Quantum communication relies on the phenomenon of quantum entanglement, where two or more particles are linked in such a way that the state of one particle is instantaneously affected by the state of the other, regardless of the distance between them.

In quantum cryptography, entangled particles are used to transmit secret keys securely. By measuring the properties of the entangled particles, the sender and receiver can establish a shared key that is known only to them, making it virtually impossible for an eavesdropper to intercept and decode the communication.

The BB84 protocol is the most widely used protocol for quantum key distribution (QKD). It involves the exchange of randomly polarized photons between two parties, Alice and Bob, who use their measurements to establish a shared secret key.

The primary advantage of quantum communication over classical communication is enhanced security. Quantum communication protocols, such as quantum cryptography, allow for the transmission of information in a way that is provably secure against eavesdropping, thanks to the fundamental properties of quantum mechanics.

Quantum entanglement is the phenomenon where two or more particles are linked in such a way that the state of one particle is instantaneously affected by the state of the other, regardless of the distance between them. This non-local correlation between entangled particles is a fundamental property of quantum mechanics and is crucial for many quantum communication applications.

In quantum teleportation, entangled particles are used to transmit the quantum state of one particle to another, even if they are physically separated. This is achieved by entangling one particle with the particle whose state is to be teleported, and then measuring the entangled particle in a specific way, which instantaneously affects the state of the other particle.

The Quantum Communication Satellite (QCSat) is a satellite-based quantum communication network that aims to provide secure communication links between different parts of the world. QCSat uses entangled photons to transmit information in a way that is provably secure against eavesdropping.

Implementing quantum communication systems poses several challenges, including developing efficient sources of entangled particles, maintaining entanglement over long distances, and building practical quantum repeaters. These challenges are actively being addressed by researchers in the field.

The GHZ protocol is the protocol used in quantum teleportation to transmit the quantum state of one particle to another. It involves the use of three entangled particles, with one particle being measured in a specific way to instantaneously affect the state of the other two particles.

A quantum repeater is a device used to amplify the signal in a quantum communication system. It works by entangling a new pair of particles with the incoming entangled particles, effectively extending the distance over which entanglement can be maintained.

Quantum superposition is the phenomenon where a particle can exist in multiple states simultaneously. This means that the particle does not have a definite state until it is measured, at which point it collapses into a single state.

Quantum tunneling is the phenomenon where a particle can tunnel through a barrier even if it does not have enough energy to overcome it classically. This is a consequence of the wave-like nature of particles in quantum mechanics.

The BB84 protocol is the protocol used in quantum cryptography to establish a secure key between two parties. It involves the exchange of randomly polarized photons between two parties, Alice and Bob, who use their measurements to establish a shared secret key.

The GHZ protocol is the protocol used in quantum teleportation to transmit the quantum state of one particle to another. It involves the use of three entangled particles, with one particle being measured in a specific way to instantaneously affect the state of the other two particles.

A quantum repeater is a device used to amplify the signal in a quantum communication system. It works by entangling a new pair of particles with the incoming entangled particles, effectively extending the distance over which entanglement can be maintained.