The tunable coupling of two remotely located superconducting spin qubits

The tunable coupling of two remotely located superconducting spin qubits

The entire 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 computers in some complex optimization and processing tasks. In quantum computers, classical information units (bits), which can have a value of 1 or 0, are replaced by quantum bits or qubits, which can have a combination of 0 and 1 at the same time.

Qubits have so far been realized using various physical systems, ranging from electrons to photons and ions. In recent years, some quantum physicists have experimented 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 spin qubits. Their article, published in Natural physicscould pave the way for the effective realization of two-qubit gates between far spins.

“The recent work is essentially a continuation of our work published last year Natural physicsChristian Kraglund Andersen, corresponding author of the paper, told “In this earlier work, we studied a new type of qubit, an Andreev spin qubit, which was also previously demonstrated by researchers at Yale.”

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

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

The tunable coupling of two remotely located superconducting spin qubits

A zoom-in on the device. A superconducting qubit (red) is shown on the left, linked to readout and control lines. The two Andreev spin qubits are in the small dotted box. On the right is a zoom in on the part with the two Andreev spins in the two superconducting loops. 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.

“Because of the way we designed the circuit, the combined nanowires and superconducting circuits created two superconducting loops,” Andersen explains. “The special thing about these loops is that part of each loop is a semiconductor quantum dot. We can capture an electron in the quantum dot. The nice thing is that the current flowing through the loops now depends on the spin of the trapped electron. This effect is interesting because it allows us to control a supercurrent of billions of Cooper pairs with a single spin.”

The combined current of the two coupled superconducting loops that the researchers realize ultimately depends on the spin in both quantum dots. This also means that the two spins are coupled via this supercurrent. Remarkably, this coupling can also be easily controlled, either via the magnetic field running through the loops or by modulating the gate voltage.

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

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

This recent study could soon open new possibilities for the development of high-performance quantum computing devices. In their next studies, the researchers plan to extend their proposed approach to a larger number of qubits.

“We have very good reasons to think that our approach could provide significant architectural advances for the coupling of multiple spin 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. So we would like to move to a cleaner platform, for example based on germanium, to increase coherence times.”

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

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