Researchers in Delft have succeeded in teleporting quantum information over a rudimentary network. This first of its kind is an important step towards a future quantum internet. This breakthrough was made possible by greatly improved quantum memory and improved quality of the quantum connections between the three nodes of the network. The researchers, who work at QuTech, a collaboration between TU Delft and TNO, are publishing their findings today in the scientific journal Nature.
The power of a future quantum internet is based on the ability to transmit quantum information (quantum bits) between the nodes of the network. This enables a variety of applications, such as securely sharing confidential information, linking multiple quantum computers together to increase their computing capacity, and using highly accurate, linked quantum sensors.
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Send Quantum Information
The nodes of such a quantum network consist of small quantum processors. Sending quantum information between these processors is no small feat. One possibility is to send quantum bits with light particles, but due to the unavoidable losses in fiber optic cables, especially over long distances, the light particles will most likely not reach their destination. Since it is basically impossible to simply copy quantum bits, the loss of a light particle means that the quantum information is irretrievably lost.
Teleportation provides a better way to transmit quantum information. The quantum teleportation protocol gets its name from similarities to teleportation in science fiction movies: the quantum bit disappears on the sender side and appears on the receiver side. Since the quantum bit does not have to travel through the intervening space, there is no chance of it being lost. This makes quantum teleportation a crucial technique for a future quantum internet.
Good control over the system
To be able to teleport quantum bits, various ingredients are needed: a quantum-entangled link between sender and receiver, a reliable method for reading quantum processors and the capacity to temporarily store quantum bits. Previous research at QuTech showed that it is possible to teleport quantum bits between two adjacent nodes. The QuTech researchers have now demonstrated for the first time that they can meet the package of requirements and have demonstrated teleportation between non-adjacent nodes, i.e. over a network. They teleported quantum bits from node “Charlie” to node “Alice”, using an intermediate node “Bob”.
Teleport in three steps
The teleportation consists of three steps. First, the “teleporter” must be prepared, which means that an entangled state must be created between Alice and Charlie. Alice and Charlie have no direct physical connection, but they are both directly connected to Bob. For this, Alice and Bob create an entangled state between their processors. Bob then saves his share of the entangled state. Then Bob creates a confused state with Charlie. A quantum mechanical “sleight of hand” is then performed: by making a special measurement in his processor, Bob redirects the entanglement, as it were. Results: Alice and Charlie are now entangled and the teleporter is ready to be used!
The second step is to create the “message” – the quantum bit – to be teleported. This could be, for example, ‘1’ or ‘0’ or several other intermediate quantum values. Charlie is preparing this quantum information. To demonstrate that the teleportation works generically, the researchers repeated the entire experiment for different quantum bit values.
Step three is the actual teleportation from Charlie to Alice. To this end, Charlie performs a joint measurement with the message on his quantum processor and on his 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 on Charlie’s side and immediately appears on Alice’s side.
You would think that then everything is complete, but nothing could be further from the truth. In fact, the quantum bit is encrypted on transfer; the key is determined by Charlie’s measurement result. So Charlie sends the measurement result to Alice, after which Alice performs the relevant quantum operation to decipher the quantum bit. For example via a “bit flip”: 0 becomes 1 and 1 becomes 0. After Alice has performed the correct operation, the quantum information is suitable for further use. The teleportation succeeded!
Teleport multiple times
Follow-up research will focus on reversing steps one and two of the teleportation protocol. This means that first the quantum bit to be teleported is created (or received) and only then the teleporter is prepared to perform the teleportation. Reversing the sequence is particularly challenging because the quantum information to be teleported must be stored while the entanglement is being created. However, it has an important advantage because the teleportation can then be performed completely “on demand”. This is relevant, for example, if the quantum information contains the result of a difficult calculation or if multiple teleports have to be made. In the long run, this type of teleportation will therefore form the backbone of the quantum internet.
Qubit teleportation between non-neighboring nodes in a quantum networkSLN Hermans, M. Pompili, HKC Beukers, S. Baier, J. Borregaard and R. Hanson, Nature, 2022, DOI: 10.1038/s41586-022-04697-y
Financial support comes from the EU Flagship on Quantum Technologies through the Quantum Internet Alliance project (EU Horizon 2020, Grant Agreement No. 820445); from the European Research Council (ERC) through an ERC Consolidator Grant (Grant Agreement No. 772627 to R. Hanson); from the Netherlands Organization for Scientific Research (NWO) via a VICI grant (project no. 680-47-624) and the Gravitation program Quantum Software Consortium (project no. 024.003.037/3368) and from an Erwin- Schrödinger fellowship (QuantNet , no. J 4229-N27) from the Austrian National Science Foundation (FWF).
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