Scientists have gotten one step closer to a quantum internet by creating the world's first multinode quantum network.
Researchers at the QuTech research center in the Netherlands created the system, which is made up of three quantum nodes entangled by the spooky laws of quantum mechanics that govern subatomic particles. It is the first time that more than two quantum bits, or "qubits," that do the calculations in quantum computing have been linked together as "nodes," or network endpoints.
Researchers expect the first quantum networks to unlock a wealth of computing applications that can't be performed by existing classical devices — such as faster computation and improved cryptography.
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"It will allow us to connect quantum computers for more computing power, create unhackable networks and connect atomic clocks and telescopes together with unprecedented levels of coordination," Matteo Pompili, a member of the QuTech research team that created the network at Delft University of Technology in the Netherlands, told Live Science. "There are also loads of applications that we can't really foresee. One could be to create an algorithm that will run elections in a secure way, for instance."
In much the same way that the traditional computer bit is the basic unit of digital information, the qubit is the basic unit of quantum information. Like the bit, the qubit can be either a 1 or a 0, which represent two possible positions in a two-state system.
But that's just about where the similarities end. Thanks to the bizarre laws of the quantum world, the qubit can exist in a superposition of both the 1 and 0 states until the moment it is measured, when it will randomly collapse into either a 1 or a 0. This strange behavior is the key to the power of quantum computing, as it allows a qubit to perform multiple calculations simultaneously.
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The biggest challenge in linking those qubits together into a quantum network is in establishing and maintaining a process called entanglement, or what Albert Einstein dubbed "spooky action at a distance." This is when two qubits become coupled, linking their properties so that any change in one particle will cause a change in the other, even if they are separated by vast distances.
You can entangle quantum nodes in a lot of ways, but one common method works by first entangling the stationary qubits (which form the network's nodes) with photons, or light particles, before firing the photons at each other. When they meet, the two photons also become entangled, thereby entangling the qubits. This binds the two stationary nodes that are separated by a distance. Any change made to one is reflected by an instantaneous change to the other.
"Spooky action at a distance" lets scientists change the state of a particle by altering the state of its distant entangled partner, effectively teleporting information across big gaps. But maintaining a state of entanglement is a tough task, especially as the entangled system is always at risk of interacting with the outside world and being destroyed by a process called decoherence.
This means, first, that the quantum nodes have to be kept at extremely cold temperatures inside devices called cryostats to minimize the chances that the qubits will interfere with something outside the system. Second, the photons used in the entanglement can't travel very long distances before they are absorbed or scattered, — destroying the signal being sent between two nodes.
"The problem is, unlike classical networks, you cannot amplify quantum signals. If you try to copy the qubit, you destroy the original copy," Pompili said, referring to physics' "no-cloning theorem," which states that it is impossible to create an identical copy of an unknown quantum state. "This really limits the distances we can send quantum signals to the tens of hundreds of kilometers. If you want to set up quantum communication with someone on the other side of the world, you'll need relay nodes in between."
To solve the problem, the team created a network with three nodes, in which photons essentially "pass" the entanglement from a qubit at one of the outer nodes to one at the middle node. The middle node has two qubits — one to acquire an entangled state and one to store it. Once the entanglement between one outer node and the middle node is stored, the middle node entangles the other outer node with its spare qubit. With all of this done, the middle node entangles its two qubits, causing the qubits of the outer nodes to become entangled.
But designing this weird quantum mechanical spin on the classic "river crossing puzzle" was the least of the researchers' troubles — weird, for sure, but not too tricky an idea. To make the entangled photons and beam them to the nodes in the right way, the researchers had to use a complex system of mirrors and laser light. The really tough part was the technological challenge of reducing pesky noise in the system, as well as making sure all of the lasers used to produce the photons were perfectly synchronized.
"We're talking about having three to four lasers for every node, so you start to have 10 lasers and three cryostats that all need to work at the same time, along with all of the electronics and the synchronization," Pompili said.
The three-node system is particularly useful as the memory qubit allows researchers to establish entanglement across the network node by node, rather than the more demanding requirement of doing it all at once. As soon as this is done, information can be beamed across the network.
Some of the researchers' next steps with their new network will be to attempt this information beaming, along with improving essential components of the network's computing abilities so that they can work like regular computer networks do. All of these things will set the scale that the new quantum network could reach.
They also want to see if their system will allow them to establish entanglement between Delft and The Hague, two Dutch cities that are roughly 6 miles (10 kilometers) apart.
"Right now, all of our nodes are within 10 to 20 meters [32 to 66 feet] of each other," Pompili said. "If you want something useful, you need to go to kilometers. This is going to be the first time that we're going to make a link between long distances."
The researchers published their findings April 16 in the journal Science.
Originally published on Live Science.