arXiv subjects

D. A. Ritchie

Publications and source records attributed to D. A. Ritchie.

At least 19 recordsLinked to original sources

arXiv2026

On-chip quantum sensing of Kondo spins in a high-mobility quasi-one-dimensional nanoconstriction

The precise nature of Kondo spins has remained enigmatic when extended to multiple spin impurities or, more intriguingly, when the localized spin itself may already be the consequence of many-body interactions in a presumably-delocalized open nanoconstriction, such as a quantum point contact (QPC). It is experimentally challenging to distinguish the Kondo state from other coexisting many-body spin states in such a strongly correlated system. Here we lithographically define an all-on-chip electronic resonator (ER) and a QPC in a high-mobility GaAs/AlGaAs heterostructure transistor. Local Kondo screening of the QPC spin and nonlocal spin singlet across the ER-QPC integration are controllable in response to ER occupancy parity. We also show that the 0.7 anomaly, another strongly-correlated state in QPCs, not only has a different physical origin but furthermore counteracts the Kondo spin singlet. These results demonstrate a noninvasive quantum method for sensing spontaneous magnetic impurities within an open nanoconstriction.

cond-mat.mes-hall↗

arXiv2025

The heating and cooling of 2D electrons at low temperatures

We present measurements of the cooling length $\ell_E$ for hot electrons in a GaAs-based high mobility two-dimensional electron gas (2DEG). The thermal measurements are performed on a long 60 $μ$m-wide channel, which is Joule-heated at one end, along which there are three similar hot-electron thermocouples, spaced 30 $μ$m apart. The thermocouples measure an exponentially decaying temperature profile with a characteristic length $\ell_E$, which decreases from 23 to 16 $μ$m as the lattice temperature increases from 1.8 to 5 K. From a simple one-dimensional model of heat diffusion, we measure an inelastic scattering time which decreases from $τ_i \approx$ 0.36 to 0.18 ns. The measured $τ_i$ has a magnitude and temperature dependence consistent with acoustic phonon scattering times. We discuss how the sample design can be varied for further thermal investigations. Knowledge of the temperature profile and its gradient will prove useful in measurements of the thermal conductivity and the Nernst effect.

cond-mat.mes-hall↗

arXiv2025

Probing Fermi surface parity with spin resolved transverse magnetic focussing

Measurements of the Fermi surface are a fundamental technique for determining the electrical and magnetic properties of solids. In 2D systems, the area and diameter of the Fermi surface is typically measured using Shubnikov-de Haas oscillations and commensurability oscillations respectively. However, these techniques are unable to detect changes in the parity of the Fermi surface (i.e. when +k $\neq$ -k). Here, we show that transverse magnetic focussing can be used to detect such changes, because focussing only measures a well defined section of the Fermi surface and does not average over +k and -k. Furthermore, our results show that focussing is an order of magnitude more sensitive to changes in the Fermi surface than other 2D techniques, and could be used to investigate similar Fermi surface changes in other 2D systems.

cond-mat.mes-hall↗

arXiv2025

Landau Level Single-Electron Pumping

We present the first detailed study of the effect of a strong magnetic field on single-electron pumping in a device utilising a finger-gate split-gate configuration. In the quantum Hall regime, we demonstrate electron pumping from Landau levels in the leads, where the measurements exhibit pronounced oscillations in the lengths of the pumping plateaus with the magnetic field, reminiscent of Shubnikov-de Haas oscillations. This similarity indicates that the pumping process is dependent on the density of states of the 2D electron gas over a narrow energy window. Based on these observations, we develop a new theoretical description of the operation of single-electron pumps which for the first time allows for the determination of the physical parameters of the experiment; such as the capture energy of the electrons, the broadening of the quantised Landau levels in the leads, and the quantum lifetime of the electrons.

cond-mat.mes-hall↗

arXiv2024

Coulomb sensing of single ballistic electrons

While ballistic electrons are a key tool for applications in sensing and flying qubits, sub-nanosecond propagation times and complicated interactions make control of ballistic single electrons challenging. Recent experiments have revealed Coulomb collisions of counterpropagating electrons in a beam splitter, giving time resolved control of interactions between single electrons. Here we use remote Coulomb interactions to demonstrate a scheme for sensing single ballistic electrons. We show that interactions are highly controllable via electron energy and emission timing. We use a weakly-coupled `sensing' regime to characterise the nanoscale potential landscape of the beam splitter and the strength of the Coulomb interaction, and show multi-electron sensing with picosecond resolution.

