The Bridge - February 2018 - 8

Feature

Quantum State Generation in Optical Frequency
Combs for Quantum Computing
by: Yanbing Zhang, Piotr Roztocki, Christian Reimer, Stefania Sciara, Michael Kues, David J. Moss, and Roberto Morandotti

Key words: optical frequency combs, integrated
quantum sources, high-dimensional quantum states.
Quantum states represent a key resource for
quantum computing, with the potential to
advance practical implementations beyond proof
of concept demonstrations. Optical frequency
combs (broadband light sources spanning a large
bandwidth of equidistantly-spaced spectral lines,
which were first developed for classical optical
applications), are a promising approach to generate
the required quantum states. The advantages of
quantum frequency combs have been supported
by the development and application of quantum
photonic sources in both bulk systems and
integrated platforms. In combination with
the benefits of integrated photonics, which
is increasingly perceived as a practical,
scalable platform for implementing
quantum technology, integrated quantum
frequency combs allow the generation
of scalable, complex quantum states
in a low-cost and ultra-compact footprint,
as well as their manipulation using
standard telecommunications signal
processing techniques.

INTRODUCTION
In the last few decades, research towards achieving
the holy grail of universal quantum computers has
greatly intensified, with the promise of being able
to perform calculations that are completely beyond
the capability of conventional electronic computers.
To implement a quantum computer, a physical
system is required that can support the preparation,
manipulation, and measurement of quantum bits
(qubits), units of quantum information analogous to

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classical bits. Since quantum states are so delicate
that their surroundings can quickly deteriorate
the quantum information, the host media should
be capable of shielding them from the outside
environment [1]. Also, the physical qubits should be
scalable and able to be precisely controlled in order
to realize a universal set of quantum logic gates [2].
Furthermore, the errors induced in these qubits due
to inevitable disturbances need to be effectively
corrected during operations to preserve the quantum
information. Quantum technologies are being
advanced in several platforms including electronic,
ions, solid state, nuclear magnetic resonance, and
superconducting systems [1].

Fig.1 Schematic of a classical optical frequency comb,
which consists of a series of discrete, equally-spaced
frequency lines. The comb may function as a frequency
ruler since unknown frequencies can be measured
relative to a precisely known n-th order frequency (fn),
with the offset frequency (f0) and the repetition frequency
(fr) known and stabilized.

Photons are one of the best candidates for
quantum system realizations. They exhibit very
low decoherence, which improves the capability
to preserve the quantum states sufficiently long
enough to perform an operation. Qubits can easily
be encoded in a photon via their different degrees

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