Magnetic resonance force microscopy : harnessing nuclear spin fluctuations

Over the past few years, a wide variety of nuclear spin preparation techniques using hyperfine interaction-mediated
dynamics have been developed in systems including gate-defined double quantum dots, self-assembled
single quantum dots and nitrogen-vacancy centers in diamond. Here, we present a novel approach to nuclear
spin state preparation by harnessing the naturally occuring stochastic fluctuations in nanoscale ensembles of
nuclear spins in a semiconductor nanowire. Taking advantage of the excellent sensitivity of magnetic resonance
force microscopy (MRFM) to monitor the statistical polarization fluctuations in samples containing very
few nuclear spins, we develop real-time spin manipulation protocols that allow us to measure and control the
spin fluctuations in the rotating frame. We focus on phosphorus and hydrogen nuclear spins associated with
an InP and a GaP nanowire and their hydrogen-containing adsorbate layers. The weak magnetic moments of
these spins can be detected with high spatial resolution using the outstanding sensitivty of MRFM. Recently,
MRFM has been used to image the proton spin density in a tobacco mosaic virus with a sensitivity reaching
up to 100 net polarized spins. We describe how MRFM together with real-time radio frequency (RF) control
techniques can also be used for the hyperpolarization, narrowing and storage of nuclear spin fluctuations and
discuss how such nuclear spin states could potentially be harnessed for applications in magnetic resonance and
quantum information processing.
In addition to presenting the experimental results on nuclear spin order, the theory of nuclear spin resonance
and nanomechanical resonators is briefy discussed. The physical concepts explained provide the necessary
background for the understanding of our MRFM experiments. The MRFM experimental apparatus, both sample-on-
cantilever and magnet-on-cantilever, is also presented in considerable detail.