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Extremely large lamb shift in a deep-strongly coupled circuit QED system with a multimode resonator
Ziqiao Ao1,2,3, Sahel Ashhab4, Fumiki Yoshihara5,6
1Department of Applied Physics, Waseda University, Okubo 3-4-1, Shinjuku-ku, Tokyo, 169-8555, Japan. zqao@asagi.waseda.jp.
Scientific Reports
|July 13, 2023
Summary
We observed an extremely large Lamb shift in a multimode circuit quantum electrodynamics system operating in the deep-strong coupling regime. This phenomenon, driven by coupling to numerous resonator modes, significantly renormalizes qubit energy without suppressing it entirely.
Area of Science:
- Quantum Electrodynamics
- Superconducting Circuits
- Quantum Computing Hardware
Background:
- Circuit Quantum Electrodynamics (cQED) systems are crucial for quantum information processing.
- The deep-strong coupling (DSC) regime, where qubit-resonator coupling is comparable to or exceeds their frequencies, presents unique phenomena.
- Understanding Lamb shift in multimode systems is essential for accurate qubit characterization and control.
Purpose of the Study:
- To experimentally and theoretically investigate the extremely large Lamb shift in a multimode cQED system.
- To analyze the influence of coupling to numerous resonator modes on qubit energy renormalization.
- To develop theoretical formulas for calculating Lamb shift in such complex systems.
Main Methods:
- Utilized a superconducting flux qubit (FQ) inductively coupled to a coplanar waveguide resonator (CPWR) in the DSC regime.
- Performed spectroscopy around the fundamental mode frequency of the CPWR.
- Fitted the spectrum using a single-mode quantum Rabi Hamiltonian and derived theoretical formulas for multimode Lamb shift.
Main Results:
- Observed an extremely large Lamb shift, with the fundamental mode contributing 82.3% and all modes contributing 96.5%.
- Demonstrated that coupling to higher modes in the CPWR leads to a high-frequency cut-off effect, suppressing coupling above a certain frequency.
- Showed that this cut-off prevents qubit energy from being suppressed to zero, a critical finding for system stability.
Conclusions:
- Multimode coupling in cQED systems, particularly in the DSC regime, results in a substantial Lamb shift.
- The high-frequency cut-off mechanism is vital for maintaining non-zero qubit energy despite strong coupling.
- These findings offer crucial insights for designing and optimizing superconducting quantum processors.
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