Charge and magnetic flux

Let's take a look at the laboratory of Dr. Nakamura of the University of Tokyo which conducts the latest research.

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 (|1rangle) state, and the absence of the pair as the (|0rangle) 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 (|0rangle) state and counterclockwise as our (|1rangle) (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

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。