Information Technology Reference
In-Depth Information
1.
Mechanical cells coupled by electrostatic contact potential forces. These would
suffer from the slower response of atoms compared with electrons, but remain
largely unexplored.
2.
Magnetic cells (collective nuclear or electronic spins) coupled by magnetic fields.
This is the basis of nanomagnetic QCA described elsewhere in this volume.
3.
Electronic cells coupled by Coulomb multipole-multipole interactions. The
charge distribution could be the result of either mobile atoms or mobile electrons,
and could involve a few or several charges.
Few-electron QCA cells, as has been noted above, have an intrinsic bistability due to
charge quantization. If there are many charges forming the charge multipole, the
bistability must be provided by another mechanism. One example is a CMOS cell that
is an analog to QCA and switches adiabatically [
77
].
4
Issues in QCA Development
4.1
The Role of Quantum Mechanics in QCA
To achieve robustness against fabrication variations, the QCA paradigm uses only a
classical degree of freedom, the electric (or magnetic) quadrupole moment of the cell.
It does not use quantum phase information nor interference effects. QCA involves bits
not qubits. It is quantum mechanical precisely in that it relies on quantum tunneling
for cell switching. This is crucial because if quantum mechanics were ''turned off''
(h = 0) there would be no tunneling and a QCA cell could not switch. If the barriers
to tunneling were removed so that classical switching were allowed, a QCA cell would
oscillate and settle into a particular configuration randomly depending on the details of
the trajectory and energy dissipation. Switching a classical double-well system is
much more prone to error because the system can get caught in a metastable state if
the timing is not perfect. Reliance on quantum tunneling stabilizes the bit information.
4.2
Power Gain
In molecular, metal dot (discussed above), or other implementations, power gain is
crucial because there is always some dissipation of energy as information moves from
stage to stage in a computation. This dissipation is the microscopic version of friction
in mechanical devices. It can be minimized, and by moving gradually can be reduced
to whatever level is desired, but cannot be completely eliminated. Therefore, unless
there is a way to restore the signal energy, it will eventually be completely attenuated.
In conventional devices the source of the energy is usually the constant-voltage power
supply. In QCA the restoring energy is provided by the clock; it automatically supplies
enough energy to restore the signal levels.
4.3
Metastability, Memory, and Coherence
For a physical system to act as a memory its state cannot be determined by only its
boundary conditions. A Hamiltonian system in a unique ground state, for example,
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