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In this short review (written to celebrate David Campbell's 80th birthday), we provide a theoretical description of quantum transport in nanoscale systems in the presence of single-electron excitations generated by Lorentzian voltage drives, termed Levitons. These excitations allow us to realize the analog of quantum optics experiments using electrons instead of photons. Importantly, electrons in condensed matter systems are strongly affected by the presence of different types of non-trivial correlations, with no counterpart in the domain of photonic quantum optics. After providing a short introduction about Levitons in non-interacting systems, we focus on how they operate in the presence of two types of strong electronic correlations in nanoscale systems, such as those arising in the fractional quantum Hall effect or in superconducting systems. Specifically, we consider Levitons in a quantum Hall bar of the fractional quantum Hall effect, pinched by a quantum point contact, where anyons with fractional charge and statistics tunnel between opposite edges. In this case, a Leviton–Leviton interaction can be induced by the strongly correlated background. Concerning the effect of superconducting correlations on Levitons, we show that, in a normal metal system coupled to BCS superconductors, half-integer Levitons minimize the excess noise in the Andreev regime. Interestingly, energy-entangled electron states can be realized on-demand in this type of hybrid setup by exploiting crossed Andreev reflection. The results exposed in this review have potential applications in the context of quantum information and computation with single-electron flying qubits. [ABSTRACT FROM AUTHOR]

The mass production and the practical number of cryogenic quantum devices producible in a single chip are limited to the number of electrical contact pads and wiring of the cryostat or dilution refrigerator. It is, therefore, beneficial to contrast the measurements of hundreds of devices fabricated in a single chip in one cooldown process to promote the scalability, integrability, reliability, and reproducibility of quantum devices and to save evaluation time, cost and energy. Here, we used a cryogenic on-chip multiplexer architecture and investigated the statistics of the 0.7 anomaly observed on the first three plateaus of the quantized conductance of semiconductor quantum point contact (QPC) transistors. Our single chips contain 256 split gate field-effect QPC transistors (QFET) each, with two 16-branch multiplexed source-drain and gate pads, allowing individual transistors to be selected, addressed and controlled through an electrostatic gate voltage process. A total of 1280 quantum transistors with nano-scale dimensions are patterned in 5 different chips of GaAs heterostructures. From the measurements of 571 functioning QFETs taken at temperatures T = 1.4 K and T = 40 mK, it is found that the spontaneous polarisation model and Kondo effect do not fit our results. Furthermore, some of the features in our data largely agreed with van Hove model with short-range interactions. Our approach provides further insight into the quantum mechanical properties and microscopic origin of the 0.7 anomaly in QFETs, paving the way for the development of semiconducting quantum circuits and integrated cryogenic electronics, for scalable quantum logic control, readout, synthesis, and processing applications.

A numerical simulation of a two-dimensional Gaussian wave packet of charged particles has been performed to investigate the diffraction phenomena from a single-, double-, and multi-slit grating biased with an electrostatic potential (V e 0 ). The wave packet dynamics are obtained by solving the time-dependent Schrödinger's equation using the generalized finite difference time domain (GFDTD-Q) method for quantum systems. The effect of V e 0 on transmission properties, fringe pattern, motion of the peaks, and wave number distribution in the diffracted wave has been studied. It is found that V e 0 changes the shape of the quantum point contact of diffracting constriction, which controls the allowed quantum states in the diffracted wave and the transmission coefficient T c can be tuned by V e 0 . It is observed that the number of peaks, their relative intensity, and quantization of lateral wavenumber depend upon V e 0 . This study will be helpful in optimizing the parameters for material grating-based matter–wave interferometers employing charged particle such as proton beams. [ABSTRACT FROM AUTHOR]

In this work, we present a study on the resistive memory properties of ITO/MEH-PPV/Al devices. The pristine diode-like device, highly resistive, needs an electroforming process to turn the device into a memory with resistive switching. During electroforming, I-V sweeps are described using Schottky Emission (SE) and Fowler-Nordheim (FN) models, resulting in an injection barrier around ~ 0.8 to 0.9 eV that may be associated with injection through polymer/Al interface. At high resistance sate (HRS), the linear coefficient of the log I x V curve is n ≅ 1 at low voltages and present a transition to n ≅ 2 when the device is close to switching voltage. At low resistance state (LRS), we observe n ≅ 1 up to the negative differential resistance (NDR). In our devices, exploring the reverse bias NDR at different levels, it is possible to observe multilevel conducting states in direct bias sweeps (opposite polarizations), unlike unipolar memories, that show multilevel states when exploring NDR at different levels in the same polarization direction. I-V multilevel sweeps are in accordance with the Quantum Point Contact Model (QPC). The resulting fitting parameters show that the number of filaments increases from one (at HRS) to 70 as multilevel state approach LRS. Also, the tgap decreases from (0.59 ± 0.02) nm to (0.44 ± 0.01) nm . The conductance found in terms of G 0 ( = 2 e 2 / h = 7.75 x 10 - 5 S ) was 1 G 0 up to 11.5 G 0 , compatible with values found from QPC model on I-V multilevel. Our results are consistent with the soft breakdown of an Al2O3 oxide layer at the polymer/metal interface. [ABSTRACT FROM AUTHOR]

