Qubits along with bits can be another combination based on which quantum
processors are developed [282], that has capabilities well beyond the reach of
classical computations. Therefore, exploring possible synergies of bits +
qubits + neurons will shape the future of computing. With this, the
capabilities of bits + neurons + qubits must be extensively researched and
will remain a topic of active research in coming years. However, some advanced
computing systems and use cases are already operating at the intersection
between the pairs of these computing methods.

###  VII-F Exploration of Solid-state memories

Solid-state qubits are formed by electron spins [95] followed by
superconducting circuits [96], that are defined as superconducting circuits
comprising of Josephson junctions, interconnects, and passive elements, that
are designed to behave as a quantum mechanical two-stage system with good
isolation. Solid state quantum memories are more advantageous and attractive
as they can maintain its coherence even at cryogenic temperatures. For
example, crystalline-solid spin ensembles formed by implanting defects in
lattice; such as implanting nitrogen-vacancy (NV) centers into diamonds, or by
rare-earth-doped crystals, can be proved coherent for hours at cryogenic
temperatures. Hence, comprehensive efforts are needed to explore the union of
superconducting processors and solid-state quantum memories to enhance the
performance of transmission and generation of microwave photons. Accordingly,
steps must be taken to investigate on-chip teleportation between a
superconducting qubit and NV in a local quantum memory. If these integrations
are successful, this hybrid technology would become the most encouraging
design to be scaled up into a comprehensive quantum networks. Therefore,
employing such hybrid quantum computers as end nodes in quantum networking,
the next 5-10 years could see the implementation of a hybrid-technology
quantum internet globally.

##  VIII CONCLUSION

In this survey, we have discussed the quantum internet and its technological
advances that covers the basic concepts to understand quantum internet
technology. We have introduced the readers to the basic physics underlying
this technology and the unmatched security and privacy that it offers due to
laws of inherently secure quantum mechanics. The future quantum internet will
exchange security based on various quantum key distribution (QKD) algorithms,
and enables networking of classical and quantum nodes that are located
remotely. The most important and challenging requirement for quantum internet
is quantum teleportation based on entanglement generation and distribution
between a network of far-away nodes. For this, quantum repeaters are employed
that generate entanglement between far-away nodes with the help of
entanglement swapping among pairwise intermediary nodes, where the quantum
information is stored in local quantum memories. The quantum information is
fragile, which means that environmental effects such as decoherence, where
quantum information is lost with time, will negatively impact quantum
communication. Due to decoherence, quantum states looses its properties and
enters the world of classical physics. Therefore, realization of quantum
internet technology will come with lots of design challenges and complexities.
Consequently, multidisciplinary efforts will be required that will pave the
way for the realization of a full-fledged quantum internet. A breakthrough has
been achieved since the launch of the quantum satellites that have achieved
QKD between two far-away ground stations, that has motivated various research
groups to speed-up the efforts to build a full-blown quantum internet in near
future.

## Acknowledgment

The authors would like to thank all the contributors who shared data with us
for this survey.

