Publications

Fractional quantum statistics are a signature prediction of fractional quantum Hall states, which have long been elusive in experiments. Here, we present the observation of anyonic interference and exchange phases in a novel co-propagating 'optical-like' Mach-Zehnder Interferometer. This architecture is free of charging and backscattering effects that often plague the widely used Fabry-Perot interferometer, thus exhibiting pristine Aharonov-Bohm (AB) interference without fractional phase slips. We studied the three lowest Jain filling factors, {\nu}=1/3, 2/5, and 3/7, which host quasiparticles with fractional charges e*=e/3, e/5, and e/7, respectively. The observed AB interference patterns, plotted as a function of magnetic field B and modulation-gate voltage, VMG (known as pajamas), exhibited the expected flux periodicities: 3{\Phi}0, 5{\Phi}0, and 7{\Phi}0, with {\Phi}0 being the flux quantum. A small biased top gate (TG) deposited in the center of the interferometer induces local quasiparticles that are spatially isolated from the interfering modes. At non-zero TG voltage VTG, quantized phase slips appear in the AB pajamas approximately with every flux quantum that pierces the effective area below the TG. Moreover, when tuning VTG, at a constant magnetic field, abrupt phase jumps corresponding to adding one localized quasiparticle at a time under the top gate appear in the B-VTG pajamas.

We study the interaction of a dark exciton Bose-Einstein condensate with the nuclei in gallium arsenide/aluminum gallium arsenide coupled quantum wells and find clear evidence for nuclear polarization buildup that accompanies the appearance of the condensate. We show that the nuclei are polarized throughout the mesa area, extending to regions that are far away from the photoexcitation area and persisting for seconds after the excitation is switched off. Photoluminescence measurements in the presence of radio frequency radiation reveal that the hyperfine interaction between the nuclear and electron spins is enhanced by two orders of magnitude. We suggest that this large enhancement manifests the collective nature of the N-exciton condensate, which amplifies the interaction by a factor of √N.

Interferometry is a vital tool for studying fundamental features in the quantum Hall effect. For instance, Aharonov-Bohm interference in a quantum Hall interferometer can probe the wave-particle duality of electrons and quasiparticles. Here, we report an unusual Aharonov-Bohm interference of the outermost edge mode in a quantum Hall Fabry-Pérot interferometer, whose Coulomb interactions were suppressed with a grounded drain in the interior bulk of the interferometer. In a descending bulk filling factor from νb=3 to νb≈(5/3), the magnetic field periodicity, which corresponded to a single "flux quantum,"agreed accurately with the enclosed area of the interferometer. However, in the filling range, νb≈(5/3) to νb=1, the field periodicity increased markedly, a priori suggesting a drastic shrinkage of the Aharonov-Bohm area. Moreover, the modulation gate voltage periodicity decreased abruptly at this range. We attribute these unexpected observations to edge reconstruction, leading to area changing with the field and a modified modulation gate-edge capacitance. These reproducible results support future interference experiments with a quantum Hall Fabry-Pérot interferometer.

The remarkable Cooper-like pairing phenomenon in the Aharonov-Bohm interference of a Fabry-Perot interferometer - operating in the integer quantum Hall regime - remains baffling. Here, we report the interference of paired electrons employing "interface edge modes."These modes are born at the interface between the bulk of the Fabry-Perot interferometer and an outer gated region tuned to a lower filling factor. Such a configuration allows toggling the spin and the orbital of the Landau level of the edge modes at the interface. We find that electron pairing occurs only when the two modes (the interfering outer and the first inner) belong to the same spinless Landau level.

In samples of field-effect transistors based on GaAs/AlGaAs heterostructures with an electron system in a single 50-nm-wide GaAs quantum well, a transition stimulated by a quantizing magnetic field has been detected from a bilayer state of the system in zero magnetic field to a single-layer state when only the lowest Landau level is filled. In contrast to the results for the 60-nm-wide quantum well obtained in [S. I. Dorozhkin, A. A. Kapustin, I. V. Fedorov, V. Umansky, and J. H. Smet, Phys. Rev. V 102, 235307 (2020)], the single-layer state is observed not only in incompressible quantum Hall effect states of the electron system at filling factors of 1 and 2, but also in compressible states between these filling factors. The spatial location of the single-layer system in the quantum well has been established; it appears to be independent of the electron distribution over the layers in a low magnetic field. A possible qualitative explanation for this observation has been proposed. The detected transition is supposedly due to the negative compressibility of two-dimensional electron systems caused by exchange-correlation contributions to the electron-electron interaction.

Multielectron semiconductor quantum dots (QDs) provide a novel platform to study the Coulomb interaction-driven, spatially localized electron states of Wigner molecules (WMs). Although Wigner-molecularization has been confirmed by real-space imaging and coherent spectroscopy, the open system dynamics of the strongly correlated states with the environment are not yet well understood. Here, we demonstrate efficient control of spin transfer between an artificial three-electron WM and the nuclear environment in a GaAs double QD. A LandauZener sweep-based polarization sequence and low-lying anticrossings of spin multiplet states enabled by Wigner-molecularization are utilized. Combined with coherent control of spin states, we achieve control of magnitude, polarity, and site dependence of the nuclear field. We demonstrate that the same level of control cannot be achieved in the non-interacting regime. Thus, we confirm the spin structure of a WM, paving the way for active control of correlated electron states for application in mesoscopic environment engineering.

Thermal conductance measurements are sensitive to both charge and chargeless energy flow and are an essential measurement technique in condensed-matter physics. For two-dimensional topological insulators, the determination of thermal Hall (transverse) conductance and thermal longitudinal conductance is crucial for the understanding of topological order in the underlying state. Such measurements have not been accomplished, even in the extensively studied quantum Hall effect regime. Here we report a local power measurement technique that we use to reveal the topological thermal Hall conductance, going beyond the ubiquitous two-terminal conductance. For example, we show that the thermal Hall conductance is approximately zero in the v = 2/3 particlehole conjugated state. This is in contrast to the two-terminal thermal conductance that gives a non-universal value that depends on the extent of thermal equilibration between the counter-propagating edge modes. Moreover, we demonstrate the utility of this technique in studying the power carried by the current fluctuations of a partitioned edge mode with an out-of-equilibrium distribution.

A bilayer electron system that is formed in a 60-nm-wide GaAs quantum well and has a large difference of the electron densities in the layers has been studied. It has been found that, when a magnetic field is tilted from the normal to the plane of the system, integer quantum Hall effect states at the filling factors of Landau levels of 1 and 2 disappear; instead, fractional quantum Hall effect states in the interval between these filling factors appear at the filling factors ν_F = 4/3, 10/7, and 6/5 with odd denominators and at the filling factor ν_F = 5/4. Several different states can be observed under the variation of the magnetic field. The detected fractional quantum Hall effect states are interpreted as combined states with the same filling factor 1 in the layer with the higher density and with the filling factors ν_F 1 in the layer with the lower density. These states are formed because of the redistribution of electrons between the layers, which occurs under the variation of the magnetic field. The appearance of the state with the filling factor ν_F = 5/4 with the even denominator is presumably attributed to the dominance of the interlayer electronelectron interaction over the intralayer one for electrons in the layer with the lower density.

The topological order of a quantum Hall state is mirrored by the gapless edge modes owing to bulk-edge correspondence. The state at the filling of ν = 5/2, predicted to host non-abelian anyons, supports a variety of edge modes (integer, fractional, neutral). To ensure thermal equilibration between the edge modes and thus accurately determine the states nature, it is advantageous to isolate the fractional channel (1/2 and neutral modes). In this study, we gapped out the integer modes by interfacing the ν = 5/2 state with integer states ν = 2 and ν = 3 and measured the thermal conductance of the isolated-interface channel. Our measured half-quantized thermal conductance confirms the non-abelian nature of the ν = 5/2 state and its particle-hole Pfaffian topological order. Such an isolated channel may be more amenable to braiding experiments.

We report energy-selective tunneling readout-based Hamiltonian parameter estimation of a two-electron spin qubit in a GaAs quantum dot array. Optimization of readout fidelity enables a single-shot measurement time of 16 μs on average, with adaptive initialization and efficient qubit frequency estimation based on real-time Bayesian inference. For qubit operation in a frequency heralded mode, we observe a 40-fold increase in coherence time without resorting to dynamic nuclear polarization. We also demonstrate active frequency feedback with quantum oscillation visibility, single-shot measurement fidelity, and gate fidelity of 97.7%, 99%, and 99.6%, respectively, showcasing the improvements in the overall capabilities of GaAs-based spin qubits. By pushing the sensitivity of the energy-selective tunneling-based spin to charge conversion to the limit, the technique is useful for advanced quantum control protocols such as error mitigation schemes, where fast qubit parameter calibration with a large signal-to-noise ratio is crucial.

