Spin qubits for quantum simulations

The investigation of quantum mechanical systems mostly concentrates on single elementary particles. If we combine such particles into a composite quantum system, the number of degrees of freedom of the combined system grows exponentially with the number of particles. This is a major difficulty when we try to describe the dynamics of such a system, since the computational resources required for this task also grow exponentially. In the context of quantum information processing, this difficulty becomes the main source of power: in some situations, information processors based in quantum mechanics can process information exponentially faster than classical systems. From the perspective of a physicist, one of the most interesting applications of this type of information processing is the simulation of quantum systems. We call a quantum information processor that simulates other quantum systems a quantum simulator. This review discusses a specific type of quantum simulator, based on nuclear spin qubits, and using nuclear magnetic resonance for processing. We review the basics of quantum information processing by nuclear magnetic resonance (NMR) as well as the fundamentals of quantum simulation and describe some simple applications that can readily be realized by today's quantum computers. In particular, we discuss the simulation of quantum phase transitions: the qualitative changes that the ground states of some quantum mechanical systems exhibit when some parameters in their Hamiltonians change through some critical points. As specific examples, we consider quantum phase transitions where the relevant ground states are entangled. Chains of spins coupled by Heisenberg interactions represent an ideal system for studying these effects: depending on the type and strength of interactions, the ground states can be product states or they can be maximally entangled states representing different types of entanglement.