Electronic structure methods can be used to model the dynamics of two-qubit gates for quantum computing.
The Journal of Physics Condensed matter has selected an article by LCN and UCL researchers as one of the 2007 papers based on referees recommendation, reader downloads and board member selection.
"Just as classical computers use networks of standard gates to manipulate bits, quantum computers use networks of quantum gates to manipulate qubits. But quantum computing uses entanglement as a resource, the special quantum dance that correlates the behaviours of these qubits.
A Kerridge, A H Harker and A M Stoneham (University College, London) have looked at the dynamics of quantum gate operation. They consider an important class of solid-state gates — those employing optically-controlled electron spin qubits. Such gates form the basis of a possible realisation of the basic component of a proposed quantum information processor that might even operate at useful temperatures.
Their approach took a time-dependent configuration interaction method to study how the electronic structure of two electron spin qubits evolved when they interacted with a third, optically-excited, control spin in an applied magnetic field. They could identify unitary operations which approximately disentangle the control spin, and use these operations to construct high-accuracy two-electron operations that were locally equivalent to the standard CNOT, SWAP, and root-SWAP operations. They could then estimate the accuracy of a set of candidate quantum gates, evaluating the residual entanglement of the control electron and overall gate operation times. Their results attest to the feasibility of the silicon-based quantum gates proposed by Stoneham, Fisher and Greenland in 2003.
Whilst it is important to show that high accuracy gates are possible, what is particularly novel is their demonstration that state-of-the-art electronic structure methods can be used to model the dynamics of two-qubit gates, a significant advance over previous analytical studies. Their approach can be generalised to multiqubit systems, and is the basis of a powerful tool to optimise a number of solid-state routes to quantum information processing."