Alexander Sushkov

Nuclear magnetic resonance detection and spectroscopy of single proteins using quantum logic

Sensing single proteins with diamonds Nuclear magnetic resonance is a powerful technique for medical imaging and the structural analysis of materials, but is usually associated with large-volume samples. Lovchinsky et al. exploited the magnetic properties of a single spin associated with a defect in diamond and manipulated it with a quantum-logic protocol. They demonstrated the magnetic resonance detection and spectroscopy of multiple nuclear species within individual ubiquitin proteins attached to a specially treated diamond surface at room temperature. Science, this issue p. 836 The quantum properties of diamond are used for magnetic resonance spectroscopy of single proteins. Nuclear magnetic resonance spectroscopy is a powerful tool for the structural analysis of organic compounds and biomolecules but typically requires macroscopic sample quantities. We use a sensor, which consists of two quantum bits corresponding to an electronic spin and an ancillary nuclear spin, to demonstrate room temperature magnetic resonance detection and spectroscopy of multiple nuclear species within individual ubiquitin proteins attached to the diamond surface. Using quantum logic to improve readout fidelity and a surface-treatment technique to extend the spin coherence time of shallow nitrogen-vacancy centers, we demonstrate magnetic field sensitivity sufficient to detect individual proton spins within 1 second of integration. This gain in sensitivity enables high-confidence detection of individual proteins and allows us to observe spectral features that reveal information about their chemical composition.

Observation of the thermal Casimir force

A thermal Casimir force—an attraction between two metal surfaces caused by thermal, rather than quantum, fluctuations in the electromagnetic field—has now been identified experimentally between a flat and a spherical gold plate.

Nanoscale NMR spectroscopy and imaging of multiple nuclear species

Nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) provide non-invasive information about multiple nuclear species in bulk matter, with wide-ranging applications from basic physics and chemistry to biomedical imaging. However, the spatial resolution of conventional NMR and MRI is limited to several micrometres even at large magnetic fields (>1 T), which is inadequate for many frontier scientific applications such as single-molecule NMR spectroscopy and in vivo MRI of individual biological cells. A promising approach for nanoscale NMR and MRI exploits optical measurements of nitrogen-vacancy (NV) colour centres in diamond, which provide a combination of magnetic field sensitivity and nanoscale spatial resolution unmatched by any existing technology, while operating under ambient conditions in a robust, solid-state system. Recently, single, shallow NV centres were used to demonstrate NMR of nanoscale ensembles of proton spins, consisting of a statistical polarization equivalent to ∼100-1,000 spins in uniform samples covering the surface of a bulk diamond chip. Here, we realize nanoscale NMR spectroscopy and MRI of multiple nuclear species ((1)H, (19)F, (31)P) in non-uniform (spatially structured) samples under ambient conditions and at moderate magnetic fields (∼20 mT) using two complementary sensor modalities.

Quantum error correction for metrology.

We propose and analyze a new approach based on quantum error correction (QEC) to improve quantum metrology in the presence of noise. We identify the conditions under which QEC allows one to improve the signal-to-noise ratio in quantum-limited measurements, and we demonstrate that it enables, in certain situations, Heisenberg-limited sensitivity. We discuss specific applications to nanoscale sensing using nitrogen-vacancy centers in diamond in which QEC can significantly improve the measurement sensitivity and bandwidth under realistic experimental conditions.

A multiferroic material to search for the permanent electric dipole moment of the electron

We describe the first-principles design and subsequent synthesis of a new material with the specific functionalities required for a solid-state-based search for the permanent electric dipole moment of the electron. We show computationally that perovskite-structure europium barium titanate should exhibit the required large and pressure-dependent ferroelectric polarization, local magnetic moments and absence of magnetic ordering at liquid-helium temperature. Subsequent synthesis and characterization of Eu(0.5)Ba(0.5)TiO(3) ceramics confirm the predicted desirable properties.

Surface Contact Potential Patches and Casimir Force Measurements

We present calculations of contact potential surface patch effects that simplify previous treatments. It is shown that, because of the linearity of Laplaces equation, the presence of patch potentials does not affect an electrostatic calibration of a two-plate Casimir measurement apparatus. Using models that include long-range variations in the contact potential across the plate surfaces, a number of experimental observations can be reproduced and explained. For these models, numerical calculations show that if a voltage is applied between the plates which minimizes the force, a residual electrostatic force persists, and that the minimizing potential varies with distance. The residual force can be described by a fit to a simple two-parameter function involving the minimizing potential and its variation with distance. We show the origin of this residual force by use of a simple parallel capacitor model. Finally, the implications of a residual force that varies in a manner different from

Cosmic Axion Spin Precession Experiment (CASPEr)

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Measurement of the Short-Range Attractive Force between Ge Plates Using a Torsion Balance

on the accuracy of previous Casimir measurements is discussed.

All-Optical Sensing of a Single-Molecule Electron Spin

We propose an experiment to search for QCD axion and axionlike-particle dark matter. Nuclei that are interacting with the background axion dark matter acquire time-varying C P -odd nuclear moments such as an electric dipole moment. In analogy with nuclear magnetic resonance, these moments cause precession of nuclear spins in a material sample in the presence of an electric field. Precision magnetometry can be used to search for such precession. An initial phase of this experiment could cover many orders of magnitude in axionlike-particle parameter space beyond the current astrophysical and laboratory limits. And with established techniques, the proposed experimental scheme has sensitivity to QCD axion masses m a ≲ 1 0 - 9 eV , corresponding to theoretically well-motivated axion decay constants f a ≳ 1 0 16 GeV . With further improvements, this experiment could ultimately cover the entire range of masses m a ≲ μ eV , complementary to cavity searches.

Quantum Metrology Enhanced by Repetitive Quantum Error Correction.

We have measured the short-range attractive force between crystalline Ge plates, and found contributions from both the Casimir force and an electrical force possibly generated by surface patch potentials. Using a model of surface patch effects that generates an additional force due to a distance dependence of the apparent contact potential, the electrical force was parametrized using data at distances where the Casimir force is relatively small. Extrapolating this model, to provide a correction to the measured force at distances less than 5 microm, shows a residual force that is in agreement, within experimental uncertainty, with five models that have been used to calculate the Casimir force.