A high-electron-mobility GaAs/Al0.36Ga0.64As heterostructure is processed by electron-beam lithography to fabricate samples with a separation of the current-supplying Ohmic contacts of between 28μm and 1.3 mm. Voltage pulses of 20-ms duration, with a 4% filling factor and an amplitude up to 30 V, stimulate terahertz emission from samples cooled to 4 K. The radiation spectrum, centered at about 390 GHz (about 1.6 meV), registered with a Fabry-Perot interferometer, shows almost no dependence on the applied voltage or on the shape of the sample. The emission appears in a threshold manner and is accompanied by a jump of current by more than 2 orders of magnitude. The energy of the emitted photons is about three times smaller than in the case of any previously reported impurity-related emission from either bulk GaAs or GaAs-based quantum wells. The emission is interpreted as resulting from a two-step ionization-recombination process involving shallow donors in the (Al,Ga)As barrier, which create bound states of electrons in the GaAs quantum well. Thus, the frequency of emission is argued to be tuned by the width of the spacer layer. We present these as small convenient sources that can be used for calibration and check-up operation of cooled bolometric systems used in astronomical observations.

A novel experimental optical method, based on photoluminescence and photo-induced resonant reflection techniques, is used to investigate the spin transport over long distances in a new, recently discovered collective statemagnetofermionic condensate. The given BoseEinstein condensate exists in a purely fermionic system (ν = 2 quantum Hall insulator) due to the presence of a non-equilibrium ensemble of spin-triplet magnetoexcitonscomposite bosons. It is found that the condensate can spread over macroscopically long distances of approximately 200 µm. The propagation velocity of long-lived spin excitations is measured to be 25 m/s.

The individual confinement and two-axis qubit operations of two-electron spin qubits in a GaAs gate-defined sextuple quantum dot array with an integrated micromagnet are reported in this study. As a first step toward multiple-qubit operations, we demonstrate coherent manipulations of three singlet-triplet qubits showing underdamped Larmor and Ramsey oscillations in all double dot sites. An accurate measurement of site-dependent field gradients as well as root-mean-squared electric and magnetic noise is provided, which is followed by a discussion of the adequacy of a simple rectangular micromagnet for practical use in multiple quantum dot arrays. Current limitations and possible strategies for achieving simultaneous multi-qubit operations in extended linear arrays are also presented.

The realization of integrated quantum circuits requires precise on-chip control of charge carriers. Aiming at the coherent coupling of distant nanostructures at zero magnetic field, here we study the ballistic electron transport through two quantum point contacts (QPCs) in series in a three terminal configuration. We enhance the coupling between the QPCs by electrostatic focusing using a field effect lens. To study the emission and collection properties of QPCs in detail we combine the electrostatic focusing with magnetic deflection. Comparing our measurements with quantum mechanical and classical calculations we discuss generic features of the quantum circuit and demonstrate how the coherent and ballistic dynamics depend on the details of the QPC confinement potentials.

Fast and high-fidelity quantum state detection is essential for building robust spin-based quantum information processing platforms in semiconductors. The Pauli spin blockade (PSB)-based spin-to-charge conversion and its variants are widely used for the spin state discrimination of two-electron singlet-triplet (ST0) qubits; however, the single-shot measurement fidelity is limited by either the low signal contrast, or the short lifetime of the triplet state at the PSB energy detuning, especially due to strong mixing with singlet states at large magnetic field gradients. Ultimately, the limited single-shot measurement fidelity leads to low visibility of quantum operations. Here, we demonstrate an alternative method to achieve spin-to-charge conversion of ST(0)qubit states using energy-selective tunneling between doubly occupied quantum dots (QDs) and electron reservoirs. We demonstrate a single-shot measurement fidelity of 90% and an S-T(0)oscillation visibility of 81% at a field gradient of 100 mT (similar to 500MHz h (g*center dot mu(B))(-1)); this allows single-shot readout with full electron charge signal contrast and, at the same time, long and tunable measurement time with negligible effect of relaxation even at strong magnetic field gradients. Using an rf-sensor positioned opposite to the QD array, we apply this method to two ST(0)qubits and show high-visibility readout of two individual single-qubit gate operations is possible with a single rf single-electron transistor sensor. We expect our measurement scheme for two-electron spin states can be applied to various hosting materials and provides a simplified and complementary route for multiple qubit state detection with high accuracy in QD-based quantum computing platforms.

Quantum point contacts (QPCs) are fundamental building blocks of nanoelectronic circuits. For their emission dynamics as well as for interaction effects such as the 0.7 anomaly the details of the electrostatic potential are important, but the precise potential shapes are usually unknown. Here, we measure the one-dimensional subband spacings of various QPCs as a function of their conductance and compare our findings with models of lateral parabolic versus hard-wall confinement. We find that a gate-defined QPC near pinch-off is compatible with the parabolic saddle-point scenario. However, as the number of populated subbands is increased, Coulomb screening flattens the potential bottom and a description in terms of a finite hard-wall potential becomes more realistic.

We propose a new design microwave radiation sensor based on a selectively doped semiconductor structure of asymmetrical shape (so-called bow-tie diode). The novelty of the design comes down to the gating of the active layer of the diode above different regions of the two-dimensional electron channel. The gate influences the sensing properties of the bow-tie diode depending on the nature of voltage detected across the ungated one as well as on the location of the gate in regard to the diode contacts. When the gate is located by the wide contact, the voltage sensitivity increases ten times as compared to the case of the ungated diode, and the detected voltage holds the same polarity of the thermoelectric electromotive force of hot electrons in an asymmetrically shaped n-n(+) junction. Another remarkable effect of the gate placed by the wide contact is weak dependence of the detected voltage on frequency which makes such a microwave diode to be a proper candidate for the detection of electromagnetic radiation in the microwave and sub-terahertz frequency range. When the gate is situated beside the narrow contact, the two orders of sensitivity magnitude increase are valid in the microwaves but the voltage sensitivity is strongly frequency-dependent for higher frequencies.

We report raster scan multiplexed charge-stability diagram measurements for tuning multiple gate-defined quantum dots in GaAs/AlGaAs heterostructures. We evaluate the charge sensitivity of the quantum point contact (QPC) in both radio frequency (rf)-reflectometry and direct current-transport modes, where we measure the signal-to-noise ratio (SNR) of 40 for rf-QPC with an integration time per pixel of 10 ms, corresponding to 1.14 ms for resolving single electron transition in the few electron regime. The high SNR for reasonable integration time allows fast two-dimensional (2D) scanning, which we use to facilitate double and triple quantum dot (TQD) tuning processes. We configure a highly stable raster scan multiplexed quantum dot tuning platform using a switching matrix and transformer-coupled alternating current ramp sources with software control. As an example of high-throughput multiple quantum dot tuning, we demonstrate systematic TQD formation using this platform in which a multiplexed combination of 2D scans enables the identification of the few electron regime in multiple quantum dots in just a few minutes. The method presented here is general, and we expect that the tuning platform is applicable to more complex multiple quantum dot arrays, allowing efficient quantum dot system Hamiltonian parameter calibration.

Three different methods of experimental mesoscopic physics, namely, weak antilocalization effects, universal conductance fluctuations, and Aharonov-Bohm oscillations, have been used to extract the electron phase-coherence scattering rate in two-dimensional gas of InGaAs/AlInAs heterostructures. The Aharonov-Bohm oscillations reveal strong beating effects, indicating the existence of two similar periodicities of the flux dependencies. As suggested by certain theoretical models, such a behavior might be expected from the so-called Berry phase acquired by electrons with different spin orientations in the presence of strong spin-orbit coupling and Zeeman splitting. In our paper we deduce the experimental values of the dephasing length and try to compare the observed beating pattern with possible scenarios for the appearance of the Berry phase.

Topological edge-reconstruction occurs in hole-conjugate states of the fractional quantum Hall effect. The frequently studied filling factor, ν = 2/3, was originally proposed to harbor two counter-propagating modes: a downstream v = 1 and an upstream v = 1/3. However, charge equilibration between these two modes always led to an observed downstream v = 2/3 charge mode accompanied by an upstream neutral mode. Here, we present an approach to synthetize a v = 2/3 edge mode from its basic counter-propagating charged constituents, allowing a controlled equilibration between the two counter-propagating charge modes. This platform is based on a carefully designed double-quantum-well, which hosts two populated electronic sub-bands (lower and upper), with corresponding filling factors, v _l and v _u . By separating the 2D plane to two gated intersecting halves, each with different fillings, counter-propagating chiral modes can be formed along the intersection line. Equilibration between these modes can be controlled with the top gates voltage and the magnetic field.

