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**Quantum networks are an integral part of quantum computing and quantum communications. They are similar to classical computing systems and use qubits as opposed to the conventional computer bit. As these bits are built up, they form quantum networks, and these quantum networks can be built together into complex quantum networks which can be used as functional systems. In this article, we look at what quantum networks are and how they’re used in quantum computing and quantum communications.**

**It Starts with the Qubit**

At the heart of any quantum network is the most fundamental building block—the qubit. A qubit—a shortened word for a quantum bit—is much like a classical computing bit, although there are some marked differences between the two types of bit. Quantum networks function because of the qubits and the subsequent interactions between qubits. Computing bits, otherwise known as binary bits can adopt two values in the form of a 0 or a 1. Qubits are slightly different in that they can adopt one of three values— a 0 value, a 1 value and a superimposed 0 or 1 value. In real terms, this means that quantum operations can be performed in both values simultaneously, and this is where qubits and quantum networks significantly vary (in fundamental principles) to classical networks.

Individual qubits can have an infinite value within the defined 0, 1 and 0 or 1 sets, and this often leads to a continuum of states within the network. The information within the quantum network is stored quantum mechanically by utilizing the ½ spin electron state (up and down spins) and photon polarizations (horizontal and vertical). Aside from the basic functions, the qubits can undergo certain beneficial phenomena, namely quantum entanglement, which makes all the qubits in a network to become one and indistinguishable from each other. In other words, each individual entangled network is described as whole and complete system as opposed to individual qubits.

What the qubits, and in turn the quantum network, is made of is very important. Quantum systems are created from physical materials, so the properties and characteristics of these materials is very important. Semiconductors are one popular choice. The materials which are used to build the quantum network need to possess long-lived spin states, have the ability to control these spin states and their subsequent interactions, and operate multiple qubit networks in parallel.

**Building Up the Network**

For any quantum network to work, there needs to be a series of interconnected quantum communication lines which are run between end nodes. Just like in classical computing, the nodes represent the information contained within a single quantum network when it is part of a larger, and more complex, quantum network. In many cases, these end nodes are a quantum processor of at least one qubit, but they can possess a quantum memory as well. In other cases, these end nodes can be beamsplitters, photodetectors, or telecommunication lasers if some form of encryption is required. There are many other variations depending on the intended application and include the implementation of quantum logic gates (a small quantum circuit of a few qubits) and ion traps.

There are two other important components of a quantum network. These are the physical communication lines and the quantum repeaters. The communication lines come in two forms, which are fiber optic networks and free space networks. Fiber optic cables can be modified to send a single photon along the communication path by attenuating a telecommunication laser. The path is controlled by interferometers and beam splitters and photons are received by photodetectors. Free space networks, on the other hand, are much faster, have higher transmission rates and don’t suffer any scrambling effects (as sometimes seen with fiber optics). Instead, they rely on the line of sight between the two communicating ends and can be used over long distances.

Repeaters are needed because long-distance communications can become compromised in a quantum network, namely the networks experience signal loss or decoherence. Many classical networks will use an amplifier to boost the signal, but this is not possible with a quantum network. Instead, quantum networks utilize trusted repeaters, quantum repeaters, error correctors and entanglement purifying mechanisms to prevent signal loss. Trusted and quantum repeaters test the communication infrastructure and enable the continued generation of entangled qubits, respectively, whereas, error correctors detect any localized and short-range communication errors and the entanglement purifying mechanism is in place to maximise the degree of entanglement between qubits and minimize the degree of decoherence.

**Computation or Communication?**

Quantum networks can be used for both computation and communications purposes. On the computation front, quantum networks can be used to send qubits between quantum processors and form a quantum computing cluster—much in the way that you can connect multiple classical computers together to form a computing cluster. This is typically done over short distances. These quantum computing systems, otherwise known as networked quantum computers are much more powerful than a single computing unit, and because they are modular, more can be added over time.

Now, if a quantum network wants to send qubit information over a long distance, then it needs to do so by being connected to a quantum internet. In networks where there are many entangled qubits, the information can be transferred between quantum processors remotely. The applications that are set to benefit from this type of quantum communication are quantum key distribution and quantum cryptography for the secure transfer of information.

**Sources**

- Technische Universität München- http://wwwmayr.informatik.tu-muenchen.de/konferenzen/Jass05/courses/5/Talks/Fehr_pres.pdf
- American Physical Society: https://www.aps.org/units/fiap/meetings/presentations/upload/kane.pdf
- Cambridge University: https://www.cl.cam.ac.uk/teaching/0910/QuantComp/notes.pdf
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*NANOCOM '18 Proceedings of the 5th ACM International Conference on Nanoscale Computing and Communication*, **2018, **DOI: 10.1145/3233188.3233224
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*Quantum Science and Technology, ***2017**, DOI: 10.1088/2058-9565/aa6994
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*Quantum Science and Technology, ***2017**, 10.1088/2058-9565/aa7154
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*New Journal of Physics*, **2002**, DOI: 10.1088/1367-2630/4/1/346
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