Controlling the interaction of a single quantum system with its environment is a fundamental challenge in quantum science and technology. We strongly suppressed the coupling of a single spin in diamond with the surrounding spin bath by using double-axis dynamical decoupling. The coherence was preserved for arbitrary quantum states, as verified by quantum process tomography. The resulting coherence time enhancement followed a general scaling with the number of decoupling pulses. No limit was observed for the decoupling action up to 136 pulses, for which the coherence time was enhanced more than 25 times compared to that obtained with spin echo. These results uncover a new regime for experimental quantum science and allow us to overcome a major hurdle for implementing quantum information protocols.

Crystal defects can confine isolated electronic spins and are promising candidates for solid-state quantum information. Alongside research focusing on nitrogen-vacancy centres in diamond, an alternative strategy seeks to identify new spin systems with an expanded set of technological capabilities, a materials-driven approach that could ultimately lead to ‘designer’ spins with tailored properties. Here we show that the 4H, 6H and 3C polytypes of SiC all host coherent and optically addressable defect spin states, including states in all three with room-temperature quantum coherence. The prevalence of this spin coherence shows that crystal polymorphism can be a degree of freedom for engineering spin qubits. Long spin coherence times allow us to use double electron–electron resonance to measure magnetic dipole interactions between spin ensembles in inequivalent lattice sites of the same crystal. Together with the distinct optical and spin transition energies of such inequivalent states, these interactions provide a route to dipole-coupled networks of separately addressable spins.

Quantum registers of nuclear spins coupled to electron spins of individual solid-state defects are a promising platform for quantum information processing [1][2][3][4][5][6][7][8][9][10][11][12][13]. Pioneering experiments selected defects with favourably located nuclear spins having particularly strong hyperfine couplings [4][5][6][7][8][9][10]. For progress towards large-scale applications, larger and deterministically available nuclear registers are highly desirable. Here we realize universal control over multi-qubit spin registers by harnessing abundant weakly coupled nuclear spins. We use the electron spin of a nitrogen-vacancy centre in diamond to selectively initialize, control and read out carbon-13 spins in the surrounding spin bath and construct high-fidelity single-and two-qubit gates. We exploit these new capabilities to implement a three-qubit quantum-error-correction protocol [14][15][16][17] and demonstrate the robustness of the encoded state against applied errors. These results transform weakly coupled nuclear spins from a source of decoherence into a reliable resource, paving the way towards extended quantum networks and surface-code quantum computing based on multi-qubit nodes [11,18,19].Electron and nuclear spins associated with defects in solids provide natural hybrid quantum registers [3][4][5][6][7][8][9][10][11]. Fullycontrolled registers of multiple spins hold great promise as building blocks for quantum networks [18] and fault-tolerant quantum computing [19]. The defect electron spin enables initialization and readout of the register and coupling to other (distant) electron spins [11,18], whereas the nuclear spins provide well-isolated qubits and memories with long coherence times [8,9,11]. Previous experiments relied on selected defects having nuclear spins with strong hyperfine couplings that exceed the inverse of the electron spin dephasing time (1/T * 2 ). With these strongly coupled spins, singleshot readout [9,10,[20][21][22] and entanglement [9,11] were demonstrated. However, the number of strongly coupled spins varies per defect and is intrinsically limited, so that universal control has so far been restricted to two-qubit registers [4,7] and the required control of multi-qubit registers has remained an open challenge.Here we overcome this challenge by demonstrating universal control of weakly coupled nuclear spins (unresolved hyperfine coupling 1/T * 2 ). We use the electron spin of single nitrogen-vacancy (NV) centres in room-temperature diamond to selectively control multiple carbon-13 ( 13 C) nuclear spins in the surrounding spin bath (Fig. 1a). With this new level of control we realize multi-qubit registers by constructing high-fidelity unconditional and electroncontrolled gates, implementing initialization and readout, and creating nuclear-nuclear entangling gates through the electron spin. Finally, we demonstrate the power of this approach by implementing the first quantum-error-correction protocol with individual solid-state spins.We have used dynamical decoupling spect...