Floquet Drives Extend Coherence in Superconducting Fluxonium Qubits

Researchers demonstrated enhanced coherence in fluxonium qubits using two-tone Floquet drives on magnetic flux. Optimised drive parameters, determined via perturbation theory and simulations, broadened dephasing time peaks without significantly altering relaxation. This approach also facilitates improved phase gate implementation compared to single-tone drives.

Superconducting qubits represent a leading technology in the development of quantum computers, but their sensitivity to environmental noise remains a significant obstacle to building stable and scalable systems. Researchers are actively investigating methods to shield these qubits from disruptive influences, and a promising approach involves driving them with time-varying electromagnetic fields – a technique known as ‘Floquet engineering’ – to create protected operating regimes. A team led by Joachim Lauwens (KU Leuven) and Kristof Moors (Imec), with contributions from Bart Sorée (KU Leuven and University of Antwerp), detail in their work the optimisation of these Floquet drives, specifically employing two distinct frequencies to enhance qubit coherence. Their findings, published under the title “Optimization of Floquet fluxonium qubits with commensurable two-tone drives”, demonstrate improved control over the qubit’s energy spectrum and a pathway towards more robust quantum gate operations.

Enhanced Coherence and Gate Fidelity in Fluxonium Qubits via Two-Tone Floquet Drives

Recent research details improvements in the coherence and control of fluxonium qubits achieved through the application of specifically tailored, time-periodic magnetic flux drives, utilising principles of Floquet theory. The study focuses on employing two-tone drives of the form ϕ(t) = ϕ0 cos(ω1t) + ϕ0 cos(ω2t) to manipulate qubit behaviour and mitigate the detrimental effects of low-frequency noise. Researchers identify optimal drive parameters through a combination of perturbation theory and numerical calculations, revealing that two-tone drives offer increased tunability of the quasi-energy spectrum. This tunability results in broader and higher peaks in dephasing times – a critical metric for qubit performance – without substantially impacting relaxation times.

This investigation extends to the implementation of phase gates – fundamental components of quantum computation – and demonstrates that utilising a second, commensurable drive tone enhances performance compared to implementations relying on a single drive tone. Monte Carlo simulations corroborate this improvement, directly translating to increased fidelity in quantum operations and paving the way for more complex quantum algorithms.

Early investigations identified low-frequency noise as a primary source of qubit decoherence, prompting researchers to explore methods for shielding qubits from environmental fluctuations. They discovered that operating qubits on dynamical sweet-spot manifolds – regions where qubits exhibit reduced sensitivity to flux noise – effectively extends coherence times. This approach, however, faced limitations in terms of tunability and optimisation, motivating the exploration of more sophisticated control techniques.

Researchers employed a combination of perturbation theory and numerical calculations to identify optimal drive parameters for achieving extended coherence. Perturbation theory provides an analytical framework for understanding the effects of the time-periodic drive on the qubit’s energy levels, while numerical calculations allow for the accurate determination of the optimal drive frequencies and amplitudes. This allows scientists to shape the quasi-energy spectrum in a way that minimises the effects of low-frequency noise. Maintaining coherence for longer periods is crucial for performing complex quantum computations, as it reduces the accumulation of errors and improves the overall fidelity of the results.

The findings highlight the potential of Floquet engineering and parametric modulation as powerful tools for advancing superconducting qubit technology and realising robust quantum computation. This approach offers a pathway to optimise qubit performance beyond the limitations of static sweet spots, enabling the development of more resilient and scalable quantum technologies.

Researchers validated the improved performance of phase gates through Monte Carlo simulations, which model the probabilistic behaviour of the qubit under various noise conditions. These simulations demonstrate that the second commensurable drive tone enhances the accuracy of the gate, reducing the probability of errors and improving the overall fidelity of the quantum computation. The simulations also reveal that the improved gate performance is robust to variations in the qubit parameters and noise conditions, suggesting that this approach is practical for building scalable quantum computers.

This research demonstrates that the application of two-tone drives, guided by the principles of Floquet theory, offers a promising avenue for enhancing the coherence and fidelity of superconducting qubits. By precisely controlling the time-dependent magnetic flux, scientists can engineer the qubit’s energy landscape, shielding it from environmental fluctuations and extending its coherence.

Future research will focus on exploring the optimal parameters for the two-tone drive, as well as investigating the potential for applying this technique to other types of qubits. The ultimate goal is to develop a robust and scalable quantum computer that can solve problems intractable for classical computers. This research represents a step towards that goal, demonstrating the power of Floquet engineering and parametric modulation for advancing superconducting qubit technology. The continued development of these techniques will be crucial for realising the full potential of quantum computing.

Jargon Explanation:

• Fluxonium Qubit: A type of superconducting qubit known for its relatively long coherence times and robustness to charge noise.
• Floquet Theory: A mathematical framework used to analyse the behaviour of systems subjected to time-periodic driving forces.
• Dephasing Time (T₂^*): A measure of how long a qubit maintains its quantum phase coherence. Shorter dephasing times indicate faster decoherence.
• Relaxation Time (T₁): A measure of how long a qubit maintains its excited state before decaying to its ground state.
• Dynamical Sweet Spot: A specific operating point for a qubit where its sensitivity to certain types of noise is minimised.
• Parametric Modulation: A technique for controlling a system by varying its parameters in a time-dependent manner.
• Monte Carlo Simulation: A computational technique that uses random sampling to obtain numerical results.