A silicon based nuclear spin quantum computer.pdf

Nature ? Macmillan Publishers Ltd 1998 8 NATURE | VOL 393 | 14 MAY 1998 133 articles A silicon-based nuclear spin quantum computer B. E. Kane Semiconductor Nanofabrication Facility, School of Physics, University of New South Wales, Sydney 2052, Australia . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quantum computers promise to exceed the computational efficiency of ordinary classical machines because quantum algorithmsallow theexecution of certain tasks in fewersteps. But practical implementationof thesemachinesposesa formidable challenge. Here I present a scheme for implementing a quantum-mechanical computer. Information is encoded onto the nuclear spins of donor atoms in doped silicon electronic devices. Logical operations on individual spins are performed using externally applied electric fields, and spin measurements are made using currents of spin-polarizedelectrons.The realization of suchacomputer is dependenton future refinementsof conventional silicon electronics. Although the concept of information underlying all modern com- puter technology is essentially classical, phsyicists know that nature obeys the laws of quantum mechanics. The idea of a quantum computer has been developed theoretically over several decades to elucidate fundamental questions concerning the capabilities and limitations of machines in which information is treated quantum mechanically1,2. Specifically, in quantum computers the ones and zeros of classical digital computers are replaced by the quantum state of a two-level system (a qubit). Logica