Charge qubit
In quantum computing, a charge qubit is a qubit whose basis states are charge states (e.g. states which represent the presence or absence of excess Cooper pairs in the island). In superconducting quantum computing, a charge qubit is formed by a tiny superconducting island (also known as a Cooper-pair box) coupled by a Josephson junction to a superconducting reservoir (see figure). The state of the qubit is determined by the number of Cooper pairs which have tunneled across the junction. In contrast with the charge state of an atomic or molecular ion, the charge states of such an "island" involve a macroscopic number of conduction electrons of the island. The quantum superposition of charge states can be achieved by tuning the gate voltage U that controls the chemical potential of the island. The charge qubit is typically read-out by electrostatically coupling the island to an extremely sensitive electrometer such as the radio-frequency single-electron transistor.
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.