Images encoded with larger alphabets safely transferred over a quantum network

Breakthrough in Quantum Communication

Nature Communications has published research done by an international team of scientists from the University of the Witwatersrand (WITS, Johanesbourg, South Africa) and ICFO – The Institute of Photonic Sciences, which demonstrates the transport of an image imprinted on a beam of light across a network without physically sending the image, an important step towards realising a quantum network for the transmission of information written with a high-dimensional alphabet.

Quantum communication over long distances is integral to information security. Researchers have demonstrated it with two-dimensional states (qubits) over vast distances between satellites. If we compare it with its classical counterpart, i.e., sending bits that we can encode in 1s (signal) and 0s (no signal), one at a time, this may seem enough. However, quantum optics allows us to increase the alphabet and securely describe more complex systems, such as a unique fingerprint or a face, in a single shot.

“Traditionally, two communicating parties physically send the information from one to the other, even in the quantum realm,” says Prof. Andrew Forbes, one of the authors of the paper.  “Now, it is possible to teleport information so that it never physically travels across the connection – a “Star Trek” technology made real.”

Researchers have so far only demonstrated teleportation between two parties using low dimensional alphabets, which requires several entangled photons to send complex images.

High-Dimensional Quantum Teleportation Achieved with Two Entangled Photons

In this research, the team performed the first experimental demonstration of quantum transport of high-dimensional states with just two entangled photons as a quantum resource. The team achieved this advancement by using a nonlinear optical detector that eliminates the need for additional photons while remaining compatible with any image requiring transmission. They report a new state-of-the-art communication protocol that can send information written in an alphabet of 15 dimensions, with the scheme scalable to even higher dimensions, paving the way for quantum network connections with higher information capacity.

In their experiment, the researchers devised an elegant method to securely transfer high-dimensional spatial information between Alice and Bob using a teleportation-inspired scheme. Unlike previous experiments that successfully teleported 3-dimensional states (using path entanglement) and required extra entangled photons, the team only used two entangled photons to form the quantum channel.

They first encoded the information for teleportation with a 15-element alphabet. Concurrently, they created a photon pair entangled in all 15 dimensions. From this pair, the second entangled photon traveled from Bob to Alice. Alice then measured it in a nonlinear spatial detector through interaction with the patterned light source, performing a Bell State Measurement (BSM).

This measurement mixed the states of the second photon and the light source photons in a second nonlinear crystal, performing a specific spatial projection on the resulting single photon. Due to the initial entanglement of the first and second photons (their joint state highly correlated), the BSM transferred the encoded information from the coherent light source to the first photon. This photon remained with Bob and never directly interacted with the source.

Practical Applications in a Banking setting

The attached figure illustrates the potential of this new quantum transport protocol. Imagine a customer wishing to send sensitive information, perhaps a fingerprint, to a bank. Traditional quantum communication requires physically sending the information from the customer to the bank, always risking interception (even if secure). In the newly proposedquantum transport scheme, the bank sends a single photon (one of an entangled pair) with no information to the customer. The customer overlaps it on a nonlinear detector with the information they want to send. As a result, the information appears at the bank exactly as if teleported. This method never physically sends information between the two parties, so interception is fruitless, while the exchange of quantum entangled photons establishes the quantum link connecting the parties.

“This protocol possesses all the hallmarks of teleportation except for one essential ingredient: it requires a bright laser beam to increase the efficiency of the nonlinear detector. This allows the sender to know what they are sending, without actually needing to know,” Forbes explains. “In this sense, it doesn’t strictly qualify as teleportation, but it could become so in the future if we can improve the efficiency of the nonlinear detector.” Even as it stands now, it opens a new pathway for connecting quantum networks, ushering in nonlinear quantum optics as a resource.

Quantum Teleportation Experiment Heralds High-Dimensional Secure Communication and Future Quantum Networks

“We hope that these results validating the feasibility of the process motivate further advances in the nonlinear optics field, pushing the limits towards a full quantum implementation,” says Dr Adam Vallés from ICFO (Barcelona), one of the leads on the project who worked on the experiment during his postdoctoral fellowship at Wits. “We have to be cautious now, as this configuration could not prevent a cheating sender from keeping better copies of the information to be teleported, which means we could end up with many Mr Spock clones in the Star Trek world if that is what Scotty wanted. From a practical point of view, the configuration that we currently demonstrate can already be used to establish a high-dimensional secure channel for quantum communications between two parties, provided that the protocol does not need to be fed with single photons, as would be the case for quantum repeaters.”

Vallés adds: “Performing such proof-of-concept experiments with currently available technology has been an interesting journey, and we have Dr Bereneice Sephton from Wits to thank for her determination and the comprehensive skills set needed to tame such an experimental beast. This is a true laboratory endeavour for which she should be lauded.”

Forbes echoes the sentiment: “This was an heroic experiment and Dr Bereneice Sephton must be recognised as she is the one who got the system to work and performed the key experiments.”

The team plan to continue working in this direction, with the next step focusing on quantum transport across an optical fibre network.