The tunable coupling of two distant superconducting spin qubits

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The complete chip mounted on a printed circuit board. Credit: Pita-Vidal, Wesdorp et al.

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The complete chip mounted on a printed circuit board. Credit: Pita-Vidal, Wesdorp et al.

Quantum computers, computing devices that use the principles of quantum mechanics, could outperform classical computing in some complex optimization and processing tasks. In quantum computers, classical units of information (bits), which can have a value of 1 or 0, are replaced by quantum bits, or qubits, which can be a combination of 0 and 1 at the same time.

So far, qubits have been realized using a variety of physical systems, ranging from electrons to photons and ions. In recent years, some quantum physicists have been experimenting with a new kind of qubits known as Andreev spin qubits. These qubits use the properties of superconducting and semiconductor materials to store and manipulate quantum information.

A team of researchers at Delft University of Technology, led by Marta Pita-Vidal and Jaap J. Wesdorp, recently demonstrated the strong and tunable coupling between two distant Andreev qubits. Their article, published in Natural physicsmay pave the way to the efficient realization of two-qubit gates between remote spins.

“The recent work is essentially a continuation of our work published last year in Natural physics” Christian Kraglund Andersen, corresponding author of the paper, told Phys.org. “In this earlier work, we investigated a new type of qubit called the Andreev spin qubit, which had also been previously demonstrated by researchers at Yale.”

Andreev spin qubits simultaneously take advantage of the advantageous properties of both superconducting and semiconductor qubits. These qubits are essentially created by embedding a quantum dot in a superconducting qubit.

“With the establishment of the new qubit, the natural next question was whether we could connect two of them,” Andersen said. “A theoretical paper published in 2010 proposed a method to connect two such qubits, and our experiment is the first experiment to realize this proposal in the real world.”


Increase the device. A superconducting qubit (red) connected to sense and control lines is shown on the left. The two Andreev spin qubits are located in the small dashed box. On the right is a magnification of the part with the two Andreev spins located in the two superconducting chains. Credit: Pita-Vidal, Wesdorp et al.

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Increase the device. A superconducting qubit (red) connected to sense and control lines is shown on the left. The two Andreev spin qubits are located in the small dashed box. On the right is a magnification of the part with the two Andreev spins located in the two superconducting chains. Credit: Pita-Vidal, Wesdorp et al.

As part of their research, Andersen and his colleagues first fabricated a superconducting circuit. They then placed two semiconductor nanowires on top of this circuit using a precisely controlled needle.

“The way we designed the circuit, the combined nanowires and superconducting circuits created two superconducting circuits,” Andersen explained. “The special part of these circuits is that part of each circuit is a semiconductor quantum dot. In the quantum dot, we can trap an electron. The cool thing is that the current that flows around the circuits will now depend on the spin of the trapped electron. This effect is interesting because it allows us to control an overcurrent of billions of Cooper pairs in a single spin.”

The combined current of the two connected superconducting circuits, the researchers realized, ultimately depends on the spin in the two quantum dots. This also means that the two turns are connected through this overcurrent. It should be noted that this coupling can be easily controlled either by the magnetic field passing through the loops or by modulating the gate voltage.

“We have demonstrated that we can indeed couple spins over ‘long’ distances using a superconductor,” Andersen said. “Typically, spin-spin coupling only occurs when two electrons are very close. When comparing semiconductor-based qubit platforms to those based on superconducting qubits, this proximity requirement is one of the architectural drawbacks of semiconductors.”

Superconducting qubits are known to be bulky, thus taking up a lot of device space. The new approach pioneered by Andersen and his colleagues allows greater flexibility in the design of quantum computers by allowing qubits to be linked over long distances and packed closer together.

This recent research may soon open up new opportunities for the development of high-performance quantum computing devices. In their next studies, the researchers plan to extend their proposed approach to larger numbers of qubits.

“We have very good reason to believe that our approach can offer significant architectural advances for connecting multiple spinning qubits,” Andersen added. “However, there are also experimental challenges. The current coherence times are not very good and we expect that the nuclear spin bath of the semiconductor we used (InAs) is to blame for this. So we would like to move to a cleaner platform, for example germanium-based, to increase the coherence times.”

More info:
Marta Pita-Vidal et al., Strongly tunable coupling between two distant superconducting spin qubits, Natural physics (2024). DOI: 10.1038/s41567-024-02497-x

Log information:
Natural physics

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