# Quantum Communication

**Introduction**

Einstein called quantum entanglement as spooky action at a distance. Quantum entanglement is one of the strangest observation in quantum mechanics. When two particles are allowed to interact, they influence each other’s properties. The particle properties influenced are spin, polarization, and momentum. A change to a particle impacts a change to the other particle at the same time. Researchers have applied quantum entanglement to transfer information between two nodes. One of the nodes is the sender who holds half of the entangled photons. The other node is related to the receiver. Quantum communication is achieved by the manipulation of the photons. This impacts an instantaneous change in the entangled photons.

Every node of a quantum network has quantum processors. Quantum network node relies on qubits. Qubits can exist in multiple states 0 or 1 and in a superposition of 0 and 1. The superposition allows executing multiple calculations at once. A quantum processor changes the states of its photons. The entangled photons are changed in a quantum processor in the quantum network while transferring the qubits.

This helps in creating a safe system of communication. Eavesdrop attempt or intercepting the information will disentangle the particles in a quantum network. Hence, it would modify the message. The hacking attempt is known when it happens. This principle is being used in quantum key distribution. Decoupling is related to the disentanglement of the entangled particles due to interaction with the environment. Due to this reason, long distance communication is difficult. Quantum repeaters are used to avoid this problem. Many quantum repeaters are used in quantum networks.

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The problem to overcome is to create a quantum network which is global. Since the long distances are the real problem, the entangled photons need to be beamed through the satellite or optic fiber. More than a couple of hundred kilometers, 99.99 percent of communicated information will be gone. The signal will be very weak for communication usage.

**Quantum Communication**

A fully connected quantum network architecture has a single entangled photon source which distributes quantum states to multiple users. Quantum network tries to minimize the resources required. Quantum networks and the quantum internet will impact the communication world. Quantum networks help in communicating at higher speeds. These speeds are in higher orders of magnitude.

The quantum internet is a part of the regular internet network. It can be useful for quantum computing schemes. The users can use a quantum computer through a quantum cloud on the internet. Many quantum devices will be connecting to the quantum network. Any connected device will be able to talk to the other device in the quantum network. The quantum internet can operate in parallel to the internet which is in use today. This network can connect quantum processors which has capabilities better than the classical means.

The first stage of the quantum network development is to ensure unbreakable privacy and security in communications. The next stage is to have a mature network. The mature quantum will have quantum sensors which can detect gravitational waves. Users can receive quantum-generated codes. They will not be able to send or receive quantum states. Two end users can share an encryption key on a quantum network. They can receive and measure quantum states. They can share a private key and have their password verified. They can obtain entangled states which are not for saving. This helps in having the strongest quantum encryption in the quantum network The users can access and save entangled qubits. They can teleport quantum information to each other on the quantum cloud. The quantum computers are the devices on the quantum network.

Quantum communication techniques can be used to coordinate and reach a consensus in a large group. For instance, they can be used to check and approve electronic currency based transactions such as Bitcoin.

**Conclusion**

There are plans to develop large-scale graph-state photon generators and ultra-low loss photonic circuits. These will help in allowing the time-reversed adaptive Bell measurement on a large number of photons. These circuits enable the scientists to scale up an all-optical quantum network. A complete working quantum internet which has functional quantum computers is farther away on the roadmap. The first long-range quantum networks are in the planning phase.

**References**