How do qubits interact to form, say, CNOT gates? There are very few comprehensive explanations of how quantum computing works. They all use obscure metaphors that don’t make sense (like Schrödinger’s cat). And even then, they never show the rest of the story, with how the qubits interact.
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5$\begingroup$ Please try to be more specific. You started by (what seems to be) a question on how the CNOT gate works; ending with a wide general intuition for quantum mechanics. $\endgroup$– Daniele CuomoCommented Sep 17, 2023 at 13:55
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1$\begingroup$ There are many comprehensive explanations of how quantum computing works, of varying quality. This site has many pointers to lectures and books. I've found SandboxAQ to be particularly patient. To your question, depending on the technology one fires lasers at ions or pulses microwaves at superconductors or otherwise applies some other control to the qubits, to perform the specific gates of interest. "Gates" and "wires" are holdover terms so don't think of them as transistors or etchings of metal or anything. $\endgroup$– Mark SpinelliCommented Sep 17, 2023 at 14:03
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$\begingroup$ @MarkSpinelli I said there were few comprehensive explanations, not that there were none. $\endgroup$– WhatsYourIQ192Commented Sep 18, 2023 at 12:01
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1$\begingroup$ Dear @WhatsYourIQ192 I'm sorry if you perceived my comment as a bit snarky. But I disagree with your assertion that there are "very few comprehensive explanations of how quantum computing works", and gave you one nice lecture series that I like, that doesn't refer to Schrodinger's cat. I also gave you a suggestion about how quantum gates work. You can edit your question otherwise to clarify if you'd like; people here do like volunteering to help. Otherwise, good luck! $\endgroup$– Mark SpinelliCommented Sep 18, 2023 at 13:03
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1$\begingroup$ does quantumcomputing.stackexchange.com/q/1262/55 answer your question? $\endgroup$– glS ♦Commented Sep 18, 2023 at 15:11
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1 Answer
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You may be aware that a classical NAND gate operates with two pullup transistors in parallel while concurrently operating with two pulldown transistors in series, to pull up the output signal iff the two inputs are grounded. But, these transistors are physical and built in silicon to realize the NAND gate. While, in quantum computing most of the time the quantum gates are more akin to pulses of electromagnetism and are not (necessarily) physical structures but are more akin to opcodes.
That is, depending on the modality of the qubits (e.g., how they are implemented), quantum gates are generally sequences of electromagnetic pulses applied to the qubits that induce changes to the wavefunction that the qubits represent. Two of the most popular modalities are superconducting qubits and ion-trap qubits - the former uses microwave pulses while the latter uses lasers fired at individual ions.
Because you asked about the CNOT gate in particular, I would point you to the Cirac–Zoller CNOT gate. By carefully controlling various properties (polarization, etc.) of the laser pulses one can change the state of one of the ions - to move it into various superpositions of a ground state and an excited state. One can also have the two-qubit CNOT gate realized by acting concurrently on two of the qubits.
The laser pulses induce the coupling between ions - so that one ion will be flipped from the excited state to the ground state (or from the ground state to excited state) iff another ion is in the excited state.
answered Sep 19, 2023 at 20:03
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$\begingroup$ So if the CNOT gate entangles qubits, is entanglement purely conceptual (like you know the state of qubit B because of qubit A) or does it persist after the logical operation? (like after you determine the state of qubit B it continues to change based on qubit A) $\endgroup$ Commented Sep 22, 2023 at 11:20
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$\begingroup$ Entanglement persists (or rather, is created) after the CNOT, and will last up until you measure one of the qubits. $\endgroup$ Commented Sep 22, 2023 at 11:40
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$\begingroup$ Note that you may not be the one to measure the qubits to break the entanglement; any interaction with the environment likewise has the same effect as stopping the entanglement. This is the theory of decoherence. $\endgroup$ Commented Sep 22, 2023 at 12:28