We consider two one-dimensional quantum XX magnets linked by a periodically driven quantum point contact. If magnets are initially polarized in opposite directions, one expects that a spin current through the quantum point contact will establish. It has been shown recently [Phys. Rev. B 103, L041405 (2021)] that, in fact, when the driving frequency exceeds a critical value, the current halts completely, the quantum point contact being effectively insulating. Here we enquire how this picture is affected by quantum dephasing. Our findings reveal that any nonzero dephasing restores the current. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of quantum Hall edge transmission, especially a hole-conjugate of a Laughlin state such as a 2/3 state, is critical for advancing fundamental quantum Hall physics and enhancing the design of quantum Hall edge interferometry. In this study, we show a robust intermediate 2h/e2 resistance quantization in a quantum point contact (QPC) when the bulk is set at the fractional filling 2/3 quantum Hall state. Our results suggest the occurrence of two equilibration processes. First, the co-propagating 1/3 edges moving along a soft QPC arm confining potential fully equilibrate and act as a single 2/3 edge mode. Second, the 2/3 edge mode is further equilibrated with an integer 1 edge mode formed in the QPC. The complete mixing between them results in a diagonal resistance value quantized at 2h/e2. Similar processes occur for a bulk filling 5/3, leading to an intermediate (2/3)h/e2 resistance quantization. This finding highlights the importance of understanding the equilibration mechanisms that occur between different edge modes, offering insights into the processes of edge equilibration. In the quantum Hall regime, electrical current flows along the edges in a chiral fashion and they determine the Hall resistance plateaus. This work reports on experiments on fractional and integer quantum Hall edge channel mixing in a quantum point contact, which lead to unexpectedly anomalous resistance plateaus, shedding light onto the edge reconstruction and equilibration processes. [ABSTRACT FROM AUTHOR]

One-dimensional conductors are described by Luttinger liquid theory, which predicts a power-law suppression of the single-electron tunneling density of states at low voltages. The scaling exponent is predicted to be quantized when tunneling into a single isolated chiral edge state of the fractional quantum Hall effect. We report conductance measurements across a point contact linking integer and fractional quantum Hall edge states (at fillings 1 and 1 3, respectively). At weak coupling, we observe the predicted universal quadratic scaling with temperature and voltage. At strong coupling, we demonstrate perfect Andreev reflection of fractionalized quasiparticles at the point contact. We use the strong coupling physics to realize a nearly dissipationless direct current voltage step-up transformer, whose gain arises directly from topological fractionalization of electrical charge. [ABSTRACT FROM AUTHOR]

The article presents diverse range of scientific topics, which includes unique behavior of one-dimensional systems of electrons and the study of chiral Luttinger liquids in fractional quantum Hall states; explores the structural and functional aspects of nucleolus organizer regions in Arabidopsis thaliana; and the efficacy of fecal microbiota transfer (FMT) in eradicating multidrug-resistant organisms (MDROs) among renal transplantation recipients.

The scattering of exotic quasiparticles may follow different rules than electrons. In the fractional quantum Hall regime, a quantum point contact (QPC) provides a source of quasiparticles with field effect selectable charges and statistics, which can be scattered on an 'analyzer' QPC to investigate these rules. Remarkably, for incident quasiparticles dissimilar to those naturally transmitted across the analyzer, electrical conduction conserves neither the nature nor the number of the quasiparticles. In contrast with standard elastic scattering, theory predicts the emergence of a mechanism akin to the Andreev reflection at a normal-superconductor interface. Here, we observe the predicted Andreev-like reflection of an e/3 quasiparticle into a − 2e/3 hole accompanied by the transmission of an e quasielectron. Combining shot noise and cross-correlation measurements, we independently determine the charge of the different particles and ascertain the coincidence of quasielectron and fractional hole. The present work advances our understanding on the unconventional behavior of fractional quasiparticles, with implications toward the generation of novel quasi-particles/holes and non-local entanglements. Quantum transport of fractional quasiparticles can drastically differ from conventional charge transport. Here the authors demonstrate Andreev-like reflection of a fractional quasiparticle incident on a barrier in the fractional quantum Hall regime. [ABSTRACT FROM AUTHOR]

We propose a source of purely electronic energy-entangled states implemented in a solid-state system with potential applications in quantum information protocols based on electrons. The proposed device relies on the standard tools of electron quantum optics and exploits entanglement of the Cooper pairs of a BCS superconductor. The latter is coupled via an adjustable quantum point contact to two opposite spin-polarized electron wave-guides, which are driven by trains of Lorentzian pulses. This specific choice for the drive is crucial to inject purely electronic entangled states devoid of spurious electron–hole pairs. In the Andreev regime, a perturbative calculation in the tunnel coupling confirms that entangled electrons states are generated at the output of the normal side. For arbitrary tunnel coupling and for a periodic drive, direct current and noise (auto and cross correlations) are computed numerically using a Keldysh–Nambu–Floquet formalism. Importantly, for a periodic drive, the production of these states can be controlled in time, thus implementing an on-demand source of entangled states. We exploit realistic experimental parameters for our device to identify its optimal functioning point. [ABSTRACT FROM AUTHOR]

