Magnetic resonance goes nano

The magnetic resonance imaging devices that hospitals use to diagnose illnesses provide detailed pictures of the insides of the human body by measuring the unique responses of the atoms and molecules in specific types of tissue to particular sequences of radio waves and magnetic pulses.

The technology also gives scientists a way to control the spins, or magnetic orientations of atoms; this ability has led to several prototype quantum computers. Although nuclear magnetic resonance quantum computer prototypes have been among the most advanced quantum devices built, such systems are generally limited to about 10 quantum bits, which is well short of the thousands needed for practical systems.

Researchers from NTT Basic Research Labs in Japan and the Japan Science and Technology Agency have built a nuclear magnetic resonance device that has the potential to overcome the limit because it is small enough to fit on a computer chip. It could also be tapped to allow nuclear magnetic resonance devices used in chemistry, biology and medicine to examine smaller samples, according to the researchers.

Quantum computers use properties like spin to represent the 1s and 0s of digital information. In theory, quantum computers would be able to solve certain types of very large problems, including those underpinning today's encryption technologies, many orders of magnitude faster than today's classical computers.

The researchers' device measures spin by measuring electrical resistance across a 200-by-200-nanometer area of semiconductor material rather than using a centimeter-scale coil to pick up radio waves. This allows it to control and measure a much smaller number of atomic spins and to control and measure six distinct types of spin.

The researchers' next step is to fabricate a quantum integrated circuit by connecting several nuclear magnetic resonance devices. Even without links to each other, the devices could be used as quantum memory, according to the researchers.

It will be 10 or 20 years before quantum computers that contain 100 to 10,000 qubits are ready for commercial use, according to the researchers. The work appeared in the April 21, 2005 issue of Nature (Controlled Multiple Quantum Coherences of Nuclear Spins in a Nanometer-Scale Device).