cond-mat.mes-hall↗

arXiv2024

Simultaneous study of acoustic and optic phonon scattering of electrons and holes in undoped $\mathrm{GaAs}$/$\mathrm{Al_{x} Ga_{1-x} As}$ heterostructures

The study of phonon coupling in doped semiconductors via electrical transport measurements is challenging due to unwanted temperature-induced effects such as dopant ionisation and parallel conduction. Here, we study phonon scattering in 2D electrons and holes in the $1.6-92.5$K range without the use of extrinsic doping, where both acoustic and longitudinal optic (LO) phonons come into effect. We use undoped GaAs/$\mathrm{Al_{x} Ga_{1-x} As}$ heterostructures and examine the temperature dependence of the sample resistivity, extracting phonon coupling constants and the LO activation energy. Our results are consistent with results obtained through approaches other than transport measurements, and highlight the benefit of this approach for studying electron-phonon and hole-phonon coupling.

cond-mat.mes-hall↗

arXiv2024

Fast characterization of multiplexed single-electron pumps with machine learning

We present an efficient machine learning based automated framework for the fast tuning of single-electron pump devices into current quantization regimes. It uses a sparse measurement approach based on an iterative active learning algorithm to take targeted measurements in the gate voltage parameter space. When compared to conventional parameter scans, our automated framework allows us to decrease the number of measurement points by about an order of magnitude. This corresponds to an eight-fold decrease in the time required to determine quantization errors, which are estimated via an exponential extrapolation of the first current plateau embedded into the algorithm. We show the robustness of the framework by characterizing 28 individual devices arranged in a GaAs/AlGaAs multiplexer array, which we use to identify a subset of devices suitable for parallel operation at communal gate voltages. The method opens up the possibility to efficiently scale the characterization of such multiplexed devices to a large number of pumps.

cond-mat.mes-hall↗

arXiv2024

Formation of a lateral p-n junction light-emitting diode on an n-type high-mobility GaAs/Al$_{0.33}$Ga$_{0.67}$As heterostructure

We have fabricated a device which includes two lateral p-n junctions on an n-type GaAs/Al$_{0.33}$Ga$_{0.67}$As heterostructure. A section of the n-type material has been converted to p-type by removing dopants and applying a voltage to a gate placed in this region. Controlled electroluminescence from both of the p-n junctions has been demonstrated by varying the applied bias voltages. An emission peak with a width of ~8 nm is observed around 812 nm. The electroluminescence seen from both junctions is considered to originate from the GaAs quantum well layer in the device. The lithographic techniques that we have developed are compatible for further integration of gated quantum devices such as single-electron pumps to build on-demand single-photon sources.

physics.app-ph↗

arXiv2024

Statistical study and parallelisation of multiplexed single-electron sources

Increasing electric current from a single-electron source is a main challenge in an effort to establish the standard of the ampere defined by the fixed value of the elementary charge $e$ and operation frequency $f$. While the current scales with $f$, due to an operation frequency limit for maintaining accurate single-electron transfer, parallelisation of singleelectron sources is expected to be a more practical solution to increase the generated electric current $I = Nef$, where $N$ is a number of parallelised devices. One way to parallelise single-electron sources without increasing the complexity in device operation is to use a common gate. Such a scheme will require each device to have the same operation parameters for single-electron transfer. In order to investigate this possibility, we study the statistics for operation gate voltages using single-electron sources embedded in a multiplexer circuit. The multiplexer circuit allows us to measure 64 single-electron sources individually in a single cooldown. We also demonstrate the parallelisation of three single-electron sources and observe the generated current enhanced by a factor of three.

cond-mat.mes-hall↗

arXiv2024

Photon emission by hot electron injection across a lateral \textit{pn} junction

We demonstrate a method to generate photons by injecting hot electrons into a {\it pn} junction within a \ce{GaAs/AlGaAs} heterostructure. Hot electrons are generated by biasing across a mesoscopic potential in {\it n}-type region and travel toward {\it p}-type region through quantum Hall edge channel in the presence of magnetic field perpendicular to the substrate. The {\it p}-type region is created several microns away from the hot electron emitter by inducing interfacial charges using a surface gate. The energy relaxation of the hot electrons is suppressed by separating the orbitals before and after longitudinal-optical (LO) phonon emission. This technique enables the hot electrons to reach the {\it p}-type region and to recombine with induced holes followed by photon emissions. Hot electron-induced hole recombination is confirmed by a peak around \qty{810}{nm} in an optical spectrum that corresponds to excitonic recombination in a \ce{GaAs} quantum well. An asymmetric structure observed in the optical spectrum as a function of the magnetic field originates from the chiral transport of the hot electrons in the Hall edge channel. We propose the combination of our technology and on-demand single-electron source would enable the development of an on-demand single photon source that is an essential building block to drive an optical quantum circuit and to transfer quantum information for a long distance.

