During the past decade, research into superconducting quantum bits (qubits) based on Josephson junctions has made rapid progress. Many foundational experiments have been performed, and superconducting qubits are now considered one of the most promising systems for quantum information processing. However, the experimentally reported coherence times are likely to be insufficient for future large-scale quantum computation. A natural solution to this problem is a dedicated engineered quantum memory based on atomic and molecular systems. The question of whether coherent quantum coupling is possible between such natural systems and a single macroscopic artificial atom has attracted considerable attention since the first demonstration of macroscopic quantum coherence in Josephson junction circuits. Here we report evidence of coherent strong coupling between a single macroscopic superconducting artificial atom (a flux qubit) and an ensemble of electron spins in the form of nitrogen-vacancy colour centres in diamond. Furthermore, we have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3 × 10(7) such colour centres. This provides a foundation for future quantum memories and hybrid devices coupling microwave and optical systems.

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Single-crystalline Sr1−xLaxCuO2 thin films of electron-doped infinite-layer compounds were grown by molecular-beam epitaxy. Crucial to our success was the use of KTaO3 substrates. The best film showed Tconset=41.5 K and Tczero=39.0 K, which is close to the highest Tconset of 43 K for the bulk value. The resistivity of the optimum-doped film exhibited metallic temperature dependence with a low resistivity of 320 μΩ cm at room temperature and 120 μΩ cm just above Tc.

We report the growth of n-type superconducting T′-(La,Ce)2CuO4 and infinite-layer (IL) (Sr,La)CuO2 thin films by means of molecular beam epitaxy (MBE). The bulk synthesis of T′-(La,Ce)2CuO4 and IL-(Sr,La)CuO2 requires complicated techniques: synthesis at low temperatures below 600 °C for the former and at high pressures above 3 GPa for the latter. This makes it difficult to grow bulk single crystals. We have found, however, that high-quality single-crystalline films of both compounds can be rather easily prepared by thin-film processes. Single-phase T′-(La,Ce)2CuO4 films can be obtained for a wide range of x (0.0 ≤ x ≤ ∼0.4). The best Tcend is over 30 K, which is the highest in the T′ family. For IL-(Sr,La)CuO2, by using KTaO3 substrates, high Tcend over 39 K and also metallic resistivity were achieved for the first time to our knowledge. We describe the key parameters in the growth and the properties of the resultant films.

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