3.3

## Keio University

Skip to 0 minutes and 3 secondsHi. In this step, we are here with Professor Kohei Itoh from Keio’s Faculty of Science and Technology. Hey, nice to see you again. Thank you. Nice to see you. Tell me what is spin? Spin! Okay it’s a – spin is like a tiny magnet. Okay And so each, for example electron has, is also a tiny magnet and magnetic, direction up or down can be defined if the electron spin is placed in the magnetic field. So spin up means that it’s pointing in the direction of the field. And the spin down means the other direction. And up and down can be, you know, uh, can be regarded as 0 and 1 state or 0 or 1 state, of bit number.

Skip to 1 minute and 1 secondSo we can use the spin of an electron as our qubit, as our 0 state or 1 state. That’s our state variable. And we can also do the same thing with the spin of a nucleus? So nuclear spin or nuclear nucleus also as a spin 0 up state or 1 down state. So we can use that as well. I see, so that gives us two different kinds of state variables.

# Electron spin

Electrons have a characteristic known as spin. Spin has a particular direction, and it creates a magnetic field; if a large number of electrons have their spins pointing in the same direction, they create a large magnetic field and you have a macroscopic magnet. In this video, Professor Kohei Itoh of Keio University explains how the spin of individual electrons can be used as qubits.

For a single electron, the spin can be either aligned with the surrounding magnetic field, which we call spin up, or anti-aligned with it, which we call spin down. We can write these states using the ket notation we introduced when we talked about entanglement and Bell pairs, $|\uparrow\rangle$ and $|\downarrow\rangle$. We then assign one of these to be our $|0\rangle$ state and the other to be our $|1\rangle$ state, so that we can use them as variables.