Nuclear magnetic resonance on a single quantum dot and a quantum dot in a nanowire system: quantum photonics and opto-mechanical coupling

Self-assembled semiconductor quantum dots (QD) are excellent single photon sources and possible hosts for electron spin qubits, which can be initialized, manipulated and read-out optically. The nuclear spins in nano-structured semiconductors play a central role in quantum applications. The nuclear spins represent a useful resource for generating local magnetic fields but nuclear spin noise represents a major source of dephasing for spin qubits. Controlling the nuclear spins enhances the resource while suppressing the noise. Nuclear magnetic resonance (NMR) techniques are challenging: the group-III and group-V isotopes have large spins with widely different gyromagnetic-ratios; in strained material there are large atom-dependent quadrupole-shifts; nano-scale NMR is hard to detect. We report NMR on 100,000 nuclear spins of a quantum dot using chirped radio-frequency pulses. Following
polarization, we demonstrate a reversal of the nuclear spin. We can flip the nuclear spin back-and-forth a hundred times. We demonstrate that chirped-NMR is a powerful way of determining the chemical composition, the initial nuclear spin temperatures and quadrupole frequency distributions
for all the main isotopes. The key observation is a plateau in the NMR signal as a function of sweep-rate: we achieve inversion at the first quantum transition for all isotopes simultaneously. These experiments represent a generic technique for manipulating nano-scale inhomogeneous nuclear spin ensembles and open the way to probe the coherence of such mesoscopic systems. For most solid state electron spin qubits in GaAs one unmastered source of decoherence is the hyperfine interaction with the nuclear spins, whose coherence is inevitably limited by nuclear dipole-dipole interactions. Resent work on uncharged QDs showed that in strained nano-structures quadrupolar effects suppress dipole-dipole interactions and prolong nuclear spin coherence times up to a few ms. It has been argued this would also lead to enhanced electron spin coherence times. However, the effect of actually loading the QD with an electron on nuclear spin coherence has so far only been investigated theoretically. Here we measure the nuclear spin ensemble coherence for a single InGaAs quantum dot embedded in a charge tunable device. For an empty dot we confirm Hahn echo coherence times T2 of a few ms. In contrast, on charging with a single electron T2 drops by more than a factor 100 down to a few tens of µs. The reduction of coherence is explained by electron mediated coupling between nuclear spins due to the hyperfine interaction, an example of RKKY-type interaction. Charging the QD with two electrons (a singlet state) recovers the T2 times of the empty dot, ruling out any systematic errors resulting from the switching process itself.
Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. While the top-down fabrication of such structures remains a technological challenge, their bottom up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-in-nanowire system which reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometer precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells. We show that optically-active quantum dots (QDs)embedded in MBE-grown GaAs/AlGaAs core-shell nanowires (NWs) are coupled to the NW mechanical motion. Oscillations of the NW modulate the QD emission energy in a broad range exceeding 14 meV. Furthermore, this opto-mechanical interaction enables the dynamical tuning of two neighbouring QDs into resonance, possibly allowing for emitter-emitter coupling. Both the QDs and the coupling mechanism - material strain - are intrinsic to the NW structure and do not depend on any functionalization or external field. Such systems open up the prospect of using QDs to probe and control the mechanical state of a NW, or conversely of making a quantum non-demolition readout of a QD state through a position measurement.