Dynamics of integrable systems, such as Tomonaga-Luttinger (TL) liquids, is deterministic, and the absence of stochastic thermalization processes provides unique characteristics, such as long-lived non-thermal metastable states with many conserved quantities. Here, we show such non-thermal states can emerge even when the TL liquid is excited with extremely high-energy hot electrons in chiral quantum-Hall edge channels. This demonstrates the robustness of the integrable model against the excitation energy. Crossover from the single-particle hot electrons to the many-body TL liquid is investigated by using on-chip detectors with a quantum point contact and a quantum dot. The charge dynamics can be understood with a single-particle picture only for hot electrons. The resulting electron-hole plasma in the TL liquid shows a non-thermal metastable state, in which warm and cold electrons coexist without further thermalization. The multi-temperature constituents are attractive for transporting information with conserved quantities along the channels. The Dynamics of Tomonaga-Luttinger (TL) liquids is deterministic, and the absence of stochastic thermalization processes provides unique characteristics, such as long-lived non-thermal metastable states with many conserved quantities. Here, we show such non-thermal states can emerge even when the TL liquid is excited with extremely high-energy hot electrons in chiral quantum-Hall edge channels. [ABSTRACT FROM AUTHOR]

Quantum structures are ideal objects by which to discover and study new sensor mechanisms and implement advanced approaches in sensor analysis to develop innovative sensor devices. Among them, one of the most interesting representatives is the Yanson point contact. It allows the implementation of a simple technological chain to activate the quantum mechanisms of selective detection in gaseous and liquid media. In this work, a portable device for multipurpose research on dendritic Yanson point contacts and quantum sensing was developed and manufactured. The device allows one to create dendritic Yanson point contacts and study their quantum properties, which are clearly manifested in the process of the electrochemical cyclic switchover effect. The device tests demonstrated that it was possible to gather data on the compositions and characteristics of the synthesized substances, and on the electrochemical processes that influence the production of dendritic Yanson point contacts, as well as on the electrophysical processes that provide information on the quantum nature of the electrical conductance of dendritic Yanson point contacts. The small size of the device makes it simple to integrate into a micro-Raman spectrometer setup. The developed device may be used as a prototype for designing a quantum sensor that will serve as the foundation for cutting-edge sensor technologies, as well as be applied to research into atomic-scale junctions, single-atom transistors, and any relative subjects. [ABSTRACT FROM AUTHOR]

The conductance of a trench-type quantum point contact (QPC) with side gates has been experimentally investigated over a wide range of gate voltages. The performed measurements, in which the asymmetric gate bias modifies the confinement potential while the sum of the gate voltages populates it with electrons, made it possible to scan the electron states in the QPC. Analysis of the experimental data revealed an unusual four-well shape of the confining potential in a single QPC. The rather complicated transconductance plot measured can be divided into its component parts—the contributions of the four separate conducting channels. Different electron states observed in the experiment have been associated with a certain number of filled one-dimensional (1D) subbands belonging to different channels. A whole network of degeneration events of 1D subbands in parallel channels has been found. Almost every such event was experimentally manifested by anticrossings observed both for small and large numbers of filled 1D subbands. [ABSTRACT FROM AUTHOR]

We present the design and implementation of a measurement system that enables parallel drive and detection of small currents and voltages at numerous electrical contacts to a multi-terminal electrical device. This system, which we term a feedback lock-in, combines digital control-loop feedback with software-defined lock-in measurements to dynamically source currents and measure small, pre-amplified potentials. The effective input impedance of each current/voltage probe can be set via software, permitting any given contact to behave as an open-circuit voltage lead or as a virtually grounded current source/sink. This enables programmatic switching of measurement configurations and permits measurement of currents at multiple drain contacts without the use of current preamplifiers. Our 32-channel implementation relies on commercially available digital input/output boards, home-built voltage preamplifiers, and custom open-source software. With our feedback lock-in, we demonstrate differential measurement sensitivity comparable to a widely used commercially available lock-in amplifier and perform efficient multi-terminal electrical transport measurements on twisted bilayer graphene and SrTiO3 quantum point contacts. The feedback lock-in also enables a new style of measurement using multiple current probes, which we demonstrate on a ballistic graphene device. [ABSTRACT FROM AUTHOR]

The Andreev bound state spectra of multi-terminal Josephson junctions form an artificial band structure, which is predicted to host tunable topological phases under certain conditions. However, the number of conductance modes between the terminals of a multi-terminal Josephson junction must be few in order for this spectrum to be experimentally accessible. In this work, we employ a quantum point contact geometry in three-terminal Josephson devices to demonstrate independent control of conductance modes between each pair of terminals and access to the single-mode regime coexistent with the presence of superconducting coupling. These results establish a full platform on which to realize tunable Andreev bound state spectra in multi-terminal Josephson junctions. Multiterminal Josephson junctions may provide a novel way to realize topologically non-trivial band structures in an n-dimensional phase space. Here, the authors experimentally demonstrate the proposed necessary conditions to measure these states. [ABSTRACT FROM AUTHOR]

A quantum system implanted with monolayer transition metal dichalcogenide (TMDC) is proposed as a valley filter based on a ballistic point contact with zigzag edges. A quantum point contact (QPC) structure is formed in the middle of scattering region and the geometrical size of the QPC is varied by electrostatic gate potentials. The energy band structures and electronic transport properties are investigated using a two-bands k ⋅ p Hamiltonian by using parameters from the density functional theory (DFT) calculations. The magnetic proximity effect from an Eu-terminated ferromagnetic substrate (EuS) generates a giant valley splitting in the energy of the monolayer WS 2 nanoribbon. The conductance is limited by reducing the width and by increasing the length of the QPC because conductance is strongly related to the number of transmission channels and the amounts of back scatterings at the QPC. The polarity of valley current can be manipulated by applying a local gate potential in the QPC region, where the manipulated polarity is limited to well defined K ′ polarization only. The lateral hetero materials on both sides of the QPC or lateral hetero ferromagnetic substrate is necessary to have desired valley and spin polarities of current by manipulating the local gate potential. The experimental realization of this quantum system will make it possible to control the valley degree of freedom in addition to controlling charge and spin degree of freedom of carriers in future electronic devices. [Display omitted] • The magnetic proximity effects of the substrate can break the spin and valley degeneracy of the system. • Using the two-band k ⋅ p Hamiltonian approximation, the band structures near the K and K' valleys are obtained. • Eu-terminated substrate can provide exchange interactions of 400 T B-field in the system. • Valley filtering device is designed as a bowtie-shaped system with a local gate in the center. • A heterojunction channel is required to have selective valley polarization. [ABSTRACT FROM AUTHOR]

