Neutrons Reveal Quantum Order in Solid-State System

It's a truism that the world looks very different through the eyes of quantum mechanics. In a new paper appearing in Science, an international team of scientists working at the National Institute of Standards and Technology's Center for Neutron Research (NCNR) and the Rutherford Appleton Laboratory in Britain have detailed a particular striking example—a system that looks completely random to classical physics but that at the quantum level displays a hidden, relatively long-range, coherence. Their work demonstrates a unique solid-state device—basically a row of nickel atoms in a ceramic crystal—that might form the basis for a key element in a future quantum computer.

Classical magnetism is a function of the orientation of electron "spins" in a material that has a discrete set of quantum states. If the spins in a material tend to line up in the same direction, it's ferromagnetic, and if they tend to alternate, cancelling out any magnetic field, then it's antiferromagnetic. These new experiments studied magnetism in linear chains of up to 100 nickel atoms that, because of their arrangement in the ceramic, have the curious property of forming a "spin fluid" where the spin directions remain in constant flux rather than assuming a fixed configuration as in a ferromagnet or an antiferromagnet.

It's seemingly a completely disorganized system, but, according to team member Collin Broholm, a physics professor at the Johns Hopkins' Krieger School of Arts and Sciences, the chain has "a beautiful, underlying quantum order." Although the spin directions may be random, the underlying wave functions that describe the quantum mechanical nature of the atoms are coherent—perfectly synchronized over the span of almost 100 atoms and 30 nanometers. In the quantum mechanical world, that's a significant distance for coherence, hitherto seen only in exotic materials such as superconductors, superfluids and Bose-Einstein condensates.

The team was able to demonstrate this by pinging the chains with neutrons that induce small packets of magnetic excitation to propagate along the coherent segment—which can be measured indirectly by observing the scattered neutrons. They also demonstrated that they could limit the coherent region by introducing defects in the chain, either by adding chemical impurities or raising the temperature to induce thermal breaks (the experiments were conducted from 10 degrees to 120 degrees above absolute zero). Their results show that relatively large-scale regions in a solid-state material can be placed in a coherent quantum magnetic state, which might function as a "q-bit" (quantum bit) in a quantum computer.

The research team included scientists from Johns Hopkins, the Department of Energy's Brookhaven National Laboratory, the NIST Center for Neutron Research, Dartmouth College, University College London, Louisiana State University, the Rutherford Appleton Laboratory, Japan's National Institute of Advanced Industrial Science and Technology, and the University of Tokyo.

The work was funded by the Office of Basic Energy Sciences within the U.S. Department of Energy's Office of Science, the National Science Foundation, the Wolfson-Royal Society (U.K.), and by the Basic Technologies program of the U.K. Research Councils. For additional details on this work, see "Discovery of 'Hidden' Quantum Order Improves Prospects for Quantum Super Computers" www.jhu.edu/news_info/news/home07/jul07/quantum.html.

G. Xu, C. Broholm, Y.-A. Soh, G. Aeppli, J. F. DiTusa, Y. Chen, M. Kenzelmann, C. D. Frost, T. Ito, K. Oka and H. Takagi. Mesoscopic phase coherence in a quantum spin fluid. ScienceExpress. Published on-line 26 July 2007 (10.1126/science.1143831).