This introductory chapter is aimed at adopting a wide view on the physics steps that create the “artificial atom”.

We ask:

• Why is a cavity bounded by superconducting walls even a circuit?
• What makes this circuit quantum and describable by a macroscopic quantum wavefunction?
• What really is the degree of freedom that we create and manipulate in a superconducting qubits?

We will follow Devoret’s description throughout Ref[1] of the plasmon excitation being a single degree of freedom resonating in a real metal, motivating our ability to quantize a Hamiltonian of a macroscopic circuit.

Essentially we try to provide some new elements on this question of how we can have a reduction of the number of degrees of freedom from Avogadro’s number to essentially 1.

We are setting up one degree of freedom in a metal:

Can a collective variable involving billions of atoms be as perfectly quantum as a single atom?

this essentially reformulates the key question of John Martinis’s PhD thesis in the early 1980s, “Do macroscopic variables obey quantum mechanics”, as referenced here in the following talk: [2].

The Nobel Prize has rewarded Martinis’s contribution as foundational to the field of quantum devices today. Martinis redid his thesis experiment years later at Google and demonstrated that there is quantum supremacy. Quantum computation is possible on a very large Hilbert space $(2^{53}=10^{16} \text{ states})$

Martinis work built on fundamental work on macroscopic quantum phenomena, it was among others A.J Leggett’s , Ref [3 ] , discussion on how an electrical circuit is quantum as well as the following paper, Ref [4**],** on the quantum network theory of electric circuits. This set the context for Martinis’s PhD Thesis.

“The idea is that we can take an electrical circuit which is represented by a circuit diagram and then we do quantum mechanics with it. So it means that we are going to quantize the degrees of freedom of the circuit. What are the degrees of freedom of a circuit? How do you find them? What does it mean to quantize or control them?” - Devoret May 2025 quantum design workshop introductory lecture: