For the NV center, nature provides us with a quantum two-level system that we exploit to make it operate as a qubit. However, this is not the only approach to realize a qubit. In this video, prof. Leo DiCarlo presents about the superconducting qubit, more specifically the superconducting - or transmon - qubit, which they study at his research group in QuTech.
Superconducting qubits generally consist of superconducting islands that are interconnected by Josephson junctions. These form a multilevel system, which is fundamentally different from the quantum two-level system of the NV center, as you will see in the video!
- The NV center qubit gives a more general introduction to qubits.
- Superconducting islands
- Josephson junctions
- Quantum observables
- Classical LC circuits
- Simple harmonic oscillator
So for a specific parameter space, the behavior of these superconducting circuits can be simplified to a harmonic oscillator (which operates as a two-level system). Simplifying a complex system to a well-known, smaller system is crucial in many other fields of physics. Can you think of another example?
These superconducting qubits were used in the quantum computer used by Google to claim the first quantum supremacy!
Additionally, this review paper by W.B. Oliver and P.B. Welander provides a good overview of the different kinds of superconducting qubits.
Also cool to see is the first theoretical paper by J. Koch et al. suggesting these qubits might have exciting properties, back in 2007.