Nat. Phys. The perfect qubit that merges superconductivity and semiconductor technology

Spin qubits in semiconductors are a promising platform for producing highly scalable quantum computing devices. However, it is difficult to achieve multi-qubit interactions over long distances.

Recently, researchers from QuTech, a collaboration between the Delft University of Technology and TNO, the Netherlands organization for Applied Scientific Research, have improved the so-called Andreev spin qubit in a crucial way, and believe it could be a prime candidate in the quest for the perfect qubit. Compared to previous versions, this new type of qubit is created by combining the advantages of the other two types of qubits in a way that is more reliable and inherently stable. They published their work in Nature Physics.

On May 22nd, The research results are entitled Direct manipulation of a superconducting spin qubit strongly coupled to a transmon qubit), in the journal Nature Physics.

Unlike the world of traditional computers, where the bits are based on very mature and reliable technology; Right now, perfect qubits haven't been invented yet. Will future quantum computers contain qubits based on superconducting transmon qubits, spin qubits in silicon, NV color centers in diamond, or some other quantum phenomenon?

Each type of qubit has its own advantages and disadvantages. One is more stable, the other has higher fidelity, and the others are easier to mass produce. Today, perfect qubits don't exist.

In this work, researchers from QuTech, together with international collaborators, made clever combinations of existing technologies to store quantum information.

Schematic diagram of Andreev spin qubits in hybrid superconductor semiconductor nanowires. This qubit is formed in a gated quantum dot with an odd number of electrons and coupled to a superconducting wire.

Co-first author Marta Pita-Vidal explains, "The two most promising types of qubits are spin qubits in semiconductors and transmon qubits in superconducting circuits. However, each type has its own challenges."

"For example," continued Pita-Vidal, "spin qubits are small and compatible with current industrial techniques, but they are difficult to interact with over long distances; transmon qubits, on the other hand, can be effectively controlled and read over long distances, but they have an inherent operating speed limit and are relatively large."

The researchers behind this study aim to take advantage of both types of qubits by developing a hybrid architecture that combines them.

"In our experiments, we managed to directly manipulate the spin of qubits using microwave signals," says Arno Bargerbos, another co-first author. "Ultimately, we achieved a very high 'Rabi frequency,' which is a measure of the speed at which a qubit can be controlled. Next, we embed this' Andreev spin qubit 'into a superconducting transmon qubit, which allows us to quickly measure the qubit's state."

The researchers characterized the Andreev spin qubit's coherence time -- a measure of how long the qubit lasts. They observed that its "lifetime" was affected by the magnetic field of the surrounding material.

"Finally," says Bargerbos, "we demonstrated for the first time a direct strong coupling between spin qubits and superconducting qubits, which means that we can get the two qubits to interact in a controlled way." This suggests that Andreev spin qubits could be a key element in the interconnection of quantum processors based on completely different qubit technologies: semiconductor spin qubits and superconducting qubits.

Coherent ASQ-transmon coupling

The research is a key step towards a hybrid architecture that promises to combine the beneficial aspects of superconductivity and semiconductor qubits.

However, lead researcher Christian Andersen said: "The current Andreev spin qubits are not perfect. It still needs to prove multi-qubit operation - which is what a general-purpose quantum computer needs."

"Its coherence time is also suboptimal. This can be improved by using another material. Fortunately, this qubit is comparable in scalability to semiconductor qubits, raising our hopes that we can reach a point where quantum algorithms become limiting factors rather than quantum hardware."

Reference link:

[1]https://qutech.nl/2023/05/22/new-qubit-from-delft-is-more-stable-and-prime-candidate-for-universal-quantum-computer/? cn-reloaded=1

[2]https://phys.org/news/2023-05-stable-qubit-prime-candidate-universal.html

[3]https://www.nature.com/articles/s41567-023-02071-x