PORTLAND, Ore.—The ultimate memory chips of the future will encode bits on individual atoms, a capability recently demonstrated for iron atoms by IBM's Almaden Research Center in San Jose, Calif., which unveiled a new pulsed technique for scanning tunneling microscopes (STMs).
Pulsed-STMs yield nanosecond time-resolution, a requirement for designing the atomic-scale memory chips, solar panels and quantum computers of the future.
"My hope is that we can spawn a great following doing nanosecond time resolution and atomic-scale spatial resolution with their STMs," said Andreas Heinrich, a physicist in the IBM's Almaden Lab.
STMs, invented at IBM in the 1980s, have become the workhorse of the semiconductor materials industry. Their resolution extends all the way to the atomic scale, allowing individual atoms to be imaged. Unfortunately, STMs are slow at making such delicate measurements. Now IBM has perfected a new pulsed-STM technique that puts its ability to measure time on par with the nanoscale accuracy as its distance measurements.
IBM's pump-probe technique works in a manner similar to the way a pulsed laser works. First a pump signal is passed into the material from the STM tip to put the atom's electron spin in a known state, then after a waiting period a smaller probe signal is used to make a measurement. By repeating the process, each time extending the time between the pulses by a few nanoseconds, the process was able to accurately measure the electron spin relaxation time—or how long a bit of information is retained by a single iron atom.
Today's DRAM cells must have their bits refreshed every 50 milliseconds or so, but by using its new pulsed-STM technique, IBM has now observed that single iron atoms will need to be refreshed about every 250 nanoseconds—about 200,000 times faster.
Scanning tunneling microscope topograph of an iron atom (yellow) on a nitride-covered substrate (blue) which may someday enable single-atom bit-cells for memory ICs. Next to the iron are two more atoms and a missing atom defect in the nitride.
"We now know the answer to the question, 'What happens when you try to store information on a single iron atom?' And we hope that in the longer-term future we can make similar progress in answering questions about solar cell efficiency and quantum computers," said Heinrich.
The pulsed-STM technique will be adapted to measuring the efficiency of individual solar cells by using a light pulse as the pump to stimulate the solar cell, then probing it with the STM tip. Heinrich also hopes to reveal the inner workings of quantum computer gates using the pulsed-STM technique.
"If we can put quantum bits on surfaces so they have to interact with each other, then basically we will be showing a new way of performing quantum computations truly on the atomic scale. That's my vision of the future of quantum mechanics," said Heinrich.