The detection of single nuclear spins is an important goal in magnetic resonance spectroscopy. Optically detected magnetic resonance can detect single nuclear spins that are strongly coupled to an electron spin, but the detection of distant nuclear spins that are only weakly coupled to the electron spin has not been considered feasible. Here, using the nitrogen-vacancy centre in diamond as a model system, we numerically demonstrate that it is possible to detect two or more distant nuclear spins that are weakly coupled to a centre electron spin if these nuclear spins are strongly bonded to each other in a cluster. This cluster will stand out from other nuclear spins by virtue of characteristic oscillations imprinted onto the electron spin decoherence profile, which become pronounced under dynamical decoupling control. Under many-pulse dynamical decoupling, the centre electron spin coherence can be used to measure nuclear magnetic resonances of single molecules. This atomic-scale magnetometry should improve the performance of magnetic resonance spectroscopy for applications in chemical, biological, medical and materials research, and could also have applications in solid-state quantum computing.

We experimentally investigate the protection of electron spin coherence of nitrogen vacancy (NV) center in diamond by dynamical nuclear polarization. The electron spin decoherence of an NV center is caused by the magnetic field fluctuation of the 13 C nuclear spin bath, which contributes large thermal fluctuation to the center electron spin when it is in equilibrium state at room temperature. To address this issue, we continuously transfer the angular momentum from electron spin to nuclear spins, and pump the nuclear spin bath to a polarized state under Hartman-Hahn condition. The bath polarization effect is verified by the observation of prolongation of the electron spin coherence time (T * 2). Optimal conditions for the dynamical nuclear polarization (DNP) process, including the pumping pulse duration and depolarization effect of laser pulses, are studied. Our experimental results provide strong support for quantum information processing and quantum simulation using polarized nuclear spin bath in solid state systems. PACS numbers: 76.70.Fz, 03.65.Yz,03.67.Lx,67.30.hj Quantum information processing requires qubits that can be initialized, controlled and readout with high fidelity. Furthermore , the quantum coherence of qubits should persist for a long time to realize multiple gate operations on them [1]. However there exists inevitable noise from the environment that causes decoherence of the qubits. Many efforts have been done to protect the qubit from the noise. Two major strategies have been proposed to enhance the coherent time of qubits, namely, dynamical decoupling (DD) [2]-[5] and dynamical nuclear polarization (DNP)[6]-[8]. DD can average out the fluctuation of the spin bath by flipping center spin state, thus could effectively cut off the interaction between the center spin and its surrounding spin bath. For DNP method, in the ideal case, the nuclear spin bath is prepared in a spin polarized state, and the thermal fluctuation is completely suppressed. In this case, the electron spin could have long coherence time even in the absence of spin echo control (i.e. T * 2 ∼ T 2). DD can be used to protect the coherence of NV electron spin [9], [10] but, in general, DD sequences like CPMG or UDD, do not commute with quantum gate operations, so one cannot realize gate operations on the protected spin during DD sequence unless special designed pulses are applied [11], [12]. On the contrary, if one uses DNP to generate a polarized nuclear spin bath, the center spin state can be well protected and manipulated under the polarized surrounding bath, leading to full exploitation of the center spin coherence [6], [13] and [14]. Furthermore, the small magnetic moment of nucleus (3 orders smaller than electron spin) makes the bath polarization persist for a very long time. The polarized nuclear spins are important quantum resources for quantum information processing and quantum simulation applications [15]. Figure 1 shows the general idea of this paper. Firstly, a FIG. 1: (Color online) General Schematic. (a) Polarizatio...

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