Photon Blockade in the Ultrastrong Coupling Regime

Abstract: We explore photon coincidence counting statistics in the ultrastrong-coupling regime where the atom-cavity coupling rate becomes comparable to the cavity resonance frequency. In this regime usual normal order correlation functions fail to describe the output photon statistics. By expressing the electric-field operator in the cavity-emitter dressed basis we are able to propose correlation functions that are valid for arbitrary degrees of light-matter interaction. Our results show that the standard photon blockade scenario is significantly modified for ultrastrong coupling. We observe parametric processes even for two-level emitters and temporal oscillations of intensity correlation functions at a frequency given by the ultrastrong photon emitter coupling. These effects can be traced back to the presence of two-photon cascade decays induced by counter-rotating interaction terms.

• The paper demonstrates that ultrastrong coupling disrupts conventional photon blockade by enabling two-photon cascades through counter-rotating interactions.
• It introduces a new formalism using the cavity-emitter dressed basis to compute photon statistics beyond traditional normal order methods.
• Numerical simulations show rapid oscillations in the intensity correlation function that correlate with the coupling constant, offering insights for quantum device design.

Photon Blockade in the Ultrastrong Coupling Regime

The paper "Photon Blockade in the Ultrastrong Coupling Regime" by Ridolfo et al. investigates the alterations in photon blockade effects when entering the ultrastrong coupling (USC) regime of cavity quantum electrodynamics (QED). The work addresses the inadequacy of traditional normal order correlation functions in describing photon statistics when the coupling strength between an emitter and a cavity reaches levels comparable to the resonance frequency of the cavity.

Key Findings and Contributions

The authors propose a formalism for correlating photo-detection statistics in scenarios involving ultrastrong couplings, where conventional operator methods fail. By formulating correlation functions within the cavity-emitter dressed basis, they provide a more accurate description of photon statistics for varying degrees of light-matter interaction. Notably, the paper emphasizes the inability of the photon blockade to persist under ultrastrong coupling due to the activation of two-photon cascade processes by counter-rotating interaction terms.

Numerical Results and Experimental Significance

The study delineates significant deviations from the traditional photon blockade. For their conditional parameter set, observed phenomena include alternations between subpoissonian and antibunched emission characteristics at one polaritonic resonance, and superpoissonian and bunched statistics at another. These contrasting behaviors rooted in the ultrastrong coupling regime offer intriguing insights into the quantum optical behavior induced by parametric processes — characteristics notably different from those of the Jaynes-Cummings model.

The numerical simulations reveal temporal oscillations in the intensity correlation function $g^{(2)}(\tau)$ at much faster rates than those typically observed in standard blockade scenarios. This oscillation frequency correlates with the ultrastrong emitter-photon coupling constant $g$, emphasizing the novel dynamics induced by USC conditions. Such detailed insights in these regimes were previously unexplored but are crucial for predicting experimental outcomes as systems breach conventional interaction strengths.

Theoretical and Practical Implications

From a theoretical standpoint, the proposed methodology offers substantial advancements in understanding photon statistics in complex couplings. By overcoming traditional limitations, these extended correlation functions not only inform foundational quantum optical theory but also practical developments in emerging photonic technologies.

Practically, the paper's findings suggest pathways for designing higher yield parametric down-converters and potentially inform future nonlinear optical systems leveraging ultrastrong coupling. By applying their model to existing superconducting circuit technologies, the authors assert the feasibility of observing these phenomena with current experimental setups.

Future Directions

The implications of this study extend to the design and analysis of new quantum devices manipulated within the USC regime, especially in computing and secure communication. Future exploration could involve frequency-resolved detection methodologies or applications in multi-cavity setups, promising further evolution in quantum information science.

Overall, Ridolfo et al. make significant contributions to the understanding of how ultrastrong coupling alters traditional quantum optical phenomena, thereby laying groundwork for advanced applications of cavity QED technologies.