The effect of microwave radiation on the amplitudes of the quantum magnetocapacitance oscillations in field-effect transistors has been studied. It has been found that the microwave-induced variation of the oscillation amplitude depends nonmonotonically on the magnetic field including the amplitude enhancement effect. It has been shown that this behavior is due to the band bending near the edge of the gate. The effect has been attributed to the interference of quantum oscillations with radiation-induced magneto-oscillations of the screening length of a static electric field.

The temperature dependence of the microwave photovoltage has been studied in microwave-induced states of a two-dimensional electron system, which are characterized by an almost dissipationless flow of a low-frequency current. At decreasing temperature, a smooth transition has been found from a bistable state, where the photovoltage demonstrates switching between two levels, which are due to reversals of the spontaneous electric field in a domain structure, to a steady state. The transition occurs as the shift of one of the levels of the bistable photovoltage to the other level accompanied by a decrease in the switching frequency. The results indicate the freezing of the dynamic domain structure in the state corresponding to the more stable configuration of the electric field.

Topological states of matter are characterized by topological invariants, which are physical quantities whose values are quantized and do not depend on the details of the system (such as its shape, size and impurities). Of these quantities, the easiest to probe is the electrical Hall conductance, and fractional values (in units of e ²/h, where e is the electronic charge and h is the Planck constant) of this quantity attest to topologically ordered states, which carry quasiparticles with fractional charge and anyonic statistics. Another topological invariant is the thermal Hall conductance, which is harder to measure. For the quantized thermal Hall conductance, a fractional value in units of κ ₀ (κ ₀ = π ²k _B²/(3h), where k _B is the Boltzmann constant) proves that the state of matter is non-Abelian. Such non-Abelian states lead to ground-state degeneracy and perform topological unitary transformations when braided, which can be useful for topological quantum computation. Here we report measurements of the thermal Hall conductance of several quantum Hall states in the first excited Landau level and find that the thermal Hall conductance of the 5/2 state is compatible with a half-integer value of 2.5κ ₀, demonstrating its non-Abelian nature.

Electronic systems harboring one-dimensional helical modes, where spin and momentum are locked, have lately become an important field of its own. When coupled to a conventional superconductor, such systems are expected to manifest topological superconductivity; a unique phase hosting exotic Majorana zero modes. Even more interesting are fractional helical modes, yet to be observed, which open the route for realizing generalized parafermions. Possessing non-abelian exchange statistics, these quasiparticles may serve as building blocks in topological quantum computing. Here, we present a new approach to form protected one-dimensional helical edge modes in the quantum Hall regime. The novel platform is based on a carefully designed double-quantum-well structure in a GaAs based system hosting two electronic sub-bands; each tuned to the quantum Hall effect regime. By electrostatic gating of different areas of the structure, counter-propagating integer, as well as fractional, edge modes with opposite spins are formed. We demonstrate that due to spin-protection, these helical modes remain ballistic for large distances. In addition to the formation of helical modes, this platform can serve as a rich playground for artificial induction of compounded fractional edge modes, and for construction of edge modes based interferometers.

In recent interference experiments with an electronic Fabry-Pérot interferometer (FPI), implemented in the integer quantum Hall effect regime, a flux periodicity of h/2e was observed at bulk fillings νB>2.5. The halved periodicity was accompanied by an interfering charge e∗=2e, determined by shot-noise measurements. Here, we present measurements demonstrating that, counterintuitively, the coherence and the interference periodicity of the interfering chiral edge channel are solely determined by the coherence and the enclosed flux of the adjacent edge channel. Our results elucidate the important role of the latter and suggest that a neutral chiral edge mode plays a crucial role in the pairing phenomenon. Our findings reveal that the observed pairing of electrons is not a curious isolated phenomenon, but one of many manifestations of unexpected edge physics in the quantum Hall effect regime.

We study the exciton gas-liquid transition in GaAs/AlGaAs coupled quantum wells. Below a critical temperature, T-C = 4.8 K, and above a threshold laser power density the system undergoes a phase transition into a liquid state. We determine the density-temperature phase diagram over the temperature range 0.1-4.8 K. We find that the latent heat increases linearly with temperature at T less than or similar to 1.1 K, similarly to a Bose-Einstein condensate transition, and becomes constant at 1.1 less than or similar to T

The physics of itinerant electrons in condensed matter is by and large governed by repulsive Coulomb forces. However, attractive interactions may emerge and prevail in determining the ground state despite the pervasive Coulomb repulsion. A notable example is electron pairing and superconductivity. The interplay of attractive and repulsive interactions may also instigate spontaneous symmetry lowering and clustering of charges in geometric patterns even without net attraction. Both types of attractive interaction triggered physics - pairing and charge ordering - are at play in two-dimensional electron systems exposed to a quantizing magnetic field. The charge ordering has been concluded indirectly from transport behaviour. Here we report the observation of negative permittivity present solely when bubble and stripe phases form. In conjunction with a theoretical model, the negative permittivity provides evidence for the underlying attractive exchange-correlation energy which sufficiently countervails Coulomb repulsion at small distances to enable and mediate charge clustering.

We have studied the absorption of monochromatic microwave radiation in high-quality two-dimensional electron systems for the frequency span from 10 to 380 GHz using a bolometric method. For frequencies above 100 GHz the absorption exhibits an anomalous magnetic field dependence. Minima form at harmonics of the cyclotron resonance frequency. The results contrast previously reported data for other frequency ranges. Quasiclassical memory effects originating from the non-Markovian dynamics of electrons in a disorder potential containing short-range scatterers on top of a smooth potential background favorably account for the observed behavior.

The nature of edge reconstruction in the quantum Hall effect (QHE) and the issue of where the current flows have been debated for years. Moreover, the recent observation of proliferation of 'upstream' neutral modes in the fractional QHE has raised doubts about the present models of edge channels. Here, we present a new picture of the edge reconstruction in two of the hole-conjugate states. For example, while the present model for 1/2 = (2/3) consists of a single downstream chiral charge channel with conductance (2/3)(e 2 /h) and an upstream neutral mode, we show that the current is carried by two separate downstream chiral edge channels, each with conductance (1/3)(e 2 /h). We uncover a novel mechanism of fragmentation of upstream neutral modes into downstream propagating charge modes that induces current fluctuations with zero net current. Our unexpected results underline the need for better understanding of edge reconstruction and energy transport in all fractional QHE states.

The authors compare four methods to investigate the threading dislocations (TDs) observed in metamorphic buffers used in the growth of InSb quantum well on GaAs (001) substrates. Three types of buffers with varying number of Al0.24In0.76Sb interlayers (N = 0, 1, and 3) were studied. Cross-sectional scanning transmission electron microscopy (STEM) revealed an effective dislocation filtering by the interlayers. Individual TDs were identified with atomic-force microscopy (AFM) as distinct morphological features of dislocation outcrops on the surface. Threading dislocation density (TDD) is reduced by 1 order of magnitude with three interlayers, consistent with the STEM observation. TDD measured with a scanning electron microscope in electron channeling contrast imaging (ECCI) mode agrees closely with the AFM analysis, except for the N = 0 buffer where the ECCI gives TDD lower by more than a factor of two. The etch pit density of N = 3 buffer, measured with a Nomarski differential interference contrast microscope after defect selective etching (DSE), is almost 1 order of magnitude lower than the TDD measured by AFM and ECCI. Due to the large pit size, the used etching recipe only works well for samples with TDD lower than 10(7) cm(-2). AFM, ECCI, and DSE are excellent alternatives to transmission electron microscopy in the process of metamorphic buffer optimization. The AFM technique offers the additional advantage of high vertical resolution morphology mapping. Such capability is of great importance for the optimization of metamorphic buffers from the perspective of surface smoothness improvement. (C) 2017 American Vacuum Society.

In a two-dimensional electron system, microwave radiation may induce giant resistance oscillations. Their origin has been debated controversially and numerous mechanisms based on very different physical phenomena have been invoked. However, none of them have been unambiguously experimentally identified, since they produce similar effects in transport studies. The capacitance of a two-subband system is sensitive to a redistribution of electrons over energy states, since it entails a shift of the electron charge perpendicular to the plane. In such a system, microwave-induced magnetocapacitance oscillations have been observed. They can only be accounted for by an electron distribution function oscillating with energy due to Landau quantization, one of the quantum mechanisms proposed for the resistance oscillations.

This Rapid Communication was motivated by the quest for observing interference of fractionally charged quasiparticles. Here, we study the behavior of an electronic Mach-Zehnder interferometer at the integer quantum Hall effect regime at filling factors greater than 1. Both the visibility and the velocity were measured and found to be highly correlated as a function of the filling factor. As the filling factor approached unity, the visibility quenched, not to recover for filling factors smaller than unity. Alternatively, the velocity saturated around a minimal value at the unity filling factor. We highlight the significant role interactions between the interfering edge and the bulk play as well as that of the defining potential at the edge. Shot-noise measurements suggest that phase averaging (due to phase randomization), rather than single-particle decoherence, is likely to be the cause of the dephasing in the fractional regime.

