Quantum computing is advancing in small and giant steps. Tiny, because what is achieved involves unique overcoming of limitations; giant, because each stride opens a world of endless possibilities.
A research team led by Ronald Hanson, a physicist at the QuTech lab at Delft University in the Netherlands, has taken such a step and succeeded in teleporting quantum information through a rudimentary network with no direct connection between sender and receiver. receiver.
This quantum leap published on Wednesday in Nature, it means we are one step closer to a quantum internet that would then lead to an entirely new type of computer that could carry out tasks that would take supercomputers thousands of years in a matter of minutes, paving the way for great progress in the field. Of medicine. and artificial intelligence.
Basic teleportation had already been achieved (Hanson’s team and others have already demonstrated this), but only between two points, or adjacent nodes, named Alice and Bob. Now, Hanson’s team has managed to link this long-distance couple to a third party, Charlie. The trio forms the first network that, although rudimentary, allows us to contemplate a quantum internet, with infinite possibilities for computing and observing a world hitherto unknown.
Explaining the relevance of this primordial quantum network, Hanson says: “Regarding quantum communication, our work shows how teleportation can be used in a real network environment, with nodes that are not directly connected. In a quantum Internet of the future, such teleportation will be the main way to transfer quantum information over great distances. Our network can be seen as a modular quantum computer [where the nodes are the modules]; our work shows that nodes can exchange quantum information, even if they are not on a single chip.”
To understand the progress made by the Hanson team, we must look back at previous achievements in this field. The first of these was the demonstration that teleportation is possible in the quantum world. The late Israeli physicist Asher Peres, who died in 2005, anticipated the phenomenon in the Physical Review Letters in 1993. When asked by a journalist if quantum teleportation could transport both the soul and the body, the physicist replied: “Not the body, only the soul.”
This anecdote is significant for understanding quantum teleportation, where it is not the matter that is transported through a medium, but the information that gives it its properties. As Hanson explains: “The key feature of quantum teleportation is that the quantum information itself is actually teleportable: it doesn’t travel through space or fiber. The pair of entangled qubits, which is the resource to execute the teleportation, the ‘teleporter’, is prepared by using a signal through fiber”. Another term for this phenomenon was coined by Albert Einstein: “Creepy action at a distance.” He himself considered it impossible.
Thus, when one particle is previously entangled with another, both cease to be individual particles with defined eigenstates and become a system with a single wave function. And whatever action occurs in Alice is instantly replicated in Bob. This is how Hanson explains it: “The measurement of one of them immediately causes the other to also choose a state; in a sense, measuring one also measures the other. This is very different from manipulating qubits: if we rotate Alice, nothing happens to Bob. Therefore, the spooky action at a distance [that Einstein spoke of] it only refers to the instantaneous correlation in the measurement results. If things other than measurement were also transferred instantly, it would be possible to send messages faster than the speed of light.”
Teleportation has been experimented with for a quarter of a century, starting with photons and moving on to atoms and more complex systems. Five years ago, Jian-Wei Pan, the leading researcher in this field working at the University of Science and Technology of China, together with his team managed to teleport photons from Earth to the artificial satellite Micius, which orbits at an altitude of 1,400 kilometers.
Jian-Wei Pan himself explained to EL PAÍS how these achievements, fundamental to quantum computing, have to overcome a “formidable challenge” in the form of the presence of noise and imperfections. “We need to use quantum error correction and fault-tolerant operations to overcome noise and scale up the system,” he said earlier this year.
If he presence of noise and imperfections can cause a quantum operation to fail on a single computer under laboratory conditions, the problem is multiplied in a network operation. And this has been the main achievement of Hanson and his team: efficient quantum teleportation between non-adjacent nodes in a network.
The Dutch researcher explains: “Noise and imperfections are a challenge for quantum information processing. In a quantum network, sending information through the nodes could be done through the intervening fibers, but that would make the quantum information subject to noise and losses in the fiber channel. In contrast, teleportation allows quantum information to be sent between distant nodes without interference from these noise sources. Teleportation requires entanglement as a resource. Bob helps create that entanglement between Alice and Charlie, who don’t share a direct physical connection.”
