To exploit the quantum coherence of electron spins in solids in future technologies such as quantum computing, it is first vital to overcome the problem of spin decoherence due to their coupling to the noisy environment. Dynamical decoupling, which uses stroboscopic spin flips to give an average coupling to the environment that is effectively zero, is a particularly promising strategy for combating decoherence because it can be naturally integrated with other desired functionalities, such as quantum gates. Errors are inevitably introduced in each spin flip, so it is desirable to minimize the number of control pulses used to realize dynamical decoupling having a given level of precision. Such optimal dynamical decoupling sequences have recently been explored. The experimental realization of optimal dynamical decoupling in solid-state systems, however, remains elusive. Here we use pulsed electron paramagnetic resonance to demonstrate experimentally optimal dynamical decoupling for preserving electron spin coherence in irradiated malonic acid crystals at temperatures from 50 K to room temperature. Using a seven-pulse optimal dynamical decoupling sequence, we prolonged the spin coherence time to about 30 mus; it would otherwise be about 0.04 mus without control or 6.2 mus under one-pulse control. By comparing experiments with microscopic theories, we have identified the relevant electron spin decoherence mechanisms in the solid. Optimal dynamical decoupling may be applied to other solid-state systems, such as diamonds with nitrogen-vacancy centres, and so lay the foundation for quantum coherence control of spins in solids at room temperature.

Precise control of quantum systems is of fundamental importance in quantum information processing, quantum metrology and high-resolution spectroscopy. When scaling up quantum registers, several challenges arise: individual addressing of qubits while suppressing crosstalk, entangling distant nodes and decoupling unwanted interactions. Here we experimentally demonstrate optimal control of a prototype spin qubit system consisting of two proximal nitrogen-vacancy centres in diamond. Using engineered microwave pulses, we demonstrate single electron spin operations with a fidelity FE0.99. With additional dynamical decoupling techniques, we further realize high-quality, on-demand entangled states between two electron spins with F40.82, mostly limited by the coherence time and imperfect initialization. Crosstalk in a crowded spectrum and unwanted dipolar couplings are simultaneously eliminated to a high extent. Finally, by high-fidelity entanglement swapping to nuclear spin quantum memory, we demonstrate nuclear spin entanglement over a length scale of 25 nm. This experiment underlines the importance of optimal control for scalable room temperature spin-based quantum information devices.

Atomic-size spin defects in solids are unique quantum systems. Most applications require nanometre positioning accuracy, which is typically achieved by low-energy ion implantation. A drawback of this technique is the significant residual lattice damage, which degrades the performance of spins in quantum applications. Here we show that the charge state of implantation-induced defects drastically influences the formation of lattice defects during thermal annealing. Charging of vacancies at, for example, nitrogen implantation sites suppresses the formation of vacancy complexes, resulting in tenfold-improved spin coherence times and twofold-improved formation yield of nitrogen-vacancy centres in diamond. This is achieved by confining implantation defects into the space-charge layer of free carriers generated by a boron-doped diamond structure. By combining these results with numerical calculations, we arrive at a quantitative understanding of the formation and dynamics of the implanted spin defects. These results could improve engineering of quantum devices using solid-state systems.

Ab initio computation of molecular properties is one of the most promising applications of quantum computing. While this problem is widely believed to be intractable for classical computers, efficient quantum algorithms exist which have the potential to vastly accelerate research throughput in fields ranging from material science to drug discovery. Using a solid-state quantum register realized in a nitrogen-vacancy (NV) defect in diamond, we compute the bond dissociation curve of the minimal basis helium hydride cation, HeH(+). Moreover, we report an energy uncertainty (given our model basis) of the order of 10(-14) hartree, which is 10 orders of magnitude below the desired chemical precision. As NV centers in diamond provide a robust and straightforward platform for quantum information processing, our work provides an important step toward a fully scalable solid-state implementation of a quantum chemistry simulator.

§ These authors contributed equally to this work Building a quantum repeater network for long distance quantum communication requires photons and quantum registers that comprise qubits for interaction with light, good memory capabilities and processing qubits for storage and manipulation of photons. Here we demonstrate a key step, the coherent transfer of a photon in a single solid-state nuclear spin qubit with an average fidelity of 98% and storage over 10 seconds. The storage process is achieved by coherently transferring a photon to an entangled electron-nuclear spin state of a nitrogen vacancy centre in diamond, confirmed by heralding through high fidelity single-shot readout of the electronic spin states. Stored photon states are robust against repetitive optical writing operations, required for repeater nodes. The photon-electron spin interface and the nuclear spin memory demonstrated here constitutes a major step towards practical quantum networks, and surprisingly also paves the way towards a novel entangled photon source for photonic quantum computing.A quantum repeater network is intended to distribute entanglement between distant nodes realizing an elementary quantum network [1]. Building up such a network requires photon sources (single or entangled pairs), processing nodes with the ability to make (i) optical or spin Bell-state measurements, (ii) long coherence times and (iii) ability for entanglement purification or quantumerror correction [2,3] . With such strong requirements it is hard to find physical systems meeting all of the above criteria. In this regard ensembles of atomic gases, trapped ions and solid state systems are intensively studied [1,2,[4][5][6][7]. While e.g., atomic systems provide high interaction efficiency with photons, rare earth systems on the other hand show long coherence times, all are required for processing the absorbed/emitted photons. As opposed to ensembles, single particles though typically have a significantly less interaction efficiency with photons, however are useful for quantum networks due to their ability for in situ information processing [8][9][10][11], like entanglement purification [12,13].For this reason solid state devices with well controllable spins are recently proposed to be promising candidates for quantum repeater networks [15,16]. The nitrogenvacancy (NV) defect centre in diamond does show significant potential in this respect. It provides a hybrid spin system in which electron spins are used for fast [17], high-fidelity control [18] and readout [19,20], and nuclear spins which are well-isolated from their environment yielding ultra-long coherence time [21]. Electron and nuclear spins could form a small-scale quantum register allowing for e.g. necessary high-fidelity quantum error correction. Furthermore, the NV electron spin can be entangled with an emitted optical photon [22,23]. Quantum entanglement and quantum teleportation between two remote NV centres have already been experimentally demonstrated [24,25]. A further and significant step towa...