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I'v read published books, articles and papers about Quantum-Computing.

I found that all the materials I've seen are, instead of describing quantum gate from basic physics to abstraction, trying hard to avoid talking about the implementation details of quantum-gates.

I firstly questioned myself: Am I searching in the wrong area where only formal mathematics are concerned?

But I found those papers and books do have explained ion-traps, optical-switches and even laser-fibre in great detail.

When it comes to quantum-gates they claimed to have used in their research, only matrices, equations, formulations and black-box components are shown.

We all know about the unitaries and matrix computation. But if you print a quantum-gate matrix operator on an A4-paper, that ain't going to do anything when photons or electrons are thrown to it.

So, does anyone know what is a quantum gate, concretely so that I can know:

  • Is Quantum Gate a device using magnetic field to do element arithmetics?
  • Is Quantum Gate a device using laser-beam?
  • Is Quantum Gate a device using metal wire?
  • When scientists happily multiply eigenstates by observables, do electrons collide in the gate when "multiplied"?

Thanks in advance.

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You seem to have the idea that a quantum gate is a physical thing rather than just a conceptual thing. It doesn't necessarily work that way. While CMOS gates are usually actual physical devices, quantum gates may be just conceptual.

Consider an ion trap. The ions represent qubits by using one electronic state as a $|0\rangle$ and another as a $|1 \rangle$. A quantum gate is performed by applying a $4 \times 4$ unitary matrix to two of these ions. This is done by shining a sequence of laser pulses on the ions. It's not a physical device into which two ions are input, in which they interact, and out of which the ions come with their states changed.

Quantum gates are performed by shining laser pulses on the ions, but there's not necessarily one laser per quantum gate—the same lasers are used for many different quantum gates. Thus, you can't point to any specific piece of hardware and say "this is the quantum gate".

And if you use flux qubits in superconductors for your quantum computer, the quantum gates don't use lasers at all, and they look completely different (and slightly more like CMOS devices).

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  • $\begingroup$ Thank you so much！I was just wondering the physical fundaments behind gate concept. Now from your answer I know it is laser that actually does the job. That means a lot and helps me out of the illusory mathematics where, even for physical operations, few physics is explained without which I have nowhere to base my mind upon. But I still feel confused about what character of laser leads to the conclusion that matrix must be unitary. If it won't bother, would you please throw a little bit more light on me. Thank you again! $\endgroup$
    – Chiron
    Dec 22, 2016 at 6:32
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    $\begingroup$ All closed systems in physics are unitary — something is non-unitary only if information about the system leaks out. So it's not too difficult to design sequences of laser pulses which perform unitary operations on qubits. Having said that, this is really not something which can be answered without using more physics than belongs in the theoretical computer science SE. $\endgroup$
    – Peter Shor
    Dec 22, 2016 at 14:28

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