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.
Qutrit
The qubit's orthonormal basis states formula_6 span the two-dimensional complex Hilbert space formula_7, corresponding to spin-up and spin-down of a spin-1/2 particle. Qutrits require a Hilbert space of higher dimension, namely the three-dimensional formula_8 spanned by the qutrit's basis formula_9
, which can be realized by a three-level quantum system. Note, however, that not all three-level quantum systems are qutrits.