A quantum leap forward

Have you ever wondered what the big tech companies do with your personal data? Although we usually encrypt our data when sending them via the internet, we have no means of knowing what the recipient does with our data after decryption. There is however a way out of this dilemma, called “Blind Quantum Computing”. Here, your data is quantum-encrypted, sent through a telecom network to the tech company, and then processed on a quantum computer. When implemented correctly, it is mathematically proven that there is no means for the quantum computer to know any details about your data. Only you can decrypt the data. On a (very) small laboratory scale, Blind Quantum Computing has already been demonstrated, and efforts towards a connection with the Quantum Internet are on the way.

The key resource for Blind Quantum Computing is the distribution of quantum entanglement, e.g., distributing an entangled photon pair between a sender and a receiver. What now happens is truly astonishing: Although a measurement on each individual photon leads to a completely random result, both photon measurements always give the exact same random result. Even more bizarre: this randomness is synchronized instantaneously, even if the photons are lightyears apart from each other.

“You are not allowed to think of the phenomenon “classically”,” says Florian Kaiser who is the group leader of the Quantum Materials Team within the Materials Research and Technology department at LIST. “We are naive in our everyday experience which does not teach us better. We would think that there needs to be a medium, a channel for these photons to connect and to communicate information. But the set of physics rules that we experience in our everyday macroscopic world is flawed, since it does not account for the underlying microscopic details of the quantum world.”

Kaiser joined LIST at the end of last year and has been working on a funded EU-wide project that could revolutionize communicating over the internet.

Entanglement for Blind Quantum Computing

Imagine a coordination between two partners that is in all sense perfect. In a quantum entanglement, that is exactly how two particles behave. They are always in tune with each other. If a measurement is carried out on one particle (a measurement is like asking any random question), the result will be exactly the same for the other, and that too instantaneously. And, what’s more, quantum entanglement is purely monogamous. There is never a place for a third particle or third measurement, since it immediately destroys the synchronization. This exclusivity feature of entanglement is the basis of an un-hackable internet.

“Current communication over the internet is never a hundred percent secure,” continues Kaiser. Indeed, current encryption methods – even the toughest of them – are fallible and can be cracked given enough time and computational power, especially when quantum computers become available. What is s even worse, we can never know if somebody intercepts our communication. This is where the head-twisting laws of quantum mechanics come into play. While quantum communication cannot prevent an eavesdropper from attempting to intercept our communication, the monogamy of entanglement allows us to clearly detect the presence of a third party by simply comparing some of our measurement results. It they are not perfectly identical, it would be advised to change the communication channel.

Blind quantum computing is a special use case of quantum communication. Here, you are the sender and the receiver at the same time, thus you possess both entangled photons. As a temporary lend, you send one photon to a powerful quantum computer via the quantum internet. A quantum computer has the capability to perform computations on the photon without knowing anything about its properties. Therefore, it can return to you the photon without learning anything about your important calculation. Again, you cannot prevent that the operator of a quantum computer wants to know about your calculations. However, you can interleave “dummy data” into your photon stream, on which you perform entanglement monogamy tests using the second entangled photon, which you have always kept with you. If these tests fail, you should probably resort to another provider of quantum computing hardware.

Quantum Communication

The notion of using quantum modes of communication is not new. The idea was first brought to light by physicist Stephen Wiesner in the 1970s. He recognized the potential of a fundamental principle in quantum mechanics: the inability to measure a system's property without altering it; and was the one to suggest encoding information in quantum bits, or qubits. Unlike classical bits, which can either be in the state “0” or “1”, qubits also exist in any intermediate (superposition) state. However, a measurement on a qubit enforces an immediate collapse to “0” or “1”. In other words, a measurement inherently changes the qubit state, which fundamentally prevents the development of a qubit cloning machine. This is intuitively clear, since any form of cloning requires some sort of a measurement, which alters the original superposition states. This restriction serves as a foolproof solution for security concerns, as it ensures that quantum information cannot be copied without leaving behind a trace.