Nanoscale device fabrication has enabled remarkable scientific advances. Yet a single broken electrode can render a complex device useless. The authors consider local electron-beam-induced deposition (EBID) of platinum as a method for restoring function to devices with damaged gate electrodes. The authors find that platinum deposits written with EBID at low acceleration voltage (350V) remain conductive down to millikelvin temperatures, if they are annealed after deposition in the presence of oxygen. The authors apply this technique to a complex quantum dot device based on a GaAs/AlGaAs heterostructure.

The central-spin problem is a widely studied model of quantum decoherence. Dynamic nuclear polarization occurs in central-spin systems when electronic angular momentum is transferred to nuclear spins and is exploited in quantum information processing for coherent spin manipulation. However, the mechanisms limiting this process remain only partially understood. Here we show that spin-orbit coupling can quench dynamic nuclear polarization in a GaAs quantum dot, because spin conservation is violated in the electron-nuclear system, despite weak spin-orbit coupling in GaAs. Using Landau-Zener sweeps to measure static and dynamic properties of the electron spin-flip probability, we observe that the size of the spin-orbit and hyperfine interactions depends on the magnitude and direction of applied magnetic field. We find that dynamic nuclear polarization is quenched when the spin-orbit contribution exceeds the hyperfine, in agreement with a theoretical model. Our results shed light on the surprisingly strong effect of spin-orbit coupling in central-spin systems.

The nuclear spin relaxation rate (1/T₁) is measured for GaAs two-dimensional (2D) electron systems in the quantum Hall regime with an all-electrical technique for agitating and probing the nuclear spins. A "tilted plateau" feature is observed near the Landau level filling factor v = 1 in 1/T₁ versus v. Both the width and magnitude of the plateau increase with decreasing electron density. At low temperatures, 1/T₁ exhibits an Arrhenius temperature dependence within the tilted plateau regime. The extracted energy gaps are up to two orders of magnitude smaller than the corresponding charge transport gaps. These results point to a nontrivial mechanism for the disorder-enhanced nuclear spin relaxation, in which microscopic inhomogeneities play a key role for the low energy spin excitations related to skyrmions.

Terahertz spectroscopy experiments at magnetic fields and low temperatures were carried out on samples of different gate shapes processed on a high electron mobility GaAs/AlGaAs heterostructure. For a given radiation frequency, multiple magnetoplasmon resonances were observed with a dispersion relation described within a local approximation of the magnetoconductivity tensor. The second harmonic of the cyclotron resonance was observed and its appearance was interpreted as resulting from a high-frequency, inhomogeneous electromagnetic field on the border of a two-dimensional electron gas with a metallic gate and/or an ohmic contact.

Unwanted interaction between a quantum system and its fluctuating environment leads to decoherence and is the primary obstacle to establishing a scalable quantum information processing architecture. Strategies such as environmental and materials engineering, quantum error correction and dynamical decoupling can mitigate decoherence, but generally increase experimental complexity. Here we improve coherence in a qubit using real-time Hamiltonian parameter estimation. Using a rapidly converging Bayesian approach, we precisely measure the splitting in a singlet-triplet spin qubit faster than the surrounding nuclear bath fluctuates. We continuously adjust qubit control parameters based on this information, thereby improving the inhomogenously broadened coherence time (T∗₂) from tens of nanoseconds to >2 μs. Because the technique demonstrated here is compatible with arbitrary qubit operations, it is a natural complement to quantum error correction and can be used to improve the performance of a wide variety of qubits in both meteorological and quantum information processing applications.

In order to characterize magnetic field (B) tunable THz plasmonic detectors, spectroscopy experiments were carried out at liquid helium temperatures and high magnetic fields on devices fabricated on a high electron mobility GaAs/AlGaAs heterostructure. The samples were either gated (the gate of a meander shape) or ungated. Spectra of a photovoltage generated by THz radiation were obtained as a function of B at a fixed THz excitation from a THz laser or as a function of THz photon frequency at a fixed B with a Fourier spectrometer. In the first type of measurements, the wave vector of magnetoplasmons excited was defined by geometrical features of samples. It was also found that the magnetoplasmon spectrum depended on the gate geometry which gives an additional parameter to control plasma excitations in THz detectors. Fourier spectra showed a strong dependence of the magnetoplasmon resonance amplitude on the conduction-band electron filling factor which was explained within a model of the electron gas heating with THz radiation. The study allows to define both the advantages and limitations of plasmonic devices based on high-mobility GaAs/AlGaAs heterostructures for THz detection at low temperatures and high magnetic fields.

The fractional quantum Hall effect is a canonical example of topological phases. While electric currents flow downstream in edge modes, neutral edge modes, observed only in hole-conjugate states and in v 1/2=5/2, flow upstream. It is believed that the latter transport results from multiple counter-propagating channels-mixed by disorder that is accompanied by Coulomb interaction. Here we report on sensitive shot noise measurements that reveal unexpected presence of neutral modes in non-hole-conjugate fractional states; however, not in the integer states. Furthermore, the incompressible bulk is also found to allow energy transport. While density reconstructions along the edge may account for the energy carrying edge modes, the origin of the bulk energy modes is unidentified. The proliferation of neutral modes changes drastically the accepted transport picture of the fractional quantum Hall effects. Their apparent ubiquitous presence may explain the lack of interference of fractional quasiparticles-preventing observation of fractional statistics.

We report an observation, via sensitive shot noise measurements, of charge fractionalization of chiral edge electrons in the integer quantum Hall effect regime. Such fractionalization results solely from interchannel Coulomb interaction, leading electrons to decompose to excitations carrying fractional charges. The experiment was performed by guiding a partitioned current carrying edge channel in proximity to another unbiased edge channel, leading to shot noise in the unbiased edge channel without net current, which exhibited an unconventional dependence on the partitioning. The determination of the fractional excitations, as well as the relative velocities of the two original (prior to the interaction) channels, relied on a recent theory pertaining to this measurement. Our result exemplifies the correlated nature of multiple chiral edge channels in the integer quantum Hall effect regime.

Magnetic-field tunable semiconductor detectors are used in THz spectroscopy due to their sensitivity and possibility to respond to photons in a broad frequency range. We compare THz detectors processed on high electron mobility GaAs/GaAlAs and CdTe/CdMgTe quantum wells. Transmission, photocurrent and photovoltage measurements were carried out as a function of the magnetic field at a constant energy of incident THz photons from a THz laser. The samples investigated were grid-gated and grid-free. The spectra show features resulting from excitation of the cyclotron resonance and magnetoplasmons. Theoretical models allow to analyze quantitatively the frequency of observed excitations and determine plasmon dispersion relations. This study allows to point at advantages and disadvantages of THz cyclotron-resonance and plasmonic detectors fabricated on GaAs-and CdTe-based quantum wells as well as to compare these two types of devices.

The quantum eraser is a device that illustrates the quantum principle of complementarity and shows how a dephased system can regain its lost quantum behavior by erasing the "which-path" information already obtained about it. Thus far, quantum erasers were constructed predominantly in optical systems. Here, we present a realization of a quantum eraser in a mesoscopic electronic device. The use of interacting electrons, instead of noninteracting photons, allows control over the extracted information and a smooth variation of the degree of quantum erasure. The demonstrated system can serve as a first step toward a variety of more complex setups.

We experimentally investigate the charge (isospin) frustration induced by a geometrical symmetry in a triangular triple quantum dot. We observe the ground-state charge configurations of sixfold degeneracy, the manifestation of the frustration. The frustration results in omnidirectional charge transport, and it is accompanied by nearby nontrivial triple degenerate states in the charge stability diagram. The findings agree with a capacitive interaction model. We also observe unusual transport by the frustration, which might be related to elastic cotunneling and the interference of trajectories through the dot. This work demonstrates a unique way of studying geometrical frustration in a controllable way.

Controlled dephasing of electrons, via "which path" detection, involves, in general, coupling a coherent system to a current driven noise source. However, here we present a case in which a nearly isolated electron puddle within a quantum dot, at thermal equilibrium and in millikelvin range temperature, fully dephases the interference in a nearby electronic interferometer. Moreover, the complete dephasing is accompanied by an abrupt π phase slip, which is robust and nearly independent of system parameters. Attributing the robustness of the phenomenon to the Friedel sum rule-which relates a system's occupation to its scattering phases-proves the universality of this powerful rule. The experiment allows us to peek into a nearly isolated quantum dot, which cannot be accessed via conductance measurements.

