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
Although there are many possible ways to create entangled Bell states through quantum circuits, the simplest takes a computational basis as the input, and contains a Hadamard gate and a CNOT gate (pictured below). As an example, the quantum circuit pictures takes the two qubit input formula_7and transformed it to the first Bell state (1). Explicitly, the Hadamard gate transforms formula_7 into a superposition of formula_9. This will then act as a control input to the CNOT gate, which only inverts the target when the control is 1. Thus, the CNOT gate transforms the second qubit as follows formula_10.