Magnets could be the latest materials to be used in quantum information systems following the discovery of unusual spin effects in a fluorine-based compound. Tom Rosenbaum of the University of Chicago in the US and colleagues found that the spins of clusters of atoms in the magnetic compound became aligned – or coherent – when a magnetic field was applied, in contrast with the behaviour of similar materials. This coherence persisted for up to ten seconds, and the researchers say that the clusters could have information ‘imprinted’ on them (S Ghosh et al 2002 Science 296 2195).
Physicists have long known that lithium holmium fluoride is a ferromagnet, in which the atomic spins are permanently aligned even when there is no external magnetic field. But if the holmium ions are gradually replaced with yttrium ions, this ferromagnetism is suppressed and eventually disappears. This happens because the material becomes increasingly disordered, and eventually becomes a ‘spin glass’ in which the alignment of the spins is random.
But previous studies have shown that – in contrast with theory – the addition of further yttrium ions destroys this ‘glassy’ state, and that the material becomes progressively more ordered, especially if its temperature is reduced to near absolute zero. This unusual state is dubbed an ‘anti-glass’.
Rosenbaum and colleagues set out to investigate the magnetic properties of this anti-glass state by cooling a centimetre-sized crystal of lithium holmium yttrium fluoride to temperatures of just tens of millikelvins. Then they switched on an oscillating magnetic field and measured the magnetization of the sample for a range of oscillation frequencies, and at several temperatures.
In most disordered magnets, the magnetic susceptibility falls as the temperature drops. But Rosenbaum and colleagues found that the susceptibility of their crystal increased, indicating that the atomic spins had become more closely aligned, or coherent. The shape of the spectra also suggested that the spins in small clusters of atoms had aligned to form ‘oscillators’, which could take on either an ‘up’ or ’down’ collective spin.
These oscillators – each of which contained about 260 atoms – kept the same spin for up to ten seconds. The researchers think that this alignment arises because the oscillators can flip between the two possible spin states by ‘tunnelling’ through the potential barrier that separates them. This could be evidence for quantum behaviour because it cannot be explained by classical theories of magnetism.
Rosenbaum and co-workers now suggest that it could be possible to encode bits of information onto these two-state oscillators using a magnetic field, and that the states could be ‘entangled’ and used in quantum information processing.
