Single-electron Pump Research Articles

Single-Electron Pumping by Parallel SINIS Turnstiles for Quantum Current Standard

As a step toward realizing the quantum current standard, single-electron pumping of parallel SINIS turnstiles was performed. Eleven SINIS turnstiles were fabricated on the same chip, and the number of operating SINIS turnstiles was increased one by one. As a result, 11 turnstiles were operated simultaneously up to 100 MHz, resulting in a pumped dc of 176.2 pA. We also did a linearity check toward parallelization. This paper for increasing the pumped current by parallelization will lead to practical use of stable and accurate dc sources with tiny values of the output current and the metrology triangle experiment for SI redefinition.

Precision measurement of a potential-profile tunable single-electron pump

We performed a precision measurement of the current from a single-parameter electron pump, where the potential-profile for a quantum dot was manipulated by multiple top-metal gates. In an optimally tuned condition, driven with a sinusoidal-waveform microwave at f = 0.95 GHz, B = 11 T, and T = 0.3 K, the relative deviation of the pump current from ef, δIp/ef ≡ (Ip − ef)/ef was measured to be (−0.92 ± 1.37) ppm. Our experiment reproduces the current quantisation accuracy of a previous measurement of a single-parameter pump, but in a device fabricated using very different geometry, thereby indicating that accurate single-parameter pumping is insensitive to device details.

Gigahertz single-trap electron pumps in silicon.

Manipulation of single electrons is the key to developing ultimate electronics such as single-electron-based information processors and electrical standards in metrology. Especially, high-frequency and high-accuracy single-electron pumps are essential to realize practical current standards. While electrically defined quantum dots are widely used to build single-electron pumps, a localized state in semiconductors is also a potential candidate for accurate pumps because it can have a large activation energy for the captured electron. However, the transfer mechanism of such localized-state-mediated single-electron pumps for high-accuracy operation at a high frequency has not been well examined. Here we demonstrate a single-electron pump using a single-trap level with an activation energy of a few ten millielectron volts in Si nanotransistors. By means of gate control of capture and emission rates, the pump operates at a frequency of 3 GHz with an accuracy of better than 10−3 at 17 K, indicating that an electric field at the trap level lowers the capture and emission time to less than 25 ps.

Improvement of electron pump accuracy by a potential-shape-tunable quantum dot pump

We have investigated the accuracy dependence of a single-electron pump on the confinement-potential shape of a quantum dot (QD) pump. A uniquely designed QD, which employs multiple gates to control the shape of the QD potential well, is utilized for electron pumping. It has been observed that the accuracy of the pump can be dramatically enhanced by achieving smaller QD size and greater decoupling from the electrodes, which is supported by the so-called decay-cascade model. The accuracy of the pump current is estimated, based on the decay-cascade model, to be close to 0.1 ppm for 80-pA pump current when the confinement-potential shape is optimally tuned. This is the highest reported accuracy, although it is a theoretically estimated value, among QD-based pumps measured at 4.2 K in the absence of an external magnetic field. Our numerical calculations show that the enhancement of the estimated accuracy is mainly due to the increase of the electron addition energy in the QD as well as the increase of the height and thickness of the QD potential barrier.

Scheme for topological single electron pumping assisted by Majorana fermions

Single electron pumping based on the topological property of Majorana fermions (MFs), which specify the parity of electron number, is proposed. The setup consists of a quantum dot (QD) and two NOT gates each with four nanotopological superconductors (TSs) connected by constriction junctions and an additional vortex located in the loop of TSs. Operation is performed by gate voltages at constriction junctions. The condition of QD energy for sequential electron pumping is derived in terms of interactions among TSs, indicating that the QD energy may lie outside the energies of electrodes, which is unique as compared with other existing pumping mechanisms. The essence of the present topological single electron pumping lies in the nonlocal property of MFs, which makes crossing between quasiparticle levels possible and electrically controllable. The operation of the NOT gate is demonstrated, explicitly solving the time-dependent Bogoliubov-de Gennes equation for a microscopic model.

Charge pumping through a single donor atom

Presented in this paper is a proof-of-concept for a new approach to single electron pumping based on a single atom transistor. By charge pumping electrons through an isolated dopant atom in silicon, precise currents of up to 160 pA at 1 GHz are generated, even if operating at 4.2 K, with no magnetic field applied, and only when one barrier is addressed by sinusoidal voltage cycles.

An accurate single-electron pump based on a highly tunable silicon quantum dot.

