Single-molecule electron spin resonance by means of atomic force microscopy.

Understanding and controlling decoherence in open quantum systems is of fundamental interest in science, whereas achieving long coherence times is critical for quantum information processing1. Although great progress was made for individual systems, and electron spin resonance (ESR) of single spins with nanoscale resolution has been demonstrated2-4, the understanding of decoherence in many complex solid-state quantum systems requires ultimately controlling the environment down to atomic scales, as potentially enabled by scanning probe microscopy with its atomic and molecular characterization and manipulation capabilities. Consequently, the recent implementation of ESR in scanning tunnelling microscopy5-8 represents a milestone towards this goal and was quickly followed by the demonstration of coherent oscillations9,10 and access to nuclear spins11 with real-space atomic resolution. Atomic manipulation even fuelled the ambition to realize the first artificial atomic-scale quantum devices12. However, the current-based sensing inherent to this method limits coherence times12,13. Here we demonstrate pump-probe ESR atomic force microscopy (AFM) detection of electron spin transitions between non-equilibrium triplet states of individual pentacene molecules. Spectra of these transitions exhibit sub-nanoelectronvolt spectral resolution, allowing local discrimination of molecules that only differ in their isotopic configuration. Furthermore, the electron spins can be coherently manipulated over tens of microseconds. We anticipate that single-molecule ESR-AFM can be combined with atomic manipulation and characterization and thereby paves the way to learn about the atomistic origins of decoherence in atomically well-defined quantum elements and for fundamental quantum-sensing experiments.

Parahydrogen Hyperpolarization Allows Direct NMR Detection of α-Amino Acids in Complex (Bio)mixtures.

The scope of non-hydrogenative parahydrogen hyperpolarization (nhPHIP) techniques has been expanding over the last years, with the continuous addition of important classes of substrates. For example, pyruvate can now be hyperpolarized using the Signal Amplification By Reversible Exchange (SABRE) technique, offering a fast, efficient and low-cost PHIP alternative to Dynamic Nuclear Polarization for metabolic imaging studies. Still, important biomolecules such as amino acids have so far resisted PHIP, unless properly functionalized. Here, we report on an approach to nhPHIP for unmodified α-amino acids that allows their detection and quantification in complex mixtures at sub-micromolar concentrations. This method was tested on human urine, in which natural α-amino acids could be measured after dilution with methanol without any additional sample treatment.

Determination of hydrogen exchange and relaxation parameters in PHIP complexes at micromolar concentrations.

Non-hydrogenative para-hydrogen-induced polarization (PHIP) is a fast, efficient and relatively inexpensive approach to enhance nuclear magnetic resonance (NMR) signals of small molecules in solution. The efficiency of this technique depends on the interplay of NMR relaxation and kinetic processes, which, at high concentrations, can be characterized by selective inversion experiments. However, in the case of dilute solutions this approach is clearly not viable. Here, we present alternative PHIP-based NMR experiments to determine hydrogen and hydride relaxation parameters as well as the rate constants for para-hydrogen association with and dissociation from asymmetric PHIP complexes at micromolar concentrations. Access to these parameters is necessary to understand and improve the PHIP enhancements of (dilute) substrates present in, for instance, biofluids and natural extracts.

Parahydrogen induced hyperpolarization provides a tool for NMR metabolomics at nanomolar concentrations.

An NMR approach based on parahydrogen hyperpolarization is presented to detect and resolve specific classes of metabolites in complex biomixtures at down to nanomolar concentrations. We demonstrate our method on solid phase extracts of urine, by simultaneously observing hundreds of metabolites well below the limits of detection of thermal NMR.