Researchers teleport quantum information to a rudimentary quantum network

Delft researchers have successfully teleported quantum information through 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 better quality of quantum connections between the three network nodes. The researchers working at QuTech – a collaboration between Delft University of Technology and the Dutch Organization for Applied Scientific Research (TNO) – publish their findings today in the scientific journal Nature.

The strength of a future quantum internet lies in the ability to send quantum information (quantum bits) between network nodes. This will allow all kinds of applications such as secure sharing of confidential information, interconnection of multiple quantum computers to increase their computer capacity and use of high precision connected quantum sensors.

Sends quantum information

The nodes in 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 using light particles, but due to the inevitable losses in fiberglass cables, especially over long distances, 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 irrevocably lost.

Teleportation offers a better way to send quantum information. The quantum teleportation protocol owes its name to similarities with teleportation in science fiction movies: the quantum bit disappears on the transmitter side and appears on the receiver side. Therefore, since the quantum bit does not have to cross the space, there is no chance of it being lost. This makes quantum teleportation a crucial technique for a future quantum internet.

Good knowledge of the system

To be able to teleport quantum bits, several ingredients are needed: a quantum entangled connection between transmitter and receiver, a reliable method of reading quantum processors, and the ability to temporarily store quantum bits. Previous research at QuTech has shown that it is possible to teleport quantum bits between two adjacent knob. QuTech researchers have now for the first time shown that they can meet all the requirements and have demonstrated teleportation between not adjacent nodes, that is, on a network. They teleported quantum bits from the “Charlie” node to the “Alice” node using an intermediate “Bob” node.

Three-stage teleportation

Teleportation consists of three steps. First, the “teleporter” must be prepared, which means that a tangled 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 intricate state between their processors. Bob then saves his share of the intricate state. Then Bob creates a tangled state with Charlie. A quantum mechanical “hand game” is then performed: By performing a special measurement in his processor, Bob somehow sends the entanglement. Results: Alice and Charlie are now wrapped up and the teleporter is ready to use!

The second step is to create the “message” – the quantum bit – to be teleported. It can be, for example, ‘1’ or ‘0’ or various other intermediate quantum values. Charlie prepares this quantum information. To show that teleportation works generically, the researchers repeated the whole experiment for different quantum bit values.

The third stage is the actual teleportation from Charlie to Alice. For this, Charlie performs a common measurement with the message on his quantum processor and on his half of the tangled state (Alice has the other half). What then happens 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.

You may think it’s all over, but nothing can be further from the truth. In fact, the quantum bit was encrypted during transfer; the key is determined by Charlie’s measurement result. Charlie then sends the result of the measurement 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 performs the correct operation, the quantum information can be used later. The teleportation was successful!

Teleporter several times

Follow-up research will focus on reversing steps one and two in the teleportation protocol. This means first creating (or receiving) the quantum bit to be teleported, and only then preparing the teleporter to perform the teleportation. Reversing the order is particularly difficult because the quantum information to be teleported must be stored during the creation of the entanglement. However, it has a significant 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 the teleportation has to be performed several times. Eventually, this type of teleportation will therefore serve as the backbone of the quantum Internet.

Financing details

Financial support comes from the EU flagship project 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 Dutch Organization for Scientific Research (NWO) through a VICI scholarship (project no. 680-47-624) and the Zwaartekracht Quantum Software Consortium program (project no. 024.003.037 / 3368) and an Erwin-Schrödinger Fellowship (QuantNet , No. J 4229-N27) from the Austrian National Science Foundation (FWF).

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