Seminar: From model system to devices – Understanding solar cell materials with photoelectron spectroscopy

13.30 – 13.45 WelcomeAnnica Black-Schaffer, Uppsala University

13.45 – 14.15 Quantum Measurements: From Fundamentals to Quantum Cooling and Sensing Applications Sreenath K. Manikandan, Nordita

14.15 – 14.45 Geometry of Quantum Evolution Erik Sjöqvist, Uppsala University

14.45 – 15.15 Coherent-Error Threshold for Surface Codes from Majorana Delocalization Jan Behrends, Cambridge University

15.15 –15.45 Coffee break

15.45 – 16.15 Identification of Majorana Bound States in Josephson Junctions Jorge Cayao, Uppsala University

16.15 – 16.45 High-dimensional Quantum Correlations Roope Uola, University of Geneva

16.45 – 17. 15 Spin-Based Architectures for Quantum Information Processing Vahid Azimi Mousolou, Uppsala University

Abstracts

Title:Quantum Measurements: From Fundamentals to Quantum Cooling and Sensing Applications

Speaker: Sreenath K. Manikandan, Nordita

Abstract: Quantum measurements are inherently probabilistic. I will discuss methods to characterize the irreversibility of quantum measurements from a thermodynamic perspective, by associating a statistical arrow of time for individual realizations of the quantum measurement process. I will show that continuous quantum measurements are absolutely irreversible---similar to the free expansion of a single gas molecule in a box---and I will discuss a cold atom experiment which demonstrates this. I will conclude the talk by presenting some applications of simultaneous measurements of position and momentum of a mechanical oscillator for optimal quantum parametric feedback cooling, as well as time-continuous energy measurements of massive acoustic bar resonators for table-top tests of gravity in the quantum regime.

Title:Geometry of quantum evolution

Speaker: Erik Sjöqvist, Uppsala University

Abstract: We discuss a simplification of the notions of metric and length along evolution of mixed quantum states. We discuss a few applications of this metric, such as time-energy uncertainty, single qubit geodesics, and response in magnetic systems to changes in external parameters.

Title:Coherent-Error Threshold for Surface Codes from Majorana Delocalization

Speaker: Jan Behrends, Cambridge University

Abstract: Statistical mechanics mappings provide key insights on quantum error correction. However, existing mappings assume incoherent noise, thus ignoring coherent errors due to, e.g., spurious gate rotations. We map the surface code with coherent errors, taken as X or Z rotations (replacing bit or phase flips), to a two-dimensional (2D) Ising model with complex couplings, and further to a 2D Majorana scattering network. Our mappings reveal both commonalities and qualitative differences in correcting coherent and incoherent errors. For both, the error-correcting phase maps, as we explicitly show by linking 2D networks to 1D fermions, to a Z2-nontrivial 2D insulator. However, beyond a rotation angle ϕth, instead of a Z2-trivial insulator as for incoherent errors, coherent errors map to a Majorana metal. This ϕth is the theoretically achievable storage threshold. We numerically find ϕth≈0.14π. The corresponding bit-flip rate sin^2(ϕth)≈0.18 exceeds the known incoherent threshold pth≈0.11.

Title:Identification of Majorana bound states in Josephson junctions

Speaker: Jorge Cayao, Uppsala University

Abstract: Semiconductor-superconductor junctions represent one of the most promising platforms for realizing topological superconductivity and Majorana bound states. However, these systems also host topologically trivial zero-energy states that often exhibit properties similar to those of Majorana modes. In this talk, I will show how Josephson junctions based on semiconductor-superconductor hybrids represent an outstanding platform for detecting Majorana bound states even when topologically zero-energy states are present. In particular, I will show that supercurrents and critical currents reveal unique information about the topological phase transition and spatial nonlocality of the Majorana bound states. Furthermore, I will discuss that, by controlling the Majorana nonlocality, it is possible to enhance the efficiency of Josephson diodes, thus revealing a practical benefit of Majorana states in Josephson devices. Therefore, Josephson junctions offer a powerful route for unambiguously probing Majorana physics and also pave the way for interesting Majorana-based applications.

Title:High-dimensional quantum correlations

Speaker: Roope Uola, University of Geneva

Abstract: High-dimensional quantum systems form one of the central platforms for quantum communication. The main advantage of such systems lies in their extreme noise tolerance and loss resilience, which provide a considerable benefit over qubit-based protocols. In this talk, I will give an overview on our recent theoretical research on high-dimensional quantum systems. This includes defining a tailored version of the EPR paradox for such systems and showcasing methods for its certification. As a practical application, I will present a record-breaking benchmarking result based on experimental quantum photonics.

Title:Spin-Based Architectures for Quantum Information Processing

Speaker: Vahid Azimi Mousolou, Uppsala University

Abstract: In this talk, we discuss the quantum realm of spin-based architectures for information processing by delving into the quantum properties of magnons. We will touch upon quantum squeezing and entanglement among magnons, emphasizing their potential applications in quantum computing, quantum communication, and quantum sensing and metrology. Additionally, we will address the integration of quantum magnons with spintronic devices to create hybrid systems with enhanced functionality and performance. This may illuminate the promising prospect of magnons serving as mediators for coupling qubits in quantum computing architectures.