A team researchers from quantum-computing company QuTech in the Netherlands has established the first multi-node quantum network, by connecting three quantum processors through the mysterious process of entanglement.
This eureka-style moment – reported in the journal Science – marks the latest step in a decade of quantum computing research, with the ultimate goal of creating a safe, un-hackable and fast quantum internet: the internet of the future.
What is a quantum computer?
While a normal computer uses bits – which can be either 0s or 1s – a quantum computer uses qubits, or quantum bits. These are typically subatomic particles like electrons or photons, which can be isolated in a quantum state so that they can represent 1 and 0 at the same time.
Superposition allows a quantum computer to process data much faster than an average processor, because it can do multiple calculations simultaneously.
What is quantum entanglement?
Quantum entanglement is a phenomenon in quantum physics in which a pair or group of particles – usually generated together or existing in close proximity – share a quantum state, such that each particle’s state will not differ from the others, no matter how far away they are.
Einstein called this ‘spooky action at a distance’, because – thanks to another poorly understood quirk of quantum physics – when one particle is measured, not only will its original quantum state will be changed, but so will the quantum state of all other particles thus entangled, even if they are far apart.
Harnessing this effect in quantum computing offers a remarkably powerful opportunity for long-distance, un-hackable messages to be encoded.
Before now, connecting multiple processors using entanglement was more a theory than a reality; previously, only two quantum devices have shared a direct physical link. But this new, rudimentary example of a multi-node quantum network is the first workable version.
How did they do it?
The research team created three quantum nodes, named Alice, Bob and Charlie. The middle node, Bob, has a physical connection to the outer nodes, Alice and Charlie, so Bob can have entanglement links established with both. Bob is also equipped with an additional quantum bit to be used as memory, so previously generated quantum links can be stored while a new link is established.
Once the quantum links between Alice-Bob and Bob-Charlie are established, quantum operations at Bob can convert these links into a quantum link between Alice and Charlie, or between all three. These nodes can then be separated at some distance within the building, and function as a network.
“Once established, we were able to preserve the resulting entangled states, protecting them from noise,” says Sophie Hermans, a member of the research team from QuTech. “It means that, in principle, we can use these states for quantum key distribution [a secure communication method involving the sharing of encrypted keys], a quantum computation or any other subsequent quantum protocol.”
Critically, the network ‘announces’ the successful completion of its protocols with a ‘flag’ signal, which will be crucial for linking more processors into the network, and therefore for the network’s future scalability into a global system.
This entanglement-based quantum network will allow the researchers to develop and test quantum internet hardware, software and protocols.
“A quantum internet will open up a range of novel applications, from un-hackable communication and cloud computing with complete user privacy to high-precision time-keeping,” says Matteo Pompili, a PhD student at Delft University of Technology and a member of the research team. “And like with the Internet 40 years ago, there are probably many applications we cannot foresee right now.”