Quantum scars as a way out of thermalisation

Building a quantum computer is no easy task partly because quantum information is stored in extremely fragile quantum states, whereas data in classical computers are encoded with simple binary states corresponding to zeros and ones. The fragility of quantum resources requires many quantum computing systems to be cooled to temperatures of about -270 degrees Celsius to avoid thermalisation, which is widely regarded as the only possible fate for low-entropy states in interacting quantum systems at finite temperatures.

Thermalisation describes what happens when a well-organised quantum state becomes more chaotic over time, losing its structure and with it the quantum information stored in it. Recent experimental observations in spin chains produced surprising results incompatible with this commonly accepted picture. It is now understood theoretically that there exist idealised conditions on the Hamiltonian of a quantum system that give rise to a subspace of quantum states, known as quantum many-body scars, which don't thermalise and can instead preserve the initially stored quantum information indefinitely. Theoretical studies indicate that a broad class of pure solid-state materials are suitable for hosting quantum scars. As these exotic quantum states arise at relatively high temperatures, they may be less fragile and more practical for some future quantum computing systems.

Theoretical physicist Kiryl Pakrouski and Patrice Kolb, who embarked on a PhD in the Department of Information Technology and Electrical Engineering following his physics master's degree at ETH Zurich, wished to gather more information on what happens to quantum scars when they form in real-world conditions, that is, when the host solid-state material is perturbed. Their findings, published in PRX Quantum at the end of 2023, show that there are perturbations – applied magnetic fields, for instance – that don't affect some quantum many-body scars in the fermionic systems they consider. When quantum scars are affected, however, the researchers propose new quantitative measures that should help to characterise the extent to which the scars' stability falters. Kolb and Pakrouski hope that their results provide useful guidance for experimentalists, thus facilitating further experimental investigations of quantum scars in various solid-state systems.

This news article is based on a summary written by Mr Kolb and Dr Pakrouski for the American Physical Society, the publisher of PRX Quantum.