Two-dimensional electron gases (2DEGs) can show exceptional carrier mobility, making them promising candidates for future quantum technologies. However, impurities and defects can significantly degrade their performance, impacting transport, conductivity, and coherence times. We leverage scanning gate microscopy (SGM) and machine learning approaches to extract the potential landscape of 2DEGs from SGM data. We compare three techniques: image-to-image translation with generative adversarial networks (GANs), cellular neural networks (CNNs), and an evolutionary search algorithm. Notably, the evolutionary approach outperforms both alternatives in defect identification and analysis. This work clarifies the interaction between defects and 2DEG properties, demonstrating the potential of machine learning for understanding and manipulating quantum materials, facilitating advancements in quantum computing and nanoelectronics. • SGM is used to estimate the background potential of a 2D electron gas. • Three machine learning techniques are used to estimate the background potential. • Evolutionary search estimates align closer with the anticipated behavior of 2DEGs. [ABSTRACT FROM AUTHOR]

A physical mechanism has been proposed to explain the appearance of the multichannel electron transport regime in trench quantum point contacts. It has been shown that the redistribution of electrons in a two-dimensional electron gas, which is due to the Coulomb interaction between them and with charged donors and X-valley electrons, results in the experimentally observed multichannel regime. The numerical simulation of the redistribution of electrons and its comparison with the measurements of the conductance of trench quantum point contacts based on a GaAs/AlGaAs heterostructure demonstrate the formation of a multiwell confining potential. [ABSTRACT FROM AUTHOR]

The disappearance of the giant terahertz photoconductance of a quantum point contact under the increase in the photon energy, which was discovered experimentally (Otteneder et al., Phys. Rev. Appl. 10, 014015 (2018)) and studied by the numerical calculations of the photon-stimulated transport (O. A. Tkachenko et al., JETP Lett. 108, 396 (2018)), is explained by the momentum conservation upon absorption of photons by tunneling electrons and on the base of perturbation theory calculations. [ABSTRACT FROM AUTHOR]

The quantum speed limit (QSL) of the double quantum dot (DQD) system has been theoretically investigated by adopting the detection of the quantum point contact (QPC) in the pure dephasing environment. The Mandelstamâ€"Tamm (MT) type of the QSL bound which is based on the trace distance has been extended to the DQD system for calculating the shortest evolving time. The increase of decoherence rate can weaken the capacity for potential speedup (CPS) and delay the evolving process due to the frequently measurement localizing the electron in the DQD system. The system needs longer time to evolve to the target state as the enhancement of dephasing rate, because the strong interaction between pure dephasing environment and the DQD system could vary the oscillation of the electron. Increasing the dephasing rate can sharp the QSL bound, but the decoherence rate would weaken the former effect and vice versa. Moreover, the CPS would be raised by increasing the energy displacement, while the enhancement of the coupling strength between two quantum dots can diminish it. It is interesting that there has an inflection point, when the coupling strength is less than the value of the point, the increasing effect of the CPS from the energy displacement is dominant, otherwise the decreasing tendency of the CPS is determined by the coupling strength and suppress the action of the energy displacement if the coupling strength is greater than the point. Our results provide theoretical reference for studying the QSL time in a semiconductor device affected by numerous factors. [ABSTRACT FROM AUTHOR]

Granular aluminium is a high kinetic inductance thin film superconductor which, when formed into nanowires can undergo an intrinsic electromigration process. We use a combination of experimental and computational approaches to investigate the role of grain morphology and distribution in granular aluminium thin films, when formed into nanowire constrictions. Treating the granular aluminium film as a network of randomly distributed resistors with parameters motivated by the film microstructure allows us to model the electrical characteristics of the nanowires. This model provides estimates of the dependence of sheet resistance on grain size and distribution, and the resulting device to device variation for superconducting nanowires. By fabricating a series of different length nanowires, we study the electromigration process as a function of applied current, and then compare directly to the results of our computational model. In doing so we show that the electromigration is driven by the formation of quantum point contacts between metallic aluminium grains. [ABSTRACT FROM AUTHOR]

The resistive switching properties of HfO2 based 1T-1R memristive devices are electrically modified by adding ultra-thin layers of Al2O3 into the memristive device. Three different types of memristive stacks are fabricated in the 130 nm CMOS technology of IHP. The switching properties of the memristive devices are discussed with respect to forming voltages, low resistance state and high resistance state characteristics and their variabilities. The experimental I–V characteristics of set and reset operations are evaluated by using the quantum point contact model. The properties of the conduction filament in the on and off states of the memristive devices are discussed with respect to the model parameters obtained from the QPC fit. [ABSTRACT FROM AUTHOR]

