Scaling up the number of these things allows the controlling of noise-based error below a certain threshold according to a theorem proved by Or and Aharonov. Rapid phase shifts of these things can prevent their amplitude evolution in a procedure called "bang-bang control." Two paths through a Mach-Zender interferometer are used to distinguish states of these things in time-bin encoding. Superdense coding is a procedure for sending one of these things after forming a (*) Bell pair. These things are a superposition of the ket of zero and the ket of one, which allows unitary transformations of these things to be represented as rotations on a Bloch (block) sphere; those unitary transformations of them include the Hadamard and C-NOT gates. The fact that these things can be entangled allows them to perform several calculations simultaneously. For 10 points, name these 
Scaling up the number of these things allows the controlling of noise-based error below a certain threshold according to a theorem proved by Or and Aharonov. Rapid phase shifts of these things can prevent their amplitude evolution in a procedure called "bang-bang control." Two paths through a Mach-Zender interferometer are used to distinguish states of these things in time-bin encoding. Superdense coding is a procedure for sending one of these things after forming a (*) Bell pair. These things are a superposition of the ket of zero and the ket of one, which allows unitary transformations of these things to be represented as rotations on a Bloch (block) sphere; those unitary transformations of them include the Hadamard and C-NOT gates. The fact that these things can be entangled allows them to perform several calculations simultaneously. For 10 points, name these simplest units of information in a quantum computer.
Scaling up the number of these things allows the controlling of noise-based error below a certain threshold according to a theorem proved by
qubit [or quantum bit; prompt on "quantum computer" until mention; prompt on "quantum states"]