Photon-assisted tunneling through a quantum dot

Determination of exciton binding energy using photocurrent spectroscopy of Ge quantum-dot single-hole transistors under CW pumping

We reported exciton binding-energy determination using tunneling-current spectroscopy of Germanium (Ge) quantum dot (QD) single-hole transistors (SHTs) operating in the few-hole regime, under 405-1550 nm wavelength (λ) illumination. When the photon energy is smaller than the bandgap energy (1.46 eV) of a 20 nm Ge QD (for instance, λ = 1310 nm and 1550 nm illuminations), there is no change in the peak voltages of tunneling current spectroscopy even when the irradiation power density reaches as high as 10 µW/µm2. In contrast, a considerable shift in the first hole-tunneling current peak towards positive VG is induced (ΔVG ≈ 0.08 V at 0.33 nW/µm2 and 0.15 V at 1.4 nW/µm2) and even additional photocurrent peaks are created at higher positive VG values (ΔVG ≈ 0.2 V at 10 nW/µm2 irradiation) by illumination at λ = 850 nm (where the photon energy matches the bandgap energy of the 20 nm Ge QD). These experimental observations were further strengthened when Ge-QD SHTs were illuminated by λ = 405 nm lasers at much lower optical-power conditions. The newly-photogenerated current peaks are attributed to the contribution of exciton, biexciton, and positive trion complexes. Furthermore, the exciton binding energy can be determined by analyzing the tunneling current spectra.

Impurity screening and Friedel oscillations in Floquet-driven two-dimensional metals

We develop a theory for the non-equilibrium screening of a charged impurity in a two-dimensional electron system under a strong time-periodic drive. Our analysis of the time-averaged polarization function and dielectric function reveals that Floquet driving modifies the screened impurity potential in two main regimes. In the weak drive regime, the time-averaged screened potential exhibits unconventional Friedel oscillations with multiple spatial periods contributed by a principal period modulated by higher-order periods, which are due to the emergence of additional Kohn anomalies in the polarization function. In the strong drive regime, the time-averaged impurity potential becomes almost unscreened and does not exhibit Friedel oscillations. This tunable Friedel oscillations is a result of the dynamic gating effect of the time-dependent driving field on the two-dimensional electron system.

An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection

Many mid- and far-infrared semiconductor photodetectors rely on a photonic response, when the photon energy is large enough to excite and extract electrons due to optical transitions. Toward the terahertz range with photon energies of a few milli-electron volts, classical mechanisms are used instead. This is the case in two-dimensional electron systems, where terahertz detection is dominated by plasmonic mixing and by scattering-based thermal phenomena. Here, we report on the observation of a quantum, collision-free phenomenon that yields a giant photoresponse at terahertz frequencies (1.9 THz), more than 10-fold as large as expected from plasmonic mixing. We artificially create an electrically tunable potential step within a degenerate two-dimensional electron gas. When exposed to terahertz radiation, electrons absorb photons and generate a large photocurrent under zero source-drain bias. The observed phenomenon, which we call the "in-plane photoelectric effect," provides an opportunity for efficient direct detection across the entire terahertz range.

Single-electron spin resonance in a nanoelectronic device using a global field

Spin-based silicon quantum electronic circuits offer a scalable platform for quantum computation, combining the manufacturability of semiconductor devices with the long coherence times afforded by spins in silicon. Advancing from current few-qubit devices to silicon quantum processors with upward of a million qubits, as required for fault-tolerant operation, presents several unique challenges, one of the most demanding being the ability to deliver microwave signals for large-scale qubit control. Here, we demonstrate a potential solution to this problem by using a three-dimensional dielectric resonator to broadcast a global microwave signal across a quantum nanoelectronic circuit. Critically, this technique uses only a single microwave source and is capable of delivering control signals to millions of qubits simultaneously. We show that the global field can be used to perform spin resonance of single electrons confined in a silicon double quantum dot device, establishing the feasibility of this approach for scalable spin qubit control.

Cavity QED with hybrid nanocircuits: from atomic-like physics to condensed matter phenomena

Circuit QED techniques have been instrumental in manipulating and probing with exquisite sensitivity the quantum state of superconducting quantum bits coupled to microwave cavities. Recently, it has become possible to fabricate new devices in which the superconducting quantum bits are replaced by hybrid mesoscopic circuits combining nanoconductors and metallic reservoirs. This mesoscopic QED provides a new experimental playground to study the light-matter interaction in electronic circuits. Here, we present the experimental state of the art of mesoscopic QED and its theoretical description. A first class of experiments focuses on the artificial atom limit, where some quasiparticles are trapped in nanocircuit bound states. In this limit, the circuit QED techniques can be used to manipulate and probe electronic degrees of freedom such as confined charges, spins, or Andreev pairs. A second class of experiments uses cavity photons to reveal the dynamics of electron tunneling between a nanoconductor and fermionic reservoirs. For instance, the Kondo effect, the charge relaxation caused by grounded metallic contacts, and the photo-emission caused by voltage-biased reservoirs have been studied. The tunnel coupling between nanoconductors and fermionic reservoirs also enable one to obtain split Cooper pairs, or Majorana bound states. Cavity photons represent a qualitatively new tool to study these exotic condensed matter states.

Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot

We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantum dot in a photon cavity with a single photon mode. The quantum dot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, [Formula: see text], between the photon energy, [Formula: see text], and the cyclotron energy, [Formula: see text]. Enhancement in the electron transport with increasing electron-photon coupling is observed when [Formula: see text]. In this case the photon field dominates and stretches the electron charge distribution in the quantum dot, extending it towards the contact area for the leads. Suppression in the electron transport is found when [Formula: see text], as the external magnetic field causes circular confinement of the charge density around the dot.

Photon exchange and entanglement formation during transmission through a rectangular quantum barrier

When a quantum particle traverses a rectangular potential created by a quantum field both photon exchange and entanglement between particle and field take place. We present the full analytic solution of the Schrödinger equation of the composite particle-field system allowing investigation of these phenomena in detail and comparison to the results of a classical field treatment. Besides entanglement formation, remarkable differences also appear with respect to the symmetry between energy emission and absorption, resonance effects and if the field initially occupies the vacuum state.

Electron transport through a quantum dot assisted by cavity photons

We investigate transient transport of electrons through a single quantum dot controlled by a plunger gate. The dot is embedded in a finite wire with length Lx assumed to lie along the x-direction with a parabolic confinement in the y-direction. The quantum wire, originally with hard-wall confinement at its ends, ±Lx/2, is weakly coupled at t = 0 to left and right leads acting as external electron reservoirs. The central system, the dot and the finite wire, is strongly coupled to a single cavity photon mode. A non-Markovian density-matrix formalism is employed to take into account the full electron-photon interaction in the transient regime. In the absence of a photon cavity, a resonant current peak can be found by tuning the plunger-gate voltage to lift a many-body state of the system into the source-drain bias window. In the presence of an x-polarized photon field, additional side peaks can be found due to photon-assisted transport. By appropriately tuning the plunger-gate voltage, the electrons in the left lead are allowed to undergo coherent inelastic scattering to a two-photon state above the bias window if initially one photon was present in the cavity. However, this photon-assisted feature is suppressed in the case of a y-polarized photon field due to the anisotropy of our system caused by its geometry.

Quantum pump effect induced by a linearly polarized microwave in a two-dimensional electron gas

A quantum pump effect is predicted in an ideal homogeneous two-dimensional electron gas (2DEG) that is normally irradiated by linearly polarized microwaves (MW). Without considering effects from spin-orbital coupling or the magnetic field, it is found that a polarized MW can continuously pump electrons from the longitudinal to the transverse direction, or from the transverse to the longitudinal direction, in the central irradiated region. The large pump current is obtained for both the low frequency limit and the high frequency case. Its magnitude depends on sample properties such as the size of the radiated region, the power and frequency of the MW, etc. Through the calculated results, the pump current should be attributed to the dominant photon-assisted tunneling processes as well as the asymmetry of the electron density of states with respect to the Fermi energy.

Pumped charge and spin current in a quantum dot molecule

The effects of an ac electric field on the quantum transport behaviors in a parallel-coupled double quantum dot system are investigated theoretically. A dc charge current can be pumped at zero bias due to photon-assisted tunneling effects. The sign, magnitude and position of the pumped current peaks can be well controlled and manipulated by simply varying the gate voltage, the amplitude and frequency of the ac field. Furthermore, the possibility of electrically pumping a pure spin current in the presence of spin-orbit interaction is discussed.

Resonant tunnelling features in quantum dots

We present a systematic review of features due to resonant electron tunnelling, observable in transport spectroscopy experiments on quantum dots and single donors. The review covers features attributable to intrinsic properties of the dot (orbital, spin and valley states) as well as extrinsic effects (phonon/photon emission/absorption, features in the charge reservoirs, coupling to nearby charge centres). We focus on the most common operating conditions, neglecting effects due to strong coupling to the leads. By discussing the experimental signatures of each type of feature, we aim at providing practical methods to distinguish between their different physical origins. The correct classification of the resonant tunnelling features is an essential requirement to understand the details of the confining potential or to predict the performance of the dot for quantum information processing.

