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Superconducting systems

Earlier, we learned how to use charge and magnetic flux as a qubit. We visit Professor Nakamura again to learn about the hardware.

In an earlier Step, we visited the laboratory of Professor Yasunobu Nakamura at the University of Tokyo, and learned about using electric charge and magnetic flux as our qubit state variable. In this video, we continue that visit and learn more about how those experiments are conducted.

Experiments in a refrigerator

The large can behind Professor Nakamura in the video is a dilution refrigerator, which has several stages that are each progressively cooler. Inside the dil fridge is a large rig that passes through the stages, like this:

^{Figure provided by courtesy of Prof. Yasunobu Nakamura.}

The figure below shows how a qubit is controlled from the outside (as we saw in the video), with different parts of the circuit at different temperatures: room temperature (“RT” in the figure), 4 Kelvin, then 10 millkelvin – 1/100th of a degree above absolute zero! (Quantum dot experiments use a similar physical setup to cool their chips.)

^{Figure provided by courtesy of Prof. Yasunobu Nakamura.}

At the bottom stage is a block of aluminum with two halves, each with a single connector for a coaxial cable. Mounted inside is a small, rectangular chip, a few millimeters in size. This chip has a single transmon qubit that is about one micron in size, as you can see in the figure below. The Josephson junction itself is small, but including the resonators necessary to shield and control the qubit, the overall size is nearly a millimeter.

^{Figure provided by courtesy of Prof. Yasunobu Nakamura.}

Future designs

The figure below is a computer rendering of a new chip design. The qubits are the yellow concentric circles at each lattice site. The inner and outer rings are bridged with a Josephson junction to form a transmon qubit. The qubit has four arms to interact with its neighbors. The qubits are controlled from below the chip, as you can see in the cutaway.

The four meander lines are readout resonators, used to measure the state of a qubit. Four qubits share one readout connection which is connected to a coaxial cable beneath the chip (not shown).

The small white holes that pass through the chip connect the red sheets on the top and bottom, called ground planes, to stabilize the voltage used as ground. The yellow control lines from the bottom are coaxial cables, shown in cutaway, used to execute one-qubit rotations and couple two neighboring qubits, so that we can perform two-qubit gates like CNOT.

The picture is just a schematic and not the final design, but it gives you the idea of what it takes to create a system, rather than just a single qubit in an experiment. If this design is successful, Professor Nakamura’s team will be able to build chips with tens or hundreds of qubits on them. Designs like this may be able to run surface code error correction, which we will introduce in the next Activity.

^{Figure provided by courtesy of Prof. Yasunobu Nakamura.}

Superconducting qubits, especially transmons, are large size compared to transistors. We won’t be able to put many qubits into a single chip, and entangling two superconducting qubits in separate chips is one of the major outstanding challenges.

超電導システム

これまでの講義では東京大学の中村泰信教授の研究室を訪れ電荷と磁束を量子ビットの状態変数として扱ったシステムについて学習しました。この動画ではさらに掘り下げ、そこで行われていた実験が実際にどのように繋がってくるかを学習します。

冷却装置の中での実験

中村教授の研究室で使われている巨大な装置は希釈冷凍法と呼ばれる極低温領域での冷却法により冷却します。装置の中には大きな円材が写真のように配置されています。

^{図は中村泰信教授により提供されました。}

下の図は各量子回路と温度帯毎にどのように量子ビットを制御するかを表しています。上から室温(RT)、4ケルビン、そして10ミリケルビン(絶対零度の-1/100の温度)という順番になっています(量子ドットの実験も似たような設計になっています)。

^{図は中村泰信教授により提供されました。}

最下層には半分に分かれたアルミニウムの塊がありそれぞれケーブルのコネクターとセットになっています。内部には小さな長方形のチップが埋め込まれています。この場合では約1mm程のtransmon量子ビットが使われています(transmon量子ビットに使われているジョセフソン接合が実際に占める面積はとても小さいです)。

^{図は中村泰信教授により提供されました。}