1 Quantum Internet- Applications, Functionalities, Enabling Technologies, Challenges, and Research Directions Amoldeep Singh, Kapal Dev, Member, IEEE, Harun Siljak, Senior Member, IEEE, Hem Dutt Joshi, Member, IEEE, Maurizio Magarini, Member, IEEE. Abstract—The advanced notebooks, mobile phones, and inter- technology is administered by the laws of quantum mechanics, net applications in today’s world that we use are all entrenched in where one of the laws states that it is impossible to measure a classical communication bits of zeros and ones. Classical internet property of a system without changing its state. Consequently, has laid its foundation originating from the amalgamation of mathematics and Claude Shannon’s theory of information. But qubits cannot be copied and any attempt to do so will be today’s internet technology is a playground for eavesdroppers. detected, thus making the communication more secure and This poses a serious challenge to various applications that private [2]. Due to the extraordinary properties of qubits, it relies on classical internet technology. This has motivated the gives an edge toQI over traditional internet in many ways [3]. researchers to switch to new technologies that are fundamen- Qubits also show weird phenomena of quantum entangle- tally more secure. Exploring the quantum effects, researchers paved the way into quantum networks that provide security, ment [4], where qubits at remote nodes are correlated with privacy, and range of capabilities such as quantum computation, each other. This correlation is stronger than ever possible communication, and metrology. The realization of Quantum in the classical domain. Entanglement is inherently private, Internet (QI) requires quantum communication between various as it is not possible, because of no-cloning [5], for a third remote nodes through quantum channels guarded by quantum qubit to be entangled with either of the two entangled qubits. cryptographic protocols. Such networks rely upon quantum bits (qubits) that can simultaneously take the value of zeros and Therefore, with this weird quantum effect,QI could open up a ones. Due to the extraordinary properties of qubits such as different galaxy of applications with world-changing potential superposition, entanglement, and teleportation, it gives an edge [6], [7], [8], [9], [10], [11], [12], [13]. As qubits are prone to to quantum networks over traditional networks in many ways. environmental losses, transmitting qubits over long distances But at the same time transmitting qubits over long distances is a is a challenging task due to decoherence [14]. Therefore, formidable task and extensive research is going on satellite-based quantum communication, which will become a breakthrough extensive research is going on to acheive long-haul quantum in physically realizing QI in near future. In this paper, QI communication [15], [16]. To mitigate this noise, various functionalities, technologies, applications and open challenges techniques are employed such as Quantum Error-Correcting have been extensively surveyed to help readers gain a basic (QEC) codes [17] and fault-tolerant techniques [18], [19], understanding of the infrastructure required for the development [20]. Consequently, quantum communication can bear a finite of the global QI. amount of noise, unlike classical communication. Index Terms—Quantum mechanics, Information theory, Quan- In modern era, where internet plays a vital role in everyday tum computation, Quantum communication and networking. life, secure communication is the main concern ensuring pri- vacy between two communicating parties without any eaves- I. OUTLINE dropping. These issues are readily addressed by cryptographic UANTUM INTERNET (QI) is trending internet tech- techniques wherein data is protected with a private key. New potential in cryptography emerges with advances in quantum arXiv:2101.04427v2 [quant-ph] 1 Jun 2021 Q nology that facilitates quantum communication through quantum bits (qubits) among remote quantum devices or cryptography that exploits quantum mechanical principle of nodes. Such a technology will work in synergy with classical no-cloning. These features play a vital role in providing internet to overcome the limitations posed by traditional in- secrecy and integrity to the data to be communicated, thus terconnect technologies, as was highlighted in [1]. This novel making messages unintelligible to any unauthorized party. If a malevolent third party, such as Eve, as shown in Fig. 1, Amoldeep Singh is with the department of Electronics and Communication eavesdrops on this key distribution, privacy of the communi- Engineering, Thapar Institute of Engineering and Technology, Patiala, Punjab, cation will be compromised. This