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Skip to 0 minutes and 4 seconds 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.

Skip to 0 minutes and 51 seconds 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?

Skip to 1 minute and 25 seconds 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?

Skip to 2 minutes and 10 seconds 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.

Skip to 2 minutes and 45 seconds 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.

Charge and magnetic flux

In a superconductor, there is no electrical resistance. If you start an electrical current flowing, it will run forever. It can also maintain the state of individual quanta. In this visit with Professor Yasunobu Nakamura of the University of Tokyo, we learn about three ways of using this special capability to make the quantum states we use for qubits.

The first state variable was charge. In a superconductor, electrons form Cooper pairs, in which two electrons, which normally repel each other, become loosely bound to each other, and behave together. A charge qubit uses the presence of a Cooper pair in a small, isolated island as the \(|1\rangle\) state, and the absence of the pair as the \(|0\rangle\) state.

The second choice of state variable is magnetic flux. Electrical current flowing in a loop creates a magnetic field, the basis for all electromagnets. If we have a microscopic loop of superconductor, the current can flow either clockwise or counterclockwise around the loop, so we can use clockwise as our \(|0\rangle\) state and counterclockwise as our \(|1\rangle\) (or vice versa).

The third type of state variable is an intermediate between the two, known as a transmon.

The key to all three of these state variables is being able to control the presence or absence of Cooper pairs very precisely. A Josephson junction is a tiny gap (perhaps only a few atoms across) in the metal conductor. It might seem that such a gap would prevent current from flowing (unless the voltage is high enough to make spark across the gap, but the voltages and energies here are much, much too small for that). However, because of the way that the quantum probability amplitude waves work, there is a small probability that our Cooper pair will tunnel through this barrier. Used appropriately, at close to absolute zero, this gives us the ability to control very precisely the number of Cooper pairs, giving us the states we can use as our state variable. In an upcoming article, we will learn more about the hardware necessary.




磁荷と磁束

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\(\vert1\rangle\)状態、存在しない方を\(\vert0\rangle\)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\(\vert0\rangle\)状態、もう片方を\(\vert1\rangle\)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。

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Understanding Quantum Computers

Keio University