Nanoscale single-electron pumps can be used to generate accurate currents, and can potentially serve to realize a new standard of electrical current based on elementary charge. Here, we use a silicon-based quantum dot with tunable tunnel barriers as an accurate source of quantized current. The charge transfer accuracy of our pump can be dramatically enhanced by controlling the electrostatic confinement of the dot using purposely engineered gate electrodes. Improvements in the operational robustness, as well as suppression of nonadiabatic transitions that reduce pumping accuracy, are achieved via small adjustments of the gate voltages. We can produce an output current in excess of 80 pA with experimentally determined relative uncertainty below 50 parts per million.

Temperature dependence of single-electron pumping using a SINIS turnstile

A quantum electric current standard is the last missing piece of the quantum metrology triangle. A suitable candidate for realizing this quantum current standard is a single-electron pumping device based on a superconductor/insulator/normal metal/insulator/superconductor (SINIS) turnstile. Here, we show the temperature dependence of the single-electron pumped current of a SINIS turnstile. As the operating temperatures are increased, the current plateaus of the single-electron pumping tilt because of increased back tunneling, which originates from the thermal broadening of energy levels in the normal metal island of the SINIS turnstile. Hence, we find that to use a SINIS turnstile for the current standard at more than 300mK, it is necessary to increase the superconducting energy gaps of the lead material and the charging energy of the island.

Introducing Joint Research Project «Quantum Ampere» for the realisation of the new SI ampere

The metrology community lately has adopted the long-term aim of basing the SI unit system on fundamental constants of nature. The base electrical unit, the ampere, will then be re-defined in terms of a fixed value of the elementary charge e. The most direct realization of the new ampere definition requires controlling the number of electrons which flow in a unit time interval, and of counting the errors occurring in this process of clocking single electrons. State of the art nanofabrication technology allows the fabrication of single-electron transport devices - known as single-electron pumps - which generate electric current by moving electrons one at a time. These devices are capable of delivering currents of about 100 pA with an accuracy at the 1 part per million level. Also, ultrasensitive single-electron detectors have been explored that allow electric charge detection on a resolution level below e. The European Joint Research Project presented here, undertaken by a consortium of several research institutes, aims at further developing the best existing concepts of single-electron pumps and to combine them with single-electron detectors for creating highly accurate quantum current sources, to be used as future current standards. Furthermore, necessary current measurement instrumentation will be developed. The paper comprises the project aims and the main results achieved so far.

Information flow and optimal protocol for a Maxwell-demon single-electron pump

We study the entropy and information flow in a Maxwell-demon device based on a single-electron transistor with controlled gate potentials. We construct the protocols for measuring the charge states and manipulating the gate voltages, which minimizes irreversibility for (i) constant input power from the environment or (ii) given energy gain. Charge measurement is modeled by a series of detector readouts for time-dependent gate potentials, and the amount of information obtained is determined. The protocols optimize irreversibility that arises due to (i) enlargement of the configuration space on opening the barriers, and (ii) finite rate of operation. These optimal protocols are general and apply to all systems in which barriers between different regions can be manipulated.

Towards quantized current arbitrary waveform synthesis

The generation of ac modulated quantized current waveforms using a semiconductor non-adiabatic single electron pump is demonstrated. In standard operation, the single electron pump generates a quantized output current of I = ef, where e is the charge of the electron and f is the pumping frequency. Suitable frequency modulation of f allows the generation of ac modulated output currents with different characteristics. By sinusoidal and saw tooth like modulation of f accordingly modulated quantized current waveforms with kHz modulation frequencies and peak currents up to 100 pA are obtained. Such ac quantized current sources could find applications ranging from precision ac metrology to on-chip signal generation.

Gigahertz quantized charge pumping in graphene quantum dots

Single-electron pumps are set to revolutionize electrical metrology by enabling the ampere to be redefined in terms of the elementary charge of an electron. Pumps based on lithographically fixed tunnel barriers in mesoscopic metallic systems and normal/superconducting hybrid turnstiles can reach very small error rates, but only at megahertz pumping speeds that correspond to small currents of the order of picoamperes. Tunable barrier pumps in semiconductor structures are operated at gigahertz frequencies, but the theoretical treatment of the error rate is more complex and only approximate predictions are available. Here, we present a monolithic, fixed-barrier single-electron pump made entirely from graphene that performs at frequencies up to several gigahertz. Combined with the record-high accuracy of the quantum Hall effect and proximity-induced Josephson junctions, quantized-current generation brings an all-graphene closure of the quantum metrological triangle within reach. Envisaged applications for graphene charge pumps outside quantum metrology include single-photon generation via electron-hole recombination in electrostatically doped bilayer graphene reservoirs, single Dirac fermion emission in relativistic electron quantum optics and read-out of spin-based graphene qubits in quantum information processing.

