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

S. Mahmoodian, G. Calajó, D.E. Chang, K. Hammerer, A.S. Sørensen Phys. Rev. X 10, 031011 (2020).

Here we theoretically study the few- and many-body dynamics of photons propagating through a system of two-level atoms chirally coupled to a waveguide. We show that the system supports correlated multi-photon bound states, which propagate through the system with a photon number group velocity. 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 many-body bound states recover the well-known phenomenon of mean-field solitons in self-induced transparency.
If you are interested have also a  look at the popular article on Physics!
Corralling groups of photons.

Quantum acousto-optic control of light-matter interactions in nanophotonic networks

G. Calajo, M. J. A. Shuetz, H. Pichler, M. D. Lukin, P. Schneeweiss, J. Volz, and P. Rabl Physical Review A 99 (5), 053852, 2019. 

Here we analyzed the coupling of atom-like emitters to nanophotonic waveguides in the presence of propagating acoustic waves. We showed that strong index modulations induced by such waves can drastically modify the effective photonic density and enable a dynamical control of light-matter interactions. These results provide a versatile tool for various quantum networking applications.

Exciting a Bound State in the Continuum through Multi-Photon Scattering plus Delayed Quantum Feedback
G. Calajo, Y. L. L. Fang, H. U. Baranger, and F. Ciccarello Physical Review Letters 122 (7), 073601, 2019. 

In this work, we considered a waveguide QED setup where a single-photon bound state in the continuum (BIC) is known to exist under certain conditions. Excitation of this state through scattering is problematic since it is by definition uncoupled. Here we showed that it can be excited in a nonlinear system through multi-photon scattering and delayed quantum feedback. This result could find a direct application in photonic circuits and photon-based quantum memories. 
If you are interested have also a  look at the popular article on Phys.org!

Capturing single photons to explore fundamental physics and quantum information science

Harvesting Multiqubit Entanglement from Ultrastrong Interactions in Circuit Quantum Electrodynamics 
F. Armata, G. Calajo, T. Jaako, M. S. Kim, and P. Rabl Physical Review Letters 119 (18), 183602, 2017. 

In this work, we analyzed a multiqubit circuit QED system in the regime where the qubit-photon coupling dominates over the system's bare energy scales. Under such conditions, a manifold of low-energy states with a high degree of entanglement emerges. Here we described a time-dependent protocol for extracting these quantum correlations and converting them into well-defined entangled states of noninteracting qubits. This result can potentially be exploited as a resource for entanglement-based applications. 

Strong coupling between moving atom and slow-light Cherenkov photons 
G. Calajo, and P. Rabl Physical Review A 95 (4), 043824, 2017. 

In this work we described the coupling of moving atoms to a one-dimensional photonic waveguide in the regime where the atomic velocities are comparable to the effective speed of light. The interplay between a velocity-induced directionality and the emergence of new divergencies in the photonic density of states gives rise to a range of novel non-perturbative phenomena in the emission of photons and the resulting interactions between moving atoms. 

Control of spontaneous emission of a single quantum emitter through a time-modulated photonic band-gap environment 
G. Calajo, L. Rizzuto, R. Passante Physical Review A 96 (2), 023802, 2017. 

Here we solved the spontaneous emission of a two-level quantum emitter in a modulated time-dependent environment with a photonic band gap. Our results showed that a dynamical environment can give further possibilities to modify the spontaneous emission features, such as its spectrum and emission rate, with respect to a static one. 

Atom-field dressed states in slow-light waveguide QED
G. Calajo, F. Ciccarello, D. Chang, and P. Rabl Physical Review A 93 (3), 033833, 2016. 

In this work, we discussed the properties of atom-photon bound states in waveguide QED systems consisting of single or multiple atoms coupled strongly to a finite-bandwidth photonic channel. Such bound states are formed by an atom and a localized photonic excitation Here we presented a detailed analysis of the linear and nonlinear spectral features associated with single- and multi-photon dressed states and show how the formation of bound states affects the waveguide-mediated dipole-dipole interactions between separated atoms.