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Dynamics of Many-Body Photon Bound States in Chiral Waveguide QED

Sahand Mahmoodian, Giuseppe Calajó, Darrick E. Chang, Klemens Hammerer, and Anders S. Sørensen
Phys. Rev. X 10, 031011 – Published 14 July 2020
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Abstract

We theoretically study the few- and many-body dynamics of photons in chiral waveguides. In particular, we examine pulse propagation through an ensemble of N two-level systems chirally coupled to a waveguide. We show that the system supports correlated multiphoton bound states, which have a well-defined photon number n and propagate through the system with a group delay scaling as 1/n2. This has the interesting consequence that, during propagation, an incident coherent-state pulse breaks up into different bound-state components that can become spatially separated at the output in a sufficiently long system. For sufficiently many photons and sufficiently short systems, we show that linear combinations of n-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency. Our work thus covers the entire spectrum from few-photon quantum propagation, to genuine quantum many-body (atom and photon) phenomena, and ultimately the quantum-to-classical transition. Finally, we demonstrate that the bound states can undergo elastic scattering with additional photons. Together, our results demonstrate that photon bound states are truly distinct physical objects emerging from the most elementary light-matter interaction between photons and two-level emitters. Our work opens the door to studying quantum many-body physics and soliton physics with photons in chiral waveguide QED.

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  • Received 29 November 2019
  • Revised 7 May 2020
  • Accepted 11 June 2020

DOI:https://doi.org/10.1103/PhysRevX.10.031011

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Atomic, Molecular & Optical

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Corralling Groups of Photons

Published 14 July 2020

A proposed optical waveguide would take a light pulse and break it into sets of strongly correlated photons, which may give a leg up to certain quantum technologies.

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Authors & Affiliations

Sahand Mahmoodian1, Giuseppe Calajó2, Darrick E. Chang2,3, Klemens Hammerer1, and Anders S. Sørensen4

  • 1Institute for Theoretical Physics, Institute for Gravitational Physics (Albert Einstein Institute), Leibniz University Hannover, Appelstraße 2, 30167 Hannover, Germany
  • 2ICFO-Institut de Ciencies Fotoniques, The Barcelona Institute of Science and Technology, 08860 Castelldefels (Barcelona), Spain
  • 3ICREA-Institució Catalana de Recerca i Estudis Avançats, 08015 Barcelona, Spain
  • 4Center for Hybrid Quantum Networks (Hy-Q), Niels Bohr Institute, University of Copenhagen, Blegdamsvej 17, DK-2100 Copenhagen, Denmark

Popular Summary

When a single photon interacts with a two-level atom, the atom can delay the photon by absorbing the photon and then reemitting it. Here, we explore what happens if many photons interact with such an atom and what happens if the photons interact consecutively with more than one atom. We show that stimulated emission plays a central role and, if there are enough atoms, that a pulse of laser light can be broken apart into separate pulses, each containing a different number of bound photons.

In a photon bound state, the observation of one photon is strongly correlated with the quick arrival of other photons. We show that photon bound states have a number-dependent dispersion relation and therefore propagate with a number-dependent velocity as they interact with the atoms: Photon bound states with a greater number of photons are delayed less. In a pulse of classical laser light, which contains a distribution of photons, these larger bound states become separated from smaller ones.

Typically, the shape of a pulse of light is broadened and distorted when it linearly interacts with a dispersive element such as an optical cavity or an atom. But we show that the photon bound states propagate with significantly reduced distortion. In the classical limit of many photons, they propagate like a soliton: classical pulses of light where nonlinear effects and pulse-broadening effects compensate for each other and the pulse can propagate without changing shape.

Our work provides a link between well-known classical solitons and photon bound states and opens the door to quantum technologies such as metrology that can benefit from improved precision of photon bound states.

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Vol. 10, Iss. 3 — July - September 2020

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