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I was following the Qiskit textbook:

https://qiskit.org/textbook/ch-quantum-hardware/calibrating-qubits-pulse.html

In this, there is a statement as indicated below:

With superconducting qubits, higher energy levels are also available, but we fabricate the systems to be anharmonic so that we can control which transition we are exciting. That way, we are able to isolate two energy levels and treat each qubit as a basic two-level system, ignoring higher energy states.

I understand that there are higher energy levels available, but how are we controlling which transition we are exciting by fabricating the system to be anharmonic?

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1 Answer 1

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Superconducting qubits are a type of quantum resonator. They are fabricated to be anharmonic so that the energy difference $\hbar \omega_{01}$ between the two lowest levels $|0\rangle$ and $|1\rangle$ is different from the difference of any other pair of energy levels, letting us treat it as a two-level system to implement a qubit.

Specifically, we can use microwave pulses at $\omega_{01}$ to drive transitions between $|0\rangle$ and $|1\rangle$, giving us 1-qubit operations on a two-level system. If all of the states were evenly spaced, e.g. $\omega_{01} = \omega_{12}$, then we wouldn't be able to make a two-level system/qubit because driving at that frequency would drive transitions from $|1\rangle$ to $|0\rangle$ and $|2\rangle$, and then we'd have a multi-level quantum system (which introduces all sorts of headaches theoretically and experimentally).

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