By Kimberly Patch, Technology Research News
All hydrogen atoms are not necessarily alike -- they can contain different amounts of energy, which gives them different spins. The spin of a particle is like a top turning either clockwise or counterclockwise.
As a result, identical molecules that contain the same atoms in the same order can still be distinguishable because their atoms can have different spins.
Researchers from the University of Oklahoma have found a way use the spins of 19 hydrogen atoms contained in a liquid crystal molecule to briefly store and read 1,024 bits representing a 32-by-32-pixel black and white pattern, a method they have termed "molecular photography".
The researchers had previously used liquid crystal molecules to briefly store, then read out the sentence "the quick brown fox jumps over the lazy dog," according to Bing M. Fung, a professor of chemistry at the University of Oklahoma. Each hydrogen atom, with its two spin states, can represent one bit of information.
In theory, 19 hydrogen atoms with two spin states each can store nearly 219, or about half a million, bits of information, said Fung. A group of five bits can represent 25, or 32 bits of information, which is enough to distinguish among the letters of the alphabet.
The researchers used nuclear magnetic resonance, or radio waves -- the same process that underlies medical magnetic resonance imaging (MRI) technology-- to change the spin states of the hydrogen atoms contained in a sample of trillions of molecules of liquid crystal. Nuclear magnetic resonance machines use radio waves and magnetic fields to image substances like soft tissue. The bits representing the image were coded into a single electromagnetic pulse that contained 1,024 distinct frequencies near the 400 megahertz, or million-cycle-per-second frequency of FM radio.
The researchers put the sample in a nuclear magnetic resonance spectrometer, which provided a very strong magnetic field. "In a high magnetic field, the hydrogen nuclei can absorb radio frequency energy," said Fung. "Different spin states can absorb tiny differences in energy even though they are in the same molecule, he said.
When the researchers stopped the pulse, the spins that absorbed energy released it, and the spectrometer picked up the released energy, giving the researchers a picture of the frequencies contained in, and thus the information coded in, the pulse, said Fung.
The researchers were able to store as many as 1,024 frequencies in the 19 hydrogen atoms, but the resolution of the spectograph is best when there are fewer than about 300 frequencies, said Fung. When there were more than 300 frequencies, the researchers had to read the pattern one row at the time. "The peaks overlap so we had to break them down into two dimensions," he said.
The researchers do not know exactly how the hydrogen atoms absorbing and storing energy in spin states, said Fung. "I think each of the frequencies [affects] more than one spin state. Maybe a bunch of spin states absorb one frequency, another bunch of spin states absorb another," he said.
The technique may eventually make it easier to store and process information within molecules, said Fung. "We're developing some fundamental concepts and techniques [toward using] molecules to store and process information in a practical way, but I cannot tell you right now what it will end up to be," he said.
There is an inherent advantage in storing and processing information in molecules, said Fung. "You want to go down to smaller and smaller sizes so that you can put a lot of information in a very small device," he said.
In addition, when a device becomes as small as a few dozen molecules, the physical rules that govern the material's behavior change, and there may be advantages to the new rules, Fung said. "When the device becomes smaller, the normal physics rules break down and the quantum behavior dominates," he said. "We have to study smaller systems to understand how they work."
Even though the researchers are storing information in the spins of individual atoms, the process is not quantum computing because the spins are not correlated, said Fung. The technique falls between classical and quantum information processing, he said. Quantum computer schemes use attributes like spins of particles to compute, but require entangled, or linked particles.
The research could have interesting applications, said Noel Clark, a physics professor at the University of Colorado. "I've never heard of anyone encoding this many bits in the spin-coupled system of a single molecule," he said.
Such large collections of coherently interacting spins may be useful if the method can eventually be used to store different spins in different molecules, he said. "It's not, in the form presented here, a potential method for bulk data storage [because] it probes many molecules in parallel, each of which are doing the same thing," he said.
It's hard to say when the method could be used practically, said Fung. "It's really, really hard to use molecules to store and process information. It won't be next year, but it won't be decades either," he said.
The researchers are currently trying to find solid materials that will store information in a similar way. "Liquid crystal [has] mechanical properties like a liquid so it cannot be easily miniaturized," said Fung. "So we're working on solids, but unfortunately solids can't store that much information," he said. The researchers are working with special types of solids that have more motion in order to find solids that can store more, he said.
Fung's research colleagues Anatoly K. Khitrin and Vladimir L. Ermakov. They published the research in the October 15, 2002 issue of the Journal of Chemical Physics. The research was funded by the National Science Foundation (NSF).