Popular Summary: Magnetic resonance imaging (MRI) is a commonly used technique for medical imaging. MRI uses static and time-dependent magnetic fields to detect the collective response of large ensembles of nuclear spins from molecules localized within millimeter-scale volumes in the body. Increasing the detection resolution from the millimeter to nanometer range would be a technological dream come true. However, detection of nanoscale magnetic resonance signals within the realm of the conventional MRI faces two challenges: The magnetic field strength and configurations required are either unrealistic or impractical, and the naturally occurring quantum spin fluctuations can overwhelm the thermal spin polarization—a fundamental property of nanoscale collections of spins. It is not difficult to imagine, then, that radically different techniques that can achieve such resolution enhancement in magnetic resonance detection would be a new breakthrough. In this experimental paper, we demonstrate a new paradigm for nuclear magnetic resonance imaging and spectroscopy on the nanometer scale.
Our technique is based on two unique components: (i) a novel spin-manipulation protocol that encodes temporal correlations in the statistical polarization of nuclear spins in the sample by periodically applying radio-frequency magnetic field pulses and (ii) the generation of intense magnetic field pulses by focusing current through a nanoscale metal constriction. Together with an ultrasensitive magnetic resonance sensor based on a silicon-nanowire oscillator, our approach allows us to coherently manipulate and detect nanoscale ensembles of nuclear spins. In a proof-of-principle demonstration, we have successfully imaged proton spins in a polystyrene sample with a roughly 10-nm spatial resolution.
Although remarkably different from the conventional MRI techniques in the two fundamental aspects discussed above, our technique can easily incorporate all established pulsed magnetic resonance techniques. Looking forward, we foresee this technique becoming a paradigm for nanoscale magnetic resonance imaging and spectroscopy.