# What does a tangible Quantum-Gate look like?

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"?

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.