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The critical nature of isotopic engineering

We visit Professor Itoh again.
First, 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?
Well 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.
Uh 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.
I 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.

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.





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Understanding Quantum Computers

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