(A) study on electrical control of hyperfine interaction

Abstract

The silicon based nuclear spin quantum computer proposed by Kane is one of the most attractive quantum computer architectures because of its smart design and compatibility for implementation with mature silicon technology. In this model, $^{31}P$ nuclear spin located inside a silicon crystal with regular spacing are used as qubits and gate operation is implemented using electric field control of the hyperfine interaction. This model also requires extreme conditions, very low temperature and high magnetic field. To check the feasibility of this model, spin dynamics of electron and nuclear spins have to be brightened at such extreme conditions and the operating principles have to be experimentally demonstrated. In this study, a nuclear magnetic resonance (NMR) and an electron spin resonance (ESR) experiments were performed to investigate the spin dynamics and demonstrate qubit addressing via electrical control of the hyperfine interaction. As a first step, an NMR measurement was carried out on isolated donors in phosphorus doped silicon (Si:P) with concentration $n = 6 \times 10^{17} cm^{-3}$, but not in success because of low concentration of nuclear spins to detect. However, an NMR signal of delocalized donor in Si:P with $n = 5.6 \times10^{19} cm^{-3}$ was observed at temperature as low as 45 mK and magnetic field of 7 T. An inverted nuclear magnetization recovers its thermal equilibrium in two time steps. This implies the conduction electron does not act as a thermal bath for nuclear spins. From the $\it{T_1}$ relaxation study, it is found that the specific heat of the nuclear spins becomes lager than that of the conduction electrons at low temperature and high magnetic field limits. To investigate the spin dynamics of the isolated donor electron spin and nuclear spin, a high frequency ESR experiment on Si:P with $n = 6.5 \times 10^{16} cm^{-3}$ was carried out in a high magnetic field of 2.87 T (80 GHz) and at temperatures from 48 K down to 1.8 K. As the tem...