Molecular Scale Architectures

Quantum Cellular Automata

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Molecular Combinatorial LogicBy Jeff Kramer

Quantum Cellular Automata

Quantum Cellular Automata (or QCA) is a method of transmitting and processing data via held charges in an array of quantum dots. These electrostatic interactions allow for truly amazing data flow and processing operations at nanoscale levels.

“QCA wires, majority gates, clocked cell operation, and (recently) true power gain between QCA cells has been demonstrated.”(Lieberman)

QCA

The basic QCA cell consists of 4 quantum dots and two spare electrons. These dots are oriented in a square, and the placement of the electrons determines the polarity (left to right = 1)

Tunneling Energy is t

Nearest Neighbor Distance is a

QCA

The compensating positive charge is fixed, and the tunneling barriers between cells are assumed to be insurmountable. Adding a fifth center dot also improves the functionality of the cell.

QCA

The barriers between cells being relatively high causes the electrons to be well localized on certain dots. This cell polarization due to Columbic interaction is what allows for data transfer.

The two configurations

QCA

The cell-cell transfer via Columbic interaction is shown on the right. Cell 2 is fixed and Cell 1’s response is measured. While the response is slightly non-linear, it still is very close to perfect switching. Cell Demo

QCA

A cell can also be rotated 45 degrees. This has different special effects on the quantum tunneling and switching.

QCA

Many different nanoscale devices are possible using quantum cellular automata. These devices mimic the traditional electronic devices we’re used to, albeit in a completely different manner, both conceptually and physically.

QCA

Quantum wires are constructed by stringing together groups of CA and letting the Columbic interaction handle switching. The loss from cell to cell is very small, especially if tunneling barrier energies are high. Rotated cells have the same transmission properties but they also invert the response from every cell.

Line Demo

QCA

Inversion can also be performed by a dedicated inverter. This is constructed using the property of anti-alignment of cells.

Inverter Demo

QCA

Both a “fanout”, or branch, and a corner wire are simple to construct. The quantum cell alignment allows for transmission even around corners and to multiple cells. Fanout (top) and Corner Wire

QCA

The majority gate allows for construction of AND/OR logic. By holding an input at 1, you force an OR on the other two outputs. By holding an input at 0, you force an AND of the other two outputs. With AND/OR, you can construct complex systems like full adders.

Majority Gate Demo

QCA

This is a coplanar cross. Rotated cells do not effect the polarization of regular cells and regular cells do not effect the polarization of rotated cells. This lets strange configurations like this actually function correctly. Notice that from A to B requires a large tunnel jump, and that the horizontal rotated cells are even in number.

QCA

This is a full adder. Can you see the various parts that make it up?

Bigger Picture

QCA

• Other recent advances include the construction of quantum amplifiers, memory systems that truly take advantage of the QCA’s special nature, and conceptual computer science pipelining experiments and changes.

• However, all of these experiments have been only completed with metal dot quantum cells at low temperatures. Work on molecular scale QCA’s is progressing but is running into several technical barriers.

QCA

Any Questions?

Molecular Combinatorial Logic

• Don Eigler’s team at IBM used STM to manipulate carbon monoxide atoms on a sheet of gold, setting up a cascade that replicates the process of a three input sorter

• This all happens at a scale of 12X17 nanometers. It required only one eV to function, but had to be held near absolute zero and in hard vacuum.

Molecular Combinatorial Logic