cond-mat.mes-hall↗

arXiv2024

Low frequency resistance fluctuations in an ionic liquid gated channel probed by cross-correlation noise spectroscopy

A system in equilibrium keeps ``exploring" nearby states in the phase space and consequently, fluctuations can contain information, that the mean value does not. However, such measurements involve a fairly complex interplay of effects arising in the device and measurement electronics, that are non-trivial to disentangle. In this paper, we briefly analyse some of these issues and show the relevance of a two-amplifier cross-correlation technique for semiconductors and thin films commonly encountered. We show that by using home-built amplifiers costing less than $10$ USD/piece one can measure spectral densities as low as $\sim 10^{-18}-10^{-19}~ {\rm {V^2}{Hz^{-1}}}$. We apply this method to an ionic liquid gated Ga:ZnO channel and show that the glass transition of the ionic liquid brings about a change in the exponent of the low frequency resistance fluctuations. Our analysis suggests that a log-normal distribution of the Debye relaxation times of the fluctuations and an increased weight of the long timescale relaxations can give a semi-quantitative explanation of the observed change in the exponent.

cond-mat.mes-hall↗

arXiv2023

Spin-photon entanglement with direct photon emission in the telecom C-band

The ever-evolving demands for computational power and for a securely connected world dictate the development of quantum networks where entanglement is distributed between connected parties. Solid-state quantum emitters in the telecom C-band are a promising platform for quantum communication applications due to the minimal absorption of photons at these wavelengths, "on-demand" generation of single photon flying qubits, and ease of integration with existing network infrastructure. Here, we use an InAs/InP quantum dot to implement an optically active spin-qubit, based on a negatively charged exciton where the electron spin degeneracy is lifted using a Voigt magnetic field. We investigate the coherent interactions of the spin-qubit system under resonant excitation, demonstrating high fidelity spin initialisation and coherent control using picosecond pulses. We further use these tools to measure the coherence of a single, undisturbed electron spin in our system. Finally, we report the first demonstration of spin-photon entanglement in a solid-state system capable of direct emission into the telecom C-band.

quant-ph↗

arXiv2023

Exciton-polaritons in GaAsbased slab waveguide photonic crystals

We report the observation of band gaps for low loss exciton-polaritons propagating outside the light cone in GaAs-based planar waveguides patterned into two-dimensional photonic crystals. By etching square lattice arrays of shallow holes into the uppermost layer of our structure, we open gaps on the order of 10 meV in the photonic mode dispersion, whose size and light-matter composition can be tuned by proximity to the strongly coupled exciton resonance. We demonstrate gaps ranging from almost fully photonic to highly excitonic. Opening a gap in the exciton-dominated part of the polariton spectrum is a promising first step towards the realization of quantum-Hall-like states arising from topologically nontrivial hybridization of excitons and photons.

physics.optics↗

arXiv2023

Decoupling of the many-body effects from the electron mass in GaAs by means of reduced dimensionality

Determining the (bare) electron mass $m_0$ in crystals is often hindered by many-body effects since Fermi-liquid physics renormalises the band mass, making the observed effective mass $m^*$ depend on density. Here, we use a one-dimensional (1D) geometry to amplify the effect of interactions, forcing the electrons to form a nonlinear Luttinger liquid with separate holon and spinon bands, therefore separating the interaction effects from $m_0$. Measuring the spectral function of gated quantum wires formed in GaAs by means of magnetotunnelling spectroscopy and interpreting them using the 1D Fermi-Hubbard model, we obtain $m_0=(0.0525\pm0.0015)m_\textrm{e}$ in this material, where $m_\textrm{e}$ is the free-electron mass. By varying the density in the wires, we change the interaction parameter $r_\textrm{s}$ in the range from $\sim$1-4 and show that $m_0$ remains constant. The determined value of $m_0$ is $\sim 22$% lighter than observed in GaAs in geometries of higher dimensionality $D$ ($D>1$), consistent with the quasi-particle picture of a Fermi liquid that makes electrons heavier in the presence of interactions.

