Wet etch methods for InAs nanowire patterning and self-aligned electrical contacts

Advanced synthesis of semiconductor nanowires (NWs) enables their application in diverse fields, notably in chemical and electrical sensing, photovoltaics, or quantum electronic devices. In particular, Indium Arsenide (InAs) NWs are an ideal platform for quantum devices, e.g. they may host topological Majorana states. While the synthesis has been continously perfected, only few techniques were developed to tailor individual NWs after growth. Here we present three wet chemical etch methods for the post-growth morphological engineering of InAs NWs on the sub-100 nm scale. The first two methods allow the formation of self-aligned electrical contacts to etched NWs, while the third method results in conical shaped NW profiles ideal for creating smooth electrical potential gradients and shallow barriers. Low temperature experiments show that NWs with etched segments have stable transport characteristics and can serve as building blocks of quantum electronic devices. As an example we report the formation of a single electrically stable quantum dot between two etched NW segments.Comment: 9 pages, 5 figure

Parallel quantized charge pumping

Two quantized charge pumps are operated in parallel. The total current generated is shown to be far more accurate than the current produced with just one pump operating at a higher frequency. With the application of a perpendicular magnetic field the accuracy of quantization is shown to be $<$20 ppm for a current of $108.9$pA. The scheme for parallel pumping presented in this work has applications in quantum information processing, the generation of single photons in pairs and bunches, neural networking and the development of a quantum standard for electrical current. All these applications will benefit greatly from the increase in output current without the characteristic decrease in accuracy as a result of high-frequency operation

Progress towards GaAs multiplexed single-electron pump arrays

In this thesis we present progress towards making multiplexed GaAs single-electron pump arrays. The single-electron pump is a device for transferring an accurate integer number $n$ of electrons per cycle to generate precise current $I=nef$, where $f$ is the frequency of the periodic AC voltage applied and $e$ is the electron charge. Multiplexing electron pumps may also allow the pumps to be measured in parallel, increasing the output current and thereby enabling a higher accuracy reading. Firstly, a 4 x 32 multiplexed wide-channel electron pump array is studied and we observe a large rectified current (about 100 $\mu$A) instead of a pumping current (which would be 18 pA at 110 MHz). We designed several variations of single wide-channel electron pump devices and found out that the rectified current is from the wide-channel electron pump and not the multiplexer. We developed a model to qualitatively explain the origin of rectified current in the wide-channel electron pump devices and investigate the effects of changing the RF frequency and amplitude on the rectified current. Secondly, we characterise the transmission of RF voltage signals through the quantum multiplexer using an array of bar gates. We find that about 300 mV AC amplitude voltage can be transmitted to the bar gate device, which may be sufficiently large for an electron pump to operate. We also present the statistical study of multiplexed bar gate devices. We find that 0.1 $\mu$m wide bar gates are different from 0.2 $\mu$m wide bar gate or wider gates: more negative voltage is needed to pinch off 0.1 $\mu$m wide bar gates, because 0.1 $\mu$m is comparable with the 2DEG depth. We redesign the multiplexer structure and determine that the capacitance of the multiplexer is about 1.93 pF which will help the future multiplexed single-electron pump array design to give best RF power transmission. Thirdly, since gate insulators are required in the multiplexed electron pump design. We demonstrate electron pumping in a single-electron pump device in which the gates extend across the entire GaAs channel, and are insulated from the GaAs channel by a polyimide layer as required in the multiplexed design. We also study how design variations such as the pump gates design (quantum dot radius and tunnel barrier width), channel etch design and order of fabrication will affect the RF power required to observe clear quantised pumping. Based on the above results, we present our designs for full GaAs multiplexed electron pump arrays

Single-electron current sources: towards a refined definition of ampere

Controlling electrons at the level of elementary charge $e$ has been demonstrated experimentally already in the 1980's. Ever since, producing an electrical current $ef$, or its integer multiple, at a drive frequency $f$ has been in a focus of research for metrological purposes. In this review we first discuss the generic physical phenomena and technical constraints that influence charge transport. We then present the broad variety of proposed realizations. Some of them have already proven experimentally to nearly fulfill the demanding needs, in terms of transfer errors and transfer rate, of quantum metrology of electrical quantities, whereas some others are currently "just" wild ideas, still often potentially competitive if technical constraints can be lifted. We also discuss the important issues of read-out of single-electron events and potential error correction schemes based on them. Finally, we give an account of the status of single-electron current sources in the bigger framework of electric quantum standards and of the future international SI system of units, and briefly discuss the applications and uses of single-electron devices outside the metrological context.Comment: 55 pages, 38 figures; (v2) fixed typos and misformatted references, reworded the section on AC pump

Semiconductor nanostructures engineering: Pyramidal quantum dots

Pyramidal quantum dots (QDs) grown in inverted recesses have demonstrated over the years an extraordinary uniformity, high spectral purity and strong design versatility. We discuss recent results, also in view of the Stranski-Krastanow competition and give evidence for strong perspectives in quantum information applications for this system. We examine the possibility of generating entangled and indistinguishable photons, together with the need for the implementation of a, regrettably still missing, strategy for electrical control

Spectrographic Microfluidic Memory

Recent advancements in micro- and nanoscale fluidic manipulation have enabled the development of a new class of tunable optical structures which are collectively referred to as optofluidic devices. In this paper we will introduce our recent work directed towards the development of a spectrographic optofluidic memory. Data encoding for the memory is based on creating spectrographic codes consisting of multiple species of photoluminescent nanoparticles at discrete intensity levels which are suspended in liquids. The data cocktails are mixed, delivered and stored using a series of soft and hard-lithography microfluidic structures. Semiconductor quantum dots are ideally suited for this application due to their narrow and size tunable emission spectra and consistent excitation wavelength. Both pressure driven and electrokinetic approaches to spectral code writing have been developed and will be experimentally demonstrated here. Novel techniques for data storage and readout are also discussed and demonstrated

A highly efficient single photon-single quantum dot interface

Semiconductor quantum dots are a promising system to build a solid state quantum network. A critical step in this area is to build an efficient interface between a stationary quantum bit and a flying one. In this chapter, we show how cavity quantum electrodynamics allows us to efficiently interface a single quantum dot with a propagating electromagnetic field. Beyond the well known Purcell factor, we discuss the various parameters that need to be optimized to build such an interface. We then review our recent progresses in terms of fabrication of bright sources of indistinguishable single photons, where a record brightness of 79% is obtained as well as a high degree of indistinguishability of the emitted photons. Symmetrically, optical nonlinearities at the very few photon level are demonstrated, by sending few photon pulses at a quantum dot-cavity device operating in the strong coupling regime. Perspectives and future challenges are briefly discussed.Comment: to appear as a book chapter in a compilation "Engineering the Atom-Photon Interaction" published by Springer in 2015, edited by A. Predojevic and M. W. Mitchel