# How do we physically initialize qubits in a Quantum register?

In quantum algorithms we need to initialize the qubits at the start of our algorithm in some quantum register. Suppose that if we are working with a four qubit quantum register we can initialize the register into values such as $$|0000\rangle$$ or another value such as $$|0101\rangle$$, which means that the first and third qubits are in basis $$|0\rangle$$ state and the second and fourth qubits are in basis $$|1\rangle$$ state. Once we have initialized these qubits as such we can then proceed to apply various quantum gates on them.

My question is that in a physical quantum computer (not a simulator) where say we represent qubits with electron spin, how do we manupulate such electrons so that we can initialize a Quantum register, say with the value $$|0101\rangle$$?

Well the procedure depends on the physical system that you are using to realize the quantum register. An example I can give is of Diamond NV-Center. In simplest case, the vacancy electron is modelled as spin-1 particle here(there are reasons to model it like this) and nitrogen-14 nuclei has spin 1. So total system lies in a Hilbert space of 9-Dimensions or you can say there are 9 energy levels. Lets say you want to realise a 2-qubit register, then you would need 4-energy levels. Define the levels as $|00\rangle$, $|01\rangle$, $|10\rangle$ and $|11\rangle$. Lets say in equilibrium system is in $|00\rangle$ state, if you want to initialize the system in $|01\rangle$ state then all you would need is an electromagnetic pulse of energy being equal to the energy difference of $|01\rangle$ and $|00\rangle$ levels. In NV-Center, normally microwave and radio-frequency pulses are used.
Let's say you have a photon that is vertically or horizontally polarized. There are physical devices to create and measure polarizations of these photons. So, you can mark the horizontally polarized photons as the state $\lvert 0 \rangle$ and the vertically polarized one as the $\lvert 1 \rangle$ state.
Additionally, you could use photon polarizer to change a particular photons polarization to turn a $\lvert 0 \rangle$ to a $\lvert 1 \rangle$ or vice versa. This wiki article would help you a bit. Moreover, you could check out this cool video from professor Allan Adams where real life photon polarization is demonstrated. (link)