Qubit
In quantum computing, a qubit () or quantum bit (sometimes qbit) is the basic unit of quantum information—the quantum version of the classical binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, one of the simplest quantum systems displaying the peculiarity of quantum mechanics. Examples include: the spin of the electron in which the two levels can be taken as spin up and spin down; or the polarization of a single photon in which the two states can be taken to be the vertical polarization and the horizontal polarization. In a classical system, a bit would have to be in one state or the other. However, quantum mechanics allows the qubit to be in a coherent superposition of both states/levels simultaneously, a property which is fundamental to quantum mechanics and quantum computing.
Quantum coin flipping
Quantum coin flipping and other types of quantum cryptography communicate information through the transmission of qubits. The accepting player does not know the information in the qubit until he performs a measurement. Information about each qubit is stored on and carried by a single photon. Once the receiving player measures the photon, it is altered, and will not produce the same output if measured again. Since a photon can only be read the same way once, any other party attempting to intercept the message is easily detectable.