Magnetic resonance force microscopy : harnessing nuclear spin fluctuations

Abstract

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.