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LANTERN Report Summary

Project ID: 655701
Funded under: H2020-EU.1.3.2.

Periodic Reporting for period 1 - LANTERN (Light-Atom Interactions in Nanophotonic Structures)

Reporting period: 2016-04-01 to 2018-03-31

Summary of the context and overall objectives of the project

Quantum science is now at the spotlight of both government and industry alike, as demonstrated by the amount of both public funding (through the European Quantum Flagship) and the news-worthy investment of private companies such as Google or Microsoft. While the future is promising, there are still many challenges ahead that must be overcome before society benefits fully from robust quantum technologies.

In particular, in the context of quantum science, dissipation is a colossal problem. Losses curb our ability to realize controlled and efficient interactions between photons and atoms, which are essential for many technologies ranging from quantum information processing and quantum non-linear optics to metrology and imaging. Spontaneous emission - in which photons are first absorbed by atoms and then re-scattered into undesired channels - imposes a fundamental limit in the fidelities of many quantum applications, such as quantum memories and gates, key ingredients to realize protocols for controllable processing and storage of quantum information.

Within this context, the overall objectives of LANTERN are:

• To theoretically propose new techniques to manipulate atom-photon interactions that overcome major bottlenecks faced by current experiments to use atoms in quantum information processing and quantum simulation.
• To guide state of the art experiments with realistic photonic crystal structures (dielectric structures with a spatially-varying refractive index that guide light) that can be used to induce low-dissipation, tunable, long-range interactions between atoms.
• To understand the phenomenon of atomic collective dissipation and harness it to improve the performance of quantum information protocols.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The results obtained since the beginning of the project have been disseminated in several scientific articles, two of them published in high-impact journals (Physical Review X, and Proceedings of the National Academy of Sciences). The most significant publication, which lays the foundations for the field of collective dissipation in atomic lattices and discusses the exponential improvement in the performance of a quantum memory is:

• A. Asenjo-Garcia, M. Moreno-Cardoner, A. Albrecht, H. J. Kimble, and D. E. Chang, Physical Review X, 7, 031024 (2017)

Theoretical and experimental results concerning collective atomic interactions mediated by photonic crystals are:

• Asenjo-Garcia, J. D. Hood, D. E. Chang, and H. J. Kimble, Physical Review A, 95, 033818 (2017).
• J. D. Hood, A. Goban, A. Asenjo-Garcia, M. Lu, S.-P. Yu, D. E. Chang, and H. J. Kimble, PNAS, 113, 10507 (2016).

Further articles (under revision) regarding the realization of a 2D quantum memory, the influence of multilevel atomic structure in collective dipole interaction, and the extension of these ideas to circuit quantum electrodynamics, respectively, are:

• L. Henriet, A. Albrecht, A. Asenjo-Garcia, P. Dieterle, O. J. Painter, and D. E. Chang, arXiv: 1803.02115 (2018)
• E. Munro, A. Asenjo-Garcia, Y. Lin, L. C. Kwek, C. A. Regal, and D. E. Chang, arXiv: 1712.02061 (2017)
• M. T. Manzoni, M. Moreno-Cardoner, A. Asenjo-Garcia, J. V. Porto, A. V. Gorshkov, and D. E. Chang, arXiv: 1710.06312 (2017)

Besides the publications, the fellow has given several invited talks at international conferences, and seminars at universities including Columbia, Harvard, Rochester, Maryland, McGill, Waterloo and Innsbruck.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

During the first part of the action, the fellow has demonstrated that new opportunities for optical physics emerge from the integration of cold atoms with nanophotonic devices. Due to their small optical loss and tight field confinement, photonic crystal devices are capable of mediating strong atom-light interactions and open new avenues for quantum transport and quantum many-body phenomena. In particular, coupling atoms to the band edge of a photonic crystal waveguide provides a unique platform for generating tunable range coherent atom-atom interactions which are mediated by the guided mode photons. Due to the evanescent nature of the field in the band gap, dissipation into the structure is suppressed exponentially. Working with the group of Prof. Kimble at Caltech, the fellow has reported the first experimental observation of cooperative interaction of atoms around the band-edge of a photonic crystal. In addition, part of the efforts in this action have been directed to developing a formalism that allows to identify signatures of dispersive and dissipative interactions between the atoms. Equipped with this model, one can infer for the collective decay rates experienced by the atoms.

Furthermore, LANTERN n has demonstrated that interactions between atoms in free space can also be used to supress dissipation. Typically, it is assumed that photon loss occurs at a rate given by a single isolated atom. However, this assumption can be dramatically violated: interference in photon emission and absorption generates correlations and entanglement among atoms, thus making dissipation a collective phenomenon. Within this Marie Curie action the state of the art with respect to collective atomic dissipation has been significantly advanced. We have shown that, for atomic arrays in free space, subradiant states acquire an elegant interpretation in terms of optical guided modes, which only emit due to scattering from the ends of the finite system. By interfacing atomic chains with nanophotonic structures, these states can be excited straightforwardly. Exploiting their radiative properties allows for the realization of a quantum memory with a photon retrieval fidelity that performs exponentially better with number of atoms than previously known bounds.

In the remainder of the project, the fellow will focus in studying the many opportunities offered by subradiance. Better understanding of correlated dissipation might yield new error bounds for quantum applications in metrology and non-linear optics.

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