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The high‐pressure and high‐temperature (HPHT) diamonds with well‐controlled defects and low stress offer an ideal host substrate for the fields of quantum information science; however, fabrication of quantum‐grade HPHT diamonds remains a pressing challenge. Here, a major advance in generating highly coherent nitrogen‐vacancy (NV) centers, a promising spin defect in diamonds, via tailored HPHT synthesis and postgrown annealing treatment is reported. The resulting well‐dispersed single NV− centers in type‐IIa diamonds exhibit long spin coherence times comparable to that of the reported chemical vapor deposition diamonds. Moreover, high‐density NV− ensembles in 〈100〉‐grown type‐Ib diamonds with superb zero‐phonon lines considerably sharper than those of native NV− centers in as‐grown diamonds, together with low splitting of resonances are produced. These findings demonstrate a superior synthesis and optimization protocol for creating high‐quality NV− centers avoiding lattice damage in diamond to meet the stringent requirements for a wide range of emerging quantum technologies. [ABSTRACT FROM AUTHOR]

High‐density magnetic memories are essential components for spintronics, quantum computing, and energy‐efficient electronics. Miniaturization of magnetic storage units requires reduced dimensionality and magnetic domain stability at the nanoscale. However, inducing magnetic order and selectively tuning spin‐orbital coupling at specific locations is challenging. Here, an unprecedented approach is demonstrated to construct switchable magnetic nanoislands in a nonmagnetic matrix based on cobaltite homostructures. The magnetic and electronic states are laterally modified by epitaxial strain, which is regionally controlled by freestanding membranes. Tensile‐strained cobaltite layers exhibit ferromagnetic properties, while compressively strained cobaltite layers exhibit a small magnetic response to applied fields. The minimum size of magnetic nanoislands reaches ≈35 nm in diameter, suggesting the highest possible areal density can reach upto ≈400 Gbit/in2. This methodology provides an ideal platform for precisely controlled read/write schemes and enables scalable and patterned memories on silicon and flexible supports for various applications. [ABSTRACT FROM AUTHOR]

We investigate the Ramsey fringes of a single electron spin of the nitrogen-vacancy center in diamond with different microwave radiation frequency detunings. The fast Fourier transform demonstrates that the Ramsey fringes consist of three components caused by hyperfine interaction with 14 N nuclear spin, and the Ramsey fringes cannot be well explained without the phase term of the three components, which has not been mentioned before. Each phase is determined by the microwave frequency detuning and the resonant Rabi frequency as well as the π/2 pulse. [ABSTRACT FROM AUTHOR]

Here, we report an experimental realization of optimal phase-covariant quantum cloning machine with a single electron spin in solid state system at room temperature. The involved three states of two logic qubits are encoded physically in three levels of a single electron spin with two Zeeman sub-levels at a nitrogen-vacancy defect center in diamond. The preparation of input state and the phase-covariant quantum cloning transformation is controlled by two independent microwave fields. The average experimental fidelity reaches 85.2% which is very close to theoretical optimal fidelity 85.4% and is beyond the bound 83.3% of universal cloning. [ABSTRACT FROM AUTHOR]

Quantum coherence control usually requires low temperature environments. Even for nitrogen-vacancy center spins in diamond, a remarkable exception, the coherence signal is limited to about 700 K due to the quench of the spin-dependent fluorescence at a higher temperature. Here we overcome this limit and demonstrate quantum coherence control of the electron spins of nitrogen-vacancy centers in nanodiamonds at temperatures near 1000 K. The scheme is based on initialization and readout of the spins at room temperature and control at high temperature, which is enabled by pulse laser heating and rapid diffusion cooling of nanodiamonds on amorphous carbon films. Using the diamond magnetometry based on optically detected magnetic resonance up to 800 K, we observe the magnetic phase transition of a single nickel nanoparticle at about 615 K. This work enables nano-thermometry and nano-magnetometry in the high-temperature regime. The spins of nitrogen vacancy centres have been shown to be sensitive, high-temperature quantum probes of magnetic fields but the usual readout scheme fails above 550 K. Here the authors demonstrate coherent control and sensing up to 1000 K by using fast temperature control to reach the conditions for readout. [ABSTRACT FROM AUTHOR]

Diamond nitrogen-vacancy (NV) center-based magnetometry provides a unique opportunity for quantum bio-sensing. However, NV centers are not sensitive to parameters such as temperature and pressure, and immune to many biochemical parameters such as pH and non-magnetic biomolecules. Here, we propose a scheme that can potentially enable the measurement of various biochemical parameters using diamond quantum sensing, by employing stimulus-responsive hydrogels as a spacing transducer in-between a nanodiamond (ND, with NV centers) and magnetic nanoparticles (MNPs). The volume phase transition of hydrogel upon stimulation leads to sharp variation in the separation distance between the MNPs and the ND. This in turn changes the magnetic field that the NV centers can detect sensitively. We construct a temperature sensor under this hybrid scheme and show the proof-of-the-principle demonstration of reversible temperature sensing. Applications in the detection of other bio-relevant parameters are envisioned if appropriate types of hydrogels can be engineered. [ABSTRACT FROM AUTHOR]