# Photons: our first state variable

We visit the laboratory of Professor Akira Furusawa at the University of Tokyo to hear about quantum computing using optical systems.

Our first task in designing a quantum computer is to decide what kind of state variable we want to use. In this video, we visit the laboratory of Professor Akira Furusawa at the University of Tokyo to learn how one of the leading groups on the planet represents qubits using photons.

The state variable is how we turn some sort of physical phenomenon into data. In a classical computer chip, the state is voltage on a wire, used to turn a transistor on or off. We might, for example, define 3.5 volts to be a one, and 0 volts to be a zero. Then, with our two values 0 and 1, we can create any number we need, using binary numbers. (We have already been using binary numbers as we discussed quantum registers and algorithms.) So, what physical phenomenon do we want to use for our quantum data?

Perhaps the easiest physical phenomenon to understand is the photon. You are probably already familiar with photons from high school physics or popular discussions of science. A photon is a particle, the smallest unit of light. You know that atoms absorb and emit photons, and you might even be aware that Einsteinâs explanation of how that can create electricity helped lead to both the solar cell and the development of quantum mechanics (and to Einstein receiving the 1921 Nobel Prize in Physics).

You may even have heard that light sometimes behaves like a particle, and sometimes like a wave; that phenomenon is at the heart of quantum mechanics and allows us to use light for quantum computing. As it turns out, using individual photons we have several ways of encoding our data. In this video, we will see how polarization of a photon or the timing of its arrival, known as time bin, can be used as a qubit.

After this video, over the next several articles and videos, we will see how other physical phenomena, such as the spin of electrons, electrical current in superconductors, and the state of individual atoms can be used to represent qubits.

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