arXiv subjects

D. A. Ritchie

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

At least 163 records · Page 9Linked to original sources

arXiv2008

Biphoton interference with a quantum dot entangled light source

We demonstrate optical interferometry beyond the limits imposed by the photon wavelength using 'triggered' entangled photon pairs from a semiconductor quantum dot. Interference fringes of the entangled biphoton state reveals a periodicity half of that obtained with the single photon, and much less than that of the pump laser. High fringe visibility indicates that biphoton interference is less sensitive to decoherence than interference of two sequential single photons. The results suggest that quantum interferometry may be possible using a semiconductor LED-like device.

quant-ph↗

arXiv2008

Indistinguishable photons from a diode

We generate indistinguishable photons from a semiconductor diode containing a InAs/GaAs quantum dot. Using an all-electrical technique to populate and control a single-photon emitting state we filter-out dephasing by Stark-shifting the emission energy on timescales below the dephasing time of the state. Mixing consecutive photons on a beam-splitter we observe two-photon interference with a visibility of 64%.

quant-ph↗

arXiv2008

Energy-Dependent Electron-Electron Scattering and Spin Dynamics in a Two Dimensional Electron Gas

Measurements of spin dynamics of electrons in a degenerate two dimensional electron gas, where the Dyakonov-Perel mechanism is dominant, have been used to investigate the electron scattering time (tp*) as a function of energy near the Fermi energy. Close to the Fermi energy the spin evolution is oscillatory, indicating a quasi-collision-free regime of spin dynamics. As the energy is increased a transition to exponential, collision-dominated, spin decay occurs. The frequency and the value of tp* are extracted using a Monte Carlo simulation method. At the Fermi energy tp* is very close to the ensemble momentum relaxation time (tp) obtained from the electron mobility. For higher energies tp* falls quadratically, consistent with theoretical expectations for the onset of electron-electron scattering which is inhibited by the Pauli principle at the Fermi energy.

cond-mat.other↗

arXiv2008

Experimental position-time entanglement with degenerate single photons

We report an experiment in which two-photon interference occurs between degenerate single photons that never meet. The two photons travel in opposite directions through our fibre-optic interferometer and interference occurs when the photons reach two different, spatially separated, 2-by-2 couplers at the same time. We show that this experiment is analogous to the conventional Franson-type entanglement experiment where the photons are entangled in position and time. We measure wavefunction overlaps for the two photons as high as 94 $\pm$ 3%.

quant-ph↗

arXiv2007

Low-Temperature Collapse of Electron Localisation in Two Dimensions

We report direct experimental evidence that the insulating phase of a disordered, yet strongly interacting two-dimensional electron system (2DES) becomes unstable at low temperatures. As the temperature decreases, a transition from insulating to metal-like transport behaviour is observed, which persists even when the resistivity of the system greatly exceeds the quantum of resistivity h/e^2. The results have been achieved by measuring transport on a mesoscopic length-scale while systematically varying the strength of disorder.

cond-mat.mes-hall↗

arXiv2007

Field-tunable magnetic phases in a semiconductor-based two-dimensional Kondo lattice

We show the existence of intrinsic localized spins in mesoscopic high-mobility GaAs/AlGaAs heterostructures. Non-equilibrium transport spectroscopy reveals a quasi-regular distribution of the spins, and indicates that the spins interact indirectly via the conduction electrons. The interaction between spins manifests in characteristic zero-bias anomaly near the Fermi energy, and indicates gate voltage-controllable magnetic phases in high-mobility heterostructures. To address this issue further, we have also designed electrostatically tunable Hall devices, that allow a probing of Hall characteristics at the active region of the mesoscopic devices. We show that the zero field Hall coefficient has an anomalous contribution, which can be attributed to scattering by the localized spins. The anomalous contribution can be destroyed by an increase in temperature, source drain bias, or field range.

cond-mat.mes-hall↗

arXiv2007

Sensitivity of the magnetic state of a spin lattice on itinerant electron orbital phase

