A Reconfigurable Gate Architecture for Si/SiGe Quantum Dots

We demonstrate a reconfigurable quantum dot gate architecture that incorporates two interchangeable transport channels. One channel is used to form quantum dots and the other is used for charge sensing. The quantum dot transport channel can support either a single or a double quantum dot. We demonstrate few-electron occupation in a single quantum dot and extract charging energies as large as 6.6 meV. Magnetospectroscopy is used to measure valley splittings in the range of 35-70 microeV. By energizing two additional gates we form a few-electron double quantum dot and demonstrate tunable tunnel coupling at the (1,0) to (0,1) interdot charge transition.Comment: Related papers at http://pettagroup.princeton.ed

Scalable gate architecture for densely packed semiconductor spin qubits

We demonstrate a 12 quantum dot device fabricated on an undoped Si/SiGe heterostructure as a proof-of-concept for a scalable, linear gate architecture for semiconductor quantum dots. The device consists of 9 quantum dots in a linear array and 3 single quantum dot charge sensors. We show reproducible single quantum dot charging and orbital energies, with standard deviations less than 20% relative to the mean across the 9 dot array. The single quantum dot charge sensors have a charge sensitivity of 8.2 x 10^{-4} e/root(Hz) and allow the investigation of real-time charge dynamics. As a demonstration of the versatility of this device, we use single-shot readout to measure a spin relaxation time T1 = 170 ms at a magnetic field B = 1 T. By reconfiguring the device, we form two capacitively coupled double quantum dots and extract a mutual charging energy of 200 microeV, which indicates that 50 GHz two-qubit gate operation speeds are feasible

Book Reviews

The Case Against Oligopoly: A New Perspective-- Campaigns of corporate aggrandizement have always attracted a wide assortment of camp followers, with politicians and academics among the most persistent. To the politician, corporate size is a convenient and headline-provoking punching bag (and, during election time, a plentiful source of funding), while many a professor has established a reputation--and made money--by criticizing, extolling, or analyzing its various components. In a series of public hearings, the Senate Subcommittee on Antitrust and Monopoly, with Dr. John Blair as chief economist, produced a subdued blend of both callings. Drawing heavily but not exclusively from the record of these hearings,\u27 Economic Concentration: Structure, Behavior and Public Policy is Blair\u27s statement on corporate size. Arthur D. Austin ==================== The Austrian--German Arbitral Tribunal--A Model for Conflict Resolution Under the Austrian State Treaty, executed in 1955, Austria received from the Allies the German assets in Austria that the Allies had seized in victory. In order to prevent a renewed German predominance of the Austrian economy, Austria pledged in the treaty that the bulk of these assets would never be returned to German control. While the Soviet Union demanded payment by Austria for assets it transferred, the Western powers, including the United States, made their transfers free of charge. Germany was required to compensate its own nationals for the property in Austria that they had lost, and Austria was asked to waive on behalf of itself and its nationals those claims against Germany arising after German entry into Austria in 1938. The scheme was devised to reduce the possibility of tension generated by claims crossing state lines. Despite this effort to reduce conflict, Austria and Germany foresaw persisting problems in claim settlements, and to settle them they executed the Property Treaty on June 15, 1957. John N. Hazar

Investigation of Mobility Limiting Mechanisms in Undoped Si/SiGe Heterostructures

We perform detailed magnetotransport studies on two-dimensional electron gases (2DEGs) formed in undoped Si/SiGe heterostructures in order to identify the electron mobility limiting mechanisms in this increasingly important materials system. By analyzing data from 26 wafers with different heterostructure growth profiles we observe a strong correlation between the background oxygen concentration in the Si quantum well and the maximum mobility. The highest quality wafer supports a 2DEG with a mobility of 160,000 cm^2/Vs at a density 2.17 x 10^11/cm^2 and exhibits a metal-to-insulator transition at a critical density 0.46 x 10^11/cm^2. We extract a valley splitting of approximately 150 microeV at a magnetic field of 1.8 T. These results provide evidence that undoped Si/SiGe heterostructures are suitable for the fabrication of few-electron quantum dots.Comment: Related papers at http://pettagroup.princeton.ed

Language learning experiences of postgraduate research students in the UK

This article reports on the findings of a pioneering study of formal, non-formal and informal language learning experiences of postgraduate research students in the UK. The research involved a large-scale survey followed by semi-structured interviews. Quantitative and qualitative data analysis shows that the language needs and interests of postgraduate students are barely met due to limited institutional provision of formal language learning opportunities. The study confirms some known facts, such as the composition of the student body (e.g. mostly young, fully-funded full-time students with a significant proportion of international students), and reveals other possibly less-known realities, from a remarkable interest in language learning among research students to the reasons for their interest and the potential impact greater access to language learning could have on their research and beyond. Half of the respondents reported having funded provision of language tuition but one third were not even aware of the existence of a Language Centre at their institution. Student suggestions for alternative language provision are included as these can inform policy and decision-making at postgraduate level in the UK and serve as a basis for further research in this area

Contrasting arbuscular mycorrhizal communities colonizing different host plants show a similar response to a soil phosphorus concentration gradient

High soil phosphorus (P) concentration is frequently shown to reduce root colonization by arbuscular mycorrhizal (AM) fungi, but the influence of P on the diversity of colonizing AM fungi is uncertain. We used terminal restriction fragment length polymorphism (T-RFLP) of 18S rDNA and cloning to assess diversity of AM fungi colonizing maize (Zea mays), soybean (Glycene max) and field violet (Viola arvensis) at three time points in one season along a P gradient of 10–280 mg l−1 in the field. Percentage AM colonization changed between sampling time points but was not reduced by high soil P except in maize. There was no significant difference in AM diversity between sampling time points. Diversity was reduced at concentrations of P > 25 mg l−1, particularly in maize and soybean. Both cloning and T-RFLP indicated differences between AM communities in the different host species. Host species was more important than soil P in determining the AM community, except at the highest P concentration. Our results show that the impact of soil P on the diversity of AM fungi colonizing plants was broadly similar, despite the fact that different plants contained different communities. However, subtle differences in the response of the AM community in each host were evident

Dancing to the Partisan Beat: A First Analysis of Political Communication on TikTok

TikTok is a video-sharing social networking service, whose popularity is increasing rapidly. It was the world's second-most downloaded app in 2019. Although the platform is known for having users posting videos of themselves dancing, lip-syncing, or showcasing other talents, user-videos expressing political views have seen a recent spurt. This study aims to perform a primary evaluation of political communication on TikTok. We collect a set of US partisan Republican and Democratic videos to investigate how users communicated with each other about political issues. With the help of computer vision, natural language processing, and statistical tools, we illustrate that political communication on TikTok is much more interactive in comparison to other social media platforms, with users combining multiple information channels to spread their messages. We show that political communication takes place in the form of communication trees since users generate branches of responses to existing content. In terms of user demographics, we find that users belonging to both the US parties are young and behave similarly on the platform. However, Republican users generated more political content and their videos received more responses; on the other hand, Democratic users engaged significantly more in cross-partisan discussions.Comment: Accepted as a full paper at the 12th International ACM Web Science Conference (WebSci 2020). Please cite the WebSci version; Second version includes corrected typo