D. A. Williams

Charge-Qubit Operation of an Isolated Double Quantum Dot

We have investigated coherent time evolution of pseudomolecular states of an isolated (leadless) silicon double quantum dot, where operations are carried out via capacitively coupled elements. Manipulation is performed by short pulses applied to a nearby gate, and measurement is performed by a single-electron transistor. The electrical isolation of this qubit results in a significantly longer coherence time than previous reports for semiconductor charge qubits realized in artificial molecules.

Tunneling Anisotropic Magnetoresistance in Multilayer-(Co/Pt )/AlOx/Pt Structures

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a nonmagnetic Pt counterelectrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. The discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.

Electrically pumped single-photon sources in lateral p-i-n junctions

Electrically pumped single-photon sources using semiconductor quantum dots are of interest as they can be integrated with other semiconductor devices, using standard processing techniques. In this letter, we report electroluminescence from single quantum dots in a lateral p-i-n junction. Exciton and biexciton emission from a single quantum dot can be achieved under different electrical bias conditions. Antibunching effects from exciton and biexciton emission are observed using cw and pulsed electrical injection, indicating single-photon emission; this can be used for quantum information processing.

Strongly coupled single quantum dot in a photonic crystal waveguide cavity

Cavities embedded in photonic crystal waveguides offer a promising route toward large scale integration of coupled resonators for quantum electrodynamics applications. In this letter, we demonstrate a strongly coupled system formed by a single quantum dot and such a photonic crystal cavity. The resonance originating from the cavity is clearly identified from the photoluminescence mapping of the out-of-plane scattered signal along the photonic crystal waveguide. The quantum dot exciton is tuned toward the cavity mode by temperature control. A vacuum Rabi splitting of ∼140 μeV is observed at resonance.

Hole transport in coupled SiGe quantum dots for quantum computation

We describe transport measurements on double quantum dot structures formed by trench isolation in a SiGe:Si heterostructure. Three different device geometries are described, and a number of phenomena are observed. Transport measurements at 4.2 K reveal a carrier energy filtering effect accompanying a period doubling in Coulomb oscillations, showing that tunnel barriers can be raised and lowered by application of a gate voltage. Peak splitting in Coulomb oscillations is also observed at 4.2 K, indicating interdot capacitive coupling. The stability diagram for a double dot is mapped out at dilution refrigerator temperatures. In another device, single hole electrometers are fabricated 50 nm away from a double quantum dot, and the ability to measure a single excess charge on the double dot is demonstrated at dilution refrigerator temperatures.

Nanoscale solid-state quantum computing

Most experts agree that it is too early to say how quantum computers will eventually be built, and several nanoscale solid–state schemes are being implemented in a range of materials. Nanofabricated quantum dots can be made in designer configurations, with established technology for controlling interactions and for reading out results. Epitaxial quantum dots can be grown in vertical arrays in semiconductors, and ultrafast optical techniques are available for controlling and measuring their excitations. Single–walled carbon nanotubes can be used for molecular self–assembly of endohedral fullerenes, which can embody quantum information in the electron spin. The challenges of individual addressing in such tiny structures could rapidly become intractable with increasing numbers of qubits, but these schemes are amenable to global addressing methods for computation.

Isolated double quantum dot capacitively coupled to a single quantum dot single-electron transistor in silicon

We report electron transport measurements on a single-island single-electron transistor capacitively coupled to an isolated double quantum dot at 4.2 K. The structure is fabricated through trench isolation in silicon-on-insulator. We detect single-electron polarization of the isolated double quantum dot using the single-electron transistor as a sensitive electrometer, and estimate its charging energy. We observe a large suppression of current and modulation of Coulomb blockade peak heights as a function of applied gate voltages.

“Plug and play” single-photon sources

The authors report a “plug and play” source of single photons, with full integration to a single-mode optical fiber. One end of the fiber is attached to the top of an InGaAs∕GaAs quantum dot wafer. The other end is connected via a wavelength-division multiplexing system to two separate fibers: one for carrying excitation light and the other for emitted light. A Hanbury-Brown and Twiss [Nature (London) 77, 27 (1956)] measurement was performed on the emission from single excitons recombining in the quantum dots. A second-order correlation function at zero time delay of approximately 0.01 indicates a nearly ideal source of single photons. The maximum variation of peak position over 24days is less than 0.1nm.

Registration of single quantum dots using cryogenic laser photolithography

We have registered the position of single InGaAs quantum dots using a cryogenic laser photolithography technique. This is an important advance towards the reproducible fabrication of solid-state cavity quantum electrodynamic devices, a key requirement for commercial exploitation of quantum information processing. The quantum dot positions were registered with an estimated accuracy of 50 nm by fabricating metal alignment markers around them. Photoluminescence spectra from quantum dots before and after marker fabrication were identical except for a small redshift (~1 nm), probably introduced during the reactive ion etching.

Lateral p-n Junction in Modulation Doped AlGaAs/GaAs.

Lateral p-n junctions have advantages for monolithic integration with other electronic devices and can improve the modulation bandwidth of light emitting devices. An alternative approach to existing fabrication methods is presented. Simulations show that recombination takes place inside the undoped channel and that the lateral position of the junction can be defined via etching. In order to demonstrate the concept, a simple device was fabricated and characterized.