Quantum information teleported across a rudimentary network

Researchers working at QuTech — a collaboration between Delft University of Technology and the Netherlands Organisation for Applied Scientific Research (TNO) — have succeeded in teleporting quantum information across a rudimentary network, in an important step towards a future quantum internet. Their breakthrough has been published in the journal Nature.

The power of a future quantum internet is based on the ability to send quantum information (quantum bits) between the nodes of the network, enabling applications such as securely sharing confidential information and linking several quantum computers together to increase their computing capability. The nodes of such a quantum network consist of small quantum processors — but sending quantum information between these processors is no easy feat.

One possibility is to send quantum bits using light particles but, due to the inevitable losses in glass fibre cables, particularly over long distances, the light particles will very likely not reach their destination. As it is fundamentally impossible to simply copy quantum bits, the loss of a light particle means that the quantum information is irrecoverably lost.

Teleportation offers a better way of sending quantum information. The protocol for quantum teleportation owes its name to similarities with teleportation in science-fiction films: the quantum bit disappears on the side of the sender and appears on the side of the receiver. As the quantum bit therefore does not need to travel across the intervening space, there is no chance that it will be lost. This makes quantum teleportation a crucial technique for a future quantum internet.

In order to be able to teleport quantum bits, several ingredients are required: a quantum entangled link between the sender and receiver, a reliable method for reading out quantum processors, and the capacity to temporarily store quantum bits. Previous research at QuTech demonstrated that it is possible to teleport quantum bits between two adjacent nodes; QuTech researchers have now shown that they can meet the package of requirements and have demonstrated teleportation between non-adjacent nodes — in other words, over a network. They teleported quantum bits from node ‘Charlie’ to node ‘Alice’, with the help of the intermediate node ‘Bob’.

Artist’s impression of the quantum teleportation protocol in a network setting. Image credit: Scixel for QuTech.

The teleportation consists of three steps. First, the ‘teleporter’ has to 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 stores his part of the entangled state. Next, Bob creates an entangled state with Charlie. A quantum mechanical ‘sleight of hand’ is then performed: by carrying out a special measurement in his processor, Bob sends the entanglement on as it were. The result is that Alice and Charlie are now entangled, and the teleporter is ready to be used.

The second step is creating the ‘message’ — the quantum bit — to be teleported. This can, for example, be ‘1’ or ‘0’ or various other intermediate quantum values. Charlie prepares this quantum information. To show that the teleportation works generically, the researchers repeated the entire experiment for various quantum bit values.

Step three is the actual teleportation from Charlie to Alice. For that purpose, Charlie carries out a joint measurement with the message on his quantum processor and on his half of the entangled state (Alice has the other half). What then happens is something that is possible only in the quantum world: as a result of this measurement, the information disappears on Charlie’s side and immediately appears on Alice’s side.

The quantum bit is encrypted upon transfer; the key is determined by Charlie’s measurement result. So Charlie sends the measurement result to Alice, after which Alice carries out the relevant quantum operation for decrypting the quantum bit, for example via a ‘bit flip’ — so 0 becomes 1 and 1 becomes 0. After Alice has carried out the correct operation, the quantum information is suitable for further use. The teleportation has succeeded.

Follow-up research will focus on reversing steps one and two of the teleportation protocol. This means first creating (or receiving) the quantum bit to be teleported and only then preparing the teleporter for carrying out the teleportation. Reversing the order is particularly challenging as the quantum information to be teleported must be stored while the entanglement is being created. However, it comes with a significant advantage as the teleportation can then be carried out completely ‘on request’. This is relevant, for example, if the quantum information contains the result of a difficult calculation or if teleportation must be done multiple times. In the long run, this type of teleportation is expected to serve as the backbone of the quantum internet.

Top image caption: Researchers work on one of the quantum network nodes. Image credit: Marieke de Lorijn for QuTech.

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