A hallmark of topological superconductivity is the non-Abelian statistics of Majorana bound states (MBS), its chargeless zero-energy emergent quasiparticles. The resulting fractionalization of a single electron, stored nonlocally as a two spatially-separated MBS, provides a powerful platform for implementing fault-tolerant topological quantum computing. However, despite intensive efforts, experimental support for MBS remains indirect and does not probe their non-Abelian statistics. Here we propose how to overcome this obstacle in mini-gate controlled planar Josephson junctions (JJs) and demonstrate non-Abelian statistics through MBS fusion, detected by charge sensing using a quantum point contact, based on dynamical simulations. The feasibility of preparing, manipulating, and fusing MBS in two-dimensional (2D) systems is supported in our experiments which demonstrate the gate control of topological transition and superconducting properties with five mini gates in InAs/Al-based JJs. While we focus on this well-established platform, where the topological superconductivity was already experimentally detected, our proposal to identify elusive non-Abelian statistics motivates also further MBS studies in other gate-controlled 2D systems. The current efforts to look for Majorana bound states (MBS) still cannot probe the hallmark property, the non-Abelian statistics. Here, the authors propose to realize non-Abelian statistics through MBS fusion in mini-gate controlled planar Josephson junctions. [ABSTRACT FROM AUTHOR]

Improving resistive memory reliability requires understanding some bottlenecks, i.e., control and stability of the memory parameters during endurance cycling and physical degradation that shows up in memories that contain metal oxides. The physical degradation of indium tin oxide/poly[2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylene vinylene]/aluminum (ITO/MEH-PPV/Al) devices was studied through I–V sweeps and cycle endurance with different compliance currents (I C), in addition to scanning electron microscopy (SEM), energy-dispersive X-Ray spectroscopy (EDS), and Raman spectroscopy. The devices showed a dependence of the conductance quantization G 0 in current compliance I C , i.e., the G 0 histogram shows different distribution shape after changing I C. After many cycles, the devices presented partial destruction in the active area (physical alteration) followed by memory parameter degradation and finally failing (no commuting anymore). SEM/EDS measurements in the deteriorated region showed that the aluminum electrode was removed and only remnants of the polymer remained. The Raman spectroscopy analyses focused on changes in the polymeric structure caused by the endurance cycle, such as changes in chain planarity, which is related to the band around 966 cm−1 that was shifted to lower wavenumbers (965 cm−1) together with the increase in intensity which indicates less planar conformations when compared to the non-degraded area. Alteration of the conjugation length was evidenced by the contribution around 1581 cm−1. In this case, we can see a shift: of 1581 cm−1 to 1584 cm−1, for the non-degraded area and degraded area, respectively. Both results indicate a decrease in the conjugation length. All these changes show fragmentation of the polymer chain as a result of the endurance cycle. [Display omitted] • I-V sweeps after electroforming confirmed two conductive states: High Resistance State (HRS) and Low Resistance States (LRS). • Cycle endurance depends on current compliance. • Conductance quantization moves toward higher values as current compliance increases. • Before losing memory properties, Long-term cycle endurance, shows HRS states moving toward LRS state. • High enough current compliance developed holes in the active area and device failure. • SEM measurements in the deteriorated region showed that the aluminum electrode was removed and only remnants of the polymer remained. • EDS images show that the amount of In decreased in the degraded region. • Raman spectra on different regions of the hole indicate a decrease in the conjugation length and fragmentation of MEH-PPV. [ABSTRACT FROM AUTHOR]

The observation of a 0.5 x (2e²/h) conductance plateau in asymmetrically biased quantum point contacts (QPCs) with in-plane side gates (SGs) has been attributed to the onset of spin-polarized current through these structures. For InAs QPCs with the same width but a longer channel length, there is roughly a fourfold increase in the range of common sweep voltage applied to the SGs over which the 0.5 x (2e²/h) plateau is observed when the QPC aspect ratio (ratio of length over the width of the narrow portion of the structure) is increased by a factor 3. Non-equilibrium Green's function simulations indicate that the increase in the size of the 0.5 x (2e²/h) plateau is due to an increased importance, over a larger range of common sweep voltage, of the effects of electronelectron interactions in QPC devices with a larger aspect ratio. The use of asymmetrically biased QPCs with in-plane SGs and large aspect ratio could therefore pave the way to build robust spin injectors and detectors for the successful implementation of spin field effect transistors. [ABSTRACT FROM AUTHOR]

Resistively‐detected nuclear magnetic resonance (RDNMR) is a unique characterization method enabling highly sensitive NMR detection for a single quantum nanostructure, such as a quantum point contact (QPC). In many studies, dynamic nuclear polarization and RDNMR detection are used in a quantum Hall breakdown regime of a local QPC filling factor of 1 (νqpc = 1). However, the RDNMR lineshapes are complicated and still not fully understood yet. Herein, the nuclear spins are systematically polarized by current pumping from the close vicinity of the νqpc = 1 conductance plateau all the way down to pinch‐off point, providing clear evidence that the spin‐flip scattering between two edge channels at the lowest Landau level still occurs in the constriction even when it is close to the pinch off point. The collected RDNMR spectra reveal two sets of distinguished features. First, in a strong to intermediate tunneling regime, we observe an ordinary resistance dip lineshape with snake‐like transition frequencies, indicative of spatial modulation of electron density in the QPC. Second, in a weak tunneling regime, the spectrum turns into a dispersive lineshape, which is interpreted due to the build‐up of two sets of nuclear spin polarization that are in contact with different electron spin polarization. [ABSTRACT FROM AUTHOR]

