A quantum computer is a computing device that uses quantum bits (qubits), which can exist in multiple states simultaneously due to superposition and be correlated through entanglement, enabling it to solve certain complex problems much faster than classical computers. 

The journey to quantum computing began with the early developments in quantum mechanics. Max Planck's pioneering work revealed the concept of discreteness at sub-atomic levels. J.J. Thomson's discovery of electrons in the cathode ray tube, followed by Rutherford's description of electrons orbiting the nucleus, laid crucial foundations. An unexpected source of insight came from a bad cigar, which played a role in uncovering the concept of spin as intrinsic angular momentum in electrons (Friedrich & Herschbach, 2003).