Construction of Instantons via Tachyon Condensation

We investigate the D-brane bound states from the viewpoint of the unstable D/\bar{D}-system and their tachyon condensation. We consider two systems; a system of k D(-1)-branes and N D3-branes with open strings connecting them and a system of N D3-branes with open strings corresponding to the k-instanton flux, both of which are realized through the tachyon condensation from (N+2k) D3-branes and 2k \bar{D3}-branes with appropriate tachyon profiles. It can be shown that these systems are related with each other through a unitary gauge transformation of the D3/\bar{D3}-system. We construct an explicit form of the gauge transformation and show that the essential elements of the ADHM construction naturally arise from the explicit form of the gauge transformation. As a result, the ADHM construction is understood as an outcome of this gauge equivalence in different low energy limits. The small instanton singularities can be also understood in this context. Other kinds of solitons with different codimensions are also discussed from the view point of the tachyon condensation.