The tunable coupling of two distant superconducting spin qubits

Quantum computers, computing devices that exploit the principles of quantum mechanics, could outperform classical computing on 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 in a mixture of 0 and 1 simultaneously.

Until now, qubits have been made using various physical systems, ranging from electrons to photons and ions. In recent years, some quantum physicists have experimented with a new type of qubit, called Andreev spin qubits. These qubits exploit the properties of superconducting and semiconductor materials to store and manipulate quantum information.

A team of researchers from 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 efficient realization of two-qubit gates between distant 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 studied a new type of qubit called the Andreev spin qubit, which has also been previously demonstrated by Yale researchers. “

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

“Once the new qubit was established, the next natural question was whether we could couple two of them,” Andersen said. “A theoretical paper published in 2010 suggested a method for coupling two of these qubits, and our experiment is the first to realize this proposal in the real world.”

As part of their study, Andersen and his colleagues first fabricated a superconducting circuit. Subsequently, they deposited two semiconductor nanowires on top of this circuit using a precisely controlled needle.

“The way we designed the circuit, the combined circuits of nanowires and superconductors created two superconducting loops,” Andersen explained. “The peculiarity of these loops is that part of each loop is a semiconductor quantum dot. In the quantum dot we can trap an electron. What is interesting is that the current that flows around the loops will depend now from 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 made by the researchers ultimately depends on the spin in the two quantum dots. This also means that the two spins are coupled via this supercurrent. Notably, this coupling can also be easily controlled, either via the magnetic field passing through the loops or by modulating the gate voltage.

“We demonstrated that we can actually couple rotations 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 to those based on superconducting qubits, this proximity requirement is l “one of the architectural disadvantages of semiconductors.”

Superconducting qubits are known to be bulky, thus taking up a lot of space in a device. The new approach introduced by Andersen and colleagues allows for greater flexibility in the design of quantum computers, by allowing qubits to be coupled over long distances and brought 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 the proposed approach to a larger number of qubits.

“We have very good reasons to believe that our approach could offer 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 germanium-based, to boost consistency times.”

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