Building on Wiesner's work, in 1984, IBM's Charles Bennett and Gilles Brassard from the University of Montreal devised a method to create establish secure communication based on an unbreakable encryption key using polarized light. This approach, known as quantum key distribution (QKD), enables two users to agree upon a randomly-chosen cryptography scheme and to renew the strategy for every single bit of classical information that shall be transmitted at a later stage. As before, you cannot prevent an eavesdropper trying to get hold of your cryptography strategy, however, you can count on the monogamy of entanglement to help you identify the adversary. Today, QKD systems using these principles are already commercially available, primarily used by financial or government organizations.

Quantum repeaters

Although a solution with many advantages, one of the challenges in quantum communication is distance, explains Kaiser, as qubits can only be sent over short distances. “Although telecom optical fibres are extremely transparent, photons do eventually get absorbed. The chance for a photon to survive after 50 km distance is about the same as throwing a six with a dice. At a distance of 100 kms, it is the same chance as throwing a double-six. And at a pan-European distance of 1000 kms, you would have to throw twenty sixes at once. If you would throw these 20 dice once a second, you would have to try for about 100 millions years to be successful. Not very promising.” Moreover, the fundamental property of quantum states, which prevents them from being copied, eliminates the possibility of sending multiple copies of a qubit in the hope that at least one will reach its destination. The real challenge in building a quantum network is thus to seek alternative solutions which would make single-photon qubits travel a maximum amount of distance.

“Fortunately,” says Kaiser, “quantum physicists have found a way around this problem.” In a long-distance scenario, a network link is cut into many short segments. Any two adjacent segments are connected by a quantum repeater node, which can send and receive photonic quantum states. Once a repeater node receives a photon from another node, these two nodes are then entangled and no photons need to be sent across that segment anymore. When all pairs of repeater nodes are finally entangled, we can perform a so-called Bell-state measurement to create entanglement across the two end-stations of the network. Kaiser mentions: “Coming back to throwing dice, the quantum repeater strategy allows you to leave all dice that landed on a six on the table, and only continue with the remaining ones. With this strategy, you can get to 20 sixes in a matter of seconds instead of millions of years.”

Quantum Internet Alliance and LIST

Development of these quantum repeater systems in order to build a Europe-wide quantum internet ecosystem has been a strategic priority for the European Commission. With this objective in mind, in October 2022, an ambitious seven-year programme was initiated by the Quantum Internet Alliance (QIA), as part of a larger Quantum Flagship Project, an extensively funded research initiative by the Commission. Founded in 2017, QIA brings together academic institutions, telecom operators, system integrators, and quantum tech startups from various locations across Europe.

LIST joined the alliance with its nanotechnology expertise at the end of last year for the development of quantum materials used as repeaters, with Florian Kaiser and his team tackling one of the many aspects that would contribute towards developing this groundbreaking prototype of a vast quantum network in Europe. “We are testing the potential of quantum repeaters based on individual atomic impurities (colour centres) in otherwise perfect silicon carbide crystals.” The leading contender today are nitrogen-vacancy impurities in diamond. “However, as you can imagine, diamonds are not cheap, and notoriously difficult to fabricate. The QIA consortium has given us the key responsibility to develop the next-generation scalable quantum repeater materials platform based on silicon carbide semiconductor technology.” At the same time, the remaining partners in QIA continue with the two established diamond and trapped-ion platforms to implement a demonstration of Blind Quantum Computing, in particular to prepare user-friendly software-hardware interfaces.

How significant is joining a consortium such as the QIA? Apart from collaborating with peers and contributing to revolutionary advancements in security, computing, and science in general, Kaiser points towards the QIA map where, among the crisscross of gleaming bubbles representing various partner countries, Luxembourg now appears almost at the centre of the alliance, and the closest neighbour of the Delft University of Technology in the Netherlands, which is also at the forefront of quantum technology. “It’s a great opportunity for us,” he says, “if we were to establish a fibre connection between Delft and LIST, Luxembourg could become the first country to have a “physical” quantum internet link that connects beyond its borders and that relies on quantum repeaters developed in Luxembourg”: a literal quantum leap for the country towards realizing its potential and the vision of one day turning quantum internet into a tangible reality for everyone.