Spatially extended localized spins can interact via indirect exchange interaction through Friedel oscillations in the Fermi sea. In arrays of localized spins such interaction can lead to a magnetically ordered phase. Without external magnetic field such a phase is well understood via a "two-impurity" Kondo model. Here we employ non-equilibrium transport spectroscopy to investigate the role of the orbital phase of conduction electrons on the magnetic state of a spin lattice. We show experimentally, that even tiniest perpendicular magnetic field can influence the magnitude of the inter-spin magnetic exchange.

cond-mat.mes-hall↗

arXiv2007

Coherence of an Entangled Exciton-Photon State

We study the effect of the exciton fine-structure splitting on the polarisation-entanglement of photon pairs produced by the biexciton cascade in a single quantum dot. The entanglement is found to persist despite separations between the intermediate energy levels of up to 4 micro-eV. Measurements demonstrate that entanglement of the photon pair is robust to the dephasing of the intermediate exciton state responsible for the first order coherence time of either single photon. We present a theoretical framework taking into account the effects of spin-scattering, background light and dephasing. We distinguish between the first-order coherence time, and a parameter which we measure for the first time and define as the cross-coherence time.

quant-ph↗

arXiv2007

Quantum key distribution using a triggered quantum dot source emitting near 1.3 microns

We report the distribution of a cryptographic key, secure from photon number splitting attacks, over 35 km of optical fiber using single photons from an InAs quantum dot emitting ~1.3 microns in a pillar microcavity. Using below GaAs-bandgap optical excitation, we demonstrate suppression of multiphoton emission to 10% of the Poissonian level without detector dark count subtraction. The source is incorporated into a phase encoded interferometric scheme implementing the BB84 protocol for key distribution over standard telecommunication optical fiber. We show a transmission distance advantage over that possible with (length-optimized) uniform intensity weak coherent pulses at 1310 nm in the same system.

quant-ph↗

arXiv2007

Observation of the Purcell effect in high-index-contrast micropillar

We have fabricated pillar microcavity samples with Bragg mirrors consisting of alternate layers of GaAs and Aluminium Oxide. Compared to the more widely studied GaAs/AlAs micropillars these mirrors can achieve higher reflectivities with fewer layer repeats and reduce the mode volume. We have studied a number of samples containing a low density of InGaAs/GaAs self assembled quantum dots in a cavity and here report observation of a three fold enhancement in the radiative lifetime of a quantum dot exciton state due to the Purcell effect.

cond-mat.other↗

arXiv2007

Oxide-apertured microcavity single-photon emitting diode

We have developed a microcavity single-photon source based on a single quantum dot within a planar cavity in which wet-oxidation of a high-aluminium content layer provides lateral confinement of both the photonic mode and the injection current. Lateral confinement of the optical mode in optically pumped structures produces a strong enhancement of the radiative decay rate. Using microcavity structures with doped contact layers, we demonstrate a single-photon emitting diode where current may be injected into a single dot.

cond-mat.other↗

arXiv2007

Energy-dependent tunnelling from few-electron dynamic quantum dots

We measure the electron escape-rate from surface-acoustic-wave dynamic quantum dots (QDs) through a tunnel barrier. Rate-equations are used to extract the tunnelling rates, which change by an order of magnitude with tunnel-barrier gate voltage. We find that the tunnelling rates depend on the number of electrons in each dynamic QD because of Coulomb energy. By comparing this dependence to a saddle-point-potential model, the addition energies of the second and third electron in each dynamic QD are estimated. The scale (a few meV) is comparable to those in static QDs as expected.

cond-mat.mes-hall↗

arXiv2007

Energy-level pinning and the 0.7 spin state in one dimension: GaAs quantum wires studied using finite-bias spectroscopy

We study the effects of electron-electron interactions on the energy levels of GaAs quantum wires (QWs) using finite-bias spectroscopy. We probe the energy spectrum at zero magnetic field, and at crossings of opposite-spin-levels in high in-plane magnetic field B. Our results constitute direct evidence that spin-up (higher energy) levels pin to the chemical potential as they populate. We also show that spin-up and spin-down levels abruptly rearrange at the crossing in a manner resembling the magnetic phase transitions predicted to occur at crossings of Landau levels. This rearranging and pinning of subbands provides a phenomenological explanation for the 0.7 structure, a one-dimensional (1D) nanomagnetic state, and its high-B variants.

