Light-matter interaction in one-dimensional continuum: Waveguide QED

A superconducting qubit (marked with an arrow) embedded in a one-dimensional transmission line waveguide. A cavity is formed by two capacitive gaps in the middle conductor. (b) Without the cavity. (c) Probe light focused through a dense atomic cloud, exciting a single Rydberg atom within the blockade sphere (dashed line).

Circuit [cavity] QED set-up: (a) A superconducting qubit (marked with an arrow) embedded in a one-dimensional transmission line waveguide. A cavity is formed by two capacitive gaps in the middle conductor. Waveguide QED set-ups: (b) Without the cavity. (c) Probe light focused through a dense atomic cloud, exciting a single Rydberg atom within the blockade sphere (dashed line). Adapted from Rev. Mod. Phys. 89, 021001 (2017).

Photons, the carriers of the electromagnetic force, are elementary particles with no charge and zero rest mass. Thus, photon-photon scattering in vacuum is extremely weak. However, strong effective interactions between single photons can be realized by employing strong light-matter coupling. These interactions are a fundamental building block for quantum optics, bringing many-body physics to the photonic world and providing important resources for quantum photonic devices and for optical metrology.

Cavity quantum electrodynamics (QED) is a paradigmatic discipline exhibiting strong light-matter interactions in the quantum regime. In cavity QED, atoms (or qubits) are placed inside a high-finesse electromagnetic resonator, in which the radiation spectrum is discrete. Bouncing between the resonator mirrors, a single photon interacts with the atoms effectively many times, significantly enhancing the atom-photon coupling. This in turn can generate strong correlations between the photons. Nevertheless, the cavities used to enhance the coupling in these systems also present several disadvantages, for instance, the narrow bandwidth of the emitted photons and the problem of stochastic release of photons by the cavity. Because of these limitations, much recent work has focused on cavity-free systems. The coupling strength in these system can be quantified by the extinction of a propagating photon by a single atom or qubit. A major barrier to higher extinction in open space is the spatial-mode mismatch between the incident and scattered waves. This problem has recently been solved in two complementary ways, leading to strong coupling and photon-photon interactions in cavity-free one-dimensional (1D) systems. This recent discipline is termed as weveguide QED.

These cavity-free systems feature intrinsically-nonequilibrium, quantum many-body dynamics. The input field is driven by either a laser or microwave generator, imposing a nonequilibrium boundary condition on the propagating photons in 1D. We have developed different theoretical tools using multi-particle scattering theories, and the Heisenberg-Langevin equations approach for studying light propagation in waveguide QED systems. We studied electromagnetically induced transparency [b] and correlated light transmission in these systems [a,c,d], and proposed new all-optical devices, e.g., a nonlinear optical diode/isolator [a] and a two-photon optical probe [c].

(a) Nonlinear optical diode/isolator:

D. Roy, Phys. Rev. B 81, 155117 (2010)

D. Roy, Scientific Reports 3, 2337 (2013)

D. Roy, Phys. Rev. A 96, 033838 (2017)

Experimental Realization: Phys. Rev. Lett. 121 123601 (2018)

(b) Electromagnetically induced transparency:

D. Roy, Phys. Rev. Lett. 106, 053601 (2011)

D. Roy and N. Bondyopadhaya, Phys. Rev. A 89, 043806 (2014)

D. Roy, Phys. Rev. A 87, 063819 (2013)

P. Manasi and D. Roy, Phys. Rev. A 98, 023802 (2018)

(d) Nonlinear energy-momentum dispersion -- structured waveguides:

D. Roy, Phys. Rev. A 83, 043823 (2011)