Superconducting Quantum Computers: Experimental basics and state of the art
Abstract:
"Quantum computation is a distinctively new way of harnessing nature. It will be the first technology that allows useful tasks to be performed in collaboration between parallel universes."
This is a quote by David Deutsch, who devised the first piece of software for a quantum computer. Such a machine makes use of effects such as superposition and entanglement in order to process information at rates superior to any imaginable classical machine. Although the theoretical foundation of quantum computation is well-established, its realization poses a formidable technical challenge because it requires one to precisely control and measure a large system of well-isolated but mutually interacting quantum systems.
Around the year 2000, a new technology emerged which promises the feasibility to build solid-state integrated quantum processors. These are based on superconducting resonant circuits whose coherent quantum electrodynamics enables one to tailor their energy spectrum as needed in order to create artificial two-state systems to be used as quantum bits. This field has advanced at an impressive pace, and today's experimental prototypes are at the verge of beating the classical limit - the point where their quantum evolution will reach a level of complexity that cannot be simulated any more even by supercomputers.
This lecture course will review the concept and realization of superconducting quantum computers from an experimental viewpoint. We start with an overview of the theoretical basics of quantum computers and explain the functionality of quantum algorithms. The major experimental part starts with a short review of previous approaches to build a quantum computer, e.g. using individual photons, trapped atoms, or electrons in quantum dots. In the part on superconducting circuits, we begin with a discussion of the physics of Josephson junctions and SQUIDs that constitute the heart of quantum processors. We'll then learn about the different types of superconducting quantum bits which include charge qubits, flux qubits, phase qubits, and the so-called Transmon, hereby reviewing corresponding milestone experiments from pioneering research groups. Techniques to fight decoherence either by software (using quantum error correction) or by clever circuit designs will be discussed. The lecture arrives at a presentation of the current state-of-the-art as well as future prospects of quantum computation.