Metamaterials with negative refractive indices can manipulate electromagnetic waves in unusual ways, and can be used to achieve, for example, sub-diffraction-limit focusing, the bending of light in the wrong direction, and reversed Doppler and Cerenkov effects. These counterintuitive and technologically useful behaviours have spurred considerable efforts to synthesize a broad array of negative-index metamaterials with engineered electric, magnetic or optical properties. Here we demonstrate another route to negative refraction by exploiting the inertia of electrons in semiconductor two-dimensional electron gases, collectively accelerated by electromagnetic waves according to Newtons second law of motion, where this acceleration effect manifests as kinetic inductance. Using kinetic inductance to attain negative refraction was theoretically proposed for three-dimensional metallic nanoparticles and seen experimentally with surface plasmons on the surface of a three-dimensional metal. The two-dimensional electron gas that we use at cryogenic temperatures has a larger kinetic inductance than three-dimensional metals, leading to extraordinarily strong negative refraction at gigahertz frequencies, with an index as large as -700. This pronounced negative refractive index and the corresponding reduction in the effective wavelength opens a path to miniaturization in the science and technology of negative refraction.

We demonstrate that branching of the electron flow in semiconductor nanostructures can strongly affect macroscopic transport quantities and can significantly change their dependence on external parameters compared to the ideal ballistic case, even when the system size is much smaller than the mean free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs two-dimensional electron gas, we observe a splitting of the commensurability peaks in the magnetoresistance curve. We show that a model which includes a random disorder potential of the two-dimensional electron gas can account for the random splitting of the peaks that result from the collimation of the electron beam. The shape of the splitting depends on the particular realization of the disorder potential. At the same time, magnetic focusing peaks are largely unaffected by the disorder potential.

Quantum computers have the potential to solve certain problems faster than classical computers. To exploit their power, it is necessary to perform interqubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor because of their potential for scalability and miniaturization. However, their weak interactions with the environment, which lead to their long coherence times, make interqubit operations challenging. We performed a controlled two-qubit operation between singlet-triplet qubits using a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography, we measured the full density matrix of the system and determined the concurrence and the fidelity of the generated state, providing proof of entanglement.

Upstream neutral modes, counter propagating to charge modes and carrying energy without net charge, had been predicted to exist in some of the fractional quantum Hall states and were recently observed via noise measurements. Understanding such modes will assist in identifying the wavefunction of these states, as well as shedding light on the role of Coulomb interactions within edge modes. Here, operating mainly in the ν=2/3 state, we place a quantum dot a few micrometres upstream of an ohmic contact, which serves as a 'neutral modes source'. We show that the neutral modes heat the input of the dot, causing a net thermo-electric current to flow through it. Heating of the electrons leads to a decay of the neutral mode, manifested in the vanishing of the thermo-electric current at T>110 mK. This set-up provides a straightforward method to investigate upstream neutral modes without turning to the more cumbersome noise measurements.

Fractionally charged quasiparticles, which obey non-Abelian statistics, were predicted to exist in the ν=8/3, ν=5/2, and ν=7/3 fractional quantum Hall states (in the second Landau level). Here we present measurements of charge and neutral modes in these states. For both ν=7/3 and ν=8/3 states, we found a quasiparticle charge e=1/3 and an upstream neutral mode only in ν=8/3-excluding the possibility of non-Abelian Read-Rezayi states and supporting Laughlin-like states. The absence of an upstream neutral mode in the ν=7/3 state also proves that edge reconstruction was not present in the ν=7/3 state, suggesting its absence also in ν=5/2 state, and thus may provide further support for the non-Abelian anti-Pfaffian nature of the ν=5/2 state.

We investigate the quantum Hall stripe phase at filling factor 9/2 at the microscopic level by probing the dispersion of its collective modes with the help of surface acoustic waves with wavelengths down to 60 nm. The dispersion is strongly anisotropic. It is highly dispersive and exhibits a roton minimum for wave vectors aligned along the easy transport direction. In the perpendicular direction, however, the dispersion is featureless, although not flat as predicted by theory. Oscillatory behavior in the absorption intensity of the collective mode with a wave vector perpendicular to the stripes is attributed to a commensurability effect. It allows us to extract the periodicity of the quantum Hall stripes.

We report on the first experimental observation of neutral modes in particle-hole conjugate fractional quantum Hall states using shot noise measurements. The presence of the neutral modes, in addition to producing excess noise in a quantum point contact (QPC), was seen to affect strongly the charge of the tunneling quasiparticles of the charge mode in the QPC and to increase their apparent temperature. The observation of an upstream neutral mode in the 5/2 state may constitute an added indication of its non-abelian nature.

In many realizations of electron spin qubits the dominant source of decoherence is the fluctuating nuclear spin bath of the host material. The slowness of this bath lends itself to a promising mitigation strategy where the nuclear spin bath is prepared in a narrowed state with suppressed fluctuations. Here, this approach is realized for a two-electron spin qubit in a GaAs double quantum dot and a nearly tenfold increase in the inhomogeneous dephasing time T2* is demonstrated. Between subsequent measurements, the bath is prepared by using the qubit as a feedback loop that first measures its nuclear environment by coherent precession, and then polarizes it depending on the final state. This procedure results in a stable fixed point at a nonzero polarization gradient between the two dots, which enables fast universal qubit control.

Experimental results examining the photoluminescence spectra of selectively Si-doped GaAs/ Al_x Ga_1-x As heterostructures is presented. Possible mechanisms of carrier recombination are discussed with a special emphasis on the peculiarities of excitonic photoluminescence. Strong intensity lines in photoluminescence spectra are associated with the formation and enhancement of free exciton and exciton-polariton emission in the flat band region of an active i -GaAs layer. The excitonic PL intensity is sensitive to the excitation intensity indicating high nonlinear behavior of spectral-integrated photoluminescence intensity and exciton line narrowing. These observed phenomena may be related to the collective interaction of excitons and the interaction of excitons with emitted electromagnetic waves. The gain of the amplification of the excitonic photoluminescence intensity in the heterostructure was found to be more than 1000 times larger than the intensity of i -GaAs active layer. The quality factor of the exciton line emission and the exciton-polariton line was found to be 3800 and 7600, respectively.

The quantum Hall effect takes place in a two-dimensional electron gas under a strong magnetic field and involves current flow along the edges of the sample. For some particleĝ\u20ac"hole conjugate states of the fractional regime (for example, with fillings between 1/2 and 1 of the lowest Landau level), early predictions suggested the presence of counter-propagating edge currents in addition to the expected ones. When this did not agree with the measured conductance, it was suggested that disorder and interactions will lead to counter-propagating modes that carry only energyĝ\u20ac"the so called neutral modes. In addition, a neutral upstream mode (the Majorana mode) was expected for selected wavefunctions proposed for the even-denominator filling 5/2. Here we report the direct observation of counter-propagating neutral modes for fillings of 2/3, 3/5 and 5/2. The basis of our approach is that, if such modes impinge on a narrow constriction, the neutral quasiparticles will be partly reflected and fragmented into charge carriers, which can be detected through shot noise measurements. We find that the resultant shot noise is proportional to the injected current. Moreover, when we simultaneously inject a charge mode, the presence of the neutral mode was found to significantly affect the Fano factor and the temperature of the backscattered charge mode. In particular, such observations for filling 5/2 may single out the non-Abelian wavefunctions for the state.

We report the observation of inverse magnetic field periodic, radiation-induced magnetoresistance oscillations in GaAs/AlGaAs heterostructures prepared in W. Wegscheider's group, compare their characteristics with similar oscillations in V. Umansky's material, and describe the lineshape variation vs the radiation power, P, in the two systems. We find that the radiation-induced oscillatory resistance, ΔR_xx, in both materials, can be described by ΔR_xx=-A exp(-λ/B)sin(2πF/B), where A is the amplitude, λ is the damping parameter, and F is the oscillation frequency. Both λ and F turn out to be insensitive to P. On the other hand, A grows nonlinearly with P.

We have measured the electric transport of double quantum point contacts in series at low temperatures. Two pairs of metal gates are placed longitudinally and sequentially with an edge-to-edge distance of 600 nm. They are used to form two quantum point contacts in a GaAs / Al_x Ga_{1 - x} As heterostructure. Isolating from an insulating layer, a top gate is also fabricated on top of the quantum point contacts to modify the electron densities in the quantum point contacts and the two dimensional electron gas as well. The transport is characterized by the direct transmission probability T_d which represents the portions of electrons travelling ballistically from one quantum point contact to the other. Our results show that the parameter T_d decreases with decreasing carrier density. The transport is partially adiabatic in high 2D electron densities and transits to completely ohmic regimes in low densities. Because of the correlation between the coherence length and transmission probability, we attribute the result to the reduction of the coherence length and mean free path in the unconstricted electron gas between quantum point contacts.

