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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.: $1/\sqrt 2 |0\rangle + 1/\sqrt 2 |1\rangle$).

Mark Spinelli
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Dhanush Sai Baswa
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1 Answers1

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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

hft
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