Introduction

Gauge–string duality is arguably one of the most profound problems in modern physics [1–11]. This duality implies that the gauge-invariant observables (operators) in QCD are in one-to-one correspondence with the physical states (vertex operators) in string theory. The reason why extended objects (such as strings) appear in QCD is quite natural. If we recall standard electrodynamics, there are two ways of describing it: either in terms of local electric fields (Coulomb's approach), or in terms of the geometry of electric field lines (Faraday's approach). In the case of electromagnetic theory, Coulomb's approach turns out to be far more efficient. In the case of QCD, however, things are quite different. While the electric field lines created by the charged particles are spread over the entire space, the gluon field lines are confined to thin flux tubes. These flux tubes, connecting quarks, can be naturally interpreted as one-dimensional extended objects, known as QCD strings.