The Bridge - February 2018 - 14

Feature
can generate thousands of modes (with a FSR
of hundreds of MHz), this process requires the
additional stabilization and manipulation of individual
modes - not possible with current technology.
While on-chip χ(3)-based resonators allow stable and
scalable photon generation, as well as coherent state
control of each mode, due to the relatively large
micro-cavity FSR (on the order of tens of GHz), only
10 frequency modes have been demonstrated in
the integrated platform with the manipulation of
up to 4 modes. Reducing the FSR by increasing the
resonator circumference would, in principle, increase
the number of accessible modes; however, the new
FSR should still be large enough (>10 GHz - the
resolution of state-of-art optical filters) for photonic
components to address each mode individually.
In addition, the photons would experience higher
propagation loss inside the resonators because of
the large cavity circumference. Further increasing the
scale of quantum states by using multiple particles
and/or combining several degrees of freedom
is an option, as predicted with frequency combs
generated by both bulk optics [26, 37] and on-chip
[39, 52] approaches.
A new direction towards generating quantum
frequency combs is to use χ(2)-based micro-cavities.
This approach, which combines the advantages of
high efficiency χ(2) processes with on-chip stability
and enhancement, has been used to generate
entangled photons in LiNbO3 microdisks [53] and
heralded photons in Al-Ni microring resonators [54],
as well as being the basis of theoretical predictions
focusing on photon pair generation in AlGaAs
nanodisks [55]. Most importantly, this scheme has
the potential to create enough squeezing to reach
the fault tolerance threshold in quantum computing
with continuous-variable cluster states [56]. In
addition, since the pump is separated spectrally
far from the generated photon pairs, resonators
using χ(2) spontaneous parametric down-conversion
provide efficient pump suppression compared to
cavity resonators based on χ(3)spontaneous FWM

THE BRIDGE

processes [54]. Moreover, since this platform also
permits integrated low-loss, high-speed electrooptic phase modulation, it is promising for realizing
sufficient on-chip filtering and monolithic integration
in terms of generating and processing components.
The control and detection of quantum frequency
combs is not yet fully integrated. High-isolation filters
are indeed needed to separate the excitation field
from the photons and the detectors, while the phase
modulators processing these sources, in particular,
require discrete bulk devices until comparable
integrated components are developed [56].
Different platforms are suited to complementary
functionalities. For example, χ(2) materials permit
integrated low-loss, high-speed electro-optic phase
modulation, while χ(3) materials allow integrated
generation and efficient detection of quantum
light. Therefore, hybrid systems, where the device
material and geometry can be individually optimized,
might be an alternative option for multi-functional
integrated circuits, rather than the use of fully
monolithic platforms.
In closing, though there is still a long way before
we can apply optical quantum frequency combs
in practical quantum computing, we believe that
this approach may help address the increasing
scalability demands of quantum technologies in
the near future.



Table of Contents for the Digital Edition of The Bridge - February 2018

Contents
The Bridge - February 2018 - Cover1
The Bridge - February 2018 - Cover2
The Bridge - February 2018 - Contents
The Bridge - February 2018 - 4
The Bridge - February 2018 - 5
The Bridge - February 2018 - 6
The Bridge - February 2018 - 7
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