IBM发表STM脉冲技术可望造就原子级存储器

未来的终极存储器芯片可能是在个别原子内进行位编码；位于美国加州圣荷西的IBM阿尔玛登研究中心(Almaden Research Center)，最近发表一种扫描式穿隧显微镜(scanning tunneling microscopes，STM)应用的脉冲技术，能产生设计未来原子级存储器芯片、太阳能光电板以及量子计算机所需的、纳秒级的时间分辨率(time-resolution)。

STM是IBM在1980年代所发明，已经成为半导体材料领域的重要设备；一旦将其分辨率扩展至原子等级，就能将个别原子影像化。但遗憾的是，STM迄今尚未能达成如此精细的量测，而IBM新开发的脉冲式STM技术，则能以纳米级的精确度进行组件时间与距离的量测。

IBM所开发的泵浦探针(pump-probe)技术运作原理，与脉冲式激光类似。首先，将一个泵浦信号传递至STM尖端的材料中，使原子的电子自旋处于已知状态下；在一个等待期间之后，用一个较小的探针信号来进行量测。通过将以上程序重复，将每次脉冲的间隔时间延长数纳秒，就能精确量出电子自旋的弛豫时间(relaxation time)，或者是一个信息位被单一铁原子保留多长时间。

目前的DRAM单元必须在每50微秒(millisecond)左右的时间刷新(refreshed)位，利用新的脉冲式STM技术，IBM已经观察到单铁原子需要每250纳秒的刷新时间——也就是比现有时间快20万倍。

IBM研究中心的物理学家Andreas Heinrich表示，现在已经可以知道在单个铁原子中储存信息，会发生怎样的情况；在更远的未来，他们期望能以类似的程序，来揭开太阳能电池效率以及量子计算机的秘密。

点击进入参考原文：IBM characterizes single-atom DRAM, by R. Colin Johnson

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IBM characterizes single-atom DRAM

by R. Colin Johnson

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.

"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.