Niklas Sköld

Quantum memories : a review based on the European integrated project "Qubit Applications (QAP)"

AbstractWe perform a review of various approaches to the implementation of quantum memories, with an emphasis on activities within the quantum memory sub-project of the EU integrated project “Qubit Applications”. We begin with a brief overview over different applications for quantum memories and different types of quantum memories. We discuss the most important criteria for assessing quantum memory performance and the most important physical requirements. Then we review the different approaches represented in “Qubit Applications” in some detail. They include solid-state atomic ensembles, NV centers, quantum dots, single atoms, atomic gases and optical phonons in diamond. We compare the different approaches using the discussed criteria.

Sharp exciton emission from single InAs quantum dots in GaAs nanowires

We have performed photoluminescence spectroscopy on single GaAs nanowires with InAs quantum dots in the form of thin slices of InAs, possibly alloyed with Ga as InGaAs, incorporated into the GaAs. The nanowires were grown by chemical beam epitaxy using gold nanoparticles as catalysts. The photoluminescence measurements showed rich spectra consisting of sharp lines with energies and excitation power dependency behavior very similar to that observed for Stranski–Krastanow-grown InAs/GaAs quantum dots. By reducing the excitation power density we were able to obtain a quantum dot spectrum consisting of only one single sharp line—the exciton line.

Electric-field-induced coherent coupling of the exciton states in a single quantum dot

The ability to generate entangled photon pairs from a quantum dot critically depends on the size of the fine-structure splitting of its exciton states. A demonstration of the ability to tune this splitting with an electric field represents a promising step in the use of quantum dots to generate entangled photon pairs on demand.

Exciton-Spin Memory with a Semiconductor Quantum Dot Molecule

We report on a single photon and spin storage device based on a semiconductor quantum dot molecule. Optically excited single electron-hole pairs are trapped within the molecule, and their recombination rate is electrically controlled over 3 orders of magnitude. Single photons are stored up to 1 μs and read out on a subnanosecond time scale. By using resonant excitation, the circular polarization of individual photons is transferred into the spin state of electron-hole pairs with a fidelity above 80%, which does not degrade for storage times up to the 12.5 ns repetition period of the experiment.

Determination of diffusion lengths in nanowires using cathodoluminescence

We used cathodoluminescence imaging to determine diffusion lengths in III-V semiconductor nanowires, grown by metal-organic chemical vapor deposition seeded by gold nanoparticles. Intensity profiles were recorded either from the interface between the substrate and homogeneous nanowires, or from segments in nanowires containing axial heterostructures to determine the diffusion length. We determined diffusion lengths of 0.10 to 0.90 mu m, the shortest for uncapped wires. The reduction is attributed largely to surface recombination

Nanofluidics in hollow nanowires.

We present a novel scheme for producing nanotube membranes using free-standing hollow nanowires, with easily controllable dimensions. GaAs-AlInP core-shell nanowires were grown by metal-organic vapor phase epitaxy and were partially embedded in a polymer film. The GaAs core and substrate were etched selectively, leaving tubes with open access to both sides of the membrane. Electrophoretic transport of T4-phage DNA through the hollow nanowires was demonstrated using epifluorescence microscopy.

Nanowire growth and dopants studied by cross-sectional scanning tunnelling microscopy

Using a crystalline embedding scheme it has recently become possible to study free-standing III–V nanowires with cross-sectional scanning tunnelling microscopy (XSTM). In the present paper we discuss how this novel method can be used for direct atomically resolved imaging of the interior of nanowires. We will focus in particular on two areas where this method provides unique possibilities. First we discuss the growth of the nanowire at the substrate as studied by XSTM and determine the facets of the nanowire growth on the surface and at the onset of free-standing nanowire growth. Second, we demonstrate how individual defects can be studied inside the wires, indicating a unique way for investigating dopant structures and concentrations in nanowires. We identify a carbon defect/dopant in the nanowire positioned on arsenic sites and establish quantitative limits on the defect density in the nanowires.

Voltage tunability of single-spin states in a quantum dot

Single spins in the solid state offer a unique opportunity to store and manipulate quantum information, and to perform quantum-enhanced sensing of local fields and charges. Optical control of these systems using techniques developed in atomic physics has yet to exploit all the advantages of the solid state. Here we demonstrate voltage tunability of the spin energy-levels in a single quantum dot by modifying how spins sense magnetic field. We find that the in-plane g-factor varies discontinuously for electrons, as more holes are loaded onto the dot. In contrast, the in-plane hole g-factor varies continuously. The device can change the sign of the in-plane g-factor of a single hole, at which point an avoided crossing is observed in the two spin eigenstates. This is exactly what is required for universal control of a single spin with a single electrical gate.

All-electrical coherent control of the exciton states in a single quantum dot

Cavendish Laboratory, Cambridge University, JJ Thomson Avenue, Cambridge, CB3 0HE, U. K.(Dated: November 12, 2010)We demonstrate high- delity reversible transfer of quantum information from the polarisation ofphotons into the spin-state of an electron-hole pair in a semiconductor quantum dot. Moreover,spins are electrically manipulated on a sub-nanosecond timescale, allowing us to coherently controltheir evolution. By varying the area of the electrical pulse, we demonstrate phase-shift and spin-ipgate operations with near-unity delities. Our system constitutes a controllable quantum interfacebetween ying and stationary qubits, an enabling technology for quantum logic in the solid-state.

A quantum dot single photon source driven by resonant electrical injection

We present a demonstration of single photon emission from an entirely electrically driven resonant injection quantum dot device. We selectively measure the emission from a single dot in the ensemble by tuning the applied bias so as to induce resonant tunneling into the dot. Direct injection of carriers into the dot leads to a suppression of background light, allowing us to demonstrate single photon emission from a single dot with no spectral filtering. We study the effects limiting the linewidths of photons emitted from the device.