A superconducting alternative qubit achieves high performance for quantum computing

A superconducting alternative qubit achieves high performance for quantum computing

SEM image of a two-qubit fluxonium processor. Credit: Bao et al.

Quantum computers, devices that exploit quantum phenomena to perform computations, could eventually help tackle complex computational problems faster and more efficiently than conventional computers. These devices generally rely on basic units of information known as quantum bits, or qubits.

Researchers at Alibaba Quantum Lab, a unit of Alibaba Group’s DAMO Research Institute, recently developed a file Quantum processor using fluxonium qubits, which until now have not been the preferred choice when developing quantum computers for industry teams. Their paper was published in physical review messagesdemonstrates the potential of fluxonium to develop high-performance superconducting circuits.

“This work is a critical step for us in advancing our research in the field of quantum computing,” Yaoyun Shi, director of Alibaba’s Quantum Laboratory, told Phys.org. “When we began our research programme, we decided to explore fluconium as a building block for future quantum computers, far from the dominant choice of transmitting qubits. We believe this relatively new type of superconducting qubit could go further than transmission.”

While some previous studies have already explored the potential of fluxonium-qubit-based quantum processors, most have primarily provided proof of concept, which has been realized in university laboratories. For these “artificial atoms” to be implemented in real quantum computers and compete with transmitters (for example, widely used qubits), however, they need to be proven high performance A wide range of operations within a single machine. This is exactly the main objective of this work.

Fluxonium qubits have two characteristics that distinguish them from transmissions: their energy levels are unequal (eg, “anharmonic”) and use a large inductor to replace the capacitor used in the transmission. Both contribute to the advantage of fluxonium, at least in theory, that it is more resilient to errors, resulting in better “coherence”, i.e. longer retention of quantum information, and “higher fidelity”, i.e. accuracy, in the realization of elementary operations.

“One can imagine the energy levels that make up a ladder,” explained Chunqing Deng, who led the study. “The energy gaps are important, because each quantum instruction has a ‘tone’ or frequency, and it leads to transitions between two levels when pitch matches their energy gaps.”

Essentially, when the first two energy gaps between levels are closed, as they are in transmission, an “invitation” to go between the first two energy levels (for example, the cases “0” and “1”), it can also accidentally trigger transitions between the second and third level. This can throw the state out of the valid computational space, resulting in what is known as a leak error. On the other hand, in fluconium, the distance between the second and third energy “steps” is larger, which reduces the risk of leakage errors.

“In principle, the fluconium design is simple: it consists of two primary components – a ‘Josephson junction’ transformer with a large inductor, which, in fact, is similar to the design of the transmission, which is a Josephson junction transformer with a capacitor,” Chunking said. “The Josephson junction is the magic ingredient that creates the repulsion in the first place. The great inductor, as in our case also, is often performed by a large number (in our work, 100) of Josephson junctions.”

Replacing the capacitor with an inductor in fluconium removes the “islands” generated by the electrodes and the source of “charge noise” caused by electron charge fluctuations, making fluconium more error-resistant. This is, however, at the expense of more demanding geometry, due to the large set of Josephson junctions.

Fluxonium’s advantage in high coherence can be greatly amplified to achieve high gate accuracy if gates are used for a short time. These fast gates are already achieved by the “discipline” feature the researchers have demonstrated. More precisely, the energy or “frequency” gap between states “0” and “1” can be quickly changed, so that two qubits can be made “in resonance”, that is, they have the same frequency. The resonance occurs when the two qubits evolve together to achieve the most important building block of a quantum computer – 2-qubit gates.

In initial tests, the quantum platform designed by Chunqing and colleagues was found to reach an average single-qubit gate accuracy of 99.97% and a binary qubit gate accuracy of 99.72%. These values ​​are comparable to some of the best results achieved by quantum treatments in previous studies. Besides single- and dual-qubit gates, the team also powerfully integrated other basic operations needed for a quantum computer – reset and read.

The 2-qubit processor developed by this team of researchers could open up new possibilities for using fluxonium in quantum computing, significantly outperforming other proof-of-concept processors that have been introduced in the past. Their work could inspire other teams to develop similar designs, replacing Transmon with fluxonium qubits.

“Our study offers an alternative option for large-scale adaptive transmission,” Chunking said. “We hope that our work will inspire more interest in exploring fluxonium, so that its full potential can be unlocked to achieve significantly higher performance in fidelity, which in turn will greatly reduce the overhead of achieving fault-tolerance quantum computing. What this means is that, for the same computational task, we may A high-resolution fluconium quantum computer needs far fewer qubits.”

Essentially, Chunqing and colleagues show that fluxonium-based processors can perform much more powerful computations than transmission-based processors, using the same number of physical qubits. In their next studies, the team wants to scale up their system and try to make it fault-tolerant while still maintaining high accuracy.

“We now plan to validate our hypothesis that fluconium is indeed much better qubit From transmission and then on to the community’s next major achievement of achieving fault tolerance, using ultra-fine flexonium qubits. Theoretical limit of high-precision operation to date. It is important to continue to push this trend.”


Laser annealing transmission qubits for high-performance superconducting quantum processors


more information:
Feng Bao et al, Fluxonium: An alternative Qubit platform for high-resolution operations, physical review messages (2022). DOI: 10.1103/ PhysRevLett.129.010502

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