The Bridge - February 2018 - 12

Feature
pulse), such photon pairs show strong correlations in
time and frequency. To increase the efficiency of this
process and also provide the equidistant-line feature
of combs, cavity enhancement in on-chip resonant
structures is used, particularly in high-quality factor
microring resonators (Fig. 3) [43]. Due to the
enhancement and self-filtering of the resonances,
the generated photons exhibit high purity and
brightness, and low noise backgrounds, properties
that are desirable for quantum experiments.

the bi-photon state has a Schmidt mode number
of close to 1 (corresponding to a pure separable
state) [21]. Selecting one pair of signal and idler
photon resonances has enabled heralded sources
in Si-microrings [4, 6] and Si-microdisks [7], as
well as amorphous Si-microrings [9]. Entangled
photon pair generation has been demonstrated
in Si-microring resonators including time-bin [10,
13], energy-time [14-16], polarization [16], and
time-bin entanglement in SiN [12] resonators.

Fig.3. Quantum frequency comb generation in integrated
Hydex resonators. (a) Via spontaneous four-wave mixing
inside the nonlinear microcavity [43], two pump photons
at frequency (ωр) are converted to one signal and
one idler photon at frequencies (ωі and ωѕ), with
energy conservation demanding (ωі+ωѕ=2ωр).
Inset: an integrated Hydex photonic chip compared
to a Canadian one-dollar coin. (b) A broad measured
quantum frequency comb spectrum spanning from the
S to the L telecommunications band [11].

Path-entanglement in silicon resonators has been
demonstrated using two individual cavities [17]. In
addition, because of the multi-channel property,
these on-chip entangled quantum sources exhibit
compatibility with telecommunications wavelength
multiplexing techniques [5, 13, 15]. One challenge
to all of these approaches, however, is that these
schemes require external lasers in order to excite
the cavities. This can be resolved through the use of
an actively-modulated, nested-cavity configuration,
which additionally enables the easy increase of
the repetition rate of the excitation pulse train via
harmonic mode-locking, and thus the increase of the
pair production rates, while maintaining a low noise
background and high photon purity [46].

Such an integrated quantum frequency comb is
important for quantum information processing
because of its effective photon generation
process [44, 45]. By optically exciting a single
cavity resonance, SFWM symmetrically populates
neighboring resonances with photon pairs, creating
a highly stable source of several channels of
heralded single photons (where the measurement
of the signal heralds the presence of the idler,
and vice versa) [5]. Using photon auto-correlation
measurements, it was verified that a pure single
frequency mode photon was generated in the
signal and idler resonances, respectively, and that

THE BRIDGE

The application of quantum frequency combs
has evolved into the generation of large-scale
quantum states, which can be achieved by
increasing the number of entangled photons and/
or their dimensionality. Although multi-photon
entanglement [22, 23-26] and high-dimensional
states [19, 20] have been demonstrated using



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
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