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cannot act as a memory. A device with even short-term memory must therefore be in a
metastable state. It could be in a state representing a 1 or be in a state representing a 0.
Which state the device is in is depends not just on its boundary conditions, but also on
its past. If there is a large enough kinetic barrier between these two states they can
often be justifiably treated as distinct energetically degenerate states, but they are
actually metastable states very weakly coupled and with a very long Rabi oscillation
period.
In clocked QCA wires (i.e., shift registers), information is represented by bit
packets, a few cells in the line that are polarized in the 1 or 0 state [
78
]. Since they
could also be in the opposite state energetically, it is true that if the time evolution was
completely unitary, the bit could quantum mechanically oscillate from one state to the
other. There is a considerable kinetic barrier to doing that, however, just as in the case
of a CMOS bit. Moreover, in real systems entanglement with the environment sta-
bilizes the bit packet by loss of quantum phase in the system [
79
]. Decoherence is
precisely this sort of entanglement with the environment and, though it is detrimental
for quantum computing, it actually stabilizes QCA bits. Further exploration of the
roles of environmentally-induced decoherence and energy dissipation are part of the
broader question of the transition between the quantum and classical worlds.
4.4
Wire Crossings
QCA is naturally an in-plane technology; it does not require going out of the plane.
How can one therefore cross wires, that is move one bit independently across the path
of another? Several proposals have been made that accomplish this. (1) The original
wire-crossing proposal was to use the symmetry of cells and the second-near-neighbor
coupling (suitably strengthened by duplication) to allow one cell line to communicate
across the path of another. The limitation here is the amount of control in placement
and orientation required. (2) A permuter is a logical function which simply switches
inputs A and B to output B and A. This can be done with logic, though it does take
several cells to implement [
80
]. (3) With expanded clocking timing one can have one
bit packet cross a wire intersection horizontally at one time, and vertically at a later
time. The cost is in the added complexity of the clocking circuitry. (4) A bridge
crossover, similar to the CMOS via structure can be constructed that takes QCA cells
out of the plane to cross. (5) In many instances the crossing is for distribution of
signals to different parts of a logic array. Tougaw and Khatun have designed a general
matrix distribution scheme, again using augmented clocking patterns [
81
].
4.5
Computational Architecture
It is clear that QCA requires rethinking circuit and computer architecture on the basis
of this new device paradigm. Nevertheless because QCA still supports Boolean logic
function, it is natural that the first designs are taken over from usual logic circuitry.
Much design work is underway, supported crucially by the design tool QCADesigner
produced by the Walus group [
82
]. The goals of this important effort engaging many
research groups are both to capitalize on the functional density that QCA cells allow
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