A density-tunable GaAs-AlGaAs heterostructure is used to study the density dependence of the filling factor ν= 5 2 and other fractional and reentrant integer quantum-Hall states in the second Landau level. The activation energy at ν= 5 2 can be determined for densities between 1.3 and 2.7× 1011 / cm2 and reaches up to 310 mK. The 5/2 energy gap is calculated numerically, including finite width and Landau-level-mixing corrections, both as a function of electron density. The discrepancy between theory and experiment increases moderately with density and reaches about 1.5 K at the highest density. We argue that the activation energy is strongly influenced by disorder from ionized donors and attribute this to the surprisingly large size of the 5/2 quasiparticles. We find that the quasiparticles have a diameter of at least 12 times the magnetic length or 150 nm at a magnetic field of 4 T. Implications for heterostructure design are discussed.

One fundamental requirement for quantum computation is to carry out universal manipulations of quantum bits at rates much faster than the qubits rate of decoherence. Recently, fast gate operations have been demonstrated in logical spin qubits composed of two electron spins where the rapid exchange of the two electrons permits electrically controllable rotations around one axis of the qubit. However, universal control of the qubit requires arbitrary rotations around at least two axes. Here, we show that by subjecting each electron spin to a magnetic field of different magnitude, we achieve full quantum control of the two-electron logical spin qubit with nanosecond operation times. Using a single device, a magnetic-field gradient of several hundred millitesla is generated and sustained using dynamic nuclear polarization of the underlying Ga and As nuclei. Universal control of the two-electron qubit is then demonstrated using quantum state tomography. The presented technique provides the basis for single- and potentially multiple-qubit operations with gate times that approach the threshold required for quantum error correction.

We report the experimental results from a dark study and a photoexcited study of the high-mobility GaAs/AlGaAs system at large filling factors, ν. At large ν, the dark study indicates several distinct phase relations ("type 1," "type 2," and "type 3") between the oscillatory diagonal and Hall resistances, as the canonical integral quantum Hall effect (IQHE) is manifested in the type 1 case of approximately orthogonal diagonal and Hall resistance oscillations. Surprisingly, the investigation indicates quantum Hall plateaus also in the type 3 case characterized by approximately "antiphase" Hall and diagonal resistance oscillations, suggesting an unfamiliar and distinct class of IQHE. Transport studies under microwave photoexcitation exhibit radiation-induced magnetoresistance oscillations in both the diagonal, Rxx, and off-diagonal, Rxy, resistances. Further, when the radiation-induced magnetoresistance oscillations extend into the quantum Hall regime, there occurs a radiation-induced nonmonotonic variation in the amplitude of Shubnikov-de Haas (SdH) oscillations in Rxx vs B, and a nonmonotonic variation in the width of the quantum Hall plateaus in Rxy. The latter effect leads into the vanishing of IQHE at the minima of the radiation-induced Rxx oscillations with increased photoexcitation. We reason that the mechanism which is responsible for producing the nonmonotonic variation in the amplitude of SdH oscillations in Rxx under photoexcitation is also responsible for eliminating, under photoexcitation, the type 3 associated IQHE in the high-mobility specimen.

Optical absorption measurements are used to probe the spin polarization in the integer and fractional quantum Hall effect regimes. The system is fully spin polarized only at filling factor ν=1 and at very low temperatures (∼40mK). A small change in filling factor (δν ±0.01) leads to a significant depolarization. This suggests that the itinerant quantum Hall ferromagnet at ν=1 is surprisingly fragile against increasing temperature, or against small changes in filling factor.

We report experimental results of the effect of Ka-band microwave on the spin dynamics of electrons in a two-dimensional electron system (2DES) in a GaAs/ Al_0.35 Ga_0.65 As heterostructure via time-resolved Kerr rotation measurements. While the microwave reduces the transverse spin lifetime of electrons in the bulk GaAs, it significantly increases that in the 2DES, from 745 to 1213 ps, when its frequency is close to the Zeeman splitting of the electrons in the magnetic field. Such a microwave-enhanced spin lifetime is ascribed to the microwave-induced electron scattering which leads to a "motional narrowing" of spins via D'yakonov-Perel' mechanism.

We observe microwave-induced photocurrent and photovoltage oscillations around zero as a function of the applied magnetic field in high mobility GaAs 2D electron systems. The photosignals pass zero whenever the microwave frequency is close to a multiple of the cyclotron resonance frequency. They originate from built-in electric fields due to for instance band bending at contacts. The oscillations correspond to a suppression (screening) or an enhancement ("antiscreening") of these fields by the photoexcited electrons.

We report on the phase measurements on a quantum dot containing a single electron in the Kondo regime. Transport takes place through a single orbital state. Although the conductance is far from the unitary limit, we measure directly, for the first time, a transmission phase as theoretically predicted of π/2. As the dot's coupling to the leads is decreased, with the dot entering the Coulomb blockade regime, the phase reaches a value of π. Temperature shows little effect on the phase behavior in the range 30-600 mK, even though both the two-terminal conductance and amplitude of the Aharonov-Bohm oscillations are strongly affected. These results also suggest that previous phase measurements involved transport through more than a single level.

We report on noise measurements in a quantum dot in the presence of Kondo correlations. Close to the unitary limit, with the conductance reaching 1.8 e2 h, we observed an average backscattered charge of e* ∼5e 3, while weakly biasing the quantum dot. This result held to bias voltages up to half the Kondo temperature. Away from the unitary limit, the charge was measured to be e as expected. These results confirm and extend theoretical predictions that suggested that two-electron backscattering processes dominate over single-electron backscattering processes near the unitary limit, with an average backscattered charge e* ∼5e 3.

We report on an inelastic light scattering study in the two-dimensional electron system at filling factor nu=1. The energy and the inelastic cross section of the cyclotron spin-flip mode are analyzed. The exchange enhanced electronic g factor in the quantum Hall ferromagnet is measured. In addition to the magnetoplasmon and cyclotron spin-flip modes, inelastic scattering lines associated with spin-singlet and spin-triplet D- complexes are observed. The inelastic light scattering is shown to probe the thermodynamics of the quantum Hall ferromagnet and gives insight into the formation of ferromagnetic order in two dimensions. It is established that at temperatures above the bare Zeeman energy, the ferromagnet breaks up into domains, whose size and number change with temperature. The experimental data allow us to construct a stability diagram of the nu=1 quantum Hall ferromagnet in the temperature versus magnetic field plane.

The complementarity principle demands that a particle reveals wave-like properties only when the different paths that it can take are indistinguishable. The complementarity has been demonstrated in optics with pairs of correlated photons and in two-path solid-state interferometers with phase-coherent electrons. In the latter experiment, a charge detector embedded near one path of a two-path electron interferometer provided which-path information. Here, we report on electron dephasing in an Aharonov-Bohm ring interferometer via a charge detector adjacent to the ring. In contrast to the two-path interferometer, charge detection in the ring does not always provide path information. The interference was suppressed only when path information could be acquired, even if only in principle. Thisconfirms that dephasing is not always induced by disturbingthe interfering particle through the interferometer- environment interactions: path information of the particle must be available too. Our experiment suggests that acquisition of which-path information is more fundamental than the back-action in understanding quantum mechanical complementarity.

In a controlled dephasing experiment, an interferometer loses its coherence owing to entanglement of the interfering electron with a controlled quantum system, which effectively is equivalent to path detection. In previous experiments, only partial dephasing was achieved owing to weak interactions between many detector electrons and the interfering electron, leading to a gaussian-phase randomizing process. Here, we report the opposite extreme, where interference is completely destroyed by a few (that is, one to three) detector electrons, each of which has a strong randomizing effect on the phase. We observe quenching of the interference pattern in a periodic, lobe-type fashion as the detector current is varied, and with a peculiar V-shaped dependence on the detectors partitioning. We ascribe these features to the non-gaussian nature of the noise, which is also important for qubit decoherence. In other words, the interference seems to be highly sensitive to the full counting statistics of the detectors shot noise.

