Knight-Field-Enabled Nuclear Spin Polarization in Single Quantum Dots

Abstract

We demonstrate dynamical nuclear-spin polarization in the absence of an external magnetic field by resonant circularly polarized optical excitation of a single electron or hole charged quantum dot. Optical pumping of the electron spin induces an effective inhomogeneous magnetic (Knight) field that determines the direction along which nuclear spins could polarize and enables nuclear-spin cooling by suppressing depolarization induced by nuclear dipole-dipole interactions. Our experiments constitute a first step towards a quantum measurement of the Overhauser field.

Nuclear spin effects in semiconductor quantum dots

“…This still holds for so-called 'non-resonant' excitation when the laser is tuned up to 100-200 meV above the QD lowest energy levels [21][22][23][24][25]34,54 . Very efficient nuclear spin pumping also occurs under resonant excitation into the lowest energy states of the dot 33, 38,39,57,58 .…”

“…However, there are a number of ways to overcome the energy mismatch and to induce a preferential pumping direction, both by optical and electrical means. There have been many attempts to find an efficient way of achieving DNP [21][22][23][24][25]33,34,49,52,53 , with the main motivation to achieve a 100% polarization of nuclear spins, which would prevent nuclear-nuclear flip-flops, strongly suppressing the randomness in the nuclear field and concomitant electron spin decoherence 10 .…”

“…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 .…”

“…DNP is readily observed under excitation of QDs with circularly polarized light [21][22][23][24][25][33][34][35][36][37][38]54,55 : when σ + or σ − polarized photons are absorbed by the sample, electrons with well-defined spin orientation may be created 14,56 . This still holds for so-called 'non-resonant' excitation when the laser is tuned up to 100-200 meV above the QD lowest energy levels [21][22][23][24][25]34,54 .…”

Electron spin-flip correlations due to nuclear dynamics in driven GaAs double dots

“…The coupling of the electrons to the underlying nuclear environment plays an important role in spintronics [1] and utilization of electron spins for quantum computation [2][3][4]. More generally, the interaction between a driven electron-spin qubit and its many-body environment leads to complex dynamical phenomena which are absent if the system is assumed to be at equilibrium with its environment [5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Thus, a qubit can be utilized as a probe for studying out-of-equilibrium physics in interacting quantum systems.…”