A qubit is a quantum bit, the basic unit of quantum information. A qubit can be in a state of 0, 1, or a superposition of both. The state of a qubit can be represented on the Bloch sphere, a sphere that represents all possible states of a qubit. The North Pole of the sphere represents the state |0>, the South Pole represents the state |1>, and any other point on the sphere represents a superposition of |0> and |1>. The Hadamard gate (H) is a quantum gate that transforms a qubit from a definite state (0 or 1) into a superposition state, where the probabilities of measuring 0 or 1 are equal. The X gate (Pauli-X) is a quantum gate that flips the state of a qubit, i.e., it transforms |0> to |1> and |1> to |0>. The CNOT gate (Controlled-NOT) is a two-qubit gate that flips the target qubit if and only if the control qubit is in the state |1>.