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A series of recent experiments have shown that collision of ballistic electrons in semiconductors can be used to probe the indistinguishability of single-electron wavepackets. Perhaps surprisingly, their Coulomb interaction has not been seen due to screening. Here we show Coulomb-dominated collision of high-energy single electrons in counter-propagating ballistic edge states, probed by measuring partition statistics while adjusting the collision timing. Although some experimental data suggest antibunching behaviour, we show that this is not due to quantum statistics but to strong repulsive Coulomb interactions. This prevents the wavepacket overlap needed for fermionic exchange statistics but suggests new ways to utilize Coulomb interactions: microscopically isolated and time-resolved interactions between ballistic electrons can enable the use of the Coulomb interaction for high-speed sensing or gate operations on flying electron qubits.

Fault-tolerant spin-based quantum computers will require fast and accurate qubit read out. This can be achieved using radiofrequency reflectometry given sufficient sensitivity to the change in quantum capacitance associated with the qubit states. Here, we demonstrate a 23-fold improvement in capacitance sensitivity by supplementing a cryogenic semiconductor amplifier with a SQUID preamplifier. The SQUID amplifier operates at a frequency near 200MHz and achieves a noise temperature below 600mK when integrated into a reflectometry circuit, which is within a factor 120 of the quantum limit. It enables a record sensitivity to capacitance of 0.07 mml:mspace width=".1em"mml:mspace aF / mml:msqrt Hzmml:msqrt. The setup is used to acquire charge stability diagrams of a gate-defined double quantum dot in a short time with a signal-to-noise ration of about 38 in 1 mml:mspace width=".1em"mml:mspace mu s of integration time.

Power laws in physics have until now always been associated with a scale invariance originating from the absence of a length scale. Recently, an emergent invariance even in the presence of a length scale has been predicted by the newly-developed nonlinear-Luttinger-liquid theory for a one-dimensional (1D) quantum fluid at finite energy and momentum, at which the particle's wavelength provides the length scale. We present experimental evidence for this new type of power law in the spectral function of interacting electrons in a quantum wire using a transport-spectroscopy technique. The observed momentum dependence of the power law in the high-energy region matches the theoretical predictions, supporting not only the 1D theory of interacting particles beyond the linear regime but also the existence of a new type of universality that emerges at finite energy and momentum.

We demonstrate the generation of a spatiotemporal optical continuum in a highly nonlinear exciton–polariton waveguide using extremely low excitation powers (2-ps, 100-W peak power pulses) and a submillimeter device suitable for integrated optics applications. We observe contributions from several mechanisms over a range of powers and demonstrate that the strong light–matter coupling significantly modifies the physics involved in all of them. The experimental data are well understood in combination with theoretical modeling. The results are applicable to a wide range of systems with linear coupling between nonlinear oscillators and particularly to emerging polariton devices that incorporate materials, such as gallium nitride and transition metal dichalcogenide monolayers that exhibit large light–matter coupling at room temperature. These open the door to low-power experimental studies of spatiotemporal nonlinear optics in submillimeter waveguide devices. © 2019, The Author(s).

Disorder increasingly affects performance as electronic devices are reduced in size. The ionized dopants used to populate a device with electrons are particularly problematic, leading to unpredictable changes in the behavior of devices such as quantum dots each time they are cooled for use. We show that a quantum dot can be used as a highly sensitive probe of changes in disorder potential and that, by removing the ionized dopants and populating the dot electrostatically, its electronic properties become reproducible with high fidelity after thermal cycling to room temperature. Our work demonstrates that the disorder potential has a significant, perhaps even dominant, influence on the electron dynamics, with important implications for ‘‘ballistic’’ transport in quantum dots.

Quantum point contacts (QPCs) have shown promise as nanoscale spin-selective components for spintronic applications and are of fundamental interest in the study of electron many-body effects such as the 0.7 × 2e2/h anomaly. We report on the dependence of the 1D Landé g-factor g* and 0.7 anomaly on electron density and confinement in QPCs with two different top-gate architectures. We obtain g* values up to 2.8 for the lowest 1D subband, significantly exceeding previous in-plane g factor values in AlGaAs/GaAs QPCs and approaching that in InGaAs/InP QPCs. We show that g* is highly sensitive to confinement potential, particularly for the lowest 1D subband. This suggests careful management of the QPC’s confinement potential may enable the high g* desirable for spintronic applications without resorting to narrow-gap materials such as InAs or InSb. The 0.7 anomaly and zero-bias peak are also highly sensitive to confining potential, explaining the conflicting density dependencies of the 0.7 anomaly in the literature.

The study of electron motion in semiconductor billiards has elucidated our understanding of quantum interference and quantum chaos. The central assumption is that ionized donors generate only minor perturbations to the electron trajectories, which are determined by scattering from billiard walls. We use magnetoconductance fluctuations as a probe of the quantum interference and show that these fluctuations change radically when the scattering landscape is modified by thermally induced charge displacement between donor sites. Our results challenge the accepted understanding of quantum interference effects in nanostructures.

The study of electron motion in semiconductor billiards has elucidated our understanding of quantum interference and quantum chaos. The central assumption is that ionized donors generate only minor perturbations to the electron trajectories, which are determined by scattering from billiard walls. We use magnetoconductance fluctuations as a probe of the quantum interference and show that these fluctuations change radically when the scattering landscape is modified by thermally induced charge displacement between donor sites. Our results challenge the accepted understanding of quantum interference effects in nanostructures.