cond-mat.mes-hall↗

arXiv2007

Probing e-e interactions in a periodic array of GaAs quantum wires

We present the results of non-linear tunnelling spectroscopy between an array of independent quantum wires and an adjacent two-dimensional electron gas (2DEG) in a double-quantum-well structure. The two layers are separately contacted using a surface-gate scheme, and the wires are all very regular, with dimensions chosen carefully so that there is minimal modulation of the 2DEG by the gates defining the wires. We have mapped the dispersion spectrum of the 1D wires down to the depletion of the last 1D subband by measuring the conductance \emph{G} as a function of the in-plane magnetic field \emph{B}, the interlayer bias $V_{\rm dc}$ and the wire gate voltage. There is a strong suppression of tunnelling at zero bias, with temperature and dc-bias dependences consistent with power laws, as expected for a Tomonaga-Luttinger Liquid caused by electron-electron interactions in the wires. In addition, the current peaks fit the free-electron model quite well, but with just one 1D subband there is extra structure that may indicate interactions.

cond-mat.mes-hall↗

arXiv2007

Oscillatory D'yakonov-Perel' spin dynamics in two dimensional electron gases

Optical pump-probe measurements of spin-dynamics at temperatures down to 1.5K are described for a series of (001)-oriented GaAs/AlGaAs quantum well samples containing high mobility two-dimensional electron gases (2DEGs). For well widths ranging from 5 nm to 20 nm and 2DEG sheet densities from 1.75x1011cm-2 to 3.5x1011cm-2 the evolution of a small injected spin population is found to be a damped oscillation rather than exponential relaxation, consistent with the quasi-collision-free regime of D'yakonov-Perel spin dynamics. A Monte Carlo simulation method is used to extract the spin-orbit-induced electron spin precession frequency |W(kF)| and electron momentum scattering time tp* at the Fermi wavevector. The spin decay time passes through a minimum at a temperature corresponding to the transition from collision-free to collision-dominated regimes and tp* is found to be close to the ensemble momentum scattering time tp obtained from Hall measurements of electron mobility. The values of |W(kF)| give the Dresselhaus (BIA) coefficient of spin-orbit interaction as a function of electron confinement energy in the quantum show, qualitatively, the behaviour expected from k.p theory.

cond-mat.other↗

arXiv2007

Geometric Suppression of Single-Particle Energy Spacings in Quantum Antidots

Quantum Antidot (AD) structures have remarkable properties in the integer quantum Hall regime, exhibiting Coulomb-blockade charging and the Kondo effect despite their open geometry. In some regimes a simple single-particle (SP) model suffices to describe experimental observations while in others interaction effects are clearly important, although exactly how and why interactions emerge is unclear. We present a combination of experimental data and the results of new calculations concerning SP orbital states which show how the observed suppression of the energy spacing between states can be explained through a full consideration of the AD potential, without requiring any effects due to electron interactions such as the formation of compressible regions composed of multiple states, which may occur at higher magnetic fields. A full understanding of the regimes in which these effects occur is important for the design of devices to coherently manipulate electrons in edge states using AD resonances.

cond-mat.mes-hall↗

arXiv2007

Controlling the polarisation correlation of photon pairs from a charge-tuneable quantum dot

Correlation between the rectilinear polarisations of the photons emitted from the biexciton decay in a single quantum dot is investigated in a device which allows the charge-state of the dot to be controlled. Optimising emission from the neutral exciton states maximises the operating efficiency of the biexciton decay. This is important for single dot applications such as a triggered source of entangled photons. As the bias on the device is reduced correlation between the two photons is found to fall dramatically as emission from the negatively charged exciton becomes significant. Lifetime measurements demonstrate that electronic spin-scattering is the likely cause.

quant-ph↗

arXiv2007

Giant Fluctuations of Coulomb Drag in a Bilayer System

We have observed reproducible fluctuations of the Coulomb drag, both as a function of magnetic field and electron concentration, which are a manifestation of quantum interference of electrons in the layers. At low temperatures the fluctuations exceed the average drag, giving rise to random changes of the sign of the drag. The fluctuations are found to be much larger than previously expected, and we propose a model which explains their enhancement by considering fluctuations of local electron properties.

cond-mat.mes-hall↗