A new qubit design based on superconductors could revolutionize quantum computing. By leveraging the distinct properties of single-atom-thick layers of materials, this new approach to superconducting circuits promises to significantly extend the runtime of a quantum computer, addressing a major challenge in the field.
This limitation on continuous operation time arises because the quantum state of a qubit — the basic computing unit of a quantum computer — can be easily destabilized due to interactions with its environment and other qubits. This destruction of the quantum state is called decoherence and leads to errors in computations.
Among the various types of qubits that scientists have created, including photons, trapped ions, and quantum dots, superconducting qubits are desirable because they can switch between different states in the shortest amount of time.
Their operation is based on the fact that, due to subtle quantum effects, the power of the electric current flowing through the superconductor can take discrete values, each corresponding to a state of 0 and/or 1 (or even larger values for some designs).
For superconducting qubits to work correctly, they require the presence of a gap in the superconducting circuit called a Josephson junction through which an electrical current flows through a quantum phenomenon called tunneling — the passage of particles through a barrier that, according to the laws of classical physics, they should not be able to cross.
The problem is, the advantage of superconducting qubits in enhanced switching time comes at a cost: They are more susceptible to decoherence, which occurs in milliseconds, or even faster. To mitigate this issue, scientists typically resort to meticulous adjustments of circuit configurations and qubit placements — with few net gains.
Addressing this challenge with a more radical approach, an international team of researchers proposed a novel Josephson junction design using two, single-atom-thick flakes of a superconducting copper-based material called a cuprate. They called their design “flowermon”.
Taming superconducting qubits
In their study published in the Physical Review Letters, the team applied the fundamental laws of quantum mechanics to analyze the current flow through a Josephson junction and discovered that if the angle between the crystal lattices of two superconducting cuprate sheets is 45 degrees, the qubit exhibits more resilience to external disturbances compared to conventional designs based on materials like niobium and tantalum.
“The flowermon modernizes the old idea of using unconventional superconductors for protected quantum circuits and combines it with new fabrication techniques and a new understanding of superconducting circuit coherence,” Uri Vool, a physicist at the Max Planck Institute for Chemical Physics of Solids in Germany, explained in a press release.
The team’s calculations suggest that the noise reduction promised by their design could increase the qubit’s coherence time by orders of magnitude, thereby enhancing the continuous operation of quantum computers. However, they view their research as just the beginning, envisioning future endeavors to further optimize superconducting qubits based on their findings.
“The idea behind the flowermon can be extended in several directions: Searching for different superconductors or junctions yielding similar effects, exploring the possibility to realize novel quantum devices based on the flowermon,” said Valentina Brosco, a researcher at the Institute for Complex Systems Consiglio Nazionale delle Ricerche and Physics Department University of Rome. “These devices would combine the benefits of quantum materials and coherent quantum circuits or using the flowermon or related design to investigate the physics of complex superconducting heterostructures.”
“This is only the first simple concrete example of utilizing the inherent properties of a material to make a new quantum device, and we hope to build on it and find additional examples, eventually establishing a field of research that combines complex material physics with quantum devices,” Vool added.
Since the team’s study was purely theoretical, even the simplest heterostructure-based qubit design they proposed requires experimental validation — a step that is currently underway.
“Experimentally, there is still quite a lot of work towards implementing this proposal, concluded Vool. “We are currently fabricating and measuring hybrid superconducting circuits which integrate these van der Waals superconductors, and hope to utilize these circuits to better understand the material, and eventually design and measure protected hybrid superconducting circuits to make them into real useful devices.”
Reference: Uri Vool, et al., Superconducting Qubit Based on Twisted Cuprate Van der Waals Heterostructures, Physical Review Letters (2024). DOI: 10.1103/PhysRevLett.132.017003
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