Subatomic properties will remake computing

Imagine a data storage device the size of an atom, working at the speed of light. Imagine a microprocessor whose circuits could be changed on the fly. One minute, it would be optimized for database access, the next for transaction processing and the next for scientific number-crunching.

Finally, imagine a computer memory thousands of times denser and faster than today’s memories. And nonvolatile, so it retains its contents when the power is off.

All of these and more are on computing’s horizon, thanks to the exploding field of spintronics. Spintronics isn’t entirely new. The spintronic effect called giant magneto-resistance was introduced by IBM in 1997 in its GMR disk-read head. As a result, disk capacities have jumped by a factor of 100 in the past five years.

Electronic circuits are driven by electron flows, which have a charge that can be measured and controlled. But electrons not only flow; they also spin like tiny bar magnets. Depending on their orientation, the spins are said to be “up” or “down.” This additional variable, or “degree of freedom,” means that electrons can do more things and convey more information than they do in conventional electronics.

The most immediate research goal is to produce magnetic random-access memory (MRAM), which stores data using magnetism rather than electrical charges. Unlike the dynamic RAM in your PC, MRAM is nonvolatile.

IBM is working with Munich-based Infineon Technologies AG and says it will have MRAM in production as early as 2005. It will be 50 times faster than DRAM and 10 times denser than static RAM, and it could eventually replace both.

Others have even suggested that MRAM might replace disks for data storage. Putting logic and storage in a single chip would eliminate the slow disk I/O that’s a bottleneck in most computer processing.

IBM’s MRAM will use magnetic tunnel junctions, an application of spintronics in which electrons are allowed to “tunnel” between two ferromagnetic layers based on their spin. Each junction can store one bit. It promises a sort of universal RAM with very high performance —— high writing and reading speeds —— plus very high density and nonvolatility.

Further out, researchers are working on still more exotic applications of spin. David Awschalom, director of the Center for Spintronics and Quantum Computation at the University of California, Santa Barbara, is looking at what might be done with the spin of an atom’s nucleus, a new idea.

“The subatomic part of the atom would store the information, and the electron would act as the bus to carry information in and out of the nuclear subsystem,” Awschalom says.

He aims to build an optical-based information processor in which beams of light would transfer information to the nucleus through electrons. Such nuclear memories would be “many orders of magnitude” denser and faster than traditional semiconductor memories, he says.

Indeed, more broadly, the thrust of spintronics research will be to combine electronics and photonics with magnetism —— which traditionally involves metals —— in semiconductor materials. That will enable ultrafast and ultraefficient submicron devices that integrate computing, communications and storage. The slow interfaces between different materials that convert one kind of signal or property into another would be gone, and the latencies that typically slow the movement of data from one processing stage to another would be greatly reduced.

“You’d have everything integrated in a much simpler circuit,” says DARPA’s Wolf. “They would be much like existing semiconductor devices, except the current is spin-polarized.” That would enable, for example, the construction of very fast communication switches. “You could call it spin photonics,” he says. “They can easily operate at terahertz speeds.”

A semiconductor device can’t use spin until a way is found to get spin-polarized electrons into it, and that has proved difficult. But IBM recently demonstrated that it can use magnetic tunnel junctions to inject the current, as they do for MRAM.

IBM’s Parkin says spintronic semiconductors could be used to build reconfigurable logic devices. “So maybe your computer could be optimized for certain instructions by rearranging the way [logic] gates are connected, on the fly,” he says.

Another tough challenge has been to create magnetic semiconductors that sustain their spin states at room temperature, but physicists, materials scientists and engineers have made tremendous progress on that front just this year. The rapid development of spintronics seems likely to continue. The theory is in quite sound shape. There are many challenges, though.

亞原子特性將重構計算

想像一下數據存儲裝置只有原子大小、并以光速工作。再想像一下微處理器的電路能飛快地修改。只需一分鐘，就完成對數據庫訪問的優化，接下來對交易處理優化，接著再對科學計算優化。

最后再想像一下計算機的存儲器比今天的存儲器的密度和速度都要提高幾千倍。而且是非易失的，因此斷電時它仍能保存內容。

由于旋轉電子學研究的爆炸性進展，所有這些以及更多的新技術已經出現在計算的地平線了。旋轉電子學不全是新東西。早在1997年IBM在其GMR磁盤的讀出頭中引入了稱作巨型磁阻的旋轉電子效應。結果在過去的五年中，磁盤的容量提高了100倍。

電子電路是由電子的流動驅動的，它們擁有可測量可控制的電荷。但是，電子不僅流動，而且像微小的磁鐵那樣會旋轉。依據它們的取向，旋轉被說成“上”或者“下”。這個額外的變量，或“自由度”，意味著電子可以做比常規電子電路中更多的事和傳送更多的信息。

最直接的研究目標就是生產磁隨機存取存儲器(MRAM)，它利用磁學原理而不是電荷來儲存數據。與PC機中的動態RAM不同，MRAM是非易失的。

IBM正在與慕尼黑的Infineon技術公司合作，據稱，最早在2005年就能生產出MRAM。它比DRAM快50倍，比靜態RAM的密度高10倍，最終它能替代這兩種存儲器。

還有人認為，MRAM可能替代磁盤做數據存儲。將邏輯電路和存儲放在同一芯片中，能消除慢速的磁盤I/O，這可是多數計算機處理中的瓶頸。

IBM的MRAM利用了磁隧道結，它應用了旋轉電子學，其中電子被允許“隧道”穿過兩層基于旋轉的鐵磁層。每個結能儲存一位。它有望成為一種極高性能的通用RAM，即高的讀寫速度加上極高的密度和非易失性。

研究人員還在進一步開發旋轉的更神奇應用。加州大學圣巴巴拉分校旋轉電子學和量子計算中心主任David Awschalom正在研究利用原子核的旋轉能做些什么。

他說：“原子的亞原子部分(即原子核——譯者注)存儲信息，而電子起到運送信息進出原子核子系統的作用。”

他的目標是制造基于光學的信息處理器，其中光束通過電子向原子核傳送信息。他說，這樣的核存儲器比傳統的半導體存儲器在密度和速度上要高出很多量級。

實際上從更廣義的角度，旋轉電子學研究的迅猛進展將會在半導體材料中把電子學和光子學與磁學結合起來，而傳統上這些學科只涉及金屬。這將實現極快的、極高效的亞微米器件，將計算、通信和存儲結合在一起。不同材料之間的慢速界面(將一種信號或特性轉換成另一種)將一去不返，通常會使數據從一個處理階段過渡到另一階段步伐放慢的反應時間也將大大縮短。

美國國防高級研究計劃局(DARPA)的Wolf說:“你將所有的東西都集成在簡單得多的電路中，它們除了電流是旋轉-極化的外，很像現在的半導體器件。”例如，它們可以搭建極快速的通信交換機。他說：“你可以把它叫做旋轉光電學。它們很容易工作在太赫茲的速度。”

半導體器件不能利用旋轉，除非找到辦法把旋轉極化電子放進去，業已證明這是一件困難的事。但是最近IBM演示了能利用磁隧道結將電流注入進去，如同他們在為MRAM所做的那樣。

IBM的Parkin稱，旋轉電子學半導體可以用于制造可配置的邏輯器件。他說:“通過飛快地重新安排(邏輯)門電路連接的方式，計算機有可能針對某些指令進行優化。”

另一個嚴峻的挑戰是生成能在室溫下保持旋轉狀態的磁半導體。但在今年，物理學家、材料科學家和工程師們在這方面取得了長足的進步。旋轉電子學的快速發展有可能繼續下去，理論已經很完善，盡管還有很多挑戰。