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How do you build the next-generation internet?

From BBC - October 12, 2017

Imagine super-fast computers that can solve problems much quicker than machines today. These "quantum computers" are being developed in laboratories around the world. But scientists have already taken the next step, and are thinking about a light-based quantum internet that will have to be just as fast.

It's not easy to develop technology for a device that has not technically been invented yet, but quantum communications is an attractive field of research because the technology will enable us to send messages that are much more secure.

There are several problems that will need to be solved in order to make a quantum internet possible:

What is a quantum computer?

A quantum computer is an ultra-fast computer that will be able to factor impossibly large numbers that the classical computers of today ca not solve.

In conventional computers, the unit of information is called a "bit" and can have a value of either 1 or 0. But its equivalent in a quantum system - the qubit (quantum bit) - can be both 1 and 0 at the same time. This phenomenon opens the door for multiple calculations to be performed simultaneously.

But the qubits need to be synchronised using a quantum effect known as entanglement, which Albert Einstein termed "spooky action at a distance".

There are four types of quantum computers currently being developed, which use:

By being able to solve problems incredibly quickly, quantum computers will enable a multitude of useful applications, such as being able to model many variations of a chemical reaction to discover new medications; developing new imaging technologies for healthcare to better detect problems in the body; or to speed up how we design batteries, new materials and flexible electronics.

Pooling computing power

Quantum computers might be more powerful than classical computers, but some applications will require even more computing power than one quantum computer can provide on its own.

If you can get quantum devices to talk to each other, then you could connect several quantum computers together and pool their power to form one huge quantum computer.

However, since there are four different types of quantum computers being built today, they wo not be all be able to talk to each other without some help.

Some scientists favour a quantum internet based entirely on light particles (photons), while others believe that it would be easier to make quantum networks where light interacts with matter.

"Light is better for communications, but matter qubits are better for processing," Joseph Fitzsimons, a principal investigator at the National University of Singapore's Centre of Quantum Technologies tells the BBC.

"You need both to make the network work to establish error correction of the signal, but it can be difficult to make them interact."

It is very expensive and difficult to store all information in photons, Mr Fitzsimons says, because photons ca not see each other and pass straight by, rather than bouncing off each other. Instead, he believes it would be easier to use light for communications, while storing information using electrons or atoms (in matter).

Quantum encryption

One of the key applications of the quantum internet will be quantum key distribution (QKD), whereby a secret key is generated using a pair of entangled photons, and is then used to encrypt information in a way that is impossible for a quantum computer to crack.

This technology already exists, and was first demonstrated in space by a team of researchers from the National University of Singapore and the University of Strathclyde, UK, in December 2015.

But it's not just the encryption that we will need to build in order to secure our information in the quantum future.

Scientists are also working on "blind quantum computer protocols", because they allow the user to hide anything they want on a computer.

"You can write something, send it to a remote computer and the person who owns the computer ca not tell anything about it at all except how long it took to run and how much memory it used," says Mr Fitzsimons.

"This is important because there likely wo not be many quantum computers when they first appear, so people will want to remotely run programs on them, the way we do today in the cloud."

There are two different approaches to building quantum networks - a land-based network and a space-based network.

Both methods work well for sending regular bits of data across the internet today, but if we want to send data as qubits in the future, it is much more complicated.

To send particles of light (photons), we can use fibre optic cables in the ground. However, the light signal deteriorates over long distances (a phenomenon known as "decoherence"), because fibre optics cables sometimes absorb photons.

It is possible to get around this by building "repeater stations" every 50km. These would essentially be miniature quantum laboratories that would try to repair the signal before sending it on to the next node in the network. But this system would come with its own complexities.

Land or space?

Then there are space-based networks. Let's say you want to send a message from the UK to a friend in Australia. The light signal is beamed up from a ground station in the UK, to a satellite with a light source mounted on it.

The satellite sends the light signal to another satellite, which then beams the signal down to a ground station in Australia, and then the message can be transmitted over a ground-based quantum network or classical internet network to the other party.

"Because there's no air between the satellites, there's nothing to degrade the signal," says Dr Jamie Vicary, a senior research fellow at Oxford University's department of computer science and a member of the Networked Quantum Information Technologies Hub (NQIT).

How does quantum key distribution work?

Routing messages

Quantum memory

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