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Keio University

Skip to 0 minutes and 4 seconds Hi. We are here in the laboratory of Professor Akira Furusawa at the University of Tokyo. Professor Furusawa, thank you for joining us today. Sure. So the first thing we want to talk about, professor Furusawa’s group is one of the leading groups on the planet doing quantum computing using optical states, using photons and light. So first, tell me what’s a photon? Uhm, that is a tough question but, uh, first of all it is particle of light. Okay. And also, uh, light is wave. Um-hmm. So, it is rather complicated. Um-hmm. So a light has a nature of particle… Um-hmm. …and also wave. Just like other types of the quanta that we have been looking at so far. Okay.

Skip to 0 minutes and 59 seconds And it also has a wavelength light, so it has a certain energy? Yeah. Okay. Right. Alright, so you are working with individual photons. One photon at a time… Right. …being a qubit. Okay. So how do you represent a qubit using a photon? How do you make data with it? There are two types. One is called a polarization qubit. In that case we use a single photon. Okay. And we use two polarization. Two polarizations. One is horizontal and another one is vertical. Okay. And Another type of qubit is called time-bin qubit. In that case, we use two pulses of light and single photon. So in that case, single photon exists in first pulse or a single photon exists in second pulse.

Skip to 2 minutes and 11 seconds Okay. So that’s super position. Okay. So if it’s polarization that means that the light is vibrating in a particular direction? So either vertically or horizontally. Right. So that we might use this as our zero state and this as our one state… … or maybe the other way around. Yeah. And if they are time-bin photons, let’s see, so that means that a photon can either arrive early or it can arrive late. Right, yeah. So you might say the early state is your zero state… And the late state is your one state. Something like that. Is that correct? Yeah. Okay. And they run, in that case they run through the same physical path. They run through the same part of your system.

Skip to 2 minutes and 56 seconds Yeah. Same light beam. Okay, great. They are part of the same light beam. So I think that gives you an idea of what the state variable for photonic systems is, working with individual photons. Thank you. Sure.

Photons: our first state variable

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 Einstein’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.