Analysis of photogenerated random telegraph signal in single electron detector (photo-SET)

In this paper, we analyzed slow single traps, situated inside the tunnel oxide of small area single electron photo-detector (photo-SET or nanopixel). The relationship between excitation signal (photons) and random-telegraph-signal (RTS) was evidenced. We demonstrated that photoinduced RTS observed on a photo-detector is due to the interaction between single photogenerated charges that tunnel from dot to dot and current path. Based on RTS analysis for various temperatures, gate bias and optical power we determined the characteristics of these single photogenerated traps: the energy position within the silicon bandgap, capture cross section and the position within the Si/SiO(x = 1.5) interfaces.

Nonadiabatic two-parameter charge and spin pumping in a quantum dot

We study dc charge and spin transport through a weakly coupled quantum dot, driven by a nonadiabatic periodic change of system parameters. We generalize the model of Tien and Gordon to simultaneously oscillating voltages and tunnel couplings. When applying our general result to the two-parameter charge pumping in quantum dots, we find interference effects between the oscillations of the voltage and tunnel couplings. We show that these interference effects may explain recent measurements in metallic islands. Furthermore, we discuss the possibility to electrically pump a spin current in presence of a static magnetic field.

Microwave band on-chip coil technique for single electron spin resonance in a quantum dot

Microwave band on-chip microcoils are developed for the application to single electron spin resonance measurement with a single quantum dot. Basic properties such as characteristic impedance and electromagnetic field distribution are examined for various coil designs by means of experiment and simulation. The combined setup operates relevantly in the experiment at dilution temperature. The frequency responses of the return loss and Coulomb blockade current are examined. Capacitive coupling between a coil and a quantum dot causes photon assisted tunneling, whose signal can greatly overlap the electron spin resonance signal. To suppress the photon assisted tunneling effect, a technique for compensating for the microwave electric field is developed. Good performance of this technique is confirmed from measurement of Coulomb blockade oscillations.

Photon-assisted tunneling in a carbon nanotube quantum dot

We report on photon-assisted tunneling (PAT) experiments in a carbon nanotube quantum dot using microwave frequencies between 20 and 60 GHz. In addition to the basic PAT effect, revealed by the appearance of two extra resonances in the current through the dot, we use PAT for spectroscopy of excited states. The experimental data are in good agreement with simulations.

Current and noise in a model of an alternating current scanning tunneling microscope molecule-metal junction

The transport properties of a simple model for a finite level structure (a molecule or a dot) connected to metal electrodes in an alternating current scanning tunneling microscope (ac-STM) configuration is studied. The finite level structure is assumed to have strong binding properties with the metallic substrate, and the bias between the STM tip and the hybrid metal-molecule interface has both an ac and a dc component. The finite frequency current response and the zero-frequency photoassisted shot noise are computed using the Keldysh technique, and examples for a single-site molecule (a quantum dot) and for a two-site molecule are examined. The model may be useful for the interpretation of recent experiments using an ac-STM for the study of both conducting and insulating surfaces, where the third harmonic component of the current is measured. The zero-frequency photoassisted shot noise serves as a useful diagnosis for analyzing the energy level structure of the molecule. The present work motivates the need for further analysis of current fluctuations in electronic molecular transport.

Charge shuttle as a nanomechanical rectifier

We consider the charge shuttle proposed by Gorelik et al. driven by a time-dependent voltage bias. In the case of asymmetric setup, the system behaves as a rectifier. For pure ac drive, the rectified current shows a rather rich frequency dependent response characterized by frequency locking at fractional values of the external frequency. Because of the nonlinear dynamics of the shuttle, rectification is present also for very low frequencies. These effects could be useful to unveil the internal dynamics of nanomechanical devices.

Current switch by coherent trapping of electrons in quantum dots

We propose a new transport mechanism through tunnel-coupled quantum dots based on the coherent population trapping effect. Coupling to an excited level by the coherent radiation of two microwaves can lead to an extremely narrow current antiresonance. The effect can be used to determine interdot dephasing rates and is a mechanism for a very sensitive, optically controlled current switch.

Dynamic conductance of carbon nanotubes

The dynamic conductance of carbon nanotubes was investigated using the nonequilibrium Green's function formalism within the context of a tight-binding model. Specifically, we have studied the ac response of tubes of different helicities, both with and without defects, and an electronic heterojunction. Because of the induced displacement currents, the dynamic conductance of the nanotubes differs significantly from the dc conductance displaying both capacitive and inductive responses. The important role of photon-assisted transport through nanotubes is revealed and its implications for experiments discussed.