Skip main navigation

Charge and magnetic flux/磁荷と磁束

Charge and magnetic flux/磁荷と磁束
3.8
I am here with Professor Yasunobu Nakamura at the University of Tokyo in his laboratory. Professor Nakamura built the first superconducting qubit and now he runs his research group here at the University of Tokyo. Professor Nakamura, welcome. Hello. I want to ask you what are you doing here? Okay, here, we are running experiments on superconducting qubit, so as you see here in our cryostat, we have superconducting qubit devices at the very bottom. So inside this can, there is a vacuum and at the very end the temperature is very low, like 10 millikelvin and superconducting qubit is there and we have all the control wiring to send and read out the signal.
51.4
So we send microwave pulses to the device and then read out the signal coming back from the device through the electronics. So here we have all the control electronics to create microwave pulses and then read out microwave signals, so by doing that, we can control the superposition and entanglement of the superconducting qubit. I see, thank you Professor Nakamura. I want to ask to you how data is represented in superconducting system and what kind of state variable can we use in superconducting system?
84.9
As you know, when electrical circuit becomes superconducting, there is no resistance in the circuit, that is very important for maintaining quantum information in our circuits and if we properly design the circuit with a small Josephson junction, then we can create quantum bits in our system. There, we use either charge or flux degrees of freedom in our circuit. Charge means Cooper pair in superconducting system where two electrons are combined and flux is a quantum of magnetic field, which is confined in superconducting circuit. Okay, what is Josephson junction?
129.9
Josephson junction is a small tunnel junction, which means there is a thin insulating barrier between two superconducting electrodes and there Cooper pairs – a pair of superconducting electrons can tunnel free across the junction and the junction works as a non-linear inductive element which is very important for making Josephson superconducting qubit. Okay, which type of state variable is more popular today? Actually, neither of them.
164.9
So both kind of qubits using charge degrees of freedom or flux degrees of freedom do work properly, but each of them can cover charge noise or flux noise rather strongly, so actually the most popular design these days is kind of in the middle where we use quantum states which is rather insensitive to those noise in the environment. I see. Thank you professor Nakamura.
超伝導体内では電気抵抗がありません。なので一度電流を流すと、超伝導状態が保持される限り永遠に流れ続けることができます。これを単一量子の状態の維持にも利用することができます。この動画では東京大学の中村泰信教授に、この性質を利用して量子状態を作るための3つの方法について伺っています。

状態変数の選択肢として1つめに説明したいのは電荷です。超電導体内では、通常互いに斥力を及ぼしあう2電子間に引力が働くクーパー対という状態になっています。電荷量子ビットは独立したクーパー対が存在する方を(vert1rangle)状態、存在しない方を(vert0rangle)状態として表現しています。

2つめの選択肢として磁束が挙げられています。電流がループ状に流れると電磁石の基本原理である磁場が生じます。超伝導体内部に非常に小さな電流のループを作ることができれば、その電流は時計回りか反時計回りのいずれかになり、その一方を(vert0rangle)状態、もう片方を(vert1rangle)状態として表現することができます。

そして3つめは上記の2つの中間で、これはトランズモンとしていわれるものです。

これら3つの状態変数を実現するためのカギは、クーパー対の有無を精密にコントロールできるかにかかっています。ジョセフソン接合という金属導体間の非常に小さな隙間はたとえ電子にこの隙間を十分に通り抜けられるだけの電圧やエネルギーがなかったとしても、量子確率振幅波のクーパー対によるトンネル効果が生じ超電導電流が流れることができます。絶対零度に近くにつれてクーパー対の数のコントロールの精度が高くなり、状態変数としての利用が可能になります。

この後のステップではハードウェアについてもう少し詳しく紹介していきたいと思います。

This article is from the free online

量子コンピュータ入門

Created by
FutureLearn - Learning For Life

Our purpose is to transform access to education.

We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.

We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.

Learn more about how FutureLearn is transforming access to education