John Timler

Power gain and dissipation in quantum-dot cellular automata

Quantum-dot cellular automata (QCA) may provide a novel way to bypass the transistor paradigm to form molecular-scale computing elements. In the QCA paradigm information is represented by the charge configuration of a QCA cell. We develop a theoretical approach, based on the density matrix formalism, which permits examination of energy flow in QCA devices. Using a simple two-state model to describe the cell, and an energy relaxation time to describe the coupling to the environment, we arrive at an equation of motion well suited to the quasi-adiabatically switched regime. We use this to examine the role of power gain and power dissipation in QCA cells. We show that QCA cells can exhibit true signal power gain. The energy lost to dissipative processes is restored by the clock. We calculate the power dissipated to the environment in QCA circuits and show that it is possible to achieve the ultralow levels of power dissipation required at molecular densities.

Maxwell's demon and quantum-dot cellular automata

Quantum-dot cellular automata (QCA) involves representing binary information with the charge configuration of closed cells comprised of several dots. Current does not flow between cells, but rather the Coulomb interaction between cells enables computation to occur. We use this system to explore, quantitatively and in a specific physical system, the relation between computation and energy dissipation. Our results support the connection made by Landauer between logical reversibility and physical reversibility. While computation always involves some energy dissipation, there is no fundamental lower limit on how much energy must be dissipated in performing a logically reversible computation. We explicitly calculate the amount of energy that is dissipated to the environment in both logically irreversible “erase” and logically reversible “copy-then-erase” operations carried out in finite time at nonzero temperature. The “copy” operation is performed by using a near-by QCA cell which plays the role of Maxwells ...

Power gain in a quantum-dot cellular automata latch

We present an experimental demonstration of power gain in quantum-dot cellular automata (QCA) devices. Power gain is necessary in all practical electronic circuits where power dissipation leads to decay of logic levels. In QCA devices, charge configurations in quantum dots are used to encode and process binary information. The energy required to restore logic levels in QCA devices is drawn from the clock signal. We measure the energy flow through a clocked QCA latch and show that power gain is achieved.

Conduction band offset at the InN∕GaN heterojunction

The conduction-band offset between GaN and InN is experimentally determined. InN∕n-type GaN isotype heterojunctions grown by molecular beam epitaxy are observed to exhibit Schottky-junction like behavior based on rectifying vertical current flow. From capacitance-voltage measurements on the heterojunction, the Schottky barrier height is found to be ∼0.94eV. The photocurrent spectroscopy measurement by backside illumination reveals an energy barrier height of 0.95eV across the heterojunction, consistent with the capacitance measurement. By combining electrical transport, capacitance-voltage, and photocurrent spectroscopy measurement results, the conduction band offset between InN and GaN is estimated to be ΔEC=1.68±0.1eV.

Fanout gate in quantum-dot cellular automata

We present an experimental demonstration of a fanout gate for quantum-dot cellular automata (QCA), where a signal applied to a single input cell is amplified by that cell and sent to two output cells. Each cell is a single-electron latch composed of three metal dots, which are connected in series by tunnel junctions. Binary information is represented by an excess electron localized to one of the two peripheral metal dots of each latch. Fanout is demonstrated by writing a bit to the input latch and then simultaneously transferring the bit to both output latches using two-phase clocking.

Quantum-dot cellular automata

An introduction to of quantum-dot cellular automata (QCA) is presented. QCA is a transistorless nanoelectronic computation paradigm that addresses the issues of device and power density which are becoming increasingly important in the electronics industry. Scaling of CMOS is expected to come to an end in the next 10-15 years, with perhaps the most important limiting problem being the power density and the resulting heat generated. QCA offers the possibility of circuitry that dissipates many orders of magnitude less power than CMOS, is scalable to molecular dimensions, and provides the power gain necessary to restore signal levels. QCA cells are scalable to molecular dimensions and initial measurements have demonstrated single-electron switching within a molecule.

Power gain in a quantum-dot cellular automata (QCA) shift register

Discusses an experiment that demonstrates power gain in a quantum-dot cellular automata (QCA) shift register. Power gain is essential in any electronic system for the restoration of logic levels. The clock signal plays an important role in providing power gain in QCA devices as it can be used as a source of energy for the system. We discuss how this can be achieved in a QCA shift register and experimentally demonstrate a power gain greater than unity.

Quantum computing with quantum-dot cellular automata using coherence vector formalism

A coherence vector formalism is used to describe quantum computing with quantum-dot cellular automata, and the realizations of basic quantum gates are also discussed.

A quantum-dot cellular automata shift register

Presents a more advanced device in the family of clocked QCA systems - a two cell shift register. The device consists of two capacitively coupled QCA latches The QCA latch consists of three micron-sized aluminum islands, or dots separated by tunnel junctions. Multiple tunnel junctions are used between dots to increase the charge retention time of the latches. Two single-electron electrometers measure the state of the shift register.