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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.




状態変数としての光子

量子コンピュータを開発していくに当たって、まず最初に考えなければならないのはどの種類の状態変数を採用すべきかということでしょう。この動画では光子を用いた量子ビットの研究で世界をリードする東京大学の古澤明教授の研究室にお邪魔しています。

状態変数とはある種の物理現象をデータに変換する方法をさします。古典コンピュータのチップでは電線上の電圧の状態に基づいてトランジスターのオンオフを切り替えます。例えば仮に3.5ボルトを1、0ボルトを0とすると、二進数によって様々な数値を表現することができるようになります(二進数については量子レジスターや量子アルゴリズムの話の中で扱ってきました)。では量子データの場合どのような物理現象を利用すれば良いのでしょうか。

光子は我々が知りたい物理現象を理解するのに最も適した存在かもしれません。高校物理や一般的な科学についての知識ですでに光子について詳しい方もいらっしゃるかもしれませんが、光子というのは光が持つ最小単位の粒子だと考えてください。原子はこの光子の吸収放出をしており、この現象が電気を作るというアインシュタインの理論によって太陽電池や量子力学は発展してきました(アインシュタインはこの光量子仮説でのちにノーベル賞を受賞しています)。

光は粒子としても波としても振る舞うというのを聞いたことがある方もいらっしゃるかもしれませんが、この現象こそが量子力学の核心部分であり光を利用した量子コンピューティングを可能にしているのです。ここでわかるのは光子を用いることで、量子コンピューティングにもいくつかのデータをエンコーディングする方法があるということです。この動画では光子の偏光時間位置がどのように量子ビットとして利用されているのかを紹介しています。

また、次のステップからは、他の物理現象(電子のスピンや超伝導体の電流、1原子の状態など)がどのように量子ビットとして表現されているのかを見ていきたいと思います。

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Understanding Quantum Computers

Keio University