Very much like the ubiquitous quantum interference of a single particle with itself, quantum interference of two independent, but indistinguishable, particles is also possible. For a single particle, the interference is between the amplitudes of the particle's wavefunctions, whereas the interference between two particles is a direct result of quantum exchange statistics. Such interference is observed only in the joint probability of finding the particles in two separated detectors, after they were injected from two spatially separated and independent sources. Experimental realizations of two-particle interferometers have been proposed; in these proposals it was shown that such correlations are a direct signature of quantum entanglement between the spatial degrees of freedom of the two particles ('orbital entanglement'), even though they do not interact with each other. In optics, experiments using indistinguishable pairs of photons encountered difficulties in generating pairs of independent photons and synchronizing their arrival times; thus they have concentrated on detecting bunching of photons (bosons) by coincidence measurements. Similar experiments with electrons are rather scarce. Cross-correlation measurements between partitioned currents, emanating from one source, yielded similar information to that obtained from auto-correlation (shot noise) measurements. The proposal of ref. 3 is an electronic analogue to the historical Hanbury Brown and Twiss experiment with classical light. It is based on the electronic Mach-Zehnder interferometer that uses edge channels in the quantum Hall effect regime. Here we implement such an interferometer. We partitioned two independent and mutually incoherent electron beams into two trajectories, so that the combined four trajectories enclosed an Aharonov-Bohm flux. Although individual currents and their fluctuations (shot noise measured by auto-correlation) were found to be independent of the Aharonov-Bohm flux, the cross-correlation between current fluctuations at two opposite points across the device exhibited strong Aharonov-Bohm oscillations, suggesting orbital entanglement between the two electron beams.

The microwave response of a two-dimensional electron system with a nearby in situ grown back gate has been investigated in transport. Several resonant absorption peaks are detected in the magnetoresistance and can be assigned to the excitation of collective magnetoplasma modes. The results are compared with data measured on a similar electron system but without gate. The two-dimensional plasma spectrum is drastically altered by the gate and exhibits a linear dispersion instead of the conventional square-root dependence anticipated from theoretical considerations.

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0

Shot noise measurements provide information on particle charge and its correlations. We report on shot noise measurements in a generic quantum dot under a quantized magnetic field. The measured noise at the peaks of a sequence of conductance resonances was some 9 times higher than expected, suggesting bunching of electrons as they traverse through the dot. This enhancement might be mediated by an additional level, weakly coupled to the leads or an excited state. Note that in the absence of a magnetic filed no bunching had been observed.

We investigate commensurability oscillations in the magnetoresistances of unstressed ungated rectangular two-dimensional superlattices of different periods. The amplitude of the commensurability oscillations in these systems exhibits a nonmonotonic dependence on the applied magnetic field, which is not present in 1D or square superlattices. Furthermore, the high and low resistance directions switch between the two axial directions of the superlattices depending on the magnetic field. Our observations are explained by the drift of the cyclotron motion guiding center along contours of a magnetic-field-dependent effective potential as put forward in a recent theory. Comparison of the data with the theoretical predictions shows good agreement. For a larger modulation amplitude, we observe a flattening of commensurability oscillation minima, which is also predicted by the calculation.

Using a scanning single electron transistor we probe the individual localized states in the integer quantum Hall regime. We find that the observed states are utterly different than those predicted by the single-particle theory and are mainly determined by Coulomb interactions. They appear only when quantization of kinetic energy limits the screening ability of electrons. Consequently, the QH effect becomes more diverse acquiring new regimes and phase transitions, absent in the single-particle framework. Our experiments suggest a unified picture of localization within the QH phenomena, in which the validity of the single-particle model is constrained only to the limit of strong disorder.

We measure the absorption spectrum of a two-dimensional electron system (2DES) in a GaAs quantum well in the presence of a perpendicular magnetic field. We focus on the absorption spectrum into the lowest Landau level around [Formula presented]. We find that the spectrum consists of bound electron-hole complexes, trionlike and excitonlike. We show that their oscillator strength is a powerful probe of the 2DES spatial correlations. We find that near [Formula presented] the 2DES ground state consists of Skyrmions of small size (a few magnetic lengths).

An outstanding question pertaining to the microscopic properties of the fractional quantum Hall effect is understanding the nature of the particles that participate in the localization but that do not contribute to electronic transport. By using a scanning single electron transistor, we imaged the individual localized states in the fractional quantum Hall regime and determined the charge of the localizing particles. Highlighting the symmetry between filling factors 1/3 and 2/3, our measurements show that quasi-particles with fractional charge e*= e/3 localize in space to submicrometer dimensions, where e is the electron charge.

The magnetic vortex pinning at single point defects in submicron-sized Permalloy disks was analyzed. The effect of defects on magnetic vortex structures was investigated using micro-Hall magnetometry and micromagnetic simulations. An image reversal electron beam lithography process was used for the fabrication of ferromagnetic particles with artificial point defects. The asymmetry, which was observed in the hysteresis trace with respect to the origin, was due to asymmetric position of the defect on the the disk. The results show that point defects in the ferromagnetic disks alters the corresponding hysteresis traces.

The dephasing experiment of a quantum dot in the Kondo regime was performed. Dephasing was induced by a capacitively coupled quantum point contact (QPC), which serves as a which path detector. The qualitative dephasing is similar to that of a QD in the Coulomb blockade regime. The results show that the suppression strength is inversely proportional to the Kondo temperature.

A f-independent 1/4-cycle phase shift of the extrema in the radiation-induced oscillary magnetoresistance in GsAs/AlGaAs devices was demonstrated. The phase and the period of the magnetoresistance was examined by utilizing in situ magnetic field calibration by electron spin resonance of diphenyl-picryl-hydrazal. The phase of the R_xx oscillation were consistence with a 1/4- cycle phase shifts only at low B. The small reduction in the effective mas ration with respect to the standard value for GaAs/AlGaAs devices.

We report measurements of spin transitions for GaAs quantum dots in the Coulomb blockade regime and compare ground and excited state transport spectroscopy to direct measurements of the spin polarization of emitted current. Transport spectroscopy reveals both spin-increasing and spin-decreasing transitions, as well as higher-spin ground states, and allows [Formula presented] factors to be measured down to a single electron. The spin of emitted current in the Coulomb blockade regime, measured using spin-sensitive electron focusing, is found to be polarized along the direction of the applied magnetic field regardless of the ground state spin transition.

Fractionally charged quasiparticles in edge states, are expected to condense to a chiral Luttinger liquid (CLL). We studied their condensation by measuring the conductance and shot noise due to an artificial backscatterer embedded in their path. At sufficiently low-temperatures backscattering events were found to be strongly correlated, producing a highly nonlinear current-voltage characteristic and a nonclassical shot noiseboth are expected in a CLL. When, however, the impinging beam of quasiparticles was made dilute, either artificially via an additional weak backscatterer or by increasing the temperature, the resultant outgoing noise was classical, indicating the scattering of independent quasiparticles. Here, we study in some detail this surprising crossover from correlated particle behavior to an independent behavior, as a function of beam dilution and temperature.

We demonstrate a quantum coherent electron spin filter by directly measuring the spin polarization of emitted current. The spin filter consists of an open quantum dot in an in-plane magnetic field; the in-plane field gives the two spin directions different Fermi wavelengths resulting in spin-dependent quantum interference of transport through the device. The gate voltage is used to select the preferentially transmitted spin, thus setting the polarity of the filter. This provides a fully electrical method for the creation and detection of spin-polarized currents. Polarizations of emitted current as high as 70% for both spin directions (either aligned or anti-aligned with the external field) are observed.

Significant long-range correlations which exist in the disorder potential is shown. It focuses on broad quantum wells (QWs) in which the short-range behavior is averaged by the scanning near-field optical microscopy (SNOM) tip and studies in detail the vertical and lateral long-range order of the exciton energy distribution. It is found that the seemingly random exciton energy fluctuations exhibit a well-defined order.

An overview is given on the experimental evidence that at low temperature the unpaired spin associated with the 0.7 structure forms a Kondo-like many-body state. The evidence presented includes: 1) a narrow conductance peak at zero source-drain bias that forms at low temperature, 2) collapse of conductance data onto a single function, 3) correspondence between the Kondo scaling factor and the width of the zero-bias peak, and 4) splitting of the zero-bias peak in a magnetic field.

Shot noise measurements have been used to measure the charge of quasiparticles in the fractional quantum Hall (FQH) regime. To induce shot noise in an otherwise noiseless current of quasiparticles, a barrier is placed in its path to cause weak backscattering. The measured shot noise is proportional to the charge of the quasiparticles; for example, at filling factor v = 1/3, noise corresponding to q = e/3 appears. For increasingly opaque barriers, the measured charge increases monotonically, approaching q = e asymptotically. It was therefore believed that only electrons, or alternatively, three bunched quasiparticles can tunnel through high-potential barriers encountered by a noiseless current of quasiparticles. Here we investigate the interaction of e/3 quasiparticles with a strong barrier in FQH samples and find that bunching of quasiparticles in the strong backscattering limit depends on the average dilution of the quasiparticle current. For a very dilute current, bunching ceases altogether and the transferred charge approaches q = e/3. This surprising result demonstrates that quasiparticles can tunnel individually through high-potential barriers originally thought to be opaque for them.

