Visualizing heterogeneous dipole fields by terahertz light coupling in individual nano-junctions used in transmon qubits

The fundamental challenge underlying superconducting quantum computing is to characterize heterogeneity and disorder in the underlying quantum circuits. These nonuniform distributions often lead to local electric field concentration, charge scattering, dissipation and ultimately decoherence. It is particularly challenging to probe deep sub-wavelength electric field distribution under electromagnetic wave coupling at individual nano-junctions and correlate them with structural imperfections from interface and boundary, ubiquitous in Josephson junctions (JJ) used in transmon qubits. A major obstacle lies in the fact that conventional microscopy tools are incapable of measuring simultaneous at nanometer and terahertz, "nano-THz" scales, which often associate with frequency-dependent charge scattering in nano-junctions. Here we directly visualize interface nano-dipole near-field distribution of individual Al/AlO$_{x}$/Al junctions used in transmon qubits. Our THz nanoscope images show a remarkable asymmetry across the junction in electromagnetic wave-junction coupling response that manifests as "hot" vs "cold" cusp spatial electrical field structures and correlates with defected boundaries from the multi-angle deposition processes in JJ fabrication inside qubit devices. The asymmetric nano-dipole electric field contrast also correlates with distinguishing, "overshoot" frequency dependence that characterizes the charge scattering and dissipation at nanoscale, hidden in responses from topographic, structural imaging and spatially-averaged techniques. The real space mapping of junction dipole fields and THz charge scattering can be extended to guide qubit nano-fabrication for ultimately optimizing qubit coherence times

Disentangling the sources of ionizing radiation in superconducting qubits

Radioactivity was recently discovered as a source of decoherence and correlated errors for the real-world implementation of superconducting quantum processors. In this work, we measure levels of radioactivity present in a typical laboratory environment (from muons, neutrons, and gamma's emitted by naturally occurring radioactive isotopes) and in the most commonly used materials for the assembly and operation of state-of-the-art superconducting qubits. We develop a GEANT-4 based simulation to predict the rate of impacts and the amount of energy released in a qubit chip from each of the mentioned sources. We finally propose mitigation strategies for the operation of next-generation qubits in a radio-pure environment

Disentangling the sources of ionizing radiation in superconducting qubits

Radioactivity was recently discovered as a source of decoherence and correlated errors for the real-world implementation of superconducting quantum processors. In this work, we measure levels of radioactivity present in a typical laboratory environment (from muons, neutrons, and γ-rays emitted by naturally occurring radioactive isotopes) and in the most commonly used materials for the assembly and operation of state-of-the-art superconducting qubits. We present a GEANT-4 based simulation to predict the rate of impacts and the amount of energy released in a qubit chip from each of the mentioned sources. We finally propose mitigation strategies for the operation of next-generation qubits in a radio-pure environment

Systematic Improvements in Transmon Qubit Coherence Enabled by Niobium Surface Encapsulation

We present a novel transmon qubit fabrication technique that yields systematic improvements in T$_1$ coherence times. We fabricate devices using an encapsulation strategy that involves passivating the surface of niobium and thereby preventing the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying the surface structure, this comparative investigation examining different capping materials and film substrates across different qubit foundries definitively demonstrates the detrimental impact that niobium oxides have on the coherence times of superconducting qubits, compared to native oxides of tantalum, aluminum or titanium nitride. Our surface-encapsulated niobium qubit devices exhibit T$_1$ coherence times 2 to 5 times longer than baseline niobium qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain median qubit lifetimes above 200 microseconds. Our comparative structural and chemical analysis suggests that amorphous niobium suboxides may induce higher losses. These results are in line with high-accuracy measurements of the niobium oxide loss tangent obtained with ultra-high Q superconducting radiofrequency (SRF) cavities. This new surface encapsulation strategy enables further reduction of dielectric losses via passivation with ambient-stable materials, while preserving fabrication and scalable manufacturability thanks to the compatibility with silicon processes

The Dialogic Possibilities for Interactive Fiction in the Secondary Academy English Classroom

The literacy practices enacted in secondary school English classrooms can be influenced by the pressures acting upon teachers and students. Attention can be diverted away from the process of meaning-making when more emphasis is placed upon performance outcomes than on reading processes. This paper argues that digital forms of Interactive Fiction (IF) hold the potential to help teachers and students attend more closely to the process of meaning-making. It also argues that IF’s component parts – passages, choices and links – render it a useful resource for the scaffolding of classroom dialogue. By considering the different ways that IF could influence the choices that individuals make in the classroom, this paper suggests that works of IF could enable teachers and students to engage with texts differently, improving the literacy practices of the students involved

Pedagogical benefits of fieldwork of the students at the Faculty of Geography in the light of the Bologna Process

Students' opinion and assessment of the quality of teaching presents an important segment of the evaluation of the quality of teaching at university level in accordance with the principles of the Bologna Process. In this study, we have examined opinion of students at the Faculty of Geography, University of Belgrade on the pedagogical benefits of fieldwork, which presents an important determinant of geographers' education. A total of 215 students evaluated pedagogical benefits of fieldwork in relation to didactic-methodical aspects such as: immediate contact with objects of knowledge; interdisciplinary study of a problem; application of various methods of teaching; enhancement of motivation for learning; improvement of social relations, and development of skills necessary for fieldwork. Research results indicate that students recognize and positively evaluate benefits of fieldwork. Final-year students as well as students who had more days of fieldwork evaluate benefits of fieldwork in a more positive way. Research results indicate the need to improve the quality of fieldwork and increase its share in the curriculum of the Faculty of Geography in accordance with the constructivist paradigm in education, which places a student at the centre of educational process, and fundamental principles of the Bologna Process