Alternative superconducting qubit achieves high performance for quantum computing

Quantum computers, devices that exploit quantum phenomena to perform calculations, could eventually help tackle complex computational problems faster and more efficiently than classical computers. These devices are commonly based on basic units of information known as quantum bits, or qubits.

Researchers at Alibaba Quantum Laboratory, a unit of the Alibaba Group’s DAMO research institute, have recently developed a quantum processor using fluxonium qubits, which until now have not been the preferred choice when developing quantum computers for equipment in industry. His work, published in Physical Review Lettersdemonstrates the potential of fluxonium to develop high-performance superconducting circuits.

“This work is a critical step for us in advancing our quantum computing research,” Yaoyun Shi, director of the Alibaba Quantum Laboratory, told Phys.org. “When we started our research program, we decided to explore fluxonium as a building block of future quantum computers, deviating from the main option of the transmon qubit. We believe that this relatively new type of superconducting qubit could go much further than the transmon.”

While some previous studies had already explored the potential of fluxonium qubit-based quantum processors, most of them mainly offered proofs of concept, which were done in university labs. However, for these “artificial atoms” to be implemented in real quantum computers and compete with transmons (i.e. widely used qubits), they would need to demonstrate a high performance in a wide range of operations, within a single device. This is precisely the central objective of this work.

Fluxonium qubits have two features that distinguish them from transmons: their energy levels are much more unequal (ie “anharmonic”), and they use a large inductor to replace the capacitor used in transmons. Both contribute to fluxonium’s advantage, at least theoretically, in being more error resistant, leading to better “coherence”, i.e. keeping quantum information longer, and “higher fidelity”, i.e. , precision, in carrying out elementary operations.

“One can imagine the energy levels forming a ladder,” explained Chunqing Deng, who led the study. “Energy gaps are important because each quantum instruction has a ‘pitch’ or frequency, and it triggers transitions between two levels when the pitch matches its energy gaps.”

Essentially, when the first two energy gaps between levels close, as they are in transmon, a “call” for the transition between the first two energy levels (ie states “0” and “1”), they can also accidentally trigger transitions between the second and third levels. This can push the state out of the valid computational space, which is known as a leak error. In fluxonium, on the other hand, the distance separating the second and third energy “steps” is greater, which reduces the risk of leakage errors.

“In principle, the design of fluxonium is simple: it consists of two elementary components: a shunt ‘Josephson junction’ with a large inductor, which is similar, in fact, to that of a transmon, which is a shunt Josephson junction with a large inductor. capacitor,” Chunqing said. “The Josephson junction is the magic component that creates the disharmony in the first place. The large inducer is often, as in our case, also implemented by a large number (in our work 100) of Josephson junctions.”

Replacing the capacitor with an inductor in fluxonium eliminates the “islands” that result from the electrodes and the source of “charge noises” caused by fluctuations in charge of the electrons, making fluxonium more shockproof. mistakes. This is, however, at the expense of much more demanding engineering, due to the wide variety of Josephson crosses.

Fluxonium’s advantage in high coherence can be greatly amplified to achieve high gate fidelities if the gates use little time. In fact, these fast gates are achieved through the “tuning” function demonstrated by the researchers. More precisely, the energy or “frequency” gap between the “0” and “1” states can be changed rapidly, so that two qubits can quickly be brought into “resonance”, i.e. have the same frequency. Being in resonance is when the two qubits evolve together to make the most critical building block of a quantum computer: the 2-qubit gates.

In initial tests, the quantum platform designed by Chunqing and colleagues was found to achieve an average single-qubit gate fidelity of 99.97% and a two-qubit gate fidelity of up to 99.72%. These values ​​are comparable to some of the best results obtained by quantum processors in previous studies. In addition to the one-qubit and two-qubit gates, the team also tightly integrated other basic operations needed for a digital quantum computer: reset and read.

The 2-qubit processor developed by this team of researchers could open up new possibilities for the use of fluxonium in quantum computing, as it significantly outperformed other proof-of-concept processors introduced in the past. Their work could inspire other teams to develop similar designs, substituting fluxonium qubits for transmon.

“Our study presents an alternative option to the widely adapted transmon,” Chunqing 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 significantly reduce the overhead of performing fault-tolerant quantum computing. What this means is that, for the same computational task, a higher-fidelity fluxonium quantum computer may need significantly fewer qubits.”

Essentially, Chunqing and his colleagues showed that fluxonium-based processors could perform much more powerful computations than transmon-based ones, using the same number of physical qubits. In their next studies, the team would like to expand their system and try to make it fault tolerant while maintaining high fidelity.

“Now we plan to validate our hypothesis that fluxonium is in fact a much better qubit we transmon and then march toward the community’s next major milestone of achieving fault tolerance, using ultra-high-fidelity flxuonium qubits,” added Yaoyun. “We think that fluxonium has the potential to be more widely recognized, as we are not even close to any theoretical limits of high-fidelity operation yet. It is important to keep pushing in this direction.”

© 2022 Science X Network