A study of internal transitions of quasi-two-dimensional, negatively charged magnetoexcitons (X^-) and their evolution with excess electron density, in GaAs/AlGaAs quantum wells, was presented. The optically detected resonance spectra were dominated by bound-to-continuum bands, in dilute electron limit, in contrast to negatively charged donor system due to magnetic translational invariance. Magnetoplasmons bound to a mobile valence band hole was used to explain the blueshifted transitions.

We have investigated the magnetoresistance of high mobility two-dimensional electron gases (2DEG) under the influence of periodically arranged ferromagnetic nanopillars. A huge positive magnetoresistance and commensurability oscillations (COs) indicate that the 2DEG is subjected to a surprisingly strong modulation. Angular-dependent magnetoresistance measurements as well as the phase of the COs suggest that the modulation is dominated by electrostatic rather than by magnetic contributions.

Two-dimensional electron systems were laterally modulated using the method of in situ interferometric illumination. Magnetotransport measurements on 1D and 2D modulated systems revealed a phase change of the commensurability oscillations depending on temperature. This behaviour is surprising and cannot be explained by existing theories.

We employ shot noise measurements to characterize the effective charge of quasiparticles, at filling factor [Formula presented] of the fractional quantum Hall regime, as they scatter from an array of identical weak backscatterers. Upon scattering, quasiparticles are known to bunch, e.g., only three [Formula presented] charges, or \u201celectrons\u201d are found to traverse a rather opaque potential barrier. We find here that the effective charge scattered by an array of scatterers is determined by the scattering strength of an individual scatterer and not by the combined scattering strength of the array, which can be very small. Moreover, we also rule out intraedge equilibration of [Formula presented] quasiparticles over a length scale of hundreds of microns.

We have studied the capacitance between a two-dimensional electron system and a gate of field-effect transistors, produced from three different wafers with a single remotely doped GaAs/AlxGa1-xAs heterojunction. In the vicinity of the Landau-level filling factor v = 1/2, the increment of the capacitance relative to its zero-magnetic-field value, deltaC(1/2), was found to be insensitive to the carrier density, and close to the value as if the particles are noninteracting. This observation implies that electron-electron interaction affects the compressibility of the zero-field and composite-fermion metallic states in a very similar manner.

The near- and far-field photoluminescence (PL) spectra of a gated two-dimensional electron gas have been measured in a GaAs quantum well. Scanning near-field measurements reveal the microscopic origin of the different line shapes of the neutral (X) and negatively charged (formula presented) exciton. We find a new broadening mechanism of the exciton: local density fluctuations give rise to spatial fluctuations of the local X peak energy, and hence to inhomogeneous broadening of the far-field X line. The X linewidth is therefore proportional to the width of the electron density distribution. On the other hand, we find that the (formula presented) is homogeneously broadened, and the numerator of its Lorentzian line shape is linearly proportional to the electron density. We present a simple method to determine low electron densities from the PL spectrum.

We studied the effect of microwave (mw) irradiation on the low-temperature photoluminescence (PL) of high-quality, modulation-doped, wide GaAs/AlxGa1-xAs heterojunctions (HJ's) containing a two-dimensional electron gas (2DEG), in the density range of (0.9-4)x 10(11) cm(-2). The PL arises from excitons that recombine radiatively in the GaAs buffer layer, far from the 2DEG which is confined close to the GaAs/AlxGa1-xAs interface. We observe that the exciton PL is affected by a mw heating of the 2DEG: the mw-induced PL intensity change increases with increasing 2DEG density as well as under a perpendicular magnetic field that corresponds to the 2DEG dimensional magnetoplasma resonance (DMPR) condition. Moreover, the exciton PL intensity shows a bistability at magnetic field strengths that are close to those observed in the DMPR mw absorption. The mw-induced PL modulation effects are interpreted as being due to the interaction of the excitons with low-energy, ballistically propagating acoustic phonons that are emitted by the mw-heated 2DEG. The exciton PL quenching is associated with an exciton drag by the phonon flux towards the opposite l-IJ interface where the excitons recombine nonradiatively. The rate of phonon emission is determined by the 2DEG state, and thus the exciton PL responds to the changes of the 2DEG parameters.

The fractional quantum Hall effect occurs in the conduction properties of a two-dimensional electron gas subjected to a strong perpendicular magnetic field. In this regime, the Hall conductance shows plateaux, or fractional states, at rational fractional multiples of e²/h, where e is the charge of an electron and h is Planck's constant. The explanation of this behaviour invokes strong Coulomb interactions among the electrons that give rise to fractionally charged quasiparticles which can be regarded as noninteracting current carriers. Previous studies have demonstrated the existence of quasiparticles with one-third of an electron's charge, the same fraction as that of the respective fractional state. An outstanding ambiguity is therefore whether these studies measured the charge or the conductance. Here we report the observation of quasiparticles with a charge of e/5 in the 2/5 fractional state, from measurements of shot noise in a two-dimensional electron gas. Our results imply that charge can be measured independently of conductance in the fractional quantum Hall regime, generalizing previous observations of fractionally charged quasiparticles.

The charge of quasi-particles in the fractional quantum Hall regime was determined via quantum shot noise measurements. The noise is generated by a current flow through a partially transmitting quantum point contact in a two dimensional electron gas and is directly proportional to the charge of the quasi-particles. We measured quantum shot noise at a filling factor of 1/3 and found that the charge of the quasi-particles is e/3 as predicted by Laughlin. (C) 1998 Elsevier Science B.V. All rights reserved.

We have investigated magnetotransport through two-dimensional lateral superlattices with periods a of 100 and 120 nm prepared on a shallow, high mobility heterojunction. Measurements of the longitudinal and Hall resistance were carried out at a temperature of 50 mK with the bias voltage of the top gate as parameter. The longitudinal resistance displays an unprecedented richness in oscillatory features of both semiclassical and quantum mechanical origin. The results are discussed on the basis of the zero field miniband energy spectrum and the subband splitting of single Landau-levels.

The interactions between adsorbed organic molecules and the electronic charge carriers in specially made GaAs structures are studied by time- and wavelength-dependent measurements of the photocurrent. The adsorption of the molecules modifies the photocurrent decay time by orders of magnitude. The effects are molecularly specific, as they depend on the electronic properties and absorption spectrum of the molecules. These observations are rationalized by assuming that new surface states are created upon adsorption of the molecules and that the character of these states is controlled by the relative electronegativity of the substrates and the adsorbed molecules. The relevance for surface passivation and for construction of semiconductor-based sensors is indicated.

The effects of chemisorbed molecules on the electronic transport through an ungated high electron mobility transistor and through an ungated field effect transistor are examined, Current versus voltage measurements in the dark reveal that the adsorbed molecules and their chemical nature have a pronounced effect on the structures' performance, as they reduce the current by up to one order of magnitude. The molecular specificity of the devices is expressed in the wavelength dependence of photo current decay. The decay time, which increases by several orders of magnitude upon adsorption of the molecules, changes drastically when the excitation wavelength matches the absorption of the adsorbed molecules. Effect of Cu ions caught by adsorbed organic molecules on the photo current decay is clearly demonstrated. The observations open new possibilities in constructing semiconductor based light and chemical sensors.

Since Millikan's famous oil-drop experiments, it has been well known that electrical charge is quantized in units of the charge of an electron, e. For this reason, the theoretical prediction by Laughlin of the existence of fractionally charged 'quasiparticles'-proposed as an explanation for the fractional quantum Hall (FQH) effect-is very counterintuitive. The FQH effect is a phenomenon observed in the conduction properties of a two-dimensional electron gas subjected to a strong perpendicular magnetic field. This effect results from the strong interaction between electrons, brought about by the magnetic field, giving rise to the aforementioned fractionally charged quasiparticles which carry the current. Here we report the direct observation of these counterintuitive entities by using measurements of quantum shot noise. Quantum shot noise results from the discreteness of the current- carrying charges and so is proportional to both the charge of the quasiparticles and the average current. Our measurements of quantum shot noise show unambiguously that current in a two-dimensional electron gas in the FQH regime is carried by fractional charges-e/3 in the present case-in agreement with Laughlin's prediction.

We study the effect of spatial correlation of ionized donors on the single-particle scattering time and on spin splitting in a two-dimensional electron gas (2DEG). As the correlation is being reduced we observe a reduction in the scattering time and a collapse of the spin-splitted peaks into a single peak. We find these electronic properties to be much more sensitive than the momentum relaxation time (or mobility) in high mobility 2DEG. We compare our results with Monte Carlo simulations and find them to be in partial agreement.

We present transport measurements on surface superlattices fabricated on a GaAs/AlGaAs two-dimensional electron gas. Significant suppression of the conductance is found with increasing electric fields and with increasing temperature T. We attribute these effects to e-e scattering, which can significantly affect the resistance in the presence of a spatially modulated static potential.