Quantum spin liquids (QSLs) are long-range entangled states of matter of billions of interacting qubits or spins that develop in a Mott insulator. Remarkably QSLs harbor fractionalized excitations rather than the conventional spin waves of ordered magnets that carry integer units of angular momentum. In my talk I will identify detectable signatures of these fractionalized excitations in pump-probe spectroscopy. These fractionalized excitations are promising candidates to create logical qubits for quantum computation.