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### Would an ambient-pressure, room-temperature superconductor eliminate the need for a dil-fridge in transmon processors?

TL;DR: The reason why some qubits need to work at very low temperature is to avoid noise which would make quantum gates having poor fidelity. For this reason, a room-temperature superconductor would ...

### Could you use $|0\rangle$ as excited state and $|1\rangle$ as ground state in transmon?

It would not make any difference whatsoever, as they are simply labels. "Red" and "blue" are equally valid names to give to qubits, albeit clunky ones. A rose by any other name ...
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### In a transmon qubit, what corresponds to the ground and excited states?

The transmon is a slightly nonlinear $LC$ resonator, with a resonance frequency of a few GHz. The image below, from Bardin et al., IEEE Microwave Magazine, 2020 (of which I am an author), shows in the ...
• 388
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### Why is there a frequency difference when performing ramsey experiment with setting neighbor qubits to different state?

the difference in frequency is precisely because the interaction is always on. Let's start with a scenario where there is no ZZ interaction. In this case, the Hamiltonian during the free evolution ...
• 1,220
1 vote
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### What is the charge noise in the context of superconducting qubits?

For a complete answer (and much more!), see Charge insensitive qubit design derived from the Cooper pair box What is charge noise ? The energy of the capacitor in the transmon is $4 E_C (n-n_g)^2$ ...
1 vote
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### Frequency range of superconducting qubit

The transmon frequency $f_t$ is linked both to the charging energy $E_C$ and the inductive energy $E_L$ according to the formula: $$f_t = \sqrt{8E_JE_C}.$$ where $E_C = e^2 / 2C$ and \$E_L = (\phi_0 /...

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