Single photons role in the development of quantum science and
technology. They can carry quantum information over extended distances
to act as the backbone of a future quantum Internet(1) and can be
manipulated in advanced photonic circuits, enabling scalable photonic
quantum computing(2,3). However, more sophisticated devices and
protocols need access to multi-photon states with particular forms of
entanglement. Efficient light-matter interfaces offer a route to
reliably generating these entangled resource states(4,5). Here we
utilize the efficient and coherent coupling of a single quantum emitter
to a nanophotonic waveguide to realize a quantum nonlinear interaction
between single-photon wavepackets. We demonstrate the control of a
photon using a second photon mediated by the quantum emitter. The
dynamical response of the two-photon interaction is experimentally
unravelled and reveals quantum correlations controlled by the pulse
duration. Further development of this platform work, which constitutes a
new research frontier in quantum optics(6), will enable the tailoring of
complex photonic quantum resource states.

On-chip chiral quantum light-matter interfaces, which support
directional interactions, provide a promising platform for efficient
spin-photon coupling, nonreciprocal photonic elements, and quantum logic
architectures. We present full-wave three-dimensional calculations to
quantify the performance of conventional and topological photonic
crystal waveguides as chiral emitter-photon interfaces. Specifically,
the ability of these structures to support and enhance directional
interactions while suppressing subsequent backscattering losses is
quantified. Broken symmetry waveguides, such as the nontopological
glide-plane waveguide and topological bearded interface waveguide are
found to act as efficient chiral interfaces, with the topological
waveguide modes allowing for operation at significantly higher Purcell
enhancement factors. Finally, although all structures suffer from
backscattering losses due to fabrication imperfections, these are found
to be smaller at high enhancement factors for the topological waveguide.
These reduced losses occur because the optical mode is pushed away from
the air-dielectric interfaces where scattering occurs, and not because
of any topological protection. These results are important to the
understanding of light-matter interactions in topological photonic
crystal and design of efficient, on-chip chiral quantum devices.

Coherent photon-emitter interfaces offer a way to mediate efficient
nonlinear photon-photon interactions, much needed for quantum
information processing. Here we experimentally study the case of a
two-level emitter, a quantum dot, coupled to a single optical mode in a
nanophotonic waveguide. We carry out few-photon transport experiments
and record the statistics of the light to reconstruct the scattering
matrix elements of one- and two-photon components. This provides direct
insight to the complex nonlinear photon interaction that contains rich
many-body physics.

We report an entanglement swapping protocol implemented between
single-photon entanglement and hybrid discrete- and continuous-variables
entanglement of light, allowing the connection of disparate nodes in a
heterogeneously-structured quantum internet. (C) 2021 The Author(s)

Advanced quantum technologies, as well as fundamental tests of quantum
physics, crucially require the interference of multiple single photons
in linear-optics circuits. This interference can result in the bunching
of photons into higher Fock states, leading to a complex bosonic
behavior. These challenging tasks timely require to develop collective
criteria to benchmark many independent initial resources. Here we
determine whether n independent imperfect single photons can ultimately
bunch into the Fock state vertical bar n >. We thereby introduce an
experimental Fock-state bunching capability for single-photon sources,
which uses phase-space interference for extreme bunching events as a
quantifier. In contrast to autocorrelation functions, this operational
approach takes into account not only residual multi-photon components
but also a vacuum admixture and the dispersion of individual photon
statistics. We apply this approach to high-purity single photons
generated from an optical parametric oscillator and show that they can
lead to a Fock-state capability of at least 14. Our work demonstrates a
novel collective benchmark for single-photon sources and their use in
subsequent stringent applications. (C) 2021 Optical Society of America
under the terms of the OSA Open Access Publishing Agreement

Recent advances in quantum technologies are rapidly stimulating the
building of quantum networks. With the parallel development of multiple
physical platforms and different types of encodings, a challenge for
present and future networks is to uphold a heterogeneous structure for
full functionality and therefore support modular systems that are not
necessarily compatible with one another. Central to this endeavor is the
capability to distribute and interconnect optical entangled states
relying on different discrete and continuous quantum variables. Here, we
report an entanglement swapping protocol connecting such entangled
states. We generate single-photon entanglement and hybrid entanglement
between particle- and wave-like optical qubits and then demonstrate the
heralded creation of hybrid entanglement at a distance by using a
specific Bell-state measurement. This ability opens up the prospect of
connecting heterogeneous nodes of a network, with the promise of
increased integration and novel functionalities.

Coherent quantum optics, where the phase of a photon is not scrambled as
it interacts with an emitter, lies at the heart of many quantum optical
effects and emerging technologies. Solid-state emitters coupled to
nanophotonic waveguides are a promising platform for quantum devices, as
this element can be integrated into complex photonic chips. Yet,
preserving the full coherence properties of the coupled
emitter-waveguide system is challenging because of the complex and
dynamic electromagnetic landscape found in the solid state. Here, we
review progress toward coherent light-matter interactions with
solid-state quantum emitters coupled to nanophotonic waveguides. We
first lay down the theoretical foundation for coherent and nonlinear
light-matter interactions of a two-level system in a
quasi-one-dimensional system, and then benchmark experimental
realizations. We discuss higher order nonlinearities that arise as a
result of the addition of photons of different frequencies, more complex
energy level schemes of the emitters, and the coupling of multiple
emitters via a shared photonic mode. Throughout, we highlight protocols
for applications and novel effects that are based on these coherent
interactions, the steps taken toward their realization, and the
challenges that remain to be overcome.

