Correlated charge detection for readout of a solid-state quantum computer

The present paper discusses the investigation of electronic properties of anthracene-based single electron transistor (SET) operating in coulomb blockade region using Density Functional Theory (DFT) based Atomistix toolkit-Virtual nanolab. The charging energies of anthracene molecule in isolated as well as electrostatic SET environments have been calculated for analyzing the stability of the molecule for different charge states. Study also includes the analysis of SET conductance dependence on source/drain and gate potentials in reference to the charge stability diagram. Our computed charging energies for anthracene in isolated environment are in good agreement with the experimental values and the proposed anthracene SET shows good switching properties in comparison to other acene series SETs.

The single electron transistor (SET) is a nanometer-size device that turns on and off again every time one electron is added to it. In this article, the physics of the SET is reviewed. The consequences of confining electrons to a small region of space are that both the charge and energy are quantized. We review how the charge states and energy states of the confined electrons, sometimes called an artificial atom, are measured, and how the precision of these measurements depends on temperature. We also discuss the coupling of electrons inside the artificial atom to those in the leads of the SET, which results in the Kondo effect. We review measurements of the Kondo effect, which demonstrate that the Anderson Hamiltonian provides a quantitative description of the SET.

The radio-frequency single-electron transistor (rf-SET) possesses key requirements necessary for reading out a solid state quantum computer. This work explores the use of the rf-SET as a single-shot readout device in the presence of 1∕f and telegraph charge noise. For a typical spectrum of 1∕f noise we find that high fidelity, single-shot measurements are possible for signals Δq>0.01e. For the case of telegraph noise, we present a cross-correlation measurement technique that uses two rf-SETs to suppress the effect of random switching events on readout. We demonstrate this technique by monitoring the charge state of a metal double dot system on microsecond time scales. Such a scheme will be advantageous in achieving high readout fidelity in a solid-state quantum computer.

Recent studies of silicon spin qubits at temperatures above 1 K are encouraging demonstrations that the cooling requirements for solid-state quantum computing can be considerably relaxed. However, qubit readout mechanisms that rely on charge sensing with a single-island single-electron transistor (SISET) quickly lose sensitivity due to thermal broadening of the electron distribution in the reservoirs. Here we exploit the tunneling between two quantized states in a double-island single-electron transistor (SET) to demonstrate a charge sensor with an improvement in the signal-to-noise ratio by an order of magnitude compared to a standard SISET, and a single-shot charge readout fidelity above 99% up to 8 K at a bandwidth greater than 100 kHz. These improvements are consistent with our theoretical modeling of the temperature-dependent current transport for both types of SETs. With minor additional hardware overhead, these sensors can be integrated into existing qubit architectures for a high-fidelity charge readout at few-kelvin temperatures.

Transistors have continuously reduced in size and increased in switching speed since their invention in 1947. The exponential pace of transistor evolution has led to a revolution in information acquisition, processing and communication technologies. And reigning over most digital applications is a single device structure--the field-effect transistor (FET). But as device dimensions approach the nanometre scale, quantum effects become increasingly important for device operation, and conceptually new transistor structures may need to be adopted. A notable example of such a structure is the single-electron transistor, or SET. Although it is unlikely that SETs will replace FETs in conventional electronics, they should prove useful in ultra-low-noise analog applications. Moreover, because it is not affected by the same technological limitations as the FET, the SET can approach closely the quantum limit of sensitivity. It might also be a useful read-out device for a solid-state quantum computer.

We analyze the dynamics of a nanomechanical resonator coupled to a single-electron transistor (SET) in the regime where the resonator behaves classically. A master equation is derived describing the dynamics of the coupled system which is then used to obtain equations of motion for the average charge state of the SET and the average position of the resonator. We show that the action of the SET on the resonator is very similar to that of a thermal bath, as it leads to a steady-state probability distribution for the resonator which can be described by mean values of the resonator position, a renormalized frequency, an effective temperature, and an intrinsic damping constant. Including the effects of extrinsic damping and finite temperature, we find that there remain experimentally accessible regimes where the intrinsic damping of the resonator still dominates its behavior. We also obtain the average current through the SET as a function of the coupling to the resonator.

This paper mainly studies the multiclass stochastic user equilibrium problem considering the market share of battery electric vehicles (BEVs) and random charging behavior (RCB) in a mixed transport network containing electric vehicles and gasoline vehicles (GVs). In order to analyze the random charging and path choice behaviors of BEV users and extract the differences in travel behaviors between BEV and GV users, an improved logit-based model, multilabel algorithm, and queuing theory are applied. The influencing factors of charging possibility mainly include the initial state of charge (SOC), the SOC at the beginning of charging, and the psychologically acceptable safe SOC threshold arriving at the destination. Diversity choices of user paths and charging locations will result in changes in queuing traffic and differences in queuing time. Conversely, different stations have different queuing dwell times, which will also affect the routing and charging locations for BEVs with RCB. The path-based method of successive averages (MSA) is adopted to solve the model. Through the simulation of the test network Sioux Falls, the equilibrium traffic flow and possible charging flow under different market shares and initial SOC are predicted, and the properties of the model and the feasibility of the algorithm are verified.

We study charge states of a superconducting single-electron transistor (SSET) fabricated on a heterostructure substrate that can be switched from insulating to conductive. We probe the charge states by microwave irradiation and subsequent observation of photon-assisted Josephson quasiparticle current, which allow us to obtain the energy dispersion relation in the quasicharge space. When the substrate is insulating, the charge states show an energy-dispersion curve with a gap, manifesting that coherent superposition of the two charge states is realized. When the substrate becomes conductive, its two-dimensional electrons capacitively couple to the SSET and modify the degree of dissipation to its electromagnetic environment. The dispersion relation then becomes linear, and its slope agrees with the value estimated from the charging energy including the capacitance of the two-dimensional electrons. This indicates that quantum coherence between the two charge states is lost due to the coupling with the environment.

