# What does the unitary $[|0ranglelangle 0|otimes I+|1ranglelangle1|otimes(|1ranglelangle 0|+|0ranglelangle1|)]otimes I$ represent?

Quantum Computing Asked on August 15, 2020

Consider the following unitary defined for a system $$A$$ interacting with a bipartite system $$BB^prime$$

$$U_{AB} = Big[|0rangle langle 0|_{A} otimes mathbf{I}_{B} + |1rangle langle 1|_{A} otimes big(|1rangle langle 0|_{B} + |0rangle langle 1|_{B} big) Big] otimes mathbf{I}_{B^prime},$$ with
$$mathbf{I}_i$$ being the identity.

My question:

1. What is the physical meaning of operation $$U_{AB}$$?

2. Can one represent $$U_{AB}$$ in terms of quantum logic gates ( a circuit diagram)?

The gate/operator in the brackets is the non-local CNOT gate, frequently used to create bipartite entanglement. Given it itself is a 2 qubit gate, then the tensor of this with the identity is simply a gate that acts on 3 qubits.

This gate will take a 3 qubit state and flip the second qubit of this state when the first is $$|1rangle$$ It will take $$|110rangle to|100rangle, |111rangle to |101rangle$$ and vice-versa.

Edit: important to note is that $$|1ranglelangle 0|+|0ranglelangle 1|$$ is the Pauli X gate

Answered by GaussStrife on August 15, 2020