Optically tunable nuclear magnetic resonance in a single quantum dot

Abstract

We report optically detected nuclear magnetic resonance (ODNMR) measurements on small ensembles of nuclear spins in single GaAs quantum dots. Using ODNMR we make direct measurements of the inhomogeneous Knight field from a photo-excited electron which acts on the nuclei in the dot. The resulting shifts of the NMR peak can be optically controlled by varying the electron occupancy and its spin orientation, and lead to strongly asymmetric lineshapes at high optical excitation. The all-optical control of the NMR lineshape will enable position-selective control of small groups of nuclear spins in a dot. Our calculations also show that the asymmetric NMR peak lineshapes can provide information on the volume of the electron wave-function, and may be used for measurements of non-uniform distributions of atoms in nano-structures.Nuclear spins offer a nano-scale resource with extended spin life-times and coherence, leading to proposals to use nuclei for quantum computation [1][2][3][4] and coherent spin-memories [5]. Strong interest in nuclear spin effects in semiconductors has also been recently stimulated by research into manipulation of single spins in nano-structures, where the electron-nuclear (hyperfine) interaction plays an important role [6][7][8]. Direct control of nuclear spins by resonant techniques such as NMR is highly desirable for both electron and nuclear spin manipulation experiments. In the past NMR methods have been widely applied to large area semiconductor structures (heterojunctions, quantum wells etc) containing very large number of nuclei in the range 10 8 or more [4,[9][10][11]. Further refinement of these methods has made it possible to detect magnetic resonance of as few as 10 4 nuclei in otherwise abundant spin environments by detecting the optical response from individual GaAs quantum dot nano-structures [12,13]. These micro-ODNMR experiments revealed strong dot-to-dot variation of resonant frequencies [14], arising from interaction of small nuclear spin ensembles with random Knight fields from single spins of localized electrons [15].In this work we take advantage of the strong gradients of the Knight field inside a quantum dot produced by the localized electron spin and enter a new regime of nano-ODNMR. By employing ODNMR techniques first reported in Refs. [12,13], we measure with high precision the Knight shifts in the resonant frequencies of each individual isotope spin sub-system in individual GaAs/AlGaAs interface dots and find their dependence on the polarization and power of optical excitation. By varying the optical power, we find striking modifications of the lineshape of the NMR spectrum of the dot. These arise from the Knight field variation across the dot which in turn is determined by the spatial distribution of the electron wave-function. The interpretations are supported by calculations, which further demonstrate that by employing the inhomogeneities of the Knight shifts, it becomes possible to access selectively, by appropriate resonant frequencies, small groups of nuclear spi...

Nuclear spin effects in semiconductor quantum dots

“…Knight field gradients of the order of 10 3 T/cm can be achieved, potentially enabling determination of the nuclear spins position with resolution of 1 nm inside a single QD 16 .…”

“…The discrete exciton energy structure in QDs was successfully uti- lized: changes in the electron Overhauser shift induced by rf excitation could be measured with accuracy of a few μeV. NMR spectra were measured by simply stepping rf frequency through the resonance 12,16 . Recently, high-resolution optically detected NMR has been carried out in single self-assembled…”

“…The electron-nuclear HI results in a static effect contributing to the energies of the two spin systems, which is usually described in terms of effective magnetic fields: an Overhauser field, B nuc , acting on the electron, a result of interaction with a large number of nuclear spins 13,14,20 , and a Knight field experienced by individual nuclear spins as a result of interaction with the spin of the localized electron 13,14,16,21 . Importantly, HI also leads to a dynamical effect responsible for the transfer of spin between the two systems 13,14 .…”

“…These techniques reveal a wealth of structural information such as chemical composition and strain distribution in the volume of the wavefunction of the confined electron 19 , and present a powerful microscopy tool for non-invasive structural analysis of single QDs. In order to gain an additional enhancement in spatial resolution of NMR, the use of an effective magnetic (Knight) field of the photo-excited electron may be possible 16 .…”

“…Beyond the active research into a quiescent and controllable magnetic environment in solids, nuclear spins themselves have been suggested as a resource with extended coherence (potentially in the ms range) useful for QIP 15 . Very high nuclear polarization degrees now routinely achievable in QDs have also enabled nuclear magnetic resonance (NMR) in single QDs to be realized 12,[16][17][18] , which can be applied for non-invasive probing of chemical composition and strain in the volume occupied by the confined electron 19 .…”