The continuous downscaling of transistors results in nanoscale devices which require fewer and fewer charged carriers for their operation. The ultimate charge controlled device, the single-electron transistor (SET), controls the transfer of individual electrons. It is also the most sensitive electrometer, and as a result the electron transport through it can be dramatically affected by nearby charges. Standard direct-current characterization techniques, however, are often unable to unambiguously detect and resolve the origin of the observed changes in SET behavior arising from changes in the charge state of a capacitively coupled trap. Using a radio-frequency (RF) reflectometry technique, we are able to unequivocally detect this process, in very close agreement with modeling of the trap's occupation probability.

For the maintenance of electric vehicle batteries, a special apparatus achieving the state-of-charge (SOC) alignment of the whole cells inside a battery pack is highly promising. Specifically, when battery packs are discharged to a safe SOC level before shipping, according to battery logistic policy, and charged back to a nominal SOC level before being replaced in the swapping station, the SOC levels of all cells should be aligned to a specific target level. Conventionally, numerous cell-balancing hardware architectures and control strategies are studied; however, they only equalize the SOC between cells, and a whole pack SOC alignment feature has not been evaluated. In this article, a novel algorithm with a coordinated operation between a pack-charger and cell-equalizer is proposed as a viable SOC alignment algorithm. The full-duplex coordinated strategy, which utilizes a bidirectional pack-charger and cell-equalizer, is proposed for alignment purposes, which can minimize the operating time and the energy loss. The theoretical strategy is investigated to achieve the minimal operation time, and the performance of the proposed method is verified by a hardware prototype for a battery pack with 20- series cells. After the alignment process, the cell SOC levels are equalized within a voltage deviation of 30 mV or SOC deviation of 3%, when the pack SOC level is simultaneously adjusted to a preset level. In comparison with the half-duplex strategy, the proposed strategy has a twice faster processing time, while the energy loss during the process is 38%, 10.4%, and 26.1% lower than before. Otherwise, the proposed strategy achieves higher speed of 52% and 28.9% compared with the conventional strategies in balancing speed comparison.

Coordinated energy management for an islanded microgrid with multi-energy and multi-storage units

A recent worldwide uptake of electric vehicles (EVs) has led to an increasing interest for the EV charging situation. A proper understanding of the former is required to understand charging needs and to dimension the corresponding infrastructure. In the paper, we develop models that allow us to approximate the steady-state distribution of State-of-Charge (SoC) levels for EVs at the beginning of the day and infer its dependence regarding the daily relative range, r defined as the ratio of mean daily-driven distance to the maximum range. The framework combines: (i) a generic parametric model for decision to charge in terms of the SoC and r; and (ii) a simulation model in which we utilize longitudinal log-data for a fleet of cars to track charging events and SoC over time. The model brings about several interesting use cases, of which two stand out. Firstly, it provides a simple parametric way of circumventing transient behaviour in SoC distributions when applied to simulation frameworks. Secondly, it offers a clear method to infer crucial information regarding EV fleets and the total energy storage potential. Such information is useful for vehicle-to-grid (V2G) applications in that it provides expected lower and upper bounds for the energy that can be stored and charged. The model is applied in a Danish context, and it is suggested that while a full electrification of the vehicle fleet could lead to significant stress on the power grid, it will at the same time hold a large potential for deploying V2G as an important mechanism to dampen the effect of temporal high-demand. The potential for V2G stems from a low battery utilization between charging events of approximately 40%, which in turn provides a large storage buffer that could be harnessed with little to no impact on EV utilization. Results also indicate that tapping into just half of the available buffer could serve the EV demand and also meet close to 50% of the household consumption.

The paper discusses coherent control of charge and spin states of a biexciton system in a quantum dot via coherent two-photon transitions. Rabi oscillations between the ground state of a quantum dot and the biexciton state, as well as oscillations between the two single-exciton states induced by laser pulses with different circular or linear polarizations are studied. The effect of phonon-induced decoherence on these processes is described. System properties and driving conditions that lead to optimal coherent control are identified. It is shown that proper optimization allows one to control the two-qubit biexciton system via two-photon transitions with a high fidelity.

We review an ongoing effort to demonstrate technologies required for quantum computing with phosphorus donors in silicon. The main aspect of our research is to achieve control over charge and spin states of individual dopant atoms. This work has required the development of new techniques for engineering silicon nanodevices at the atomic level. We follow an approach for implanting single phosphorus ions into silicon substrates with integrated p–i–n detectors. Configuring our devices with radio-frequency single-electron transistors (RF-SETs) allows for charge sensing at low temperatures. In this context, we perform measurements of single-electron charge transfer between individual phosphorus donors. In a parallel effort, we employ nanoscale Schottky contacts for populating and depopulating individual dopant atoms. Of particular interest is coherent manipulation of single-electron charge and spin states on individual dopant atoms. Charge manipulation between coupled donor states may be achieved by either external microwave pumping or intrinsic tunnel coupling. Spin manipulation, on the other hand, involves magnetic resonance. In this context, we pursue electrically detected spin resonance in phosphorus-doped devices with a decreasing number of dopant atoms.

We have fabricated single-electron transistors from individual metal nanoparticles using a geometry that provides improved coupling between the particle and the gate electrode. This is accomplished by incorporating a nanoparticle into a gap created between two electrodes using electromigration, all on top of an oxidized aluminum gate. We achieve sufficient gate coupling to access more than ten charge states of individual gold nanoparticles (5–15 nm in diameter). The devices are sufficiently stable to permit spectroscopic studies of the electron-in-a-box level spectra within the nanoparticle as its charge state is varied.