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The idea is to realise that classical physics is a limiting case of quantum physics. The limits (usually) are: Number of particles, $N \to \infty$ Degrees of Freedom (DoF), $\omega \to \infty$ There are various other ways to think about it, but these are sufficient. Now, the case of a transmon circuit being a quantum system comes from the low temperatures ...


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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 main panel three wavefunctions corresponding to the first three eigenstates of harmonic $LC$ oscillator (not a transmon, but close enough for the purpose of ...


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I recommend checking out this reference if you haven't already, specifically page 28. Basically, starting from the classical Hamiltonian of the circuit described in figure 12, we see that the classical external voltage drive $V_d(t)$ ends up scaling the circuit charge variable $Q$, which, when quantized, can be expressed as the operator $Q \rightarrow \hat{...


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"If we tried to use a 20-qubit computer to calculate the ∼280 keys in the 1024-bit RSA keyspace, how long would it take?" That depends on the speed of the gates, but you are unlikely to do anything useful associated with RSA cryptography with only 20 qubits, and you are even less likely to do anything useful with IBM's gate and measurement ...


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I do not know the reliability of this site and these authors: The Next Decade in Quantum Computing—and How to Play 2018 Philipp Gerbert and Frank Ruess In the article they provide an extensive summary (2018) of the different technologies and gate fidelities, see exhibit 7:


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An honorable mention might go to Bennett, Brassard, and friends' prototype machine for implementing the BB84 protocol, built in the late '80s and described in Scientific American in 1992. In his article on the history of quantum cryptography, Brassard states: Essentially without any special budget allocated to the project, we were able, in late October 1989,...


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