Scientists are one step closer to a like the Internet by creating the world’s first multi-node quantum network.
Researchers at the QuTech Research Center in the Netherlands have created a system consisting of three quantum nodes entangled in the ghost laws of quantum mechanics which control subatomic particles. For the first time, more than two quantum bits or “qubits” that perform calculations in quantum calculations are connected together as “nodes” or endpoints of the network.
Researchers expect the first quantum networks to unlock a wealth of computer applications that cannot be run by existing classic devices ̵
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“This will allow us to connect quantum computers for more computing power, create fixed networks and connect atomic clocks and telescopes together with unprecedented levels of coordination,” said Matteo Pompili, a member of QuTech’s research team that set up the network at Delft University of Technology. in the Netherlands, said Live Science. “There are also many applications that we can’t really predict. An algorithm can be created to conduct elections in a secure way, for example.”
In 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 bits, the qubit can be 1 or 0, which represent two possible positions in a two-state system.
But that’s where the similarities end. Due to the strange laws of the quantum world, the qubit can exist in superposition of both 1 and 0 states until it is measured, when it will randomly collapse into either 1 or 0. This strange behavior is the key by virtue of quantum calculations, as it allows a qubit to perform multiple calculations simultaneously.
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The biggest challenge in connecting these qubits in a quantum network is to establish and maintain a process called entanglement, or what Albert Einstein called “ghost action from a distance.” This is when two qubits pair, connecting their properties, so that any change in one particle will cause a change in the other, even if they are separated by huge distances.
You can entangle quantum nodes in many ways, but one common method works by first entangling stationary qubits (which form the nodes of the network) with photons or light particles before launching photons at each other. When they meet, the two photons also intertwine, thus intertwining the qubits. This connects the two fixed nodes, which are separated by a distance. Every change made in one is reflected in an instantaneous change in the other.
“Ghost action from a distance” allows scientists to change the state of the particle by changing the state of their distant entangled partner, effectively teleporting information through large gaps. But maintaining a state of entanglement is a difficult task, especially since 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 quantum nodes must be maintained at extremely low temperatures in devices called cryostats to minimize the chances of qubits interfering with something outside the system. Second, the photons used in entanglement cannot travel very long distances before being absorbed or scattered – destroying the signal sent between two nodes.
“The problem is that, unlike classical networks, you can’t amplify quantum signals. If you try to copy qubits, you’re destroying the original copy,” Pompili said, referring to the “non-cloning theorem” of physics, 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 tens of hundreds of kilometers. If you want to set up quantum communication with someone on the other side of the world, you’re going to need relay nodes between them.”
To solve the problem, the team created a three-node network in which the 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 for acquiring a tangled state and one for storage. After the entanglement between one outer node and the middle node is preserved, the middle node entangles the other outer node with its spare qubit. With all this done, the middle node intertwines its two qubits, which causes the qubits of the outer nodes to intertwine.
But designing this strange quantum mechanical rotation on the classic “river crossing puzzle” was the least of the researchers’ problems – a strange, certainly, but not too complicated idea. To make the tangled photons and send them to the nodes in the right way, the researchers had to use a complex system of mirrors and laser light. The really hard part was the technological challenge of reducing annoying noise in the system, as well as ensuring that all the lasers used to produce photons were perfectly synchronized.
“We’re talking about three to four lasers for each node, so you’re starting to have 10 lasers and three cryostats that all need to work at the same time, along with all the electronics and synchronization,” Pompili said.
The three-node system is especially useful because the memory qubit allows researchers to establish entanglement through the network node by node, instead of the more demanding requirement to do everything at once. As soon as this is done, the information can be transmitted over the network.
Some of the next steps researchers will take with their new network will be to try to broadcast this information, along with improving the core components of the network’s computing power so that they can work like ordinary computer networks. All of these things will determine the scale that the new quantum network can reach.
They also want to see if their system will allow them to establish a entanglement between Delft and The Hague, two Dutch cities about 10 kilometers away.
“Currently, all our nodes are at a distance of 10 to 20 meters [32 to 66 feet] “to each other,” Pompili said. “If you want something useful, you have to go miles. This will be the first time we make a long-distance connection.”
The researchers published their findings on April 16 in the journal Science.
Originally published in Live Science.