Glossary

An atom is the smallest unit of matter that still possesses the chemical properties of an element. It consists of a central atomic nucleus containing positively charged protons and neutral neutrons, as well as a shell of negatively charged electrons orbiting the nucleus. The number of protons in the nucleus determines which chemical element it is. Atoms are the basic building blocks of all matter, and molecules are formed through bonds between atoms.

Bell's inequality: A mathematical inequality developed by John Bell to test whether quantum mechanics or classical physics better describes reality. Violations of Bell's inequality demonstrate that quantum mechanics (and thus quantum entanglement) provides more accurate predictions than classical theories. Experiments that test Bell's inequalities are fundamental tests of quantum communication.

Decoherence describes the process by which a quantum system loses its quantum mechanical properties, often due to interactions with its environment. This causes superpositions to collapse, and the system begins to behave like a classical system. In quantum communication, decoherence is one of the greatest challenges, as it can compromise the integrity of the transmitted quantum information.

An electron is a subatomic particle with a negative electric charge. It is one of the fundamental building blocks of matter and is typically found in the “shell” of an atom, where it orbits the nucleus. Electrons are crucial for chemical reactions and the conduction of electricity, as they can move relatively freely within certain materials.

Free-space links refer to a method of wireless data transmission in which light beams, typically in the infrared or visible spectrum, are transmitted through the air. This is often referred to as free-space optical communication. It is a technology that transmits data using light rather than radio signals.

“Inherent” means that something is naturally or essentially part of something else. It describes characteristics that are indispensable or inseparable from a particular object, concept, or state. For example, one might say that “change is inherent in life,” meaning that change is a fundamental and natural part of life.

The nanocosmos refers to the extremely small realm of matter on the scale of nanometers, or billionths of a meter (1 nm = 10⁻⁹ meters). This realm encompasses atoms, molecules, and structures smaller than 100 nanometers. The nanocosmos is important in fields such as nanotechnology, biology, and physics because special effects occur here that are not visible in the “normal” world, such as quantum phenomena and altered material properties.

The no-cloning theorem is a fundamental law of quantum mechanics that states it is impossible to perfectly copy an unknown quantum state. This is important for quantum communication because it ensures that information cannot be duplicated or intercepted without being detected. Any attempt to copy a qubit results in a disturbance of the system and makes the eavesdropping attempt detectable.

A photon is the smallest elementary particle of light and carries electromagnetic energy, which is why they are also called light quanta. In quantum communication, photons are used to transmit qubits over long distances, often via fiber-optic cables or satellite links. Because they interact very little with their surroundings, photons are particularly well-suited for transmitting information without loss.

Polarization refers to the orientation of the oscillations of light waves. In quantum communication, the polarization of photons is often used to represent qubits. Different polarizations can be used to encode information, which is then transmitted securely.

Post-quantum cryptography refers to cryptographic methods that are secure against attacks by quantum computers. Quantum computers could easily break many of today’s common encryption methods because they can solve certain mathematical problems (such as factoring large numbers) much faster than classical computers. Post-quantum cryptography therefore develops new algorithms that cannot be easily broken, even with the computing power of quantum computers.

Quanta are the smallest, indivisible units of certain physical quantities, such as energy or light. They form the foundation of quantum physics, which deals with the laws governing the smallest scale—that is, the realm of atoms and subatomic particles.

QKD is a method for securely transmitting encryption keys using qubits. It involves using photons (particles of light) with different polarizations to transmit the key. Since any attempt to eavesdrop alters the quantum information, the security of the transmission can be guaranteed. The best-known QKD method is the BB84 protocol, named after its developers, Bennett and Brassard.

Quantum teleportation refers to the transfer of quantum information from one location to another without moving physical particles. This transfer is made possible by quantum entanglement. Quantum teleportation transfers the properties of a quantum, such as the spin of an electron, to another particle located at a distant location.

A qubit is the smallest unit of information in quantum communication. Unlike a classical bit, which can be either 0 or 1, a qubit can exist in both states simultaneously thanks to superposition. This property allows quantum computers to perform many calculations in parallel and is crucial to the efficiency and security of quantum communication.

Quantum computer: A computer that uses the principles of quantum mechanics to perform calculations. Quantum computers use qubits and are capable of solving certain problems much faster than classical computers. In quantum communication, quantum computers could be used for encryption and secure data transmission.

Quantum interference is a phenomenon that occurs when quantum objects, such as photons or electrons, interact with one another and their wave functions interfere with each other. This interference can result in the waves being amplified or canceled out, leading to characteristic interference patterns. This is comparable to the interference patterns observed in classical waves, such as water waves or sound waves.

Quantum Internet: A hypothetical global network based on the principles of quantum communication. A quantum internet would enable the transmission of entangled qubits worldwide, making it possible to achieve extremely secure communication and to use quantum computers within a global network.

A quantum channel is the physical or virtual communication path through which quantum information (such as entangled photons) is transmitted. A quantum channel can run through fiber-optic cables, satellites, or even through the open air. A quantum channel must be particularly well shielded, as quantum information is sensitive to interference.

Quantum cryptography uses the principles of quantum mechanics to encrypt data in such a way that it cannot be intercepted or manipulated without being detected. One key approach is quantum key distribution (QKD), in which a key is exchanged between two parties. Any attempt to eavesdrop would alter the quantum state of the transmitted information, which would be immediately detected.

A quantum repeater is a device used to bridge distances in a quantum communication network. Since quantum information is susceptible to loss and errors over long distances, quantum repeaters are needed to improve signal quality and extend the range of communication without destroying the quantum information.

Quantum router: A device capable of forwarding quantum information from one quantum channel to another without destroying the quantum information. Quantum routers are essential for building quantum networks to enable quantum communication over long distances.

An encryption key generated and exchanged using quantum cryptography. Because this key is secured by quantum mechanics, it is extremely secure against eavesdropping attempts compared to classical keys.

In quantum physics, a quantum leap refers to the sudden transition of an electron from one energy state to another within an atom. In this process, the electron “jumps” from one specific energy level to another without passing through intermediate states. This process occurs when the electron absorbs or emits energy, usually in the form of light. Although the term “quantum leap” is often used to describe something significant, in physics it refers to a tiny but very precise change at the atomic level.

This principle states that a quantum object does not exist in a single, clearly defined state. Instead, it can exist in a superposition of several possible states. Imagine that a quantum object, such as an electron or a photon, can exist in multiple states at the same time—almost like a coin that could show both heads and tails at the same time before it is tossed.

Quantum entanglementQuantum entanglement describes the unique connection between two or more quantum objects, in which the states of these objects are directly linked to one another regardless of the distance between them. If the state of one entangled particle changes, the state of the other changes as well—instantly and without delay. This property is used in quantum communication to transmit information over long distances without the need for classical signal transmission.

The “subatomic level” refers to the branch of physics that deals with particles smaller than an atom. These include electrons, protons, neutrons, and other elementary particles such as quarks and neutrinos. This level describes processes and phenomena that occur on the scale of atoms and their building blocks and are governed by quantum mechanics, where classical physical laws often no longer apply.

Superposition is a central concept in quantum mechanics, which states that a quantum object (e.g., a qubit) can exist in multiple states simultaneously. A qubit can therefore take on the values 0 and 1 at the same time until a measurement is performed. In quantum communication, this property is used to transmit complex information efficiently.