Electric-field-controlled charge transport is a crucial concept of modern computers, embodied, namely in field-effect transistors. The metallic gate voltage controls the charge population. Thus, it is possible to define logical elements which are the key to computational processes. Here, we investigate a similar system defined by metallic gates inducing quasi-one-dimensional transport channels on a high-mobility electron system in the presence of a strong perpendicular magnetic field. Firstly, we solve the three-dimensional Poisson equation, self-consistently imposing relevant boundary conditions and use the output as an initial condition to calculate charge density and potential distribution in the plane of a two-dimensional electron system in the presence of an external magnetic field. Subsequently, we impose an external current and obtain the spatial distribution of the transport charges, considering the various magnetic field and gate voltage strengths at sufficiently low (<10 Kelvin) temperatures. Finally, we show that the magnetic field breaks the spatial symmetry of the current distribution, whereas voltage applied to metallic gates determines the scattering processes. [ABSTRACT FROM AUTHOR]

The hybrid circuit quantum electrodynamics architecture integrated quantum dots with a microwave resonator results in the creation of a controllable artificial system and enriches its physics through electron–photon interaction. In this study, a hybrid device is investigated, wherein a triple quantum dot (TQD) is dipole coupled with electric field of a superconducting quantum interference device array resonator. The quantum cellular automata (QCA) process of TQD, related to charge reconfiguration as a result of Coulomb interaction between each dot, is observed by probing reflectance of the resonator. The suppressed cavity reflectance signal of the QCA process may experimentally provide the evidence for the different measurement mechanism of the cavity from external electrometers like quantum point contact. Furthermore, on careful tuning of multiple energy level differences of the TQD to match the cavity photon frequency, four tunneling processes assisted by the absorption of the cavity photons occur simultaneously, contributing to "photon‐assisted" quadruple point. Consequently, these two specific phenomena are found to be closely dependent on the relationship between the various Coulomb energy in TQD and the cavity photon energy. The results are consistent with the theoretical model presented and imply that this hybrid system has the potential for investigating exotic many‐body effects and matter–light interaction. [ABSTRACT FROM AUTHOR]

We report a cryogenic transimpedance amplifier (TA) suitable for cross-correlation current-noise measurements. The TA comprises homemade high-electron-mobility transistors with high transconductance and low noise characteristics, fabricated in an AlGaAs/GaAs heterostructure. The low input-referred noise and wide frequency band of the TA lead to a high resolution in current-noise measurements. The TA's low input impedance suppresses unwanted crosstalk between two distinct currents from a sample, justifying the advantage of the TA for cross-correlation measurements. We demonstrate the high resolution of a TA-based experimental setup by measuring the shot noise generated at a quantum point contact in a quantum Hall system. [ABSTRACT FROM AUTHOR]

The origin of anomalous current-carrying edge states in quasi-two-dimensional quantum samples with an insulating interior is currently mysterious. We propose to address this issue using a hybrid setup, an interferometric phase-sensitive configuration of two independent scanning probe tips, normal and superconducting, able to realize the quantum interference effect of quasiparticle currents moving in different directions along the metallic-like one-dimensional near-boundary channels. To simulate the dissipationless edge currents, we consider a quantum material with a simple Corbino disk geometry and analyze how the differential conductance spectrum depends on the distance between the two tips, the applied voltage bias, and the presence of a magnetic field. An essential difference between classical and quantum expectations should clarify the enigmatic origin of the long-range conducting modes observed in different materials at low temperatures. Strong dependence on the applied magnetic field can be useful for practical implementation of the quantum effects associated with the phase difference of electron wave functions in the ring geometry. [ABSTRACT FROM AUTHOR]

Quantum transport is ubiquitous in physics. So far, quantum transport between terminals has been extensively studied in solid state systems from the fundamental point of views such as the quantized conductance to the applications to quantum devices. Recent works have demonstrated a cold-atom analog of a mesoscopic conductor by engineering a narrow conducting channel with optical potentials, which opens the door for a wealth of research of atomtronics emulating mesoscopic electronic devices and beyond. Here we realize an alternative scheme of the quantum transport experiment with ytterbium atoms in a two-orbital optical lattice system. Our system consists of a multi-component Fermi gas and a localized impurity, where the current can be created in the spin space by introducing the spin-dependent interaction with the impurity. We demonstrate a rich variety of localized-impurity-induced quantum transports, which paves the way for atomtronics exploiting spin degrees of freedom. Cold atoms have recently become a versatile platform for the study of quantum transport phenomena. Here the authors realize an alternative experimental scheme for quantum transport with cold atoms, by using spin-dependent impurity scattering in a spinful Fermi gas instead of spatially separated particle distributions. [ABSTRACT FROM AUTHOR]

The transconductance and effective Landé g ∗ factors for a quantum point contact defined in silicene by the electric field of a split gate is investigated. The strong spin–orbit coupling in buckled silicene reduces the g ∗ factor for in-plane magnetic field from the nominal value 2 to around 1.2 for the first- to 0.45 for the third conduction subband. However, for perpendicular magnetic field we observe an enhancement of g ∗ factors for the first subband to 5.8 in nanoribbon with zigzag and to 2.5 with armchair edge. The main contribution to the Zeeman splitting comes from the intrinsic spin–orbit coupling defined by the Kane–Mele form of interaction. [ABSTRACT FROM AUTHOR]

