A Quantum Simulator is a controllable quantum mechanical system than can be used to study more complex quantum system that cannot be directly studied nor simulated by any classical computer.
One of the most important consequences of the existence of entanglement in quantum mechanics is related to the quantity of information necessary to describe the state of a quantum system. In a classical system, the number of parameters needed to characterize its state is equal to the sum of the degrees of freedom of each of its components. However, for a quantum system, the number of parameters needed to describe its state grows exponentially with the number of constituents. And this makes the task of simulating a quantum system a very difficult one even for systems with few constituents, the computational resources needed for the simulation quickly become unreachable using computational techniques based on classical computers.
In 1982 Nobel laureate Richard Feynman conjectured that these limitations could be overcome using computers based on quantum systems. Based on this conjecture, physicists started exploiting the potential of quantum mechanics to develop new quantum computers (universal quantum simulators) capable of simulating quantum systems.
Nowadays there are several quantum systems than can be used as hardware for quantum simulators: trapped ions, cold atoms in optical lattices, liquid and solid-state NMR, photons, quantum dots, superconducting circuits, and NV centres. 
All text copyright © Marco Vinicio Sebastian Cerezo de la Roca.
The More You Know: Quantum Simulator by Marco Vinicio Sebastian Cerezo de la Roca is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.