Toward a Silicon - Based Nuclear - Spin Quantum Computer

quantum computing is to identify a system that can be scaled up to the number of qubits needed to execute nontrivial quantum algorithms. Peter Shor’s algorithm for finding the prime factors of numbers used in public-encryption systems (numbers that typically consist of more than a hundred digits) would likely require a quantum computer with several thousand qubits. Depending on the error correction scheme appropriate to the particular computer, the required number could be much larger. Although ion-trap or nuclear-magneticresonance (NMR) quantum “computers” containing a few (less than 10) qubits have been successfully demonstrated, it is not clear that those systems can be scaled up. Solid-state quantum computers may be more likely candidates. As a result, a number of solid-state schemes have been proposed, most of which fall into two categories: The physical qubits are spin states of individual electrons or nuclei, or they are charge or phase states of superconducting structures. A particularly promising scheme is the silicon-based nuclear-spin computer, proposed a few years ago by Bruce Kane (1998), then of the University of New South Wales in Sydney, Australia, and now of the University of Maryland in College Park, Maryland. Shown in Figure 1, the Kane computer architecture consists of a linear array of phosphorus atoms embedded beneath the surface of a pure silicon wafer. Each phosphorous atom serves as a qubit, and the linear array forms the computer’s quantum register.1 The entire wafer is placed in a strong, static magnetic field B0 at sub-Kelvin temperatures, and alignment of the phosphorous atom’s nuclear spin as parallel or antiparallel to B0 corresponds to the | 〉 and | 〉 states of the qubit, respectively. (Throughout this article, we will follow the convention of Kane