subspaces is only useful if you have a system [in which] your qubits are subject to collective decoherence, where the same thing is effecting all of them," said Paul G. Kwiat, a research physicist at Los Alamos National Laboratory. "Each of the qubits individually is not immune to the noise, but in this combined state the noise factors out."

The researchers have developed a fine degree of control over the decoherence of photons, and controlling the decoherence process allowed them to engineer the DFS, said Kwiat.

The Los Alamos research is the first experimental verification of DFSs, said Daniel Lidar, an assistant professor of chemistry at the University of Toronto. "It's the first step towards implementing this technique on a larger scale and helping in avoiding this vexing problem of decoherence in quantum computers."

However, the experiment using photons was a simple proof of principal. "What we demonstrated was the most rudimentary [DFS] possible," Kwiat said.

The experiment demonstrated a single Decoherence-Free State. The next major step would be demonstrating two Decoherence-Free States, which would constitute a Decoherence-Free qubit, said Lidar. That would require starting with at least three qubits, he said.

Other researchers are working on demonstrating DFSs using other, more scalable techniques. A team at MIT has demonstrated a DFS using a two-qubit Nuclear Magnetic Resonance system, said Lidar, though that work has not been published. Though few researchers expect Nuclear Magnetic Resonance systems will scale up to be useful computers, they have yielded seven-qubit processors, which could be the basis for experiments demonstrating larger DFSs, he said.

The Los Alamos researchers are planning to work with larger numbers of photonic qubits to create larger DFSs, Kwiat said. In general, researchers should be able to demonstrate simple algorithms operating within DFSs within five years, he said.

Kwiat's colleagues were Andrew J. Berglund and Joseph B. Altepeter of Los Alamos National Laboratory, and Andrew G. White of the University of Queensland in Australia. They published their work in the October 20, 2000 issue of the journal Science. The research was funded by the National Security Agency.

Timeline:   <5 years
Funding:   Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:   Technical paper "Experimental Verification of Decoherence-Free Subspaces," Science, October 20, 2000


December 6, 2000

Page One

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Safe havens offer practical quantum processing

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