Coherent manipulation of nuclear spins using spin injection from a half-metallic spin source

Abstract

We have developed a nuclear magnetic resonance (NMR) system that uses spin injection from a highly polarized spin source. Efficient spin injection into GaAs from a half-metallic spin source of Mn-rich Co 2 MnSi enabled an efficient dynamic nuclear polarization of Ga and As nuclei in GaAs and a sensitive detection of NMR signals. Moreover, coherent control of nuclear spins, or the Rabi oscillation between two quantum levels formed at Ga nuclei, induced by a pulsed NMR has been demonstrated at a relatively low magnetic field of ∼0.1 T. This provides a novel all-electrical solid-state NMR system with the high spatial resolution and high sensitivity needed to implement scalable nuclear-spin-based qubits. Nuclear spins in semiconductors are an ideal system for implementing quantum bits (qubits) for quantum computation because they have an extremely long coherence time. The nuclear magnetic resonance (NMR) technique enables the control and detection of nuclear-spin qubits, and quantum algorism with seven qubits has been demonstrated in molecules in a liquid [1]. For large-scale integration, however, the implementation of qubits in solid-state materials, especially in semiconductors, is indispensable. Moreover, in conventional NMR techniques, a strong magnetostatic field should be applied to polarize nuclear spins, making it difficult to selectively control nuclear spins located within nanometer-sized regions. Furthermore, since the magnetic moment of a nuclear spin is three orders of magnitude smaller than that of an electron spin, the sensitivity of detecting nuclear spins through a pickup coil is quite low. Thus, there is a strong need to develop a novel NMR technique with high spatial resolution and high sensitivity to enable a future large-scale quantum computing system. From this point of view, dynamic nuclear polarization (DNP), where nuclear spins are dynamically polarized through a hyperfine interaction between nuclear spins and electron spins, has attracted much interest, since it can drastically increase the NMR signal. Several solid-state NMR devices based on different DNP techniques by optical [2][3][4] or electrical means [5][6][7] have recently been reported. Furthermore, coherent manipulation of nuclear spins, or the Rabi oscillation, which is a key factor for nuclear-spin-based qubits, has been demonstrated electrically in GaAs/AlGaAs quantum Hall systems [8][9][10] and optically in GaAs/AlGaAs quantum wells [11,12]. Although the optical method is suitable for clarifying the fundamental physics relevant to nuclear spins, it is restricted in its scalability because the spatial resolution is limited by the optical wavelength. Quantum Hall systems, on the other hand, require a relatively strong magnetic field of several tesla and a low temperature below 1 K to create the highly spin-polarized electrons necessary for the DNP and the detection of nuclear-spin states.An injection of spin-polarized electrons from a ferromagnetic electrode into a semiconductor also creates spinpolarized electronic states in a ...