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level that would support QCA technology [79]. As a result, very limited experi-
mental results are available for this particular implementation of QCA. One of the
advantages of using semiconductor QCA is that it is based on materials that are
well understood and many fabrication techniques have been created to work with
them. As well, there is a large economic benefit to using a material system for
which significant infrastructure already exists.
Gardelis et al. demonstrated polarization transfer into and out of a QCA cell
defined with metallic gates patterned on top of a 2-dimensional electron gas
(2DEG) in AlGaAs/GaAs [78]. The experiment was performed in a dilution
refrigerator operating at 100mK. The use of 2DEG for implementing QCA is
limited and a scalable implementation using this method is most likely not
possible. In addition, the tolerance to fabrication variations appears to be too
sensitive for any practical application. However, such devices can be fabricated
using available techniques and are providing a valuable experimental platform to
study the fundamental physics of QCA operation.
4.12.2. Magnetic QCA
Recently, significant research effort has been put into the implementation of
magnetic QCA (MQCA) based on the interactions of magnetic nanoparticles [42,
43, 80]. The magnetization vector of these nanoparticles is analogous to the
polarization vector in electronic QCA, and information is propagated via
magnetic exchange interactions as opposed to electrostatic coupling in all other
reported implementations. Although this technology is referred to as magnetic
quantum-dot cellular automata, the term quantum in this case represents the
quantum mechanical nature of the exchange interaction.
One of the immediate advantages of considering such a technology is that
MQCA cells can operate at room temperature, even for large device features
on the order of a few hundred nanometers. A potential application of such a
technology would be to implement processing-in-memory where the QCA circuit
can be used to implement both the information processing and storage elements,
potentially eliminating the need for a read/write head, as required by conventional
hard disk drives, or the interconnects required by MRAM devices. The read/write
operation would be performed at the edges of a MQCA array and the information
would be processed and propagated inwards by the coupled magnetic field
exchange interactions. Contrary to the layout pattern of magnetic storage
elements of an MRAM, where the elements are created such that there is very
little interaction between them, the MQCA cells are designed to interact in order
to enable information processing. Because these exchange interactions are similar
to electronic QCA, the MQCA cells would also be able to perform computational
tasks using majority functions. One of the drawbacks to using MQCA as a general
computing platform is the low operating speed; studies report switching frequency
estimates at around 100MHz [43]. However, it is expected that the MQCA circuits
will operate with a much higher tolerance to displacement and rotational defects
than the electronic QCA. The QCA wires fabricated in [43] operated properly for
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