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The treatment of unused qubits is far from trivial, e.g. Shor requires "to uncompute" them - I wanted to ask what happens with qubits that are not measured in superconducting QC?

If I understand properly, in superconducting QC due to extremely low temperature we can assume the initial state is prepared as the ground state $|0\rangle$, then there is unitary evolution, and finally there is actively performed readout through coupling with additional resonators (readout/Purcell)?

But what happens with qubits for which we don't perform such readout? Looking from perspective of CPT symmetry, this extremely low temperature as mean molecule energy is the same, suggesting such no-readout qubits should be also fixed to the ground state, especially that there is no energy to excite it (in readout provided through coupling)?

So can these no-readout qubits be viewed as enforced to the ground state? (postpared to $\langle0|$)

ps. Example from IQM Spark article:

enter image description here

Update: diagram with 2x2 basic examples to test/undestand: enter image description here

Jarek Duda
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Qubits, in any realization, including superconducting qubits, decay to their ground state over time. Over a period $t$ which is larger than their "lifetime" known as $T_1$, i.e. when $t \gt\gt T_1$, a given qubit surely will be found in its ground state ($|0\rangle$ if you wish) upon measurement. So if you don't measure qubits in less than $t$ time, they will surely decay to their ground state.

In the broader context of quantum computation, if you have created entanglement between qubits, then the decay of qubit $q_1$ which is entangled to $q_2$ will surely affect their combined entangled state. Careful uncomputation can indeed resolve this issue, in the case where auxiliary qubits were used as a workspace and their state is not a matter of interest anymore. If entanglement wasn't created between qubits $q_1$ and $q_2$, then the state of $q_1$ shouldn't affect the state of $q_2$.

If we dig deeper into the actual realization of QPUs, of any modality, there are always unwanted (hopefully small) physical couplings between different qubits (and other components) of the QPU (to create a desired entanglement we engineer desired physical coupling between qubits). So it is plausible to state that even unentangled qubits affect each other due to these physical couplings, hence it is probably a good idea to reset unused qubits to their stable $|0\rangle$ ground state.

Ohad
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