Exponential improvement in photon storage fidelities using subradiance and "selective radiance" in atomic arrays

Abstract: A central goal within quantum optics is to realize efficient interactions between photons and atoms. A fundamental limit in nearly all applications based on such systems arises from spontaneous emission, in which photons are absorbed by atoms and then re-scattered into undesired channels. In typical treatments of atomic ensembles, it is assumed that this re-scattering occurs independently, and at a rate given by a single isolated atom, which in turn gives rise to standard limits of fidelity in applications such as quantum memories or quantum gates. However, this assumption can be violated. In particular, spontaneous emission of a collective atomic excitation can be significantly suppressed through strong interference in emission. Thus far the physics underlying the phenomenon of subradiance and techniques to exploit it have not been well-understood. In this work, we provide a comprehensive treatment of this problem. First, we show that in ordered atomic arrays in free space, subradiant states acquire an interpretation in terms of optical modes that are guided by the array, which only emit due to scattering from the ends of the finite chain. We also elucidate the properties of subradiant states in the many-excitation limit. Finally, we introduce the new concept of selective radiance. Whereas subradiant states experience a reduced coupling to all optical modes, selectively radiant states are tailored to simultaneously radiate efficiently into a desired channel while scattering into undesired channels is suppressed, thus enabling an enhanced atom-light interface. We show that these states naturally appear in chains of atoms coupled to nanophotonic structures, and we analyze the performance of photon storage exploiting such states. We find that selectively radiant states allow for a photon storage error that scales exponentially better with number of atoms than previously known bounds.

• The paper demonstrates that arranging atoms in ordered arrays to harness subradiance and selective radiance can exponentially boost photon storage fidelities.
• It introduces methodologies where controlled interference suppresses unwanted photon scattering, surpassing the conventional 1/D scaling in optical depth.
• The study reveals that collective emission effects enhance electromagnetically induced transparency, promising more efficient quantum memories.

Exponential Improvement in Photon Storage Fidelities Using Subradiance and Selective Radiance in Atomic Arrays

The paper "Exponential improvement in photon storage fidelities using subradiance and 'selective radiance' in atomic arrays" explores the intricate dynamics of photon-atom interactions in ordered atomic arrays and presents novel methodologies for enhancing quantum optical systems. The study leverages the concepts of subradiance and selective radiance to propose methods for significantly increasing the fidelity of photon storage, an essential requirement for efficient quantum information processing and quantum memory applications.

As its central premise, the paper revisits the limitation imposed by spontaneous emission in quantum optics, where photons absorbed by atoms are often re-scattered into non-desired modes, thus impairing the fidelity of various atomic ensemble-based technologies. Traditionally, atomic emission is assumed independent, occurring at rates similar to those of isolated atoms, which constrains the achievable fidelity to scales inversely with optical depth (OD), defined by $D \sim (\lambda_0^2/A_\text{eff})N$ where $\lambda_0$ is the transition wavelength, $A_\text{eff}$ the effective beam area, and $N$ the number of atoms.

A significant contribution of this work is the theoretical reinterpretation of subradiant states as optical modes guided by the atomic array. The concept of subradiance, although well-explored, had until now lacked clear techniques for practical exploitation. In ordered atomic arrays, subradiant states emerge due to the suppression of photon emission through destructive interference among the atoms, reminiscent of waveguide modes that only emit through scattering at the array's boundaries. The authors elucidate this phenomenon for both single and multi-excitation regimes, showing that controlled spatial overlap of excitations can preserve subradiance, particularly in 1D arrays.

The research introduces the novel concept of "selective radiance." While subradiant states are characterized by reduced coupling to all optical modes, selectively radiant states simultaneously optimize superradiant coupling to desired optical channels while suppressing scattering into unwanted ones, thereby enabling more efficient atom-light interfaces. This is particularly effective in chains of atoms placed near nanophotonic structures, such as nanofibers, where selectively radiant states store photons with error probabilities that improve exponentially with atomic number, significantly surpassing the previously estimated $1/D$ scaling.

The study further extends to investigate electromagnetically induced transparency (EIT) within both traditional and collective emission models, finding that the bandwidth-delay product—a critical measure for quantum memories—scales linearly with the number of atoms when collective effects are considered, indicating a move towards ideal, non-lossy systems.

The results presented herein challenge the conventional independent emission paradigms and provide compelling evidence for the potential of exploiting ordered atomic arrays and engineered radiative properties to push the boundaries of photonic quantum technologies. The implications of leveraging collective effects in photon storage and retrieval processes are profound, suggesting avenues for developing more robust quantum communication and computation systems.

This comprehensive study stands as a valuable contribution to advancing the field of quantum optics by thoroughly re-evaluating and enhancing our understanding of photon-atom interactions through collective emission processes. Looking forward, further investigations into the multi-excitation regimes and potential experimental realizations in varied atomic media could unlock new potentials for controllable quantum interfaces, bringing these theoretical advancements into practical realization.