Researchers in Delft have succeeded in teleporting quantum information
across a rudimentary network. This first of its kind is an important step
towards a future quantum internet. This breakthrough was made possible by a
greatly improved quantum memory and enhanced quality of the quantum links
between the three nodes of the network. The researchers, working at QuTech—a
collaboration between Delft University of Technology and the Netherlands
Organization for Applied Scientific Research (TNO)—are publishing their
findings today in the scientific 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. This
will enable all kinds of applications such as securely sharing confidential
information, linking several quantum computers together to increase their
computing capability, and the use of highly precise, linked quantum sensors.

### Sending quantum information

The nodes of such a quantum network consist of small quantum processors.
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 fiber cables, in particular 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 an crucial
technique for a future quantum internet.

### Good control over the system

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. The
researchers at QuTech have now shown for the first time 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 an intermediate
node, "Bob."

### Teleporting in three steps

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. Results: 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.

You might think that everything is then completed, but nothing could be
further from the truth. In fact, the quantum bit has been 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": 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.

### Teleporting several times

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 will therefore serve as the
backbone of the quantum internet.

### Reference:

S. L. N. Hermans et al, Qubit teleportation between non-neighbouring nodes
in a quantum network, Nature (2022).
DOI: 10.1038/s41586-022-04697-y

I’m ready: beam me up, Scotty! ðŸš€

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