Quantum computing advances with small giant steps. Small because the achievements that are reached suppose singular ruptures of the limitations. But gigantic because each stride opens up a world of infinite possibilities. This is the case of the research led by Ronald Hanson, a physicist at the QuTech laboratory at the University of Delft (Holland), who has achieved, according to post this wednesday Nature, teleport quantum information through a rudimentary network with no direct connection between sender and receiver. Basic teleportation had already been achieved (Hanson’s team and others have already demonstrated this). But only between two points, named Alice and Bob, or between adjacent nodes. Now, this couple is joined by Charlie from a distance. The three form the first network that, although rudimentary, allows us to think of a quantum internet, with infinite computing possibilities and to observe a world hitherto unknown.
Roland Hanson details the relevance of this early quantum network: “For quantum communication, our work shows how teleportation can be used in a real network environment, with nodes that are not directly connected. In the future quantum internet, 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 though they are not on a single chip.”
To understand the progress of Hanson’s team, we must start from previous achievements in which they themselves have participated along with other great researchers in this science. The first was the demonstration that teleportation is possible in the quantum world. Asher Peres (France 1934, Israel 2005) laid the foundations in Physical Review Letters in 1993. Then, a journalist asked him: “In the case of teleporting a person, would the body go before the soul?” The physicist replied: “Not the body, only the soul.”
This anecdote is significant for understanding quantum teleportation, where matter is not transferred through a medium, but rather the information that confers its properties. As Hanson explains: “The key feature of quantum teleportation is that the quantum information itself is actually teleported: it doesn’t travel through space or through fiber. The interlocked pair (entangled, in English) of qubits, which is the resource to execute teleportation (the “teleporter”), is prepared by using signal through fiber”. Another term for this phenomenon was coined Albert Einstein when he advanced this possibility in 1935 together with Boris Podolsky and Nathan Rosen, in what is known as the EPR paradox: “Phantasmagorical action at a distance”.
In this way, when a particle is previously entangled with another, both cease to be individual particles with their own defined states and become a system with a single wave function. And any measurement that occurs on Alice is instantly replicated on Bob. This is how Hanson explains it: “The measurement of one of them, immediately, makes the other 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, ghostly action at a distance [que planteó Einstein] it only refers to the instantaneous correlation in the measurement results. If things other than measurement were also transferred instantly, it would actually be possible to send messages faster than the speed of light.”
Teleportation has been tested for a quarter of a century starting with photons to move to atoms and more complex systems. Five years ago, Jian-Wei Pan, the leading researcher in this field who works for the University of Science and Technology of China, managed with his team the teleportation of photons from Earth to the artificial satellite Miciusin orbit at 1,400 kilometers altitude.
Jian-Wei Pan himself explained to this newspaper how these achievements, fundamental to quantum computing, face a “formidable challenge”: the presence of noise and imperfections. “We need to use quantum error correction and fault-tolerant operations to overcome noise and scale the system,” he says.
If the presence of noise and imperfections can cause quantum operation to fail in a single computer under laboratory conditions, the problem is multiplied in network operation. And this has been the great achievement of Hanson’s team: efficient quantum teleportation between non-neighboring nodes in a network.
In this sense, the Dutch researcher explains: “Noise and imperfections are a challenge for quantum information processing. In a quantum network, sending information across nodes could be down the intervening fibers, but that would make the quantum information subject to fiber channel noise and losses. In contrast, teleportation allows quantum information to be sent between distant nodes without suffering from these sources of noise. Teleportation requires entanglement as a resource. Bob helps create that intertwining between Alice and Charlie, who don’t share a direct physical connection.”
The process has continued previous research where 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’s and Bob’s processors prepare the process by creating an interlocked state between their processors, and Bob stores it while he creates an interlocked state with Charlie. “After setting up an intertwined state between Alice and Charlie, the state to be teleported is created and then executed,” summarizes 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,” explains the Dutch university.
“The Necessary Principles”
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-Qconsiders the work of Hanson and his team very relevant: “It is a very sophisticated experiment that demonstrates all the principles necessary for the creation of quantum communication networks.”
For García Ripoll, who did not participate in the Dutch study, “sending not only classical information (bits) but also quantum states requires a mechanism that allows an entangled state to be distributed between two distant points and a quantum memory that allows holding the information to be transmitted. while establishing that channel of communication based on interlacing”.
In Hanson’s experiment, NV-centers are used, “a type of diamond impurity that acts”, explains the Spanish scientist, “like a qubit and can be manipulated optically. This qubit, through the emission of photons, makes it possible to create long-distance entanglement.” NV-center is a defect whereby a carbon atom in the diamond crystal lattice is replaced by a nitrogen atom (N) and an adjacent vacant lattice site (V).
A single NV allows detection of the magnetic moment and has wide applications in quantum technology. According to García Ripoll, “the NV-center or color center can also to talk with the magnetic moments of surrounding atoms and in the experiment [de Hanson] they use this to have a quantum memory: transferring the information from the NV 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 the link with another communication node,” he adds.
“Apart from the quality of the experiment, the demonstration of a sophisticated quantum communication setup, with three nodes and also very elaborate communication algorithms, lays the groundwork to extend the idea to very promising scalable quantum communication and entanglement distribution setups” , Garcia Ripoll concludes.
Premature, but relevant
For Adán Cabello, a physicist at the University of Seville whose first measurement of a quantum sequence was recognized as one of the main advances in physics, it is premature to talk about the quantum internet, although the experiment as a teleportation to a new distant node is relevant.
Cabello tries to simplify the achievement to explain it: “You have a quantum state in a city that could be Seville and you want to send it to another city, Madrid. You need an entangled state of qubits between Seville and Madrid. That’s a standard teleport protocol. The interesting thing about the experiment is that the entanglement can only be established at a certain distance. Let’s say that, 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 limitation of distance. It is what Hanson has described: it is no longer Seville-Madrid, it is Seville-San Sebastián. They have doubled the distance.”
“It is a first panorama of what a network could be”, he sums up. “In other words, it is no longer point-to-point, but rather more nodes can begin to be involved. But to talk about the quantum internet is to promise too much. However, teleporting quantum information is very useful and will have many applications. No doubt”.
In the same vein, physicists Oliver Slattery and Yong-Su Kim highlight the progress achieved by Hanson and his team as an important, “critical” step for the creation of a new-generation, safe, quantum internet. They also highlight the importance of the innovations developed to achieve the process: the preparation, manipulation and reading of quantum states.
However, both physicists point out: “Further improvements to multiple system features will be needed to enable multiple rounds of teleportation and produce large-scale quantum networks.”