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 hasn't 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:
- Getting quantum computers to talk to each other
- Making communications secure from hacking
- Transmitting messages over long distances without losing parts of the message; and
- Routing messages across a quantum network
What is a quantum computer?
A quantum computer is a machine that is able to crack very tough computation problems with incredible speed - beyond that of today's "classical" computers.
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.
However, 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:
- Light particles
- Trapped ions
- Superconducting qubits
- Nitrogen vacancy centres in diamonds
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 won't 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."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 can't 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 won't 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.
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