Delft researchers have succeeded in teleporting quantum information over a rudimentary network in the laboratory. This scoop is an important step towards a future quantum internet. The breakthrough was made possible by a greatly improved quantum memory and increased quality of the quantum connections between the network’s three nodes. The researchers working at QuTech – a collaboration between TU Delft and TNO – publish their results today in the scientific journal Nature.
The power of the future quantum internet is based on the ability to share or send quantum information (quantum bits) between the nodes of the network. This enables all kinds of applications, such as secure sharing of confidential information, interconnection of multiple quantum computers to increase their computing power, and the use of concatenated, highly sensitive quantum applications.
Send quantum information
The nodes in such a quantum network consist of small quantum processors. Sending quantum information between these processors is not that easy. One possibility is to send quantum bits with light particles, but due to unavoidable losses in fiber optic cables, there is a high chance that the light particles will not reach, especially over large distances. Because simply copying quantum bits is fundamentally impossible, loss of a light particle means that the quantum information is irrevocably lost.
A better way to transmit quantum information is teleportation. The quantum teleportation protocol gets its name from similarities with 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 intermediate space, there is no longer any chance of it being lost. This makes quantum teleportation an interesting technique for a future quantum internet.
Good control of the system
Teleporting quantum bits requires a number of ingredients: a quantum entangled connection between sender and receiver, a reliable readout method for quantum processors, and the capacity to temporarily store quantum bits. Previous research from QuTech showed that it is possible to teleport quantum bits between two neighboring nodes. QuTech researchers are now demonstrating for the first time that they can meet the requirements and demonstrate teleportation between non-neighboring nodes or over a network. They teleport quantum bits from node “Charlie” to “Alice”, using an intermediate node “Bob”.
Teleport in three steps
Teleportation consists of three steps. First of all, the “teleporter” must be prepared, that is, an entangled state must be created between Alice and Charlie. Alice and Charlie do not have a direct physical connection with each other, but both have Bob. First of all, Alice and Bob create intricacies between their processors. Bob then saves his share of the intricate state. Then Bob gets entangled in Charlie. Now a quantum mechanical trick is performed: By performing a special measurement in his processor, Bob passes on the entanglement, so to speak. Result: Alice and Charlie are in a tangled state and the teleporter is ready to use!
The second step is to create the ‘message’ – the quantum bit – which will be teleported. This can be ‘1’ or ‘0’, for example, but also all kinds of quantum values in between. Charlie prepares this quantum information. To show that teleportation works generically, the researchers repeat the whole experiment for different quantum bit values.
Step three is the actual teleportation from Charlie to Alice. Charlie performs a common measurement on his quantum processor with the message and on his half of the intricate state (Alice owns the other half). As a result, something happens that is only possible in the quantum world: Through this measurement, the information on Charlie’s page disappears and immediately reappears on Alice’s page.
Then one would think that the stocking is finished, but nothing could be further from the truth. The quantum bit has been transmitted encrypted; the key is determined by Charlie’s measurement result. Charlie therefore sends the measurement result to Alice, after which Alice performs the corresponding quantum operation to decrypt the quantum bit. For example, with a ‘bit flip’: 0 becomes 1 and 1 becomes 0. If Alice has performed the correct operation, the quantum information is suitable for further use. The teleportation was successful!
Teleporter several times
Follow-up research will focus on reversing steps one and two in the teleportation protocol. That is: make (or receive) first the quantum bit to be teleported, then prepare the teleporter and perform the teleportation. This sequence is extra challenging because the quantum information to be teleported must be preserved during the creation of entanglements. But it offers great advantages, because the teleportation can then be performed completely “on request”, which is relevant, for example, if the quantum information is the result of a difficult calculation, or if more teleportations have to be made. In the longer term, this teleportation will therefore form the backbone of the quantum Internet.
Qubit teleportation between non-neighboring nodes in a quantum network, SLN Hermans, M. Pompili, HKC Beukers, S. Baier, J. Borregaard and R. Hanson, Nature, 2022, DOI: 10.1038 / s41586-022-04697-y