Cristián Tabares (CSIC-IFF)

Quantum computers and simulators are expected to provide new ways to study the properties ofcomplex quantum circuits. Since they are still prone to errors in the calculations, Variational Quantum Algorithms (VQAs) have emerged as a possible practical way to exploit current state-of-the-art quantum devices. These algorithms propose using a classical optimizer to train a parametrized quantum circuit (PQC), hence finding a way to exploit the limited quantum resources. However, most VQAs have been designed for fully digital approaches, in which the error ends up accumulating for circuits with many parameters. A possible way out is the use of analogue quantum simulators (AQS) instead, that allow a global evolution of the system and are more resilient to errors. For this reason, they have been recently pointed out as one of the most promising directions to achieve “practical quantum advantage”. However, current proof-of-principle demonstrations with trapped ions or cold atoms, like it occurs with fully digital VQAs, are ultimately limited by the connectivities that can be achieved with these devices.

In this seminar, I will discuss a variational AQS inspired by the tunable range interactions that can be obtained in waveguide-QED platforms. To this end, first I will motivate the topic of quantum simulation and introduce some of its possible approaches, including the variational ones. Then I will focus on a particular type of analogue quantum simulator, quantum emitters coupled to a waveguide, showing how some of the ideas typically considered in quantum optics can be used to build these devices. Finally, I will show that taking the range of the interaction as a variational parameter, one can design a novel class of PQCs with optimized connectivities that are hardware-inspired. These results highlight the potential of variational waveguide-QED quantum simulators as a promising platform for implementing quantum algorithms.