Tunable coupling of two distant superconducting spin qubits

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 mixture of 0 and 1 at the same time.

So far, qubits have been realized using different physical systems, from electrons to photons and ions. In recent years, some quantum physicists have experimented with a new type of qubit, known as Andreev spin qubits. These qubits exploit 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 strong and tunable coupling between two distant Andreev spin qubits. Their work, published in Physics of naturecould pave the way towards efficient realization of two-qubit gates between distant spins.

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

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

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

As part of their study, Andersen and his colleagues were the first to produce a superconducting circuit. They subsequently placed two semiconductor nanowires on top of this circuit using a precisely controlled needle.

“The way we designed the circuit, the combination of nanowires and superconducting circuits created two superconducting loops,” Andersen explained. “The special part of these loops is that part of each loop is a semiconductor quantum dot. In the quantum dot we can trap an electron. The cool thing is that the current flowing around the loops will now depend on the spin of the trapped electron is interesting, because it allows us to control the supercurrent of billions of Cooper pairs with one spin.”

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

“We have shown that we can indeed couple spins over ‘large’ distances using superconductors,” Andersen said. “Normally spin-spin coupling occurs only 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.”

It is known that superconducting qubits are bulky, so they take up a lot of space inside the device. The new approach presented by Andersen and his colleagues allows for greater flexibility in the design of quantum computers, allowing qubits to be joined over long distances and packed 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 larger numbers of qubits.

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

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