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#### Dynamics of many-body photon bound states in chiral waveguide QED

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1910.05828.pdf

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PhysRevX.10.031011.pdf

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##### Citation

Mahmoodian, S., Calajó, G., Chang, D. E., Hammerer, K., & Sørensen, A. S. (2020).
Dynamics of many-body photon bound states in chiral waveguide QED.* Physical Review X,* *10*: 031011. doi:10.1103/PhysRevX.10.031011.

Cite as: https://hdl.handle.net/21.11116/0000-0006-C14E-9

##### Abstract

We theoretically study the few- and many-body dynamics of photons in chiral

waveguides. In particular, we examine pulse propagation through a system of $N$

two-level systems chirally coupled to a waveguide. We show that the system

supports correlated multi-photon bound states, which have a well-defined photon

number $n$ and propagate through the system with a group delay scaling as

$1/n^2$. 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. For longer

systems, however, the solitons break apart through quantum correlated dynamics.

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.

waveguides. In particular, we examine pulse propagation through a system of $N$

two-level systems chirally coupled to a waveguide. We show that the system

supports correlated multi-photon bound states, which have a well-defined photon

number $n$ and propagate through the system with a group delay scaling as

$1/n^2$. 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. For longer

systems, however, the solitons break apart through quantum correlated dynamics.

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.