Physical and logical qubits
In quantum computing, a qubit is a unit of information analogous to a bit (binary digit) in classical computing, but it is affected by quantum mechanical properties such as superposition and entanglement which allow qubits to be in some ways more powerful than classical bits for some tasks. Qubits are used in quantum circuits and quantum algorithms composed of quantum logic gates to solve computational problems, where they are used for input/output and intermediate computations.
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.