Quantum Electrodynamics Near a Photonic Band-Gap
This paper by Yanbing Liu and Andrew A. Houck investigates the dynamics of quantum electrodynamics (QED) near a photonic band-gap using a system comprising a microwave photonic crystal and a superconducting transmon qubit. Photonic crystals offer a unique platform for manipulating optical dispersion and density of states due to their periodic structure, influencing light-matter interactions profoundly. This research explores the creation of localized cavity modes within a photonic band-gap, highlighting their potential applications in dissipative state preparation and long-range spin models.
A key focal point of the study is the non-perturbative strong coupling regime. The research explores how a structured vacuum, such as that of a photonic crystal, impacts atomic lifetimes, a process typically described by the Purcell effect in perturbative scenarios. However, when strong coupling is present, new physics emerges. For example, photonic crystals can host a bound state for photons within the band-gap. This bound state enables phenomena like Rabi oscillations and light trapping, even allowing for the localization of multiple photons by a single atom. The coherent photonic transport across normally forbidden zones can have strongly correlated characteristics.
Liu and Houck's experimental setup utilized a microwave photonic crystal with 14 unit cells and a deliberate impedance modulation for precise control of the band structure. By embedding the transmon qubit at an optimal position within this arrangement, the system enters a regime where strong coupling is achievable. This strong coupling is quantitatively explored through extracted bound state parameters like energy eigenvalue shifts and localization lengths. A particularly noteworthy result is the relationship between the bound state's spectral linewidth and its localization length, showcasing the leaky nature of the system due to finite crystal size.
The authors further explore the nonlinear interactions under pump-probe experiments. They use the transmon qubit's anharmonic multilevel structure to control the quantum state within the band-gap actively. Remarkably, they observe transmission peaks and novel spectral phenomena such as Autler-Townes splitting and Electromagnetically Induced Transparency (EIT) in a single atom, elucidating coherent control over light propagation within the band-gap.
Intriguingly, they document the hybridization of laser-driven dressed states with the photonic crystal environment. The dynamics observed near the band edge illustrate the potential for non-Markovian behavior due to feedback between the photonic medium and the qubit. These dynamics suggest a mechanism for controlling quantum states, enabling processes such as dressed-state cooling pivotal for potential applications in stabilizing specific quantum states.
In conclusion, this work presents a comprehensive study on the interplay between photonic crystals and quantum emitters in the strong coupling regime, leveraging state-of-the-art transmon qubit technology. The implications are significant for advances in QED systems, particularly for developing new quantum state engineering techniques and spin models mediated by photon interactions in photonic crystals. Future avenues include extending these experiments to three-dimensional architectures and exploring ultra-strong coupling regimes with other quantum systems, potentially marking a significant stride in quantum technologies.