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.
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.