Quantum point contacts (QPCs) are fabricated on modulation-doped Si/SiGe heterostructures and ballistic transport is studied at low temperatures. We observe quantized conductance with subband separations up to 4 meV and anomalies in the first conductance plateau at 4e2/h. At a temperature of T=22 mK in the linear transport regime, a weak anomalous kink structure arises close to 0.5(4e2/h), which develops into a distinct plateau-like structure as temperature is raised up to T=4K. Under magnetic field parallel to the wire up to B=14 T, the anomaly evolves into the Zeeman spin-split level at 0.5(4e2/h), resembling the "0.7 anomaly" in GaAs/AlGaAs QPCs. Additionally, a zero-bias anomaly (ZBA) is observed in nonlinear transport spectroscopy. At T=22 mK, a parallel magnetic field splits the ZBA peak up into two peaks. At B=0, elevated temperatures lead to similar splitting, which differs from the behavior of ZBAs in GaAs/AlGaAs QPCs. Under finite dc bias, the differential resistance exhibits additional plateaus approximately at 0.8(4e2/h) and 0.2(4e2 /h) known as "0.85 anomaly" and "0.25 anomaly" in GaAs/AlGaAs QPCs. Unlike the first regular plateau at 4e2/h, the 0.2(4e2 /h) plateau is insensitive to dc bias voltage up to at least VDS=80 mV, in-plane magnetic fields up to B=15 T, and to elevated temperatures up to T=25 K. We interpret this effect as due to pinching off one of the reservoirs close to the QPC. We do not see any indication of lifting of the valley degeneracy in our samples. [ABSTRACT FROM AUTHOR]

The mobility of the two-dimensional electron gas (2DEG) in shallow GaAs/ Al x Ga 1 − x As heterostructures is strongly suppressed by unwanted Coulomb scattering from surface charge, likely located in native surface oxides that form after the wafer is removed from the crystal growth system. Here, we show that this native surface oxide can be eliminated by growing an epitaxial aluminum gate before removing the wafer from the growth chamber. We fabricate accumulation mode devices on two wafers with nearly identical structures and growth conditions: one with an epitaxial aluminum gate 35 nm above the channel and another with an ex situ metal gate deposited on an aluminum oxide dielectric. Low temperature transport measurements show that the epitaxial gate design greatly reduces surface charge scattering, with up to 2.5 × increase in mobility. Despite the ultra-shallow 2DEG (35 nm), the mobility remains high even at low carrier densities. Finally, we show that the epitaxial aluminum gate can be patterned to make nanostructures by fabricating a quantum point contact that shows robust and reproducible 1D conductance quantization, with extremely low charge noise. [ABSTRACT FROM AUTHOR]

We study three-terminal thermoelectric transport in a two-dimensional Quantum Point Contact (QPC) connected to left and right electronic reservoirs, as well as a third one represented by a scanning probe tip. The latter acts as a voltage probe exchanging heat with the system but no charges on average. The thermoelectric coefficients are calculated numerically within the Landauer–Büttiker formalism in the low-temperature and linear response regimes. We find tip-induced oscillations of the local and non-local thermopowers and study their dependence on the QPC opening. If the latter is tuned on a conductance plateau, the system behaves as a perfect thermoelectric diode: for some tip positions, the charge current through the QPC, driven by a local Seebeck effect, can flow in one direction only. [ABSTRACT FROM AUTHOR]

The low-frequency microwave photoconductance of a short (100 nm) quantum point contact based on a high-mobility two-dimensional electron gas in the frequency range of 2–3 GHz is investigated for the first time. The giant photoconductance in the tunneling regime and the negative photoconductance in the open regime are observed. It is shown by numerical simulations that such response to microwave irradiation is caused by the forced oscillations of the saddle-point potential in the quantum point contact and of the probe voltage applied to the contact. [ABSTRACT FROM AUTHOR]

In this study, we fabricated quantum point contacts narrower than 100 nm by using an electrostatic potential to open the finite bandgap by applying a perpendicular electric field to bilayer graphene encapsulated between hexagonal boron nitride sheets. The conductance across the quantum point contact was quantized at a high perpendicular-displacement field as high as 1 V/nm at low temperature, and the quantization unit was 2e2/h instead of mixed spin and valley degeneracy of 4e2/h. This lifted degeneracy state in the quantum point contact indicates the presence of valley polarized state coming from potential profile or effective displacement field in one-dimensional channel. [ABSTRACT FROM AUTHOR]

We propose a new method to construct low-dimensional quantum devices consisting of the magnetic topological insulators. Unlike previous systems based on locally depleting two-dimensional electron gas in semiconductor heterojunctions, magnetization provides a simpler and rewriteable fabrication way. The motion of electrons can be manipulated through the domain wall formed by the boundary between different magnetic domains. Here, three devices designed by local magnetization are presented. For the quantum point contact, conductance exhibits quantized plateaus with the increasing silt width between two magnetic domains. For the quantum dot, conductance shows pronounced peaks as the change of gate voltage. Finally, for the Aharonov–Bohm ring, conductance oscillates periodically with the external magnetic field. Numerical results show that the transport of these local magnetization systems is identical to that of the previous systems based on depleting two-dimensional electron gas, and the only difference is the approach of construction. These findings may pave the way for realization of low-power-consumption devices based on magnetic domain walls. [ABSTRACT FROM AUTHOR]

