Paul Bernays Lectures 2024
The 2024 Paul Bernays Lectures on the topic "Information processing at the quantum scale with superconducting circuits" were delivered by Professor Michel Devoret.
Professor Michel Devoret
Michel Devoret is Professor of Physics at the University of California Santa Barbara. He also holds a position as Chief Scientist at Google Quantum AI.
Devoret's main contributions are in experimental solid state physics, with emphasis on mesoscopic phenomena that provide degrees of freedom suited for quantum information processing. Devoret was awarded numerous prizes, among them the Europhysics-Agilent Prize (2004) of the European Physical Society, the John Stewart Bell Prize (2013), the Fritz London Memorial Prize (2014), and most recently the Comstock Prize in Physics (2024) of the U.S. National Academy of Sciences. He is a member of the American Academy of Arts and Sciences (2003), the Académie des Sciences (2008) and the U.S. National Academy of Science (2023).
Video recordings
The recordings of all three lectures are available on the ETH video portal.
Information processing at the quantum scale with superconducting circuits
Lecture 1: The Physics of Information
- Date: Wednesday, 18 December 2024
- Time: 5.00 p.m.
- Room: HG F 30 (Audi Max, Zentrum Campus)
Abstract
It is often said that we live in an "information society". But what exactly is meant by information? A sequence of symbols 0 and 1? Currently, inside the most miniaturized computer, a binary digit, commonly called a bit, is a complex physical device with billions of interacting particles. What happens to information processing when each bit is carried by a single quantum particle, such as an atom, an electron or a photon? Conversely, can we see the movement of elementary particles as a calculation that the universe is performing? The physics of the last thirty years has been particularly rich in the development of ideas and experiments that have illustrated the crucial role of information in physical laws. A new type of computer, the quantum computer, still in the prototype phase, has been invented. This lecture, which is aimed at non-specialists, will explain the merits of such quantum machine and some of the questions it can tackle. In particular, one crucial aspect of its development, namely the progress in fault-tolerant operations, will be discussed.
Lecture 2: Error correction of a logical quantum bit beyond the break-even point
- Date: Thursday, 19 December 2024
- Time: 5.00 p.m.
- Room: HCI G 7 (Hönggerberg Campus)
Abstract
The accuracy of logical operations on quantum bits (qubits) must be improved for quantum computers to surpass classical ones in useful tasks. To that effect, quantum information needs to be made robust to noise that affects the underlying physical system. Rather than suppressing noise, quantum error correction aims at preventing it from causing logical errors. This approach derives from the reasonable assumption that noise is local: it does not act in a coordinated way on different parts of the physical system. Therefore, if a logical qubit is adequately encoded non-locally in the larger Hilbert space of a composite system, it is possible, during a limited time, to detect and correct the noise-induced evolution before it corrupts the encoded information. We will present an experiment implementing a logical qubit in a superconducting cavity coupled to a transmon synthetic atom – the latter employed here as an auxiliary non-linear element [1]. Error correction involves a novel primitive operation [2] and feedback control based on reinforcement learning [3]. Recently, we have stabilized in real-time a logical qubit manifold spanned by the Gottesman-Kitaev-Preskill grid states, reaching a correction efficiency such that the lifetime of the encoded information was prolonged by more than a factor of two beyond the lifetime of the best physical qubits composing our system [4].
* Now at UCSB & Google Quantum AI
[1] Campagne-Ibarcq, Eickbusch, Touzard, et al., Nature 584, 368-372 (2020).
[2] Eickbusch et al., Nature Physics 18, 1464 (2022).
[3] Sivak et al., Phys. Rev. X 12, 011059 (2022).
[4] Sivak et al., Nature 616, 50-55 (2023).
Lecture 3: Driven unwanted state transitions in superconducting quantum circuits
- Date: Friday, 20 December 2024
- Time: 4.30 p.m.
- Room: HCI G 7 (Hönggerberg Campus)
Abstract
Readout and parametric gate operations in qubits implemented in quantum superconducting circuits are performed by applying microwave drive tones to the circuit. The simultaneous pursuit of fidelity and speed of these operations by increasing drive strength is limited by unwanted drive-induced state transitions arising from the circuit non-linearity. We experimentally address the origin of these adverse state transitions in a driven transmon by measuring transition probabilities as a function of drive frequency and power. We show that there are three distinct mechanisms for adverse transitions caused by the drive: 1) AC Stark shift of qubit frequency into resonance with lossy degrees of freedom in the qubit environment, 2) excitation of intrinsic resonances linking computational and non-computational states within the transmon spectrum, 3) non-linear, Raman-like processes involving extrinsic degrees of freedoms such as packaging or transmission line modes. Our findings provide insights into the improvement of readout and gate operations on superconducting qubits.