The Bridge - February 2018 - 13

Quantum State Generation in Optical Frequency Combs for Quantum Computing

large-scale free-space optics, this has only been
achieved in an integrated platform very recently. A
double-pulse excitation of a single resonance was
used to demonstrate the generation of time-bin
entangled photon pairs over the entire frequency
comb spectrum [11]. The distinctive multimode
characteristic of the frequency comb allowed the
demonstration of four-photon time-bin entangled
photon states by post-selecting two signal and
idler pairs on different resonances simultaneously.
The realization of this four-photon entangled state
was confirmed through quantum interference as
well as quantum state tomography [11]. From a
different point of view, photon pairs (signal and
idler) can be generated in a quantum superposition
of many frequency modes. This approach leads to
the realization of two-photon frequency-entangled
quDits, and resulted in the demonstration of
a quantum system with at least one hundred
dimensions, formed by two entangled quDits with D
= 10 [21]. In order to perform deterministic quDit
gate operations, a coherent manipulation platform
with which to control frequency-entangled states was
introduced using off-the-shelf telecommunications
components (e.g. electro-optic phase modulators
and programmable optical filters). The platform was
validated by measuring Bell inequality violations and
performing quantum state tomography for D=4.
Frequency-bin qubits with 40 mode pairs were also
investigated using SiN resonators with a FSR of 50
GHz [47].
Besides using a single resonator, quantum frequency
combs have also been generated in coupledresonator optical waveguides (CROWs) consisting
of cascaded cavities [48, 49]. Due to the slow-light
enhancement effort in the CROW device, where the
group velocity of light slows down as a result of the
spatial and temporal compression of the local energy
density [50], the total number of generated photons
in each passband scales quadratically with the
resonator number N [51]. Although the bandwidth
of each passband increases with the number

of resonators, the CROW structure still provides
a significant enhancement of pair generation at
each frequency (e.g. in time-bin entanglement
[48]) compared to that in straight waveguides.
Furthermore, it has been shown that when one
carefully tunes the resonances in the individual
devices, each passband forms a quantum frequency
comb since the N resonators create "supermodes" of
its N transmission resonances within each passband
[49]. Thus this structure has the potential of creating
a double-level quantum frequency comb, where
a narrowband frequency comb (with a number
of resonances scaling with N) in each passband
is buried inside a broadband quantum frequency
comb, which is separated by the resonator FSR.

SUMMARY AND OUTLOOK
Optical quantum frequency combs are of high
interest for quantum information processing.
Due to their intrinsic multimode property, both
in bulk setups and integrated chips, they are
a scalable platform for generating qubits and
even large-scale quantum states. Their versatility
has been demonstrated by their use as singlephoton, entangled-photon, high-dimensional,
and multi-photon quantum state sources. The
frequency domain provides a unique framework
for the manipulation of quantum states in a single
spatial mode using standard telecommunications
components. Quantum information processing
with frequency-encoded photons not only offers
great potential towards building robust optical
interconnects, but also brings the ubiquitous
technology of well-established fiber optics to
quantum photonics [28]. Their applications, as
predicated in the realization of spectral linear
quantum computing and the implementation of
boson sampling, suggest they will keep playing an
important role in future practical implementations of
quantum technologies.
Manipulating a large number of frequency modes
is still very challenging. Although χ(2)-based OPO's

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