Quantum phase tomography of a strongly driven qubit.

Area of Science:

• Quantum physics
• Quantum computing
• Condensed matter physics

Background:

• Landau-Zener transitions describe quantum system evolution through avoided level crossings.
• Stückelberg oscillations are a signature of coherent dynamics in driven two-level systems.
• Understanding these oscillations is key to controlling and diagnosing quantum systems like qubits.

Purpose of the Study:

• To analyze the interference patterns arising from repeated Landau-Zener transitions in a qubit.
• To interpret the observed oscillatory patterns in terms of quantum phase evolution.
• To demonstrate the utility of these patterns for probing dephasing mechanisms.

Main Methods:

• Sweeping a qubit through an avoided level crossing to induce Landau-Zener transitions.
• Observing qubit magnetization oscillations resulting from the interference of these transitions.
• Performing two-dimensional Fourier transforms on the oscillatory patterns.
• Analyzing the resulting one-dimensional curves in Fourier space.

Main Results:

• Stückelberg oscillations were observed in qubit magnetization, confirming coherent dynamics.
• Two-dimensional Fourier transforms revealed a family of one-dimensional curves in Fourier space.
• These patterns were successfully interpreted in terms of the time evolution of the qubit state's quantum phase.
• The findings align with recent experimental observations in superconducting qubits.

Conclusions:

• The study confirms Stückelberg oscillations as a key indicator of the coherent strongly driven regime in qubits.
• Fourier analysis of these oscillations provides a powerful tool for visualizing quantum phase evolution.
• This method offers a novel approach to experimentally probe dephasing mechanisms in quantum systems.