An alternative superconducting qubit achieves high performance for quantum computing

Ingrid Fadelli is a writer for the website Phys.org.

Classical computers are more efficient than quantum computers in performing computations. Basic units of information called qubits are used in these devices.

When it comes to developing quantum computers for industry teams, the use of fluxonium qubits has not been the preferred choice due to its high cost. Their paper was published in the journal Physical Review Letters.

"This work is a critical step for us in furthering our quantum computing research," said the director of the quantum laboratory. The mainstream choice of the transmon qubit was not chosen when we started our research program. This new type of qubit could go further than transmon.

Most of the studies that explored the potential of quantum processor based on fluxonium qubits offered proof of concept, which were realized in university labs. They would need to demonstrate a high performance on a wide range of operations in a single device to compete with transmons. This is the main goal of the work.

Fluxonium qubits have two characteristics that make them different from transmons: their energy levels are more evenly distributed and they use a large inductor. Both contribute to fluxonium's advantage, at least theoretically, in being more resilient to errors, leading to better "coherence" and "higher fidelity" in realizing elementary operations.

The energy levels can be seen forming a ladder. The energy gaps are important because each quantum instruction has a 'pitch,' or Frequency, and it causes transitions between two levels when it matches their energy gaps.

When the first two energy gaps are closed, a call can be made for the transition between the first two levels. A leakage error is caused by bringing the state outside of the valid computational space. The risk of leaks is reduced by the distance between the second and third energy steps.

A Josephson junction with a large inductor is similar to a transmon, which is a Josephson junction with aCapacitor. The Josephson junction is a part of magic. Large inductors are implemented by a large number of Josephson junctions.

Replacing the capacitor with an inductor makes fluxonium more error-proof. Due to the large number of Josephson junctions, this is at the expense of more demanding engineering.

If the gates use a short time, the advantage of high coherence can be increased. The researchers demonstrated that such fast gates are possible. The energy gap between the 0 and 1 states can be quickly changed so that two qubits can be in resonance. The most important building block of a quantum computer is the 2-qubit gates.

The quantum platform was found to have an average single-qubit gate fidelity of 99% and a two-qubit gate fidelity of 99%. Some of the best results achieved by quantum processors can be found in these values. Other basic operations needed for a digital quantum computer were also integrated by the team.

The 2-qubit processor developed by this team of researchers could open new possibilities for the use of fluxonium in quantum computing, as it significantly outperformed other proof-of-conceptprocessors introduced in the past. Their work could lead to similar designs.

The study introduced an alternative choice to the transmon. "We hope that our work will inspire more interest in exploring fluxonium, so that its full potential can be unlocked to achieve a significantly higher performance in fidelity, which will in turn reduce the overhead of realizing fault tolerance quantum computing." A higher-fidelity quantum computer may need less qubits for the same computation task.

Using the same number of physical qubits, Chunqing and his colleagues showed that transmon-based processor could perform more powerful computations. The team wants to scale up their system and try to make it fault tolerant.

The community's next major milestone of realizing fault tolerance will be achieved using ultra-high fidelity Flaxuonium qubits. We don't think we're close to a theoretical limit of high-fidelity operation yet. It is important to push on.

More information: Feng Bao et al, Fluxonium: An Alternative Qubit Platform for High-Fidelity Operations, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.010502 Journal information: Physical Review Letters

There is a science network.