We have studied the spin splitting of the current in the Y-shaped semiconductor nanostructure with a quantum point contact (QPC) in a perpendicular magnetic field. Our calculations show that the appropriate tuning of the QPC potential and the external magnetic field leads to an almost perfect separation of the spin-polarized currents: electrons with opposite spins flow out through different output branches. The spin splitting results from the joint effect of the QPC, the spin Zeeman splitting, and the electron transport through the edge states formed in the nano wire at the sufficiently high magnetic field. The Y-shaped nanostructure can be used to split the unpolarized current into two spin currents with opposite spins as well as to detect the flow of the spin current. We have found that the separation of the spin currents is only slightly affected by the Rashba spin-orbit coupling. The spin-splitter device is an analogue of the optical device--the birefractive crystal that splits the unpolarized light into two beams with perpendicular polarizations. In the magnetic-field range, in which the current is carried through the edges states, the spin splitting is robust against the spin-independent scattering. This feature opens up a possibility of the application of the Y-shaped nanostructure as a non-ballistic spin-splitter device in spintronics. [ABSTRACT FROM AUTHOR]

We study the quantum fluctuations of the two quadratures of the emitted electromagnetic radiation generated by a quantum Hall device in a quantum point contact geometry. In particular, we focus our attention on the role played by the unavoidable electron–electron interactions between the two edge channels at filling factor two. We investigate quantum features of the emitted microwave radiation, such as squeezing, by studying the current fluctuations at finite frequency, accessible through a two-filters set-up placed just after the quantum point contact. We compare two different drives, respectively a cosine and a train of Lorentzian pulses, used for the injection of the excitations into the system. In both cases quantum features are reduced due to the interactions, however the Lorentzian drive is still characterized by a robust squeezing effect which can have important application on quantum information. [ABSTRACT FROM AUTHOR]

Using the innovative method of the additional Bloch vector, the electron transfer properties of a double quantum dot (DQD) system measured by a quantum point contact (QPC) in a fluctuating environment are investigated. The results show that the environmental noises in transverse and longitudinal directions play different roles in the dynamical evolution of the open quantum systems. Considering the DQD with symmetric energy level, the Fano factor exhibits a slight peak with the increase of transverse noise amplitude σT, which provides a basis for distinguishing dynamical phenomena caused by different directional fluctuation noises in symmetric DQD structures by studying the detector output. In the case of asymmetric DQD, the dependence of a detector current involving the level displacement is distinct when increasing the transverse noise damping coefficient τT and the longitudinal noise damping coefficient τε respectively. Meanwhile, the transverse noise damping coefficient τT could significantly reduce the Fano factor and enhance the stability of the quantum system compared with the longitudinal one. The Fano factors with stable values as the enhancement of noise amplitudes show different external influences from the detector measurement, and provide a numerical reference for adjusting the noise amplitudes in both transverse and longitudinal directions appropriately in a microscopic experimental process to offset the decoherence effect caused by the measurements. Finally, the research of average waiting time provides unique insights to the development of single electron transfer theory in the short-time limit. [ABSTRACT FROM AUTHOR]

We demonstrate dispersive readout of individual charge states in a gate-defined few-electron quantum dot in bilayer graphene. We employ a radio frequency reflectometry circuit, where an LC resonator with a resonance frequency close to 280 MHz is directly coupled to an Ohmic contact of the quantum dot device. The detection scheme based on changes in the quantum capacitance operates over a wide gate-voltage range and allows us to probe excited states down to the single-electron regime. Crucially, the presented sensing technique avoids the use of an additional, capacitively coupled quantum device such as a quantum point contact or single electron transistor, making dispersive sensing particularly interesting for gate-defined graphene quantum dots. [ABSTRACT FROM AUTHOR]

Studies of photon-stimulated transport through a quantum point contact based on a high-mobility two-dimensional electron gas in a GaAs quantum well are reviewed. This review includes a brief historical introduction, experimental results on giant terahertz and microwave photoconductance of the quantum point contact in the tunneling regime, and the analysis of its behavior within a developed theoretical model of photon-stimulated tunneling. Prospects of further studies and production of a new class of terahertz and microwave detectors are discussed in closing. [ABSTRACT FROM AUTHOR]

We show that a cryogenic amplifier composed of a homemade GaAs high-electron-mobility transistor (HEMT) is suitable for current-noise measurements in a mesoscopic device at dilution-refrigerator temperatures. The lower noise characteristics of our homemade HEMT lead to a lower noise floor in the experimental setup and enable more efficient current-noise measurement than is available with a commercial HEMT. We present the dc transport properties of the HEMT and the gain and noise characteristics of the amplifier. With the amplifier employed for current-noise measurements in a quantum point contact, we demonstrate the high resolution of the measurement setup by comparing it with that of the conventional one using a commercial HEMT. [ABSTRACT FROM AUTHOR]

The conductance of a single GaAs quantum point contact (QPC) with in-plane side gates separated from a channel by lithographic trenches is experimentally studied. It is shown that, in such a trench-type QPC, the conducting channel splits into two channels, each demonstrating a conductance quantization. A phase diagram of the 1D subband population in separate channels as a function of the voltages at side gates is experimentally obtained. The obtained data show that the electron transport regime undergoes the transition from the case of two independent parallel 1D conductors to the case of strongly correlated double-row transport. The transition occurs with the increase in the number of populated 1D subbands in the channels and manifests itself in the phase diagram by the change of behavior of the lines separating the regions corresponding to different numbers. The crossings of these lines at small numbers turn into anticrossings at large numbers. The anticrossings are explained by the splitting of degenerate levels in a double-well confinement potential formed inside the QPC constriction. [ABSTRACT FROM AUTHOR]