On December 8, Vlad Kurilovich successfully defended the thesis, “Criticality in Majorana wires” (advisor: Leonid Glazman).
Vlad explained, “My thesis focuses on the physics of topological superconductors tuned to the vicinity of the topological phase transition. Topological superconductors are an unusual type of superconductors that host exotic excitations – the so-called Majorana zero modes. The properties of the Majorana zero modes may be used to implement fault-tolerant quantum computing; this makes topological superconductors actively sought for. On a conceptual level, building a topological superconductor should not be hard; this can be done by combining well-established components, such as a conventional superconductor and a semiconductor. The Majorana zero modes should appear with the application of the magnetic field above a certain critical value. Experiments, however, provide conflicting evidence for the presence of Majorana zero modes. This brings about the need to better understand the physics of the field-driven transition into the topological superconducting phase. In my thesis, I explore salient features of realistic Majorana devices associated with the emergence of topological superconductivity across the critical field value. The theories developed in the thesis are directly relevant to the on-going experiments with proximitized semiconducting nanowires and with quantum Hall edge states coupled by a conventional superconductor.”
His next position will be as a research scientist at Google Quantum AI in Santa Barbara.
Thesis Abstract:
There is an on-going effort to engineer a one-dimensional topological superconductor. Such a superconductor is expected to host Majorana zero modes at its ends; the exotic properties of these excitations might be useful for the development of a fault-tolerant quantum computer. A recent theory predicts that topological superconductivity may arise in a structure combining quantum Hall and conventional superconducting states. It led to active experimental attempts of bringing the two states together. We note, however, that the prediction focuses on the idealized “clean” case whereas, in practice, only strongly disordered superconductors are compatible with high magnetic fields needed for the quantum Hall effect. Does topological superconductivity survive the presence of disorder? In this thesis defense, I will present our theory of nonlocal transport of two counter-propagating quantum Hall edge states coupled via a narrow disordered superconductor. In contrast to the clean-case predictions, we find that the coupled edge states do not turn into a topological superconductor. Instead, the disorder naturally tunes them to the critical point between trivial insulating and topological phases. The criticality manifests itself in a random character of the conductance and its unusual bias dependence, which we quantify. Our results for the transport properties of the critical state are not restricted to the quantum Hall-superconductor structure; they are universal and apply to other realizations of Majorana wires.