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Amoldeep Singh is Research Scholar in Electronics and Communication Department
at Thapar Institute of Engineering and Technology, Patiala, India. He has
received his ME degree in Electronics(VLSI design) from Punjab Engineering
college, Chandigarh. He is an exchange student at Politecnico di milano,
Milan, Italy and is associated with Erasmus KA-107 project. His research
interests includes Quantum communication, Quantum networking, VLSI
Interconnects, Optical Interconnects.
---
Kapal Dev is Senior Researcher at Munster Technological University, Ireland.
Previously, he was a Postdoctoral Research Fellow with the CONNECT Centre,
School of Computer Science and Statistics, Trinity College Dublin (TCD).
Previously, he worked as 5G Junior Consultant and Engineer at Altran Italia
S.p.A, Milan, Italy on 5G use cases. He worked as Lecturer at Indus
university, Karachi back in 2014. He is also working for OCEANS Network as
Head of Projects to manage OCEANS project processes and functions to improve
efficiency, consistency and best practice integration He was awarded the PhD
degree by Politecnico di Milano, Italy under the prestigious fellowship of
Erasmus Mundus funded by European Commission. His education Profile revolves
over ICT background i.e. Electronics (B.E and M.E), Telecommunication
Engineering (PhD) and Post-doc (Fusion of 5G and Blockchain). His research
interests include Blockchain, 6G Networks and Artificial Intelligence. He is
very active in leading (as Principle Investigator) Erasmus + International
Credit Mobility (ICM) and Capacity Building for Higher Education and H2020 Co-
Fund projects. Received a few Million Euros funding and few are in review as
well as in the writing phase. He is evaluator of MSCA Co-Fund schemes,
Elsevier Book proposals and top scientific journals and conferences including
IEEE TII, IEEE TITS, IEEE TNSE, IEEE IoT, IEEE JBHI, FGCS, COMNET, TETT, IEEE
VTC, WF-IoT. TPC member of IEEE BCA 2020 in conjunction with AICCSA 2020, ICBC
2021, DICG Co-located with Middleware 2020 and FTNCT 2020. He is also serving
as Associate Editor in Wireless Networks, IET Quantum Communication, IET
Networks, and Review Editor in Frontiers in Communications and Networks. He is
also serving as Guest Editor (GE) in COMCOM (I.F: 2.8), GE in COMNET (I.F
3.11), Lead chair in one of CCNC 2021 workshops, and editing a book for CRC
press. He is a member of the ACM, IEEE, and actively involved with various
working groups and committees of IEEE and ACM related to 5G and beyond,
Blockchain and Artificial Intelligence.
---
Harun Siljak received his bachelor and master degrees in Automatic Control and
Electronics from the University of Sarajevo in 2010 and 2012, respectively and
his PhD degree in Electrical and Electronics Engineering from the
International Burch University, Sarajevo, in 2015. He joined CONNECT as an
EDGE MSCA Research Fellow with his research project COCODIMM (Complexity and
Control of Distributed Massive MIMO) where he merged complex systems science
and reversible computation in a new toolbox for wireless communications,
mainly for control and optimisation of large antenna arrays. The tools
developed during the project found their application in quantum computation
and communications, as well as molecular and neuronal communications. He
serves as an associate editor for the EURASIP Journal on Wireless
Communications and Networking, and science communication officer for the
Western Balkans Chapter of Marie Curie Alumni Association and MAT-DYN-NET COST
Action.
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Hem Dutt Joshi received the B.Tech degree in ECE from Barkatullah University,
Bhopal, India in 1999, the ME degree in CCN from MITS, Gwalior in 2004 and the
PhD degree from JUET, Guna, India in 2012. He worked as Assistant Professor in
JUET, Guna from 2006 to 2013. Currently, he is working as Associate Professor
in the department of ECE, TIET, Patiala. Dr Joshi also worked as visiting
research fellow in the year 2019 at CONNECT, the Science Foundation Ireland
Research Centre for Future Networks, at Trinity College Dublin, Ireland. Dr.
Joshi is involved very extensively in research regarding high data rate
wireless communication systems and signal processing. He has more than 15
years of teaching and research experience. His research interests include
wireless communication, OFDM system, MIMO–OFDM, signal processing for wireless
communication and 5G communication system.
---
Maurizio Magarini received the M.Sc. and Ph.D. degrees in electronic
engineering from the Politecnico di Milano, Milan, Italy, in 1994 and 1999,
respectively. In 1994, he was granted the TELECOM Italia (now TIM) scholarship
award for his M.Sc. Thesis. He worked as a Research Associate in the
Dipartimento di Elettronica, Informazione e Bioingegneria at the Politecnico
di Milano from 1999 to 2001. From 2001 to 2018, he was an Assistant Professor
in Politecnico di Milano where, since June 2018, he has been an Associate
Professor. From August 2008 to January 2009 he spent a sabbatical leave at
Bell Labs, Alcatel-Lucent, Holmdel, NJ. His research interests are in the
broad area of communication and information theory. Topics include
synchronization, channel estimation, equalization and coding applied to
wireless and optical communication systems. His most recent research
activities have focused on molecular communications, massive MIMO, study of
waveforms for 5G cellular systems, wireless sensor networks for mission
critical applications, and wireless networks using UAVs and high-altitude
platforms. He has authored and coauthored more than 100 journal and conference
papers. He was the co-recipient of four best paper awards. He is Associate
Editor of IEEE Access and IET Electronics Letters and a member of the
Editorial Board of Nano Communication Networks (Elsevier) and MDPI Telecom. He
has been involved in several European and National research projects.
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