Uncontrollable outside influences, random fluctuations, and other disturbances could undermine the world of quantum computing. Now, researchers have turned to electron spin resonance (ESR) spectroscopy to help them home in on the sources of this decoherence in a solid-state system. They suggest that this takes the science one step toward closer to solving the problem.

Writing in the journal Nature this month, Achim Mueller and colleagues have, for the first time, monitored so-called Rabi oscillations in a vanadium-based molecular magnet. These oscillations represent the characteristic disturbances associated with coherent spin dynamics of the kind required to operate a fully functional, quantum bit, or qubit, device.

Quantum computation represents a paradigm shift away from the conventional bits and bytes of logical computation currently used in everything from the lowliest silicon chip in a washing machine to the supercomputers used as the hubs of Grid networks. Conventional computing handles bits of information. Fundamentally, pulses of electrical current, fire sequentially to carry out instructions converting an input into an output.

In contrast quantum computing exploits the phenomena of the quantum world to "calculate" all possible solutions to a problem simultaneously, through quantum entanglement, and then to produce the most probable answer based on the laws of quantum mechanics.

Computer scientists anticipate that quantum computing will allow us to solve problems, such as weather and climate prediction, fluid mechanics modelling, and the notorious travelling sales rep problem of network efficiency. Today, such colossal problems are essentially intractable even with a fleet of supercomputers working in parallel. Quantum computation could also hold the key to breaking even the strongest encrypted code built to date but also provide cryptographers with the tools to create an even stronger code still.

The fundamental building block of a quantum computer is the qubit. It is analogous to the conventional bit of 1's and 0's but its properties hinge on energy states of molecules or other entities in which spin states and charge are harnessed. In this context, spin is a quantum phenomenon that follows probabilistic rules rather than simply being on or off (1 or 0), the undisturbed wavefunction of such a spin system is key to its functionality. Mueller explains that there are many obstacles in the way of the development of a fully fledged quantum computer that exploits such properties.

"The basic problem for the development of the quantum computer is the so-called decoherence, essentially forgetfulness," he says. "The challenge lies in exploring the reasons for this 'forgetfulness' and finding ways to remedy the problem." If the qubit's surroundings interfere with its wavefunction, then decoherence occurs, the wavefunction undergoes "collapse" and the information-encoded in the system is lost.

In a milestone research paper, Mueller and his colleagues report new experimental results for a molecular magnet containing vanadium and oxygen atoms, which they explain can act as a carrier of quantum information. The research was carried out in Beer-Sheva, Grenoble, France, and Bielefeld, Germany with group leaders Boris Tsukerblat, Bernard Barbara, and Bielefeld's Mueller. The study builds on many years work in Mueller's laboratory constructing increasingly sophisticated organometallic complexes. Such entities have been discussed widely as having the potential to act as the active components in a quantum information system.

The molecule discussed in the present study comprises 15 vanadium atoms (in the IV oxidation state), attendant oxygens and organic ligands. It is more than one nanometre in diameter and has an electronic spin structure in which each of the vanadium atoms, with their net spin 1/2, couple strongly into three groups of five. Depending on how the spins are aligned, the whole molecule can have an overall net spin of 1/2 or 3/2.

With all this electronic spin, the obvious analytical tool with which to study such a system is ESR. The researchers used this technique to observe the degree of coherence possible with the vanadium molecule. They found that the prime source of decoherence is the ever-present nuclear spins associated with the fifteen vanadium nuclei as well as a much smaller contribution from attendant hydrogen atoms, protons, in the structure.

Philip Stamp of the Pacific Institute for Theoretical Physics at the University of British Columbia, Vancouver, Canada, writing in the same issue of Nature, suggests that, we pressingly need to understand what causes wavefunction collapse, how it works and how to get rid of it.

Mueller and colleagues have passed a milestone on the road to a quantum computer, he adds, having succeeded in pinpointing the sources of decoherence in their system, and so taken the first step towards eliminating them. The identification of nuclear spin as a serious decoherence issue hints at the possibility of using zero-spin isotopes in qubit materials.

"The control of complex coherent spin states of molecular magnets, in which exchange interactions can be tuned by well defined chemical changes of the metal cluster ligand spheres, could finally lead to a way to avoid the 'roadblock' of decoherence," Mueller and colleagues conclude.

Related links:

• Nature, 2008, 453, 203-206
• Mueller Page

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.