30. K. D. B. Higgins et al., Nat. Commun. 5, 4705 (2014).
31. K. Hammerer, A. S. Sørensen, E. S. Polzik, Rev. Mod. Phys.
82, 1041–1093 (2010).
We thank M. Lee and H.-G. Hong for helpful comments. This work was
supported by a grant from the Samsung Science and Technology
Foundation under project SSTF-BA1502-05. We declare no competing
financial interests. All data needed to evaluate the conclusions in the
paper are present in the paper and/or the supplementary materials.
Materials and Methods
Figs. S1 to S6
15 October 2017; accepted 7 December 2017
Published online 21 December 2017
A topological quantum
Sabyasachi Barik,1,2 Aziz Karasahin,3 Christopher Flower,1,2 Tao Cai,3
Hirokazu Miyake,2 Wade DeGottardi,1,2 Mohammad Hafezi,1,2,3 Edo Waks1,2,3*
The application of topology in optics has led to a new paradigm in developing photonic
devices with robust properties against disorder. Although considerable progress on
topological phenomena has been achieved in the classical domain, the realization of strong
light-matter coupling in the quantum domain remains unexplored. We demonstrate a
strong interface between single quantum emitters and topological photonic states. Our
approach creates robust counterpropagating edge states at the boundary of two distinct
topological photonic crystals. We demonstrate the chiral emission of a quantum emitter
into these modes and establish their robustness against sharp bends. This approach
may enable the development of quantum optics devices with built-in protection, with
potential applications in quantum simulation and sensing.
The discovery of the quantum Hall effects has inspired developments in similar topo- logical phenomena in a range of platforms, including ultracold neutral atoms (1, 2), photonics (3, 4), and mechanical structures (5–7). Like their electronic analogs, topological photonic states are distinctive in their
directional transport and reflectionless propagation along the interface of two topologically
distinct regions. Such robustness has been demonstrated in various electromagnetic systems,
ranging from the microwave (8, 9) to the optical
(10, 11) domain, opening avenues for a plethora
of applications—such as robust delay lines, slow-light optical buffers (12), and topological lasers
(13–15)—to develop optical devices with built-in
protection. Although the scope of previous work
has remained in the classical electromagnetic
regime, interesting physics could emerge by bringing topological photonics to the quantum domain.
Specifically, integrating quantum emitters into
topological photonic structures could lead to
robust, strong light-matter interaction (16) and
the generation of novel states of light and exotic
many-body states (17–19).
We experimentally demonstrated light-matter
coupling in a topological photonic crystal. We
used an all-dielectric structure (20–22) to imple-
ment topologically robust edge states at the inter-
face between two topologically distinct photonic
materials, where the light is transversally trapped
in a small area, up to half of the wavelength of
light. We show that a quantum emitter efficiently
couples to these edge modes and that the emitted
single photons exhibit robust transport, even in
the presence of a bend. Figure 1A shows the
fabricated topological photonic crystal structure.
The device is composed of a thin GaAs membrane
with epitaxially grown InAs quantum dots at the
center that act as quantum emitters (22).
The topological photonic structure comprises
two deformed honeycomb photonic crystal lattices made of equilateral triangular air holes
(fig. S2) on a GaAs membrane (21, 22). Figure 1B
shows a close-up image of the interface, where
the black dashed lines identify a single unit cell
of each photonic crystal. In each region, we perturb the unit cell by concentrically moving the
triangular holes either inward (yellow region) or
outward (blue region). The corresponding band
structures of the two regions are shown in Fig. 1,
C and D. The perturbations open two bandgaps
exhibiting band inversion at the G point (20, 21).
Specifically, the region with a compressed unit
cell, highlighted in yellow, acquires a topologically
trivial bandgap, whereas the expanded region,
highlighted in blue, takes on a nontrivial one.
We designed both regions so that their bandgaps
overlap. Photons within the common bandgap
cannot propagate into either photonic crystal.
However, because the crystals have different topological band properties, the interface between
them supports two topological helical edge modes,
traveling in opposite directions, with opposite circular polarizations at the center of the unit cell.
To show the presence of the guided edge mode,
we measured the transmission spectrum. We
illuminated the left grating (“L”) with a 780-nm
continuous-wave laser using a pump power of
1.3 m W and collected the emission from the right
666 9 FEBRUARY 2018 • VOL 359 ISSUE 6376 sciencemag.org SCIENCE
1Department of Physics, University of Maryland, College
Park, MD 20742, USA. 2Joint Quantum Institute, University
of Maryland, College Park, MD 20742, USA. 3Department of
Electrical and Computer Engineering and Institute for
Research in Electronics and Applied Physics, University of
Maryland, College Park, MD 20742, USA.
*Corresponding author. Email: email@example.com (M.H.);
firstname.lastname@example.org (E. W.)
Fig. 1. Fabricated device and band structure. (A) Scanning electron microscope image of the
device, which is composed of two regions identified by blue and yellow shading, corresponding
to two photonic crystals with different topological properties. The interface between the two
photonic crystals supports helical edge states with opposite circular polarization (s+ and s–).
Grating couplers at each end of the device scatter light in the out-of-plane direction for
collection. (B) Close-up image of the interface. Black dashed lines identify a single unit cell of
each photonic crystal. (C and D) Band structures for the transverse electric modes of the two