problem is addressed by the India, (e-mail: [email protected]). Kapal Dev is with Department of institute of intelligent systems, University best-known application of quantum networking, i.e. Quantum of Johannesburg, South Africa, (e-mail: [email protected]). Key Distribution (QKD), which provides secure access to Harun Siljak is with the Department of Electronic and Electrical Engineer- computers on the cloud [21] utilizing various protocols. QKD ing, Trinity College Dublin, Ireland, (e-mail: [email protected]). Hem Dutt Joshi is with the department of Electronics and Communication networks are commercially available [22], [23], and are studied Engineering, Thapar Institute of Engineering and Technology, Patiala, Punjab, and deployed covering metropolitan distances [24], [25], [26], India, (e-mail: [email protected]). [27]. Long distance QKD networks with trusted nodes are Maurizio Magarini is with the Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano, 20133 Milan, Italy, (e-mail: maur- also currently possible [24], [28]. Therefore, ultra-secure QKD [email protected]). protocols are developed based on well-accepted laws that 2 Fig. 1. Communication between Alice and Bob intercepted by Eve. Here channel is insecure which means that information is not encrypted by a cipher, thus vulnerable to attacks by Eve. Fig. 2. Vision of future quantum internet working in synergy with classical internet. govern quantum physics for sharing secret keys among two parties [9]. So, a lot of interdisciplinary effort is required by research groups working on this novel technology to make it with other scientists are trying to reach their goal to develop a reality [29], [30]. a blueprint for a pan-European entanglement-basedQI [39]. This blueprint will set the stage for a strong EuropeanQI industry. Outside Europe, the US government has earmarked A. Quantum race nearly 1.3 billion US dollars in funding for quantum research A tremendous amount of effort is put together by scientists via its National Quantum Initiative Act, which has to run from to realize globalQI. As a result, infrastructures to realize this 2019 to 2023 [40]. In the year 2017, a research group from technology are extensively studied and proposed. For instance, China has successfully demonstrated and observed a satellite- manufacturing of quantum computers is speeding up as various based distribution of entangled photon pairs to two separate tech giants such as IBM, Google, Intel, and Alibaba have ground stations on the earth, and with the successful launch already started working on it. A 50-qubits processor has been of the Micius satellite, they have already invested in the first built and tested by IBM in November 2017 [31]. Google has step to realizeQI as a global network [41]. announced a 72-qubit processor [32] and recently, towards the end of 2019, the company has claimed to enter a new era of It is envisioned that a future QI will somewhat look like incredible computational power by achieving something called as seen in Fig. 2. It will have a network of remote nodes ‘quantum supremacy’, i.e. solving a computational problem interconnected with each other with the help of multiparty that has not had a classical computing solution obtainable entanglement and quantum teleportation using fiber-based or in a reasonable time span [33]. Intel and Alibaba are ac- free space channels. The quantum effects of quantum me- tively working on technologies to develop double-digit-qubits chanics will make this network secure using various QKD processors. In this quantum computing race, future quantum protocols. QEC codes will be required to encode the qubits computers will come into force having disruptive potential to carrying the information that will protect them from decoher- tackle complex problems such as understanding photosynthe- ence and environmental effects. With advancements in multi sis, improvising catalysts for formulating renewable fuels [34], qubit processors, full scale quantum computers will come into [35], and complex Artificial Intelligence (AI) systems [36]. force that, along with existing classical infrastructure, will In the interim, European countries are also actively involved revolutionize the world with inherently secure global QI. in buildingQI and, to boost the research, the European Com- mission has hurled a ten-year e1-billion leading project [37]. In 2018, the European Union has announced its first grant of B. Motivation and Contribution e132-million as an initiative in the quantum flagship for the There are some specific surveys and review papers available following three years to speed up the quantum research [38]. that include the research work accomplished in this technol- Stephanie Wehner group at the Delft