The Bridge - February 2018 - 11

Quantum State Generation in Optical Frequency Combs for Quantum Computing

grating filter [18], followed up by experiments based
on programmable optical filters [37]. This spectral
filtering method has been extended to create highdimensional entanglement, including the verification
of quDits (D=4) in PPKTP using spatial light
modulators [19], the theoretical prediction of quDits
with a dimensionality of several tens in AlGaAs Bragg
reflection waveguides [38], and a demonstration
of polarization-frequency hyper-entanglement in
PPKTP using an external optical cavity filter [39].
Although this method has often been used to
remove noise and define spectral properties, the
spectral purity of the filtered photons is intrinsically
reduced due to the fundamental trade-off between
purity and heralding efficiency [40]. In addition,
since any additional optical element is a source
of losses, these applied filters not only reduce the
number of available photon pairs, but also degrade
the heralding efficiency of the device. To solve this
issue, a technique called spectrally-resolved HongOu-Mandel interference has been developed to
successfully create frequency entangled quDits with
D > 10 without the use of any spectral filters or
resonant cavities [20].
Although large complex quantum states have been
widely investigated, bulk optics based approaches
and spectral-filtering methods require large,
expensive, and very complex setups, not suitable for
real-world applications outside of the lab. Another
challenge for these systems is that when the OPO
is pumped by a pulsed laser, active stabilization is
required to ensure that the incident laser spectrum is
perfectly aligned with the cavity resonances in most
experiments [35, 36] (see Fig. 2(b)). Furthermore,
the quantum states that have been demonstrated
with this OPO approach have not yet achieved
the level of squeezing required (with a threshold
value of 20.5 dB) for fault-tolerant optical quantum
computation [41], being typically limited by loss
which degrades the squeezed states. In addition, the
spectral modes of a large OPO cannot be individually
addressed due to their small spectral separation.

Reducing the size of the resonant cavities
would allow access to individual frequency modes
and, in turn, also allow one to exploit single or
entangled photons instead of (or in addition to)
squeezed states. Therefore, a platform made of
highly nonlinear media in a small footprint is
highly desirable for the generation of large-scale
quantum states.

INTEGRATED QUANTUM
FREQUENCY COMBS
In recent years, integrated (on-chip) photonics has
stood out as a promising platform for quantum
optics [42]. Compact and mass-producible photonic
chips - particularly those compatible with the silicon
chip industry - will enable compact, cost-efficient,
and stable devices for the generation and processing
of non-classical optical states. This is highlighted by
demonstrations of on-chip photon sources, pathentangled quantum states, and basic algorithms [1,
2]. However, on-chip sources of quantum states and
quantum gates have mainly focused on the use of
polarization- or path-entangled photons, where the
dimension of each state corresponds to a waveguide
mode. These architectures are intrinsically limited,
since the state/gate complexity directly scales with
the quantum circuit footprint and complexity.
The on-chip generation of classical frequency combs
is a very active research field, and many of its
principles are reflected in the first demonstrations of
on-chip quantum combs [5]. As materials used for
on-chip integration typically exhibit third-order optical
nonlinearity, spontaneous four-wave mixing (SFWM)
is used for the generation of on-chip quantum
frequency combs. In SFWM, the nonlinearity
mediates the annihilation of two photons from an
excitation field and the simultaneous generation
of two daughter photons named signal and idler.
Since the process occurs randomly, and the required
pump power is typically very low (on the order of
10's of milliwatts, so that the probability of a pair
event is at most only a few percent per pump

HKN.ORG

11
http://www.HKN.ORG
Table of Contents for the Digital Edition of The Bridge - February 2018