Monte Carlo Simulation of Single Electronics Based on Orthodox Theory

In this paper, we present a detailed algorithm to compute the current-voltage characteristics of the single electron circuits by Monte Carlo method. Our simulator is designed to solve capacitance systems that contain tunnel junctions. The simulation process is based on orthodox theory. Single electron box, single electron transistor, electron pump and inverter circuits have been investigated, and the results are compared with the previous relevant literature

Stabilization of single-electron pumps by high magnetic fields

We study the effect of perpendicular magnetic fields on a single-electron system with a strongly time-dependent electrostatic potential. Continuous improvements to the current quantization in these electron pumps are revealed by high-resolution measurements. Simulations show that the sensitivity of tunnel rates to the barrier potential is enhanced, stabilizing particular charge states. Nonadiabatic excitations are also suppressed due to a reduced sensitivity of the Fock-Darwin states to electrostatic potential. The combination of these effects leads to significantly more accurate current quantization.

Semiconductor Quantized Voltage Source

We realize and investigate an all-semiconductor quantized voltage source which generates quantized output voltages V(out) = f(h/e) linked only to two fundamental constants, the electron's charge e and Planck's constant h, and to an applied excitation frequency f. The device is based on an integrated quantized circuit of a single-electron pump operated at pumping frequency f and a quantum Hall device monolithically integrated in series. Robust output voltages up to several μV are generated, which are expected to be scalable by orders of magnitude using present technology. The device might open a new route towards the closure of the quantum metrology triangle.

Quantized charge pumping through a carbon nanotube double quantum dot

We demonstrate single-electron pumping in a gate-defined carbon nanotube double quantum dot. By periodic modulation of the potentials of the two quantum dots, we move the system around charge triple points and transport exactly one electron or hole per cycle. We investigate the pumping as a function of the modulation frequency and amplitude and observe good current quantization up to frequencies of 18 MHz where rectification effects cause the mechanism to break down.

Donor-Based Single Electron Pumps with Tunable Donor Binding Energy

We report on single electron pumping via a tunable number of individual donors. We use a device that essentially consists of a silicon nanowire with local arsenic implantation between a set of fine gates. A temperature-dependent characterization of the pumped current allows us to extract the ionization energy of a single arsenic donor. We observe the ionization energy to be tunable by the gate electric field over a large range of energies.

Towards a quantum representation of the ampere using single electron pumps

Electron pumps generate a macroscopic electric current by controlled manipulation of single electrons. Despite intensive research towards a quantum current standard over the last 25 years, making a fast and accurate quantized electron pump has proved extremely difficult. Here we demonstrate that the accuracy of a semiconductor quantum dot pump can be dramatically improved by using specially designed gate drive waveforms. Our pump can generate a current of up to 150 pA, corresponding to almost a billion electrons per second, with an experimentally demonstrated current accuracy better than 1.2 parts per million (p.p.m.) and strong evidence, based on fitting data to a model, that the true accuracy is approaching 0.01 p.p.m. This type of pump is a promising candidate for further development as a realization of the SI base unit ampere, following a redefinition of the ampere in terms of a fixed value of the elementary charge.

Theoretical Study on the Stability of the Single-Electron-Pump Refrigerator with Respect to Thermal and Dimensional Fluctuations

We herein investigate the operation stability of the single-electron-pump (SEP) refrigerator with respect to thermal and dimensional fluctuations. The SEP refrigerator was found to successfully demonstrate single-electron extraction and injection at temperatures up to 2K. Although the dimensional fluctuation in junction capacitance will seriously affect operation, the effect of the gate capacitance fluctuation is unlikely to be severe.

Maxwell's demon based on a single-electron pump

We propose and analyze a possible implementation of Maxwell's demon (MD) based on a single-electron pump. It presents a practical realization of MD in a solid-state environment. We show that measurements of the charge states of the pump and feedback control of the gate voltages lead to a net flow of electrons against the bias voltage ideally with no work done on the system by the gate control. The information obtained in the measurements converts thermal fluctuations into energy stored in a capacitor. We derive the conditions on the detector back action and measurement time necessary for implementing this conversion.