QST, or Quantum State Tomography, is a technique used in quantum mechanics to determine the quantum state of a system. It involves measuring various properties of a quantum system multiple times to reconstruct its state. This process is essential for understanding and manipulating quantum systems, which are fundamental in fields like quantum computing and quantum cryptography. The goal of QST is to provide a complete description of a quantum state, typically represented as a mathematical object called a density matrix. By analyzing the results of measurements, researchers can gain insights into the behavior of quantum systems and improve technologies that rely on quantum phenomena.