
Letter Optics Letters 1
Quantum electrodynamics of chiral and antichiral
waveguide arrays
JEREMY G. HOSKINS1, MANAS RACHH2,AND JOHN C. SCHOTLAND3*
1Department of Statistics, University of Chicago, Chicago, Illinois 60637, USA
2Center for Computational Mathematics, Flatiron Institute, New York, New York 10010, USA
3Department of Mathematics and Department of Physics, Yale University, New Haven, Connecticut 06511, USA
*Corresponding author: john.schotland@yale.edu
Compiled January 6, 2023
We consider the quantum electrodynamics of single
photons in arrays of one-way waveguides, each con-
taining many atoms. We investigate both chiral and
antichiral arrays, in which the group velocities of the
waveguides are the same or alternate in sign, respec-
tively. We find that in the continuum limit, the one-
photon amplitude obeys a Dirac equation. In the chi-
ral case, the Dirac equation is hyperbolic, while in the
antichiral case it is elliptic. This distinction has impli-
cations for the nature of photon transport in waveguide
arrays. Our results are illustrated by numerical simula-
tions. © 2023 Optica Publishing Group
http://dx.doi.org/10.1364/ao.XX.XXXXXX
There has been considerable recent interest in the quantum
electrodynamics of light-matter interactions in waveguide sys-
tems [
1
–
6
]. The reduced dimensionality of such systems gives
rise to new physical phenomena, especially modifications of
spontaneous and stimulated emission. Moreover, the enhanced
coupling between atoms and photons can lead to the generation
of strong correlations between photons. Similarly, long-ranged
interactions between atoms bring about a variety of novel many-
body effects. We also note that architectures based on coupled
waveguide arrays have emerged as promising candidates for
quantum circuits and other quantum technologies.
Waveguides, in which the propagation of light is predomi-
nantly in one direction, allow for the intriguing possibility of
serving as one-way carriers of quantum information [
7
–
9
]. The
attendant violation of reciprocity in such waveguides is due to
enhanced spin-orbit coupling of light that is confined at sub-
wavelength scales [
10
–
12
]. A model of single photons in a one-
way waveguide containing a collection of two-level atoms was
introduced in [
13
]. It was found that the absence of backscatter-
ing prohibits the existence of band structure in periodic systems.
Moreover, single-photon transport is unaffected by position dis-
order, but not by disorder in atomic transition frequencies, where
Anderson localization obtains. Related results for three-level
atoms and electromagnetically induced transparency have also
been described [14].
In this Letter, we consider the quantum electrodynamics of a
single photon in an array of one-way waveguides. The waveg-
uides, each of which contains many two-level atoms, are ar-
ranged in a one-dimesional lattice, as illustrated in Fig. 1. Two
physical settings are distinguished. For a chiral array, the fre-
quencies alternate in value between two interpenetrating sub-
lattices. In an antichiral array, the group velocities alternate in
sign between the sublattices. In both cases, we will show that
in the continuum limit, the one-photon amplitude of a single-
excitation state obeys a two-dimensional Dirac equation. In a
chiral array, the coordinate along the waveguide is timelike and
the transverse coordinate is spacelike. In contrast, in an antichi-
ral array, both coordinates are spacelike. We will see that the
distinction between the timelike and spacelike character of the
Dirac equations has physical consequences which we illustrate
with numerical simulations.
We begin by considering a one-dimensional array of chiral
waveguides. Each waveguide is assumed to contain many two-
level atoms. We employ a real-space quantization procedure that
treats the atoms and the optical field on the same footing [
13
].
The Hamiltonian of the system is of the form
H=HA+HF+HI
.
Here the atomic Hamiltonian HAis given by
HA=ω0Zdx ∑
n
ρn(x)σ†(x)σ(x).(1)
Here we work in units where
¯h=
1,
ω0
is the atomic transition
frequency of the atoms, and
ρn(x) = ∑jδ(x−xjn )
is the number
density of atoms in the
n
th waveguide, where
xjn
is the position
of atom
j
in waveguide
n
. In addition
σ(xjn) = |
0
jnih
1
jn|
is
the lowering operator for an atom at position
x
in waveguide
n
, where
|
0
jni
and
|
1
jni
denote the corresponding ground and
excited states of the atom. It follows that
σ
obeys the anticom-
mutation relations
{σ(xin ),σ†(xjm )}=δijδnm (2)
and the commutation relations
[σ(xin ),σ(xjm )] = 0 , (3)
with all other commutators and anticommutators vanishing. The
Hamiltonian of the optical field HFis of the form
HF=Zdx ∑
nφ†
n(x)(Ωn+ivn∂x)φn(x)(4)
+J0(φ†
n(x)φn+1(x) + φ†
n+1(x)φn(x)).(5)
arXiv:2210.04082v2 [physics.optics] 5 Jan 2023