The quantum teleportation allows the transfer of quantum information (qubits or quantum bits) from one place to another, so it has very important potential applications in secure communications, quantum computing, and the development of the next generation of the internet.
Experimental demonstrations of this effect, behind which is the strange quantum entanglement (which ‘connects’ two distant particles in a quantum way), until now they had only been performed between two adjacent connected nodes, but researchers at the Dutch institute QuTech have managed to achieve between two that are not.
The advance, published in the magazine Naturerepresents an important step towards the future quantum internet, according to the authors, who have achieved it thanks to quality quantum links established in a network with three nodes: Alice, Bob and Charlie, connected online by fiber and with direct links between those who are ‘neighbors’: Alice and Bob on one side, and Bob and Charlie on the other. But Alice and Charlie are not neighbors.
“Neighbors in a quantum network means that they have a direct fiber connection and that they can create entanglement directly, i.e. without the need for another node. In this case, being neighbors or not has nothing to do with distance, but with connectivity, how the network is organized”, explains the lead author to SINC. Ronald Hansonwhich underlines: “Our work demonstrates for the first time the teleportation of quantum information in a network, between nodes that are not directly connected”.
To understand the experiment, it is worth remembering that sending the qubits between the processors that make up these quantum networks is not easy. One possibility is to send quantum bits via particles of light, but due to the inevitable losses in fiberglass cables, particularly over long distances, they are unlikely to reach their destination. The loss of a particle of light means that the quantum information hopelessly lost.
However, teleportation offers a better way to send quantum information. Its action protocol resembles teleportation in science fiction movies: the qubit disappears from the sender side and appears from the receiver side. As it does not traverse the in-between space, it cannot be lost, which is crucial in the quantum internet of the future.
“The main feature of teleportation is that it allows the reliable transfer of quantum information (qubits) between us, a key routine that in a future quantum internet will happen all the time. As with teleportation, quantum information does not travel through space, it is also not disturbed by space (a typical disturbance would be the scattering loss in an optical fiber). This is very important because quantum information cannot be copied, so if it is lost in transmission, it will be lost forever.”
In order to teleport the qubits several ingredients are needed: a entangled quantum bond between sender and receiver, a reliable method for read processors quantum and the ability to store these quantum bits temporarily.
Previous research by QuTech, a collaboration between Delft University of Technology and the Netherlands Organization for Applied Scientific Research (TNO), had already shown that it is possible to teleport qubits between two neighboring nodes. The novelty now is to achieve it for the first time between two non-adjacent ones, that is, through a network, at the rudimentary moment.
Specifically, they teleported quantum bits from Charlie to Alice, with the help of Bob, the intermediate node. All three operate through a kind of diamond impurity called nitrogen vacancy center (NV center).
Quantum teleportation in three steps
Teleportation consists of three steps. First of all, you have to prepare the ‘teleporter’, which means that an entangled state needs to be created between Alice and Charlie. These do not have a direct physical connection, but both are directly connected to Bob. To do this, Alice and Bob create a linked state between their processors. Bob then stores his part of the interlocked state and then interlocks with Charlie as well.
Then a kind of quantum mechanical “sleight of hand” is performed: through a special measurement on his processor, Bob “sends” the tangle, so Alice and Charlie are entangled and the teleporter is ready to go.
The second step is produce the ‘message’ –the qubit– that will teleport. It can be, for example, 1 or 0 or several intermediate quantum values. Charlie prepares this quantum information. To show that teleportation works in a generic way, the researchers repeated the entire experiment for various values of quantum bits.
The third step is real teleportation from Charlie to Alice. To do this, Charlie takes a joint measurement with the message in her quantum processor and in her half of the entangled state (Alice has the other half). What happens then is something that is only possible in the quantum world: as a result of this measurement, the information disappears from Charlie’s side and immediately appears from Alice’s side.
Teleportation of a qubit between non-neighboring nodes (made with diamond chips) in a quantum network. The three network nodes, Alice (A), Bob (B), and Charlie (C), are connected by fiber optic links in a line configuration. Each configuration has a communication qubit (purple) that allows it to create entanglement with its neighboring node. Also, Bob and Charlie contain a memory qubit (yellow). The teleportation protocol takes place in three steps: preparation of the ‘teleporter’, preparation of the state of the qubit to be teleported, and the teleportation itself. / Scixel for QuTech/ S. Hermans, R. Hanson et al./ Nature
You might think this completes the process, but nothing is further from the truth. In fact, the quantum bit has been encrypted as it is transferred. The key is determined by Charlie’s measurement result, and this is sent to Alice who, in turn, performs the corresponding quantum operation to decrypt the quantum bit.
This can be done, for example, by the ‘flip’ bit: 0 becomes 1 and 1 becomes 0. After Alice has done the correct operation, the quantum information is ready for further use. The teleportation was successful.
Future network enhancements
The team’s future research will focus on reversing steps one and two of the protocol to optimize it and expand its capabilities. In any case, the authors argue that the exchange of information between disconnected nodes that they demonstrated already represents an important step towards building a quantum network that communicates by teleporting quantum information.
Alice, the receiving node of the teleported quantum information. Inside the black aluminum cylinder, the diamond sample is cooled to -270°C to reduce background noise and allow for quantum control. / Marieke de Lorijn/QuTech
However, the possible ubiquitous teleportation in a quantum network is still a long way off, according to another journal article. Nature researchers Oliver Slattery (from the NIST Institute in the USA) and Yong-Su Kim (KIST, from South Korea). The authors believe that further improvements to various system features will be needed to enable multiple rounds of teleportation and produce large-scale quantum networks.
Carlos Sabin, researcher at the Institute of Fundamental Physics of the CSIC, also valued this study for SMC Spain: “It can be interpreted as a first (very preliminary) step towards a quantum communications network. However, it should be taken into account that the great experimental difficulty makes the quality of information transmission still very low. This can be measured by calculating the so-called ‘fidelity’ of the transmitted state, i.e. the similarity between the actual final state of the qubit and the state we wanted to transmit.”
Need to increase ‘fidelity’
“Ideally, this fidelity is 100%”, he adds. If it is above 66.6% we know that the process is impossible without the use of quantum physics. In the experiment, a fidelity of 70% is reached on average, but in some states it drops to 65%. This is enough to show that the process is quantum (at least, on average), but obviously still very far from any possible technological application, since the obtained state is 30% different from the original. There is a lot of work ahead to improve these percentages and be able to extend the experiment to nodes further away from the network.”
It is a necessary advance, but still insufficient to cover the possibilities that quantum teleportation will open to us in a practical and routine way, in particular, to be able to teleport to sufficiently great distances.
For your part, Miguel Angel Martin Delgadoprofessor of Theoretical Physics at the Complutense University of Madrid and coordinator of QUITEMAD (Quantum Information Technologies Madrid) adds: “Using spin qubits (quantum information units) embedded in diamonds and connected by fiber optic links, the team was able to teleport states guaranteeing its quantum character and with great efficiency. It has introduced many technical improvements that they hope will be reusable on other quantum platforms, but more information about the range of distances between nodes is lacking.”
“It is a necessary advance -he concludes-, but it is still not enough to go through all the possibilities that open up to us when we manage to master quantum teleportation even more in a practical and routine way and, in particular, to be able to teleport to sufficiently large distances”.
Reference:
Sophie Hermans, Ronald Hanson et al. “Qubit teleportation between non-neighboring nodes in a quantum network”. Nature2022