Quantum Computing

Delft Researchers Say They Find Prime Candidates For ‘The Perfect Qubit’


Insider Summary

  • A research team from QuTech says Andreev’s enhanced spin qubit is a step toward a universal quantum computer.
  • The new qubits are more reliable and stable.
  • Andreev’s spin qubits exploit the spin states of Cooper pairs to encode and process quantum information.

PRESS RELEASE — Researchers from QuTech are improving the so-called ‘Andreev spin qubit’ in a critical way and believe it could be a prime candidate in the pursuit of the perfect qubit. The new qubit type is created in a more reliable and intrinsically stable way, compared to previous versions, by combining the advantages of the other two qubit types. They publish their work on Natural Physics.

Unlike the conventional computer world, where bits are based on very well-established and reliable technologies, the perfect qubit has yet to be invented. Will the future quantum computer contain qubits based on superconducting transmon qubits, spin qubits in silicon, NV centers in diamond, or perhaps some other quantum phenomenon? Each type of qubit has its own advantages — and disadvantages. One is more stable, the second has higher fidelity, and the other is easier to mass produce. There are no perfect qubits. Not yet.

Best of both worlds

In this work, researchers from QuTech—a collaboration between Delft University of Technology and TNO—and together with international collaborators devised a clever combination of existing techniques for storing quantum information.

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

“For example,” continues Pita-Vidal, “spin qubits are small and compatible with current industrial technologies, but difficult to interact over long distances. On the other hand, qubit transmons can be efficiently controlled and read over long distances, but have built-in speed limits for operations and are relatively large. The researchers in this study aim to take advantage of the advantages of both types of qubits by developing a hybrid architecture that combines the two.”

Andreev spins the qubits

“In our experiments, we managed to manipulate the qubit’s spin directly using microwave signals,” said Arno Bargerbos, another co-author. “We achieved a very high ‘Rabbi frequency’, which is a measure of how fast they can control the qubits. Next, they embedded this ‘Andreev spin qubit’ inside a superconducting transom qubit that allows rapid measurement of the qubit’s state.”

The researchers characterized the spin coherence time of the Andreev qubit, a measure of how long a qubit can survive. They observed that its ‘longevity’ was affected by the magnetic field from the surrounding material.

“Finally,” says Bargerbos, “we demonstrated the first direct strong coupling between a spin qubit and a superconducting qubit, meaning that they can make the two qubits interact in a controlled way. This suggests that the Andreev spin qubit could be a key element for connecting quantum processors based on very different qubit technologies: semiconductor spin qubit and superconducting qubit.”

future job

Principal investigator Christian Andersen: “The current Andreev spin qubit is rudimentary. It remains to demonstrate multi-qubit operation, which is necessary for a universal quantum computer. Coherence time is also less than optimal. It can be fixed using other materials. Fortunately, the scalability of qubits is on par with semiconductor qubits, raising hopes that we can reach a point where making quantum algorithms the limiting factor and not quantum hardware.”

Paper available here: Direct manipulation of superconducting spin qubits tightly coupled to transmon qubits,


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