The process builds on previous research in which Hanson got a network to work between adjacent nodes. The challenge was to add a third node and create a state between the three that would show quantum correlations.
In the new experiment, Alice and Charlie have no direct connection to each other, but they are both connected to Bob. Alice and Bob’s quantum systems set up the process by using an entangled state between them, one that Bob stores, freeing it to create an entanglement with Charlie. “After setting up an entanglement between Alice and Charlie, the state to be teleported is created and then executed,” says Hanson. “What happens then is something that is only possible in the quantum world: As a result of the measurement, the information disappears from Charlie’s side and immediately appears from Alice’s side.”
“The necessary principles for quantum networks”
Juan José García Ripoll, research scientist at the Institute of Fundamental Physics of the Higher Council for Scientific Research (CSIC) and co-founder of Inspiration-Q, says that the work of Hanson and his team is very important: “This is a very sophisticated experiment that demonstrates all the principles necessary for the creation of quantum communication networks,” he says.
According to García Ripoll, “sending not only classic information [bits] but also quantum states [arbitrary qubit states] requires a mechanism to distribute an entangled state between two distant points and a quantum memory [memory qubit] to store the information to be transmitted while establishing this communication channel based on entanglement”.
Hanson’s experiment uses Nitrogen-Vacancy (NV) centers, “a type of diamond impurity that acts like a qubit and can be manipulated optically,” says García Ripoll. “Through the emission of photons, this qubit makes it possible to create long-distance entanglements.” An NV center is a defect whereby a carbon atom in the diamond crystal lattice is replaced by a nitrogen atom (N) and a neighboring vacancy (V).
A single NV can detect a single molecule magnetic moment and has wide applications in quantum technology. According to García Ripoll, “the NV center or color center can also talk to the magnetic moments of the surrounding atoms and, in [Hanson’s] experiment, they use this to get a quantum memory by passing the NV information to a nearby nuclear spin [in a carbon-13 isotope]. The information to be sent can be kept secure for a long time, freeing the NV to perform the task of establishing an entanglement with another communication node.
“In addition to the quality of the experiment, the demonstration of a sophisticated quantum communication configuration with three nodes and very elaborate communication algorithms lays the foundation for its extension to scalable configurations of entanglement distribution and quantum communication that are very promising”, adds García. Ripoll.
Premature but important
For Adán Cabello, a physicist at the University of Seville in southern Spain, whose first measurement of a quantum sequence was recognized as one of the great advances in physicsIt’s still too early to talk about a quantum internet, though the experiment is significant as a teleport to a distant node.
Cabello tries to explain the achievement in simplified terms. “You have a quantum state in a city that could be Seville and you want to send it to another city, let’s say Madrid,” he says. “There needs to be a state of entangled qubits between Seville and Madrid. That’s the standard teleportation protocol. The interesting thing about the experiment is that the entanglement can only be established at a certain distance. Let’s say, in the example of the cities, it is 500 kilometers. If you want to send qubits from Seville to San Sebastián, you have to overcome the distance limitation. That is what Hanson has achieved: it is no longer Seville-Madrid, it is Seville-San Sebastián. The distance has doubled.
“It’s a first look at what a network could be,” adds Cabello. “It is no longer point to point; you can now start involving more nodes. But to talk about the quantum internet is to get ahead of ourselves. However, teleporting quantum information is very useful and will have many applications, there is no doubt about that.”
Along the same lines, physicists Oliver Slattery and Yong-Su Kim highlight the breakthrough made by Hanson and his team as an important and “critical” step towards creating a secure next-generation quantum Internet. They also highlight the importance of the innovations developed to achieve the process: the preparation, handling and reading quantum states.
However, both physicists point out: “Further improvements in the characteristics of multiple systems will be needed to enable multiple rounds of teleportation and produce large-scale quantum networks.”