Nielsen explains how a search algorithm can access a classic database. I have a few questions. I hope you can help me a bit :) I work with a few quotes from the book.
The principle of operation is a means by which the binary encoded state of the quantum index (where $0$ to $2n − 1$ is represented by n qubits) is translated into a unary encoding (where $0$ to $2^n − 1$ is represented by the position of a single probe within $2n$ possible locations) which addresses the classical database.
Ok, if I understand that correctly, the quantum index register is translated into a unary system that specifies the position in the classic database. What I understand less is this statement: "represented by the position of a single probe within 2n possible locations" what is being said with "a single probe within 2n possible locations"?
The database effects a change on a degree of freedom within the probe which is unrelated to its position.
What is the "probe"? And what is behind "degree of freedom"? The sentence is not clear to me.
Each circle represents a switch, addressed by the qubit inscribed within.
OK, so the qubit controls the switch and thus the route.
The data register qubits enter at the top of the tree, and are routed down to the database
How does that work exactly? So where is the connection with the switches from earlier?
The binary to unary encoding is then reversed, leaving the data register with the desired contents.
That is, the entries in the database affect the qubits that penetrate the schema. So that the qubits can then accept their content?
The qubits are then routed back into a deﬁnite position
How does the way back work?