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.
Bell state
In this example, Alice is trying to communicate two bits of classical information, one of four two bit strings: formula_13or formula_14. If Alice chooses to send the two bit message formula_15, she would perform the phase flip formula_16to her qubit. Similarly, if Alice wants to send formula_17, she would apply a CNOT gate; if she wanted to send formula_14, she would apply the formula_19gate to her qubit; and finally, if Alice wanted to send the to bit message formula_20, she would do nothing to her qubit. Alice performs these quantum gate transformations locally, transforming the initial entangled state formula_21 into one of the four Bell states.