This grant supports theoretical research on fundamental issues related to the implementation of quantum computation in solid-state devices. Since the discovery that certain tasks could be performed with great efficiency by algorithms based on quantum mechanics, an intense effort has been made to find suitable quantum hardware. Although several proposed implementations, such as those based on nuclear magnetic resonance and atomic trapping, have passed the proof-of-principle, few-qubit phase, the path to achieving a reliable multi-qubit quantum computer is still undefined.
In this proposal we investigate the physical limitations to resilient computation with solid-state quantum bits (qubits), such as semiconductor quantum dots and superconductor junctions. While solid-state qubits seem easily scalable from the fabrication viewpoint, they also present high decoherence rates as compared to other implementations. One major concern is that such strong decoherence may lead to errors occurring at a rate too large to be controlled.
However, differently from other nuclear, atomic, and optical qubits, the interaction of solid-state quantum devices with the environment can introduce strong memory effects. As a result, temporal correlations may appear during the operation of multi-qubit systems. Current quantum error correction codes are not designed to cope with this situation, which may then invalidate any error threshold estimate for solid-state qubits based on the efficiency of those codes.
We will explore these issues in a comprehensive way. Starting from a thorough study of the mechanisms of decoherence in single- and double-qubit systems, we will study a model of multi-qubit systems in the presence of correlated noise in a variety of realistic conditions. Our results will help set up new strategies for the operation of multi-qubit systems. They will also let us understand what are the constraints that error correction codes will need to satisfy in order to achieve fault-tolerant quantum computation in large-scale solid state implementations. To achieve our goals, we have put together a team of researchers with expertise in nanoscale physics and computer science. The final outcome of our project will be a much better understanding of how a real solid-state quantum computer would behave.
