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Is it true that a single qubit is of no good use and all the cool things about quantum computing come to life only with a computer with a set of qubits?
or a set of entangled qubits...

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    $\begingroup$ Hi @BeetranDhiya, welcome to QCSE. I think your question is nice, but it's very similar to another one here. I suggest you review that question (and the answers), and if you still have other questions, maybe edit your posting to distinguish over that question. $\endgroup$ Commented Oct 13, 2022 at 13:49

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In theory a single qubit can be used to generating true random numbers (more like a true coin flip). By using a Hadamard gate on a zero ket. This takes the qubit into superposition. After applying the Hadamard gate, we measure the qubit and in theory it is 50% 0 and 50% 1.

But I don't think the physical implementation does support this right now.

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As you pointed out, an entanglement of qubits is necessary to fully exploit power of quantum computers. Naturally you need at least two qubits to prepare the entanglement.

What is more, to simulate one qubit on classical computer, you need to carry out matrix multiplication with 2x2 matrices and two-member vectors. This is pretty quick and there is no need to actually use a real qubit.

However, a single qubit can be used in development of quantum computers. As the qubit is a building block of a quantum processor, detailed knowledge of its behavior would be priceless for quantum hardware developers.

And of course, single qubit processors are used for testing and educational purposes. Until recently IBM provided single qubit QPU called Armonk. You could program the qubit on microwave pulses level. Now this processor is retired and it was replaced by multi-qubit QPU with the same functionality. But you had to start somewhere...

Lastly, as highlighted in the other answer, a single qubit is a generator of real random numbers. In comparison, classical random generators are pseudorandom one. Of course, there are physical random generators in classical computers as well. You can for example measure thermal noise in a bus. But qubits expand this arsenal of physical random generators.

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