March 1, 2024

A superconducting qubit based on twisted cuprate van der Waals heterostructures

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Right: Design of the flowermon qubit with a single d-wave junction shunted by a large capacitor. Left: Order parameter structure for torsion angles close to 45°. Credit: Brosco et al

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Right: Design of the flowermon qubit with a single d-wave junction shunted by a large capacitor. Left: Order parameter structure for torsion angles close to 45°. Credit: Brosco et al

Quantum technology could outperform conventional computers in some advanced computational and optimization tasks. In recent years, physicists have been working to identify new strategies for creating promising quantum systems and qubits (i.e., basic units of information in quantum computers).

Researchers at the CNR’s Institute for Complex Systems (Consiglio Nazionale delle Ricerche), the Max Planck Institute for Chemical Physics of Solids, and other institutes around the world recently introduced a new superconducting, capacitively shifted qubit, which they dubbed “flowermon.” This qubit, introduced in Physical Review Lettersis based on twisted cuprate van der Waals heterostructures.

“The project came about by good chance, during an attempt to combine the languages ​​of our different specialties in conversation,” Uri Vool, co-author of the paper, told Phys.org. “The initial motivation was the recent work of our collaborator Nicola Poccia, who managed to achieve a ‘twisted van der Waals heterostructure’ where they can control the angle between individual layers in the new BSCCO cuprate superconductor without ruining its unique properties.

“Nicola Poccia asked me and Valentina Brosco if this could be used in some way as a qubit or device for quantum technology. Initially I was quite skeptical, but this led to several brainstorming sessions between Valentina and I that eventually converged on idea presented in our newspaper.”

Most experiments aimed at creating quantum superconducting circuits have employed conventional, extensively studied superconducting materials such as aluminum or niobium. Around the year 2000, however, some theoretical physicists explored the idea of ​​introducing noise-shielded superconducting circuits that took advantage of the unique symmetry of unconventional superconductors.

As the implementation of this idea in experimental environments seemed unfeasible at the time, these theoretical works were abandoned for several years. The recent study by Vool, Poccia, Brosco and their colleagues brings this idea back to create a new superconducting qubit.

“As superconducting circuits developed, there were several proposals to create noise-shielded circuits by designing circuit elements in a way that achieved symmetry,” Vool said. “These ideas are very interesting, but experimental implementation has always been challenging, as imperfections, for example in the relative inductance of circuit elements or in the applied flux in the circuit they form, would break the symmetry and degrade its performance.

“In flowermon, we noticed that a simple circuit using a twisted van der Waals cuprate heterostructure also provides this protection, which comes from the symmetry of the material itself rather than the placement of the circuit.”

The unique structure and properties of flowermon, the qubit introduced by this research team, can greatly increase the robustness of a superconducting circuit as it eliminates the need for tuning or flow. Building on previous research efforts focusing on protected circuits, Vool and his colleagues demonstrated the potential of materials with an inherent symmetry for creating quantum superconducting systems.

“Our work shows that using materials with inherent symmetry, as opposed to designed symmetry, produces a robust qubit that does not require fine-tuning,” explained Vool. “Flowermon modernizes the old idea of ​​using unconventional superconductors for quantum shielded circuits and combines it with new manufacturing techniques and a new understanding of the coherence of superconducting circuits.”

The new qubit introduced by the researchers is essentially composed of a single BSCCO van der Waals Josephson junction. This junction has a twist angle of about 45°, deflected by a large capacitor and a superconducting readout resonator.

“Despite its simplicity, the unique nature of the order parameter’s twisted d-wave allows flowermon to encode information into parity-preserving eigenstates,” said Valentina Brosco, co-author of the paper. “Ideally, this brings an order of magnitude improvement in relaxation time over the well-known transmon. Furthermore, the control over the twist angle demonstrated in the experiment suggests that, unlike what happens in standard d-wave junctions, in flowermon quasi-particle induced dissipation is exponentially suppressed.”

Flowermon’s simple design takes advantage of the complex and peculiar characteristics of Josephson tunneling between two thin BSCCO flakes with a relative twist angle.

Another advantage of the new qubit is its distinct spectral structure, which allows manipulation of quantum electrodynamic circuits (cQED) and readout schemes.

“I think flowermon produces an excellent illustration of the emerging functionalities achievable through the integration of complex materials and heterostructures into quantum devices, particularly in the realm of superconducting circuits,” said Brosco. “What I found extremely interesting and fascinating is that the strength of the flowermon circuit is embedded in the many-body wave function that leads to a current-phase relationship with a dominant tunneling term of two copper pairs.”

In contrast to other parity-protected qubits that are realized through complex circuit engineering, flowermon relies on naturally occurring physical mechanisms. The reported robustness of this unique design could inspire other physicists to explore the potential of twisted van der Waals cuprate heterostructures for creating superconducting circuits.

“The idea behind flowermon can be extended in several directions: searching for different superconductors or junctions that produce similar effects, exploring the possibility of realizing new quantum devices based on flowermon,” Brosco said. “Such devices would combine the benefits of quantum materials and coherent quantum circuits or use the flowermon or related design to investigate the physics of complex superconducting heterostructures.”

Vool, Brosco and their collaborators now plan to conduct additional theoretical and experimental studies. In their theoretical work, they intend to address various aspects of the circuit they introduced.

Notably, the flowermon circuit opens up a possible new avenue for broadening the understanding of unconventional superconductors using quantum circuits. This is highly relevant, as the properties of these materials remain one of the greatest mysteries in condensed matter physics.

“This is just the first simple, concrete example of using the inherent properties of a material to make a new quantum device, and we hope to develop it and find additional examples, eventually establishing a field of research that combines complex materials physics with quantum devices. . ” Vool added.

“Experimentally, there is still a lot of work to implement this proposal. We are currently manufacturing and measuring hybrid superconducting circuits that integrate these van der Waals superconductors and hope to use these circuits to better understand the material and eventually design and measure shielded hybrid superconducting circuits to transform them into really useful devices.”

More information:
Valentina Brosco et al, Superconducting Qubit based on Twisted Cuprate Van der Waals Heterostructures, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.017003. About arXiv: DOI: 10.48550/arxiv.2308.00839

Diary information:
Physical Review Letters

arXiv

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