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Quantum Communications:  The Sense of Security that Quantum Mechanics Brings us (Part I)

Quantum Communications: The Sense of Security that Quantum Mechanics Brings us (Part I)

Quantum communication is a kind of game with quantum particles that leads to the improvement of today's communication technologies.

Communication in the form of a language which is spoken and written is one of the signs of human societies. Even lower-level forms of life and non-living creatures communicate with each other non-linguistically. The building blocks of matter, such as quarks and leptons, interact with each other through the exchange of force quanta, such as gluons and photons. Among living things, cells communicate through biochemical molecules. Languages are the most important communication tools on which our distinct societies are based.

When we talk about communications in the field of technology, we mean telecommunication, which began with the Morse code-based telegraph, progressed through radio and wireless and eventually became the Internet. Technically, communications involve the transmission and reception of messages using electromagnetic waves in the free space, such as photonic pulses in optical fiber or current flow in copper wires.

Efforts to develop a theoretical understanding of telecommunications began very soon. One crucial problem was to determine the maximum amount of information that could be transmitted on an information channel in the presence of noise. Claude Shannon solved this problem and thereby established information theory as a separate field of knowledge. Before him, the definition of information for use in telecommunications was very qualitative. He made this definition very precise and quantitative using Boolean algebra and the fundamental principles of thermodynamics.

Non-intuitive quantum principles, such as superposition and entanglement, are not only the foundation for a deeper understanding of nature, but also lead to technologies that enable tasks that are impossible by classical concepts. When we talk about quantum technologies, we mean technologies that explicitly utilize these kinds of quantum properties that have no classical counterpart. Quantum information science and quantum communications are important components of future information processing technologies. They make it possible to move a quantum state from a place to another place. In all quantum communication schemes, two (or more) parties communicate with each other, both through the classical channel and through the quantum channel. Measurements are usually performed on separate quantum systems, and the measurement bases used for each measurement are linked through the classical channel. Quantum communications are possible through both discrete variables and continuous variables.

Classical and Quantum Communications

The Shannon-Hartley theorem states that larger capacity requires large bandwidth and large signal-to-noise ratio (SNR). Limitations arise from the physical layer capabilities. Currently, physical interfaces with the highest-capacity are the common optical fibers with rate ۱۰۰ Gbps and 400 Gbps in laboratory settings. Noise in optical fiber channels is the result of many complex physical processes that prevail at higher speeds. However, many clever techniques have been developed to modulate and code the signal and to overcome the noise-induced loss. The real constraints stem from the transferee and transmitter architectures and their underlying physical principles, such as lasers and amplifiers.

There are other characteristics of communications such as reliability, security and energy efficiency that are based on nonphysical layer considerations. To solve these problems, you need to answer the questions about optimal network methods and architectures.

Classical communication is based on the properties of electromagnetic waves such as frequency, phase and polarization and does not include quantum mechanics considerations. The error-free rate available to transmit digital information is obtained from the Shannon theorem. So the improvements include discovering better error correction codes and faster data processing techniques. High-speed communications are fundamentally limited by the development of communication devices and computing processing. Security is another aspect of communication that is as important as speed, and even more important in some areas. Many cryptographic schemes and methods have been designed over the years, but none are completely secure.

As it turned out, quantum communication was immune to manipulation and eavesdropping, it gained a lot of interest, and after the principles of quantum mechanics were applied to communication, a new field of science called quantum cryptography was born.

Classical and Quantum Cryptography

Cryptography is a branch of mathematics that deals with the secure communication of messages between two parties in the presence of a third party, for example between Alice and Bob in the presence of Eve (as a third party). The basic components of a classical cryptographic system are as follows:

  • Plain text: A message that is encoded.
  • Cryptography: A method that Alice converts plain text into a cryptic text and usually involves operations on plain text through a series of mathematical actions such as transposing, substitution, etc. and turning it into something intangible.
  • Key Sharing: The way the decryption key is shared between Alice and Bob and must be such that Eve cannot interpret the key. This step is very important and the key sharing process must meet the requirements of confidentiality, security and authentication.
  • Transfer: The encrypted text is transmitted using a secure physical interface.
  • Decoding: Bob decrypts the cipher text to retrieve the initial plain text.

Out of many classical cryptographic algorithms, it turns out that only the One-Time Pad is secure and unbreakable. This is, of course, difficult because of the need to generate and exchange random secure code with the same plain text length. For other approaches, the decoding step can be extremely difficult to compute so that it cannot break code, but even then, code security against future advances in computational and mathematical techniques is not guaranteed.

Figure 1 shows the general arrangement for QKD where Alice sends an encrypted message to Bob on a classical channel such as a wireless or optical network. She also sends a secret key to decode the message, but on a quantum channel. Eve (eavesdropper) accesses both channels and attempts to interpret the quantum channel by measuring the secret key qubits. Quantum cryptography guarantees the security of coded text according to the laws of quantum mechanics, as any attempt by Eve to interpret the message will destroy its carrier qubit.

Figure 1: The standard arrangement of a QKD with the probability of Eve penetration

In the relationship between Alice and Bob, secret key sharing is a crucial step. When this step is complete, the coded text can be sent to the classical channel. Many quantum key distribution methods have been developed to accomplish this task effectively. They are grouped according to two fundamental quantum principles: The Heisenberg uncertainty principle and the entanglement.

To be continued...


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