The Bridge - February 2018 - 10

Feature
photons (signal and idler) satisfying both energy
conservation and the imposed cavity resonance
conditions (see Fig. 2). In cases where the nonlinear
crystal has a large phase-matching bandwidth, a
broadband quantum frequency comb of entangled
photon pairs is created by the OPO at the resonant
wavelengths. This process was predicted to exhibit
multi-partite entanglement [31] and multi-qubit
entanglement [32], and this was quickly realised
experimentally [24], showing that it indeed has the
potential for generating large-scale quantum states.
In particular, each cavity mode of a frequency comb
can be described by a quantum harmonic oscillator
and, analogous to the position and momentum
observables, the field's continuous-variable Hilbert
space can be described by its amplitude or phasequadrature observables. This approach has been
used to generate continuous-variable non-classical
states based on squeezed light, where the noise of
one quadrature of the electrical field is below the
optical shot noise. Since these quantum sources can
be operated by using well-known opto-electronic
modulation and homodyne techniques [33],
they allow for deterministic quantum operations.
Moreover, the squeezing property, controlled by
phase-sensitive nonlinear gain, has been used
effectively to generate single-mode squeezed
vacuum states [33], and theoretical work predicts
that they can potentially provide qubit-like behaviour
around the oscillation threshold [34].
In continuous-wave (CW) pumped schemes,
quantum state preparation using squeezed states
has been remarkably successful. Examples include
the generation of many complex states with, for
example, the simultaneous generation of 15
quadripartite entangled quantum states using
60 cavity modes [24], or the generation of one
60-mode and two 30-mode copies of a dual-rail
quantum-wire state [25]. Richer excitation spectra
and more tailored nonlinear optical interactions have
been predicted to enable larger states [24], and
even in the latter experiments the mode number

THE BRIDGE

was limited by measurement capability, rather than
by the actual generation, with the maximum number
of entangled modes predicted to be at least 6700
[25]. When the OPO is pumped by a pulse train,
these photon pairs become interlinked through
the introduction of frequency correlations beyond
purely symmetric pair creation. The injection of a
femtosecond pulse train with individual frequency
modes into the cavity can induce an intimate
connection between symmetric and asymmetric
frequency correlations [35]. Through the control of
the nonlinear crystal, the optical cavity, and the pulse
characteristics, such a system was used to realize
quantum networks with a total of 511 possible
entangled bipartitions among 10 spectral regions
[30]. A follow-up experiment demonstrated the first
full multipartite entanglement with an entangled
state for all of the 115,974 possible nontrivial
partitions of this 10-mode state [26]. In parallel,
by multiplexing pulse trains in the time domain, a
continuous-variable cluster state containing more
than 104 sequentially-entangled temporal modes
was demonstrated, although its accessibility was
limited to only two states at a time [22]. Using
an intrinsically multimode quantum comb and a
homodyne detection apparatus, both on-demand
reconfigurable multimode entanglement and the
simultaneous generation of 13 cluster states have
been reported [23]. So far, all of the demonstrated
cluster states have been scalable in one degree
of freedom - either in frequency or in time. By
entangling the quantum frequency comb in both
the time and frequency continuous variables, it has
been shown that a hybrid time-frequency square
lattice cluster state suitable for universal quantum
computing can be produced [36].
Besides using a resonant cavity to shape the
energy spectrum of the entangled photons, a
quantum frequency comb can also be created
via spectral filtering after the generation process,
as demonstrated using correlated frequency bins
in χ(2) PPLN with a narrowband fiber-based Bragg



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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