QUROPE Quantum Information Processing and Communication in Europe

Quantum information devices promise unique opportunities in information technology. Physicists are intrigued with building such devices because they probe our understanding of the nature of quantum mechanics. Quantum effects, although providing the basis of atomic, molecular, and solid state physics, usually are not observed in everyday life because the highly fragile nature of coherence and entanglement requires extensive shielding against environmental effects.

The artificial creation of shallow nitrogen-vacancy (NV) centres in diamond with 25 nm lateral resolution is performed by collimated implantation of low-energy nitrogen ions. The electron spin associated to this defect is the most promising qubit for a scalable quantum computer working at room temperature. Individual optical addressing of two single centres separated by only 16 nm is demonstrated with stimulated emission depletion (STED) microscopy.

There has been much recent interest in extending the technique of magnetic resonance imaging (MRI) down to the level of single spins with sub-optical wavelength resolution. However, the signal to noise ratio for images of individual spins is usually low and this necessitates long acquisition times and low temperatures to achieve high resolution. An exception to this is the nitrogen-vacancy (NV) color center in diamond whose spin state can be detected optically at room temperature.

Devices that harness the laws of quantum physics hold the promise for information processing that outperforms their classical counterparts, and for unconditionally secure communication. However, in particular, implementations based on condensed-matter systems face the challenge of short coherence times. Carbon materials, particularly diamond, however, are suitable for hosting robust solid-state quantum registers, owing to their spin-free lattice and weak spin–orbit coupling.

Projective measurement of single electron and nuclear spins has evolved from a gedanken experiment to a problem relevant for applications in atomic-scale technologies like quantum computing. Although several approaches allow for detection of a spin of single atoms and molecules, multiple repetitions of the experiment that are usually required for achieving a detectable signal obscure the intrinsic quantum nature of the spin’s behavior.

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.