Scientists have come one step closer to quantum internet creating the world’s first multisite quantum network.
Researchers at the QuTech Research Center in the Netherlands have created a system that consists of three quantum nodes entangled in eerie laws quantum mechanics which control subatomic particles. For the first time, more than two quantum bits or “qubits” performing computations in quantum computing were linked together as “nodes” or endpoints of a network.
Researchers expect the first quantum networks to open up many computing applications that cannot be done with existing classical devices, such as faster computation and improved cryptography.
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“This will allow us to connect quantum computers to increase computing power, create networks that cannot be hacked, and connect atomic clocks and telescopes together with an unprecedented level of coordination,” said Matteo Pompili, a member of the QuTech research group that created the network at Delft University of Technology. in the Netherlands, Live Science told. “There are also many applications that we cannot foresee. You can, for example, create an algorithm that will conduct elections in a safe way. ”
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 a bit, a qubit can have a value of 1 or 0, which corresponds to two possible positions in a two-state system.
But that’s where the similarities end. Thanks to the strange laws of the quantum world, a qubit can exist in a superposition of states 1 and 0 until the moment of measurement, when it collapses randomly to either 1 or 0. This strange behavior is the key. to the power of quantum computing, since it allows a qubit to perform multiple computations at the same time.
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The biggest challenge in connecting these qubits into a quantum network is creating and maintaining a process called entanglement, or what Albert Einstein called “spooky action at a distance.” This is when two qubits become linked, linking 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 a variety of ways, but one general method works by first entangling stationary qubits (which form network nodes) with photons or light particles, before the photons shoot each other. When they meet, the two photons also become entangled, entangling the qubits. This connects two fixed nodes separated by a distance. Any change in one of them is reflected by the instantaneous change in the other.
“Spooky action at a distance” allows scientists to alter the state of a particle by altering the state of its distant entangled partner, effectively teleporting information across large gaps. But maintaining a state of entanglement is a difficult task, especially since an entangled system is always threatened with interaction with the outside world and destruction in a process called decoherence.
This means, first, that quantum nodes must be stored at very low temperatures inside 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 they are absorbed or scattered, which destroys the signal transmitted between the two nodes.
“The problem is that, unlike classical networks, you cannot amplify quantum signals. If you try to copy a qubit, you will destroy the original copy, ”Pompili said, referring to the physical“ no-cloning theorem, ”which says that this is not possible. to create an identical copy of an unknown quantum state. “This really limits the distances over which we can send quantum signals to tens of hundreds of kilometers. If you want to establish a quantum connection with someone on the other side of the world, you need intermediate relay nodes. “
To solve this problem, the team created a three-node network in which photons essentially “transfer” entanglement from a qubit at one of the outer nodes to one at the middle node. There are two qubits in the middle node – one for getting the entangled state and the other for storing it. Once the entanglement between one outer node and the middle node is preserved, the middle node entangles the other outer node with its spare qubit. After all this, the middle node entangles two of its qubits, as a result of which the qubits of the outer nodes become entangled.
But creating this strange quantum mechanical spin based on the classic “river crossing puzzle” was the least of the researchers’ problems – a weird idea, of course, but not too difficult. To make the entangled photons and direct them to the nodes in the correct way, the researchers had to use a complex system of mirrors and laser light. The real tricky part was the technological challenge of reducing the annoying noise in the system, as well as ensuring perfect synchronization of all the lasers used to generate photons.
“We’re talking about having three to four lasers for each node, so you start to have 10 lasers and three cryostats that need to work at the same time, along with all the electronics and timing,” Pompili said.
The three-node system is especially useful because the memory qubit allows researchers to establish entanglement between nodes in a network, rather than doing it all at the same time. Once this is done, the information can be transmitted over the network.
Some of the next steps for researchers in their new network will be to try to communicate this information, along with improving the basic components of the network’s computing capabilities so that they can function like regular computer networks. All of this will set the scale that the new quantum network can reach.
They also want to see if their system will allow for a link between Delft and The Hague, two Dutch cities that are about 6 miles (10 kilometers) apart.
“Now all our nodes are within the range of 10 to 20 meters. [32 to 66 feet] each other, “said Pompili.” If you want something useful, you have to travel miles. This will be the first time we are going to connect long distances. “
The researchers published their findings on April 16 in the journal. The science…
Originally published on Live Science.