Simulation of collective effects between the qubits of a quantum computer

An international team, involving researchers from IFISC (UIB-CSIC) in collaboration with the University of Helsinki, Aalto University and the startup Algorithmiq (Finland), has successfully simulated the collective and dissipative dynamics of two qubits in a real quantum computer. These results may pave the way for quantum simulation of more complex collective dynamics on currently available quantum computers and establish a procedure for comparing the results of quantum simulations with the noise properties of experimental devices. The research represents the first fully quantum digital simulation of dissipative collective effects on a quantum computer.

The study, published in the prestigious journal PRX Quantum, consisted of simulating the dynamics of quantum systems with the smallest possible dimension and which form the basis of quantum computing: qubits. By simulating a global bath between two qubits, the researchers were able to see how their emissions interfere, both constructively (superradiance) and destructively (subradiance). These two qubits form a structured quantum system whose dynamics is "open" and "collective". The researchers also studied theoretically and experimentally the properties of the noise and established relationships between its characteristics and the accuracy of the simulation. Current quantum computers are inevitably noisy, constrained by short coherence times. This means that there are strong constraints on the depth of quantum circuits that can be implemented.

The concept behind quantum simulation is based on the idea of simulating quantum systems on a controllable physical platform whose dynamics are driven by the laws of quantum mechanics. This makes it possible to explore and obtain solutions to quantum dynamics that would otherwise be impossible to obtain on a classical computer. Controlling these quantum simulations is crucial to understand the properties of noise in real quantum computers and to explore intriguing phenomena such as dissipative quantum phase transitions, quantum synchronization or dissipative time crystals. Furthermore, characterizing the noise in quantum computers can help to understand the limitations of the devices and thus to design possible countermeasures.

Image: Schematic of the simulated system. A particle generator (gray) collides a particle with the qubit Q1 (green), then with Q2 (red) and again with Q1. These collisions generate a global interaction represented by the cloud (blue).

Cattaneo, Marco, et al. “Quantum Simulation of Dissipative Collective Effects on Noisy Quantum Computers.” PRX Quantum, vol. 4, no. 1, 2023, https://doi.org/10.1103/prxquantum.4.010324.