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I am trying to understand how are qubits created and implemented in real life. I was unable to find much information about how electrons are kept or sent into a superposition of our wish which is of course a basic need to implement any quantum circuit.

I found that we can use microwave pulses to measure the spin of a electron in the outermost shell of a phosphorous atom kept in a magnetic field. (explained in this video by Veritasium : https://youtu.be/zNzzGgr2mhk?si=oDJht0oKBqpXWPJ5)

But I am not clear how to keep the electron in a particular superposition of our wish. (for e.g.: $1/\sqrt 2 |0\rangle + 1/\sqrt 2 |1\rangle$).

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    $\begingroup$ Doesn't the video broadly explain it at about the 2:21 mark? "If you just make a new gate and stop your pulse at some specific point what you've created is a special quantum superposition of the spin-up and spin-down state with a specific phase between the two superpositions". $\endgroup$ Oct 26, 2023 at 0:28
  • $\begingroup$ @MarkSpinelli Yes it doesn't explain briefly. Sending a particular pulse(say v) will excite electron if it absorbs v, will not excite it if it doesn't absorb v. How can electron be in any in between state? Even duration for which pulse is sent isn't going to effect either as again electron will either get excited or not. $\endgroup$ Oct 26, 2023 at 15:58

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But I am not clear how to keep the electron in a particular superposition of our wish. (for e.g.: $1/\sqrt 2 |0\rangle + 1/\sqrt 2 |1\rangle$).

Consider a Stern-Gerlach apparatus with it's field in the $z$ direction. Such an apparatus splits a single electron beam produced, say, by emission of electrons from a hot filament, into two beams. The two output electron beams are: 1) a beam of "spin up" electrons; 2) a beam of "spin down" electrons. We can get pure spin-up-along-z states, also denoted by $|0\rangle$ conventionally, by just blocking the lower output port of the apparatus and only using the beam from the upper output port. We can get pure spin-down-along-z states, also denoted by $|1\rangle$ conventionally, by blocking the upper output port of the apparatus and only using the beam from the lower output port of the apparatus.

These spin-up-along-z and spin-down-along-z states are the "usual" computational basis states $|0\rangle$ and $|1\rangle$. We are all nodding along and agreeing with this, I'm sure.

OK... So, now go get a second Stern-Gerlach apparatus and point it's magnetic field along the $x$ direction instead of the $z$ direction. The upper output port of such an apparatus will output electrons in the state you want: $$ |+\rangle = \frac{1}{\sqrt{2}}\left(|0\rangle + |1\rangle\right). $$


If you are looking for more detailed information on modern quantum computers (as opposed to single-qubit quantum computers from the 1920s), this might be a useful resource: https://quantumai.google/research/publications

For example, from that above-linked webpage, this publication has some nice pictures of quantum computers and probably the references will tell you more about how to build one: https://storage.googleapis.com/pub-tools-public-publication-data/pdf/45919.pdf

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