July 20, 2011 Researchers have made a major advance in predicting and quashing environmental decoherence, a phenomenon that has proven to be one of the most formidable obstacles standing in the way of quantum computing.
The findings -- based on theoretical work conducted at the University of British Columbia and confirmed by experiments at the University of California Santa Barbara -- are published online in the July 20 issue of the journal Nature.
Quantum mechanics states that matter can be in more than one physical state at the same time -- like a coin simultaneously showing heads and tails. In small objects like electrons, physicists have had success in observing and controlling these simultaneous states, called "state superposition."
Larger, more complex physical systems appear to be in one consistent physical state because they interact and "entangle" with other objects in their environment. This entanglement makes these complex objects "decay" into a single state -- a process called decoherence.
Quantum computing's potential to be exponentially faster and more powerful than any conventional computer technology depends on switches that are capable of state superposition -- that is, being in the "on" and "off" positions at the same time. Until now, all efforts to achieve such superposition with many molecules at once were blocked by decoherence.
"For the first time we've been able to predict and control all the environmental decoherence mechanisms in a very complex system, in this case a large magnetic molecule called the 'Iron-8 molecule,'" said Phil Stamp, UBC professor of physics and astronomy and director of the Pacific Institute of Theoretical Physics. "Our theory also predicted that we could suppress the decoherence, and push the decoherence rate in the experiment to levels far below the threshold necessary for quantum information processing, by applying high magnetic fields."
In the experiment, the California researchers prepared a crystalline array of Iron-8 molecules in a quantum superposition, where the net magnetization of each molecule was simultaneously oriented up and down. The decay of this superposition by decoherence was then observed in time -- and the decay was spectacularly slow, behaving exactly as the UBC researchers predicted.
"Magnetic molecules now suddenly appear to have serious potential as candidates for quantum computing hardware," said Susumu Takahashi, assistant professor of chemistry and physics at the University of Southern California. "This opens up a whole new area of experimental investigation with sizeable potential in applications, as well as for fundamental work."
Takahashi conducted the experiments while at UC Santa Barbara and analyzed the data while at UC Santa Barbara and the University of Southern California.
"Decoherence helps bridge the quantum universe of the atom and the classical universe of the everyday objects we interact with," Stamp said. "Our ability to understand everything from the atom to the Big Bang depends on understanding decoherence, and advances in quantum computing depend on our ability to control it."
The research was supported by the Pacific Institute of Theoretical Physics at UBC, the Natural Sciences and Engineering Research Council of Canada, the Canadian Institute for Advanced Research, the Keck Foundation, and the National Science Foundation.
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- S. Takahashi, I. S. Tupitsyn, J. van Tol, C. C. Beedle, D. N. Hendrickson, P. C. E. Stamp. Decoherence in crystals of quantum molecular magnets. Nature, 2011; DOI: 10.1038/nature10314
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