The generation and manipulation of hybrid entanglement of light
involving discrete- and continuous-variable states have recently
appeared as essential resources towards the realization of heterogeneous
quantum networks. Here we investigate a scheme for the remote generation
of hybrid entanglement between particle-like and wave-like optical
qubits based on a non-local heralding photon detection. We also extend
this scheme with additional local or non-local detections. An additional
local heralding allows the resulting state to exhibit a higher fidelity
with the targeted entangled qubits while a two-photon non-local
heralding detection gives access to a higher dimensionality in the
discrete-variable subspace, resulting thereby in the generation of
hybrid entangled qutrits. The implementation of the presented schemes,
in combination with ongoing works on high-fidelity quantum state
engineering, will provide novel non-classical light sources for the
development of optical hybrid architectures.

Establishing a highly efficient photon-emitter interface where the
intrinsic linewidth broadening is limited solely by spontaneous emission
is a key step in quantum optics. It opens a pathway to coherent light
matter interaction for, e.g., the generation of highly indistinguishable
photons, few photon optical nonlinearities, and photon-emitter quantum
gates. However, residual broadening mechanisms are ubiquitous and need
to be combated. For solid-state emitters charge and nuclear spin noise
are of importance, and the influence of photonic nanostructures on the
broadening has not been clarified. We present near-lifetime-limited
linewidths for quantum dots embedded in nanophotonic waveguides through
a resonant transmission experiment. It is found that the scattering of
single photons from the quantum dot can be obtained with an extinction
of 66 +/- 4%, which is limited by the coupling of the quantum dot to
the nanostructure rather than the linewidth broadening. This is obtained
by embedding the quantum dot in an electrically contacted nanophotonic
membrane. A clear pathway to obtaining even larger single-photon
extinction is laid out; i.e., the approach enables a fully deterministic
and coherent photon-emitter interface in the solid state that is
operated at optical frequencies.

We report on high-efficiency superconducting nanowire single-photon
detectors based on amorphous tungsten silicide and optimized at 1064 nm.
At an operating temperature of 1.8 K, we demonstrated a 93% system
detection efficiency at this wavelength with a dark noise of a few
counts per second. Combined with cavity-enhanced spontaneous parametric
downconversion, this fiber-coupled detector enabled us to generate
narrowband single photons with a heralding efficiency greater than 90%
and a high spectral brightness of 0.6 x 10(4) photons/(s . mW . MHz).
Beyond single-photon generation at large rate, such high-efficiency
detectors open the path to efficient multiple-photon heralding and
complex quantum state engineering. (C) 2016 Optical Society of America.

The wave-particle duality of light has led to two different encodings
for optical quantum information processing. Several approaches have
emerged based either on particle-like discrete-variable states (that is,
finite-dimensional quantum systems) or on wave-like continuous-variable
states (that is, infinite-dimensional systems). Here, we demonstrate the
generation of entanglement between optical qubits of these different
types, located at distant places and connected by a lossy channel. Such
hybrid entanglement, which is a key resource for a variety of recently
proposed schemes, including quantum cryptography and computing, enables
information to be converted from one Hilbert space to the other via
teleportation and therefore the connection of remote quantum processors
based upon different encodings. Beyond its fundamental significance for
the exploration of entanglement and its possible instantiations, our
optical circuit holds promise for implementations of heterogeneous
network, where discrete-and continuous-variable operations and
techniques can be efficiently combined.

We present the first monolithic source that generates polarization
entangled photon pairs integrated on a silicon photonic chip. The
maximally-entangled photon pairs were generated with a state fidelity of
94% well above the classical limit.

We present a polarization-entangled photon pair source fully integrated
on a silicon photonic circuit. Using two silicon wire waveguides
connected with a silicon polarization rotator, we demonstrate a
generation of polarization-entangled photons.

Integrated photonic circuits are one of the most promising platforms for
large-scale photonic quantum information systems due to their small
physical size and stable interferometers with near-perfect lateral-mode
overlaps. Since many quantum information protocols are based on qubits
defined by the polarization of photons, we must develop integrated
building blocks to generate, manipulate, and measure the
polarization-encoded quantum state on a chip. The generation unit is
particularly important. Here we show the first integrated
polarization-entangled photon pair source on a chip. We have implemented
the source as a simple and stable silicon-on-insulator photonic circuit
that generates an entangled state with 91 +/- 2% fidelity. The source
is equipped with versatile interfaces for silica-on-silicon or other
types of waveguide platforms that accommodate the polarization
manipulation and projection devices as well as pump light sources.
Therefore, we are ready for the full-scale implementation of photonic
quantum information systems on a chip.