cond-mat.mes-hall↗

arXiv2022

Time-resolved Coulomb collision of single electrons

Precise control over interactions between ballistic electrons will enable us to exploit Coulomb interactions in novel ways, to develop high-speed sensing, to reach a non-linear regime in electron quantum optics and to realise schemes for fundamental two-qubit operations on flying electrons. Time-resolved collisions between electrons have been used to probe the indistinguishability, Wigner function and decoherence of single electron wavepackets. Due to the effects of screening, none of these experiments were performed in a regime where Coulomb interactions were particularly strong. Here we explore the Coulomb collision of two high energy electrons in counter-propagating ballistic edge states. We show that, in this kind of unscreened device, the partitioning probabilities at different electron arrival times and barrier height are shaped by Coulomb repulsion between the electrons. This prevents the wavepacket overlap required for the manifestation of fermionic exchange statistics but suggests a new class of devices for studying and manipulating interactions of ballistic single electrons.

cond-mat.mes-hall↗

arXiv2022

Spin polarisation and spin dependent scattering of holes in transverse magnetic focussing

In 2D systems with a spin-orbit interaction, magnetic focussing can be used to create a spatial separation of particles with different spin. Here we measure hole magnetic focussing for two different magnitudes of the Rashba spin-orbit interaction. We find that when the Rashba spin-orbit magnitude is large there is significant attenuation of one of the focussing peaks, which is conventionally associated with a change in the spin polarisation. We instead show that in hole systems with a $k^3$ spin-orbit interaction, this peak suppression is due to a change in the scattering of one spin state, not a change in spin polarisation. We also show that the change in scattering length extracted from magnetic focussing is consistent with results obtained from measurements of Shubnikov-de Haas oscillations. This result suggests that scattering must be considered when relating focussing peak amplitude to spin polarisation in hole systems

cond-mat.mes-hall↗

arXiv2022

Two-particle time-domain interferometry in the Fractional Quantum Hall Effect regime

Quasi-particles are elementary excitations of condensed matter quantum phases. Demonstrating that they keep quantum coherence while propagating is a fundamental issue for their manipulation for quantum information tasks. Here, we consider anyons, the fractionally charged quasi-particles 20 of the Fractional Quantum Hall Effect occurring in two-dimensional electronic conductors in high magnetic fields. They obey anyonic statistics, intermediate between fermionic and bosonic. Surprisingly, anyons show large quantum coherence when transmitted through the localized states of electronic Fabry-Pérot interferometers, but almost no quantum interference when transmitted via the propagating states of Mach-Zehnder interferometers. Here, using a novel interferometric 25 approach, we demonstrate that anyons do keep quantum coherence while propagating. Performing two-particle time-domain interference measurements sensitive to the two-particle Hanbury Brown Twiss phase, we find 53% and 60% visibilities for anyons with charges e/5 and e/3. Our results give a positive message for the challenge of performing controlled quantum coherent braiding of anyons.

cond-mat.mes-hall↗

arXiv2022

Theory of the in-plane photoelectric effect in two-dimensional electron systems

A new photoelectric phenomenon, the in-plane photoelectric (IPPE) effect, has been recently discovered at terahertz (THz) frequencies in a GaAs/Al$_x$Ga$_{1-x}$As heterostructure with a two-dimensional (2D) electron gas (W. Michailow et al., Sci. Adv. \textbf{8}, eabi8398 (2022)). In contrast to the conventional PE phenomena, the IPPE effect is observed at normal incidence of radiation, the height of the in-plane potential step, which electrons overcome after absorption of a THz photon, is electrically tunable by gate voltages, and the effect is maximal at a negative electron ``work function'', when the Fermi energy lies above the potential barrier. Based on the discovered phenomenon, efficient detection of THz radiation has been demonstrated. In this work we present a detailed theory of the IPPE effect providing analytical results for the THz wave generated photocurrent, the quantum efficiency, and the internal responsivity of the detector, in dependence on the frequency, the gate voltages, and the geometrical parameters of the detector. The calculations are performed for macroscopically wide samples at zero temperature. Results of the theory are applicable to any semiconductor systems with 2D electron gases, including III-V structures, silicon-based field effect transistors, and the novel 2D layered, graphene-related materials.

cond-mat.mes-hall↗