We have fabricated and demonstrated the operation of triggered sources of single-photons and entangled-photon pairs using self-assembled InAs quantum dots grown by MBE.

Measurements are presented of a device designed to cool a 6 mu m(2) region of 2D electron gas using quantum dots. Electrostatic effects are found to be significant in the device, and a model that accounts for them is developed. At ambient electron temperatures above 120 mK the results are consistent with the model and the base temperature of the cooled region is estimated. At an ambient electron temperature of 280 mK, the 6 mu m(2) region is found to be cooled below 190 mK. Below 120 mK the results deviate from predictions, which is attributed to reduced electron-electron scattering rates.

Quantum dot resonant tunneling diode single photon detector with independently defined absorption and sensing areas is demonstrated. The device, in which the tunneling is constricted to an aperture in an insulating layer in the emitter, shows electrical characteristics typical of high quality resonant tunneling diodes. A single photon detection efficiency of 2.1%+/- 0.1% at 685 nm was measured corresponding to an internal quantum efficiency of 14%. The devices are simple to fabricate, robust, and show promise for large absorption area single photon detectors based on quantum dot structures.

We have demonstrated spin injection from a full Heusler alloy Co2MnGa thin film into a (100) InGaAs quantum well in a semiconductor light-emitting diode structure at a temperature of 5 K. The detection is performed in the oblique Hanle geometry, allowing quantification of the effective spin lifetime and spin detection efficiency (22 +/- 4%). This work builds on existing studies on off-stoichiometric Heusler injectors into similar light-emitting-diode structures. The role of injector stoichiometry can therefore be quantitatively assessed with the result that the spin injection efficiency increases by a factor of approximately 2 as compared with an off-stoichiometric Co2.4Mn1.6Ga injector. (C) 2008 American Institute of Physics.

We present data from an induced gallium arsenide ( GaAs) quantum wire that exhibits an additional conductance plateau at 0.5(2e(2)/h), where e is the charge of an electron and h is Planck's constant, in zero magnetic field. The plateau was most pronounced when the potential landscape was tuned to be symmetric by using low-temperature scanning-probe techniques. Source-drain energy spectroscopy and temperature response support the hypothesis that the origin of the plateau is the spontaneous spin-polarization of the transport electrons: a ferromagnetic phase. Such devices may have applications in the field of spintronics to either generate or detect a spin-polarized current without the complications associated with external magnetic fields or magnetic materials.

The excitation of a quantum dot from its ground state depends on the probabilistic nature of carrier capture, and thus radiative decay can occur via a multi photon cascade. It is also possible that non-radiative initial exciton states can be formed when the spins of the component electron and heavy hole have the same sign. Time integrated and time resolved micro-photoluminescence experiments are presented that reveal the broader properties of single photon and multi-photon emission from InAs quantum dots, including single photon emission, polarisation correlated photon emission, and dark state blocking, and their implications in terms of quantum information technology are discussed.

By the application of an in-plane magnetic field, we demonstrate control of the fine structure polarization splitting of the exciton emission lines in individual InAs quantum dots. The selection of quantum dots with certain barrier composition and confinement energies is found to determine the magnetic field dependent increase or decrease of the separation of the bright exciton emission lines, and has enabled the splitting to be tuned to zero within the resolution of our experiments. Observed behavior allows us to determine g factors and exchange splittings for different types of dots.

We demonstrate control of the fine-structure splitting of the exciton emission lines in single InAs quantum dots by the application of an in-plane magnetic field. The composition of the barrier material and the size and symmetry of the quantum dot are found to determine decrease or increase in the linear polarization splitting of the dominant exciton emission lines with increasing magnetic field. This enables the selection of dots for which the splitting can to be tuned to zero, within the resolution of our experiments. General differences in the g-factors and exchange splittings are found for different types of dot. (c) 2006 Elsevier B.V. All rights reserved.

Electrical spin injection in a narrow [100] In0.2Ga0.8As quantum well in a GaAs p-i-n optical device is reported. The quantum well is located 300 nm from an AlGaAs Schottky barrier and this system is used to compare the efficiencies and temperature dependences of spin injection from Fe and the Heusler alloy Co2.4Mn1.6Ga grown by molecular-beam epitaxy. At 5 K, the injected electron spin polarizations for Fe and Co2.4Mn1.6Ga injectors are 31% and 13%, respectively. Optical detection is carried out in the oblique Hanle geometry. A dynamic nuclear polarization effect below 10 K enhances the magnetic field seen by the injected spins in both devices. The Co2.4Mn1.6Ga thin films are found to have a transport spin polarization of similar to 50% by point contact Andreev reflection conductivity measurements. (c) 2005 American Institute of Physics.

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.

By combining optical spin orientation and an externally applied longitudinal electric field, transverse charge accumulation has been detected in very high-mobility two-dimensional hole gases by measuring the transverse voltage drop across simple Hall devices. Our results indicate intrinsic band-structure (rather than extrinsic skew scattering) derived spin-orbit coupling as the underlying mechanism of this spin-polarized transport effect.

We have realized an ultrafast diode containing a single quantum dot, whose transitions can be Stark shifted in a few hundred picoseconds. Carrying out resonant excitation of this sample we demonstrate pulsed single photon emission with high spectral purity, by rapidly sweeping the neutral exciton transition into and out of the laser energy. Detailed study of the detuning dependence of the fluorescence reveals sources of decoherence in this sample, which we show can be filtered out with narrow-band excitation.