AN INTRODUCTION TO NANOCOMPUTING - Bio-Inspired and Nanoscale Integrated Computing - page 14

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Figure 1.10. Depiction of quantum-dot cellular automata (QCA). Each cell has
several quantum dots (in this case five). A cell can have two possible states, where
electrons are diagonally oriented. When the input value is changed, the change
propagates over time, this particular arrangement of cells represents a logic
function that inverts the input.

different ways, allowing for many interesting logic functions. Since there are no
wires interconnecting various cells, they can be arranged very compactly, in turn
allowing for compact logic functions. Note that quantum dots are only one of
serveral ways to implement QCA cells. QCA are discussed in Chapter 4.

1.4.4. Spin Devices
Electron spin is another interesting quantum effect that can be used to create
nanoscale switching devices. Various particles can have different types of spin, but
electrons in particular can have only two types: spin up and spin down. This is a
natural way to introduce the digital abstraction. Furthermore, electron spin is a
main nanoscale property that results in macroscopic magnetic fields. For example,
if most of the electrons in a metal object assume a polarized spin state (either all
spin up or all spin down), the metal object will be magnetic. One interesting
phenomenon that is a result of the relationship between magnetism and electron
spin is the magnetoresistive effect: the resistance of some materials can change
depending on the surrounding magnetic field.
Magnetoresistance is already widely used as the mechanism to read data from a
disk drive that stores information magnetically. Manufacturers are also considering
the possibility of magnetic random-access memory (MRAM), which would use
magnetoresistance to implement nanoscale memory cells. MRAM may have a
number of advantages over other types of memory storage. It is expected to have
the high storage density of today's dynamic RAM technology, while providing the
high speed and power savings of today's static RAM technology. Magnetoresis-
tance can also be used to create a switching device. This sort of switch is called a spin
transistor,ortranspinnor [20]. Recall that a classical transistor uses an electric field
to make it difficult for electrons to travel across the wire. Similarly, a spin transistor
uses magnetoresistance to drastically increase the resistance of the wire, effectively
blocking current like a switch. Magnetic storage is discussed in Chapter 6, and spin
devices are discussed further in Chapters 7 and 9.