applied

sciences
Editorial
Special Issue on Basics and Applications in Quantum Optics
Alessia Allevi 1,2, *,† , Stefano Olivares 3,† and Maria Bondani 2,†

1 Department of Science and High Technology, University of Insubria, Via Valleggio 11, 22100 Como, Italy
2 Institute for Photonics and Nanotechnologies, CNR, Via Valleggio 11, 22100 Como, Italy;
maria.bondani@uninsubria.it
3 INFN, Milano Section, Department of Physics “Aldo Pontremoli”, University of Milan, Via Celoria 16,
20133 Milano, Italy; stefano.olivares@fisica.unimi.it
* Correspondence: alessia.allevi@uninsubria.it; Tel.: +39-031-238-6253
† These authors contributed equally to this work.

1. Introduction
Quantum technologies are advancing very rapidly and have the potential to innovate
communication and computing far beyond current possibilities. Among the possible plat-
forms suitable to run quantum technology protocols, in the last decades quantum optics has
received a lot of attention for the handiness and versatility of optical systems. In addition
to studying the fundamentals of quantum mechanics, quantum optical states have been ex-
ploited for several applications, such as quantum-state engineering, quantum communication
and quantum cryptography protocols, enhanced metrology and sensing, quantum optical
integrated circuits, quantum imaging, and quantum biological effects. In this Special Issue,
we collect some papers and also a review on some recent research activities that show the
potential of quantum optics for the advancement of quantum technologies.



2. Quantum Optics Applications


The topics addressed in the Special Issue range from quantum computing to quantum-
Citation: Allevi, A.; Olivares, S.; state engineering, from quantum communication to quantum cryptography, from quantum
Bondani, M. Special Issue on Basics
simulation to quantum imaging, in perfect agreement with the four pillars of the European
and Applications in Quantum Optics.
Commission Quantum Technologies Flagship Program.
Appl. Sci. 2021, 11, 10028. https://
The first paper [1] of this Special Issue, authored by A. Candeloro et al., focuses on an
doi.org/10.3390/app112110028
enhanced version of an all-optical system used to implement a quantum finite automaton [2].
The considered automaton recognizes a well-known family of unary periodic languages that
Received: 15 October 2021
play a crucial role in Descriptional Complexity Theory and in the area of Formal Language
Accepted: 22 October 2021
Published: 26 October 2021
Theory. The performance of the device benefits from considering the orthogonal output
polarizations of the employed single photons. Moreover, the effect of the detector dark
Publisher’s Note: MDPI stays neutral
counts on the proper operation of the automaton is taken into account. This kind of photonic
with regard to jurisdictional claims in
quantum automaton could be hardwired into “hybrid” architectures that combine classical
published maps and institutional affil-
and quantum components to build very succinct finite-state devices operating in environments
iations. where dimension and energy absorption are particularly critical issues.
The paper written by G. Chesi et al. addresses the topic of quantum-state engineering.
The authors present the generation and characterization of Sub-Poissonian states by means of
conditional measurements performed on multi-mode twin-beam states [3]. These measure-
ments are based on the use of Silicon photomultipliers [4], a class of photon-number-resolving
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
detectors. Such detectors, very compact and cheap, can open new perspectives in the field of
This article is an open access article
quantum optics and quantum technologies, being suitable for investigating mesoscopic states
distributed under the terms and
of light [5]. In the paper, a comprehensive model taking into account all the features of the
conditions of the Creative Commons employed detectors is developed and experimentally verified.
Attribution (CC BY) license (https:// In their paper, M.-S. Kang et al. develop a quantum message authentication protocol
creativecommons.org/licenses/by/ for improving security against an existential forgery by means of single-qubit unitary oper-
4.0/). ations [6]. The protocol consists of two parts: a quantum encryption and a correspondence

Appl. Sci. 2021, 11, 10028. https://doi.org/10.3390/app112110028 https://www.mdpi.com/journal/applsci

Appl. Sci. 2021, 11, 10028 2 of 3

check. The first part is realized by means of a linear combination of wave plates [7], while
the second one is performed using the Hong–Ou–Mandel interference [8]. The successful
experimental implementation of the protocol proves that the employed optical system can
be considered as the base technology for a complete quantum cryptosystem providing
confidentiality, authentication, integrity, and nonrepudiation.
Furthermore, the paper written by K. Park et al. is devoted to quantum-state engineer-
ing [9]. Starting form the recent proposal of obtaining high-purity bi-photon states without
degrading brightness and collection efficiency by means of a nonlinear interferometer [10],
the authors experimentally investigate the fine tunability of the nonlinear interference
method to match constructive interference patterns, while maintaining the high spectral
purity of the biphoton state. Their results enrich the usefulness and practicality of the
method based on the nonlinear interferometer for the efficient generation of photon pairs
with high spectral purity, which represents an excellent practical source for quantum
information protocols.
The paper authored by A. Allevi et al. focuses on the role of losses in the degradation
of the nonclassicality of mesoscopic quantum states of light to be used for secure data
transmission in quantum communication protocols [11]. In particular, the authors investigate,
both theoretically and experimentally, the effect caused by two realistic kinds of statistically-
distributed amounts of loss, namely a Gaussian distribution and a log-normal one, on the
nonclassical photon-number correlations between the two parties of multi-mode twin-beam
states [12]. The achieved results show to what extent the involved parameters, both those
connected to loss and those describing the employed states of light, preserve nonclassicality.
In the last research paper, J. Liñares et al. present the physical simulation of the
dynamical and topological properties of atom-field quantum interacting systems by means
of integrated quantum photonic devices [13]. The photonic device consists of integrated
optical waveguides supporting two collinear modes, which are coupled by integrated
optical gratings [14]. The two-mode photonic device with a single-photon quantum state
represents the quantum system, and the optical grating corresponds to an external field.
This photonic simulator can be regarded as a basic brick for constructing more complex
photonic simulators.
Finally, in the review paper by C. Abbattista et al. the advancement of the research
toward the design and implementation of quantum plenoptic cameras is presented and dis-
cussed [15]. At variance with standard plenoptic cameras, these devices have dramatically-
enhanced features, such as diffraction-limited resolution, large depth of focus, and ultra-low
noise [16]. For the quantum advantages of the proposed devices to be effective and appeal-
ing to end-users, the authors propose to develop high-resolution single-photon avalanche
photodiode arrays and high-performance low-level programming of ultra-fast electronics,
combined with compressive sensing and quantum tomography algorithms, with the aim
of reducing both the acquisition and the elaboration time by two orders of magnitude.
These new strategies will open the way to new opportunities and applications, such as for
biomedical imaging, security, space imaging, and industrial inspection.

Author Contributions: Conceptualization, A.A., S.O. and M.B.; methodology, A.A.; writing—
original draft preparation, A.A., S.O. and M.B; writing—review and editing, A.A., S.O. and M.B. All
authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The datasets used and analysed during the current study are available
from the corresponding author on reasonable request.
Appl. Sci. 2021, 11, 10028 3 of 3

Acknowledgments: This issue would not have been possible without the contributions of several
valued authors, professional reviewers, and the Applied Sciences editorial team. We first extend our
congratulations to all the authors. Second, we would like to take this opportunity to show our sincere
gratitude to all reviewers. Finally, we thank the editorial team of Applied Sciences and especially
Patrick Han.
Conflicts of Interest: The authors declare no conflict of interest.

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