How to physically understand 'qudit'? I've heard that a qudit is a unit of quantum information described by a superposition of $d$ states (more general than qubit). However, I haven't found a formal introduction about what it is and how it might be more powerful than qubit. Also, we know that the two states $|0\rangle$ and $|1\rangle$ in qubit could correspond to spin-up and spin-down of electrons, is there a way to physically understand qudit? Could all the information in our universe be expressed in qudits? Thanks!!
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I haven't found a formal introduction about what it is

You obviously have:

a qudit is a unit of quantum information described by a superposition of $d$ states

That's basically it, really.


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how it might be more powerful than qubit.

If you have a qudit, you can choose to work exclusively with two states, giving you a qubit; as such, qudits are at least as powerful as qubits. Whether they're "more" powerful is ultimately a subjective engineering question, so it doesn't really makemore sense.


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we know that the two states $|0\rangle$ and $|1\rangle$ in qubit could correspond to spin-up and spin-down of electrons

No, we don't 'know' that. The spin of an electron is one possible instance of a qubit, but there are many other different platforms that also have two levels.


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is there a way to physically understand qudit?

As the state space of literally anything else which is restricted to a finite number of states that is bigger than two. This includes e.g. the angular momentum states at spin $s>\tfrac12$, but also path encoding in linear optics and the electronic states of atoms and ions, along with a long etcetera.


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Could all the information in our universe be expressed in qudits?

Yes, in the same way that "all the information in our universe" could be expressed in qubits and those are specific cases of qudits.
