The power and capacity of computing components such as microprocessors and memory circuits has been increasing for the last 50 years, as the size of the functional units, such as transistors, has been decreasing. This trend is now reaching a limit, however, as it is difficult to make the current functional units (such as MOSFETs) any smaller without affecting their operation.
The technology employed to manufacture conventional silicon integrated circuits has developed over the last 50 years and is today well established. Current microprocessors feature several hundreds of millions of transistors which are manufactured in high throughput lines.
Developments are ongoing to implement new types of advanced processing apparatus that can implement powerful computations using a different approach than current processors. Such advanced processing apparatus promise computational capacities well beyond current devices. For example, quantum processors are being developed which can perform computations according to the rules of quantum mechanics. Approaches to the realisation of devices for implementing quantum bits (qubits), the basic computational unit of a quantum processor, have been explored with different levels of success.
For example, semiconductor based qubits have been developed and described in a number of earlier patent publications, including U.S. Pat. No. 6,472,681 (Kane), U.S. Pat. No. 6,369,404 (Kane). These qubits are based on the exploitation of the quantum effects of a single dopant atom in a silicon crystalline lattice. Although, the properties of a single dopant atom in silicon are promising to implement qubits, the techniques for manufacturing these devices include complex nanofabrication solutions, as discussed for example in U.S. Pat. No. 7,547,648 (Ruess et al.).
It has also been proposed to encode quantum information using the spin states of semiconductor quantum dots (Loss and DiVincenzo (Loss, DiVincenzo, DP quantum computation with quantum dots. Phys Rev. A56, 120; 1998)). This proposal primarily envisaged the use of quantum dots formed using electrostatic gates on a GaAs/AlGaAs heterostructure. However, the limited coherence time and the associated fidelity of the quantum state in these systems represent a significant hurdle to application of quantum dots in a quantum processor. Experimental work has been done in GaAs/AlGaAs on quantum dot qubits, but to realise large-scale arrays of such structures will require new manufacturing process technologies to be developed. More importantly, these materials suffer from problems with fidelity and dephasing time due to the presence of nuclear spins that are inherent to the GaAs crystal lattice.