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Skip to 0 minutes and 3 secondsFirst, I have to ask you about your baseball jersey here. This is my – this is my favorite Minor League team in United States, Albuquerque Isotopes. The Albuquerque Isotopes, I see. First tell me what’s an isotope…? Why Isotopes? Yes. What’s an isotope? I were considered from isotope engineering for quantum information processing. Okay. And you know we – silicon is composed of three different isotopes, 28, 29, and 30. And only 28 and actually 28 and 30 do not have any nuclear spins, but 29 have – has nuclear spins. Ah okay. So why is it important that – that whether the atoms have or do not have nuclear spin?

Skip to 0 minutes and 47 secondsWell you know sometimes, 29 nuclear spin can be used as a qubit for quantum information processing, but sometimes we want to use other spins like electron spins for quantum information processing. Then in that case we don’t want any background nuclear spins. So we choose 28 silicon isotopes that has no nuclear spin. I see. So your team works on the isotopic technologies, is that right? Right. Isotope engineering. Isotopic engineering. Okay. So tell me what isotopic engineering is? So you know, as I said, silicon is composed of three different stable isotopes and we receive three different isotopes in a separated form. Separation is done, uh, using the huge centrifuge system somewhere in Russia. Somewhere in Russia? Right.

Skip to 1 minute and 40 secondsUh and then we receive them in a separated form and then make them into pure and also high quality silicon single crystal. That will be useful for quantum, you know information, building quantum computer. Okay. Why is isotopic engineering important in this context of controlling one single electron inside the silicon? So, electron spin is tiny magnet, as I said and nuclear spin is also a tiny magnet. So if I am an electron spin qubit then I want to sit in, in the environment that has no magnetic field around.

Skip to 2 minutes and 19 secondsI want to be sitting in a vacuum so that I can keep my mag – magnetic field direction as I am, but in the case there is actually nuclear spin nearby then I can – I start to feel the magnetic field coming from the nuclear spin that will actually deteriorate my electron spin state. I see. So we want – I don’t want anything around me when I’m trying to keep my state as it is. So your technology and your specialty helps to isolate those… Correct …spins into an environment where they can stay clean and keep good state. Yeah, exactly. Sounds very important.

The critical nature of isotopic engineering

Join us as Keio’s Professor Kohei Itoh explains how his team works to control the isotopic concentration in materials to help several research groups around the world improve the quality of their qubits.

Most elements have several different types of nuclei, known as isotopes. For example, silicon, which is a common material for making computer chips, has three common isotopes, known as silicon-28, silicon-29, and silicon-30. Silicon-28 and -30 are “spin zero”, but silicon-29 is “spin one half”, the same as a single electron.

If we want to use the nuclear spin of silicon as a qubit, we can. However, if we are using electrons as qubits, or current or magnetic flux, then isotopes with non-zero nuclear spin can interfere with the electron state and cause decoherence. We would rather have a “quiet” environment, with no nuclear spins, as the base on which to build our quantum circuits. Reducing the presence of nuclei with spin is critical for several different technologies, such as quantum dots, as it helps improve the memory lifetime of a qubit.




量子情報技術における同位体工学

慶應義塾大学の伊藤公平教授の研究グループで行われている同位体の比率制御により量子ビットの精度を高める技術の研究を紹介します。

多くの元素は、同一の原子番号に対して異なる核種を持っていてこれを同位体と呼びます。例えば、コンピュータ内部の集積回路を作っているケイ素には多数の同位体がありますが、その中でも28Si、29Si、30Siの3種類の安定同位体があります。そのうち28Siと30Siはスピン0ですが、Si29は電子などのフェルミ粒子と同じでスピンが1/2となっています。

そのため電子スピン、電流、磁束などを利用して量子ビットを作ろうとする場合、Si29のようなスピン0ではないケイ素の同位体が電子状態に干渉を引き起こし、デコヒーレンスの原因となってしまいます。つまり、量子回路の基盤の設計にはスピン0で構成された元素の方が向きます。そしてスピンを持つ同位体を減少させる技術は、量子ドットをはじめとする量子ビットの持続時間に関わってくる様々な技術にとってとても重要な働きをします。

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This video is from the free online course:

Understanding Quantum Computers

Keio University