Progress towards a quantum internet
Nature publication: successful initial proof of quantum teleportation between two different quantum dots
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| SF WRITER ARTHUR C. CLARKE |
Universität Paderborn
An international research team involving Paderborn University has achieved a crucial breakthrough on the road to a quantum internet: for the first time ever, the polarisation state of a single photon emitted from a quantum dot was successfully teleported to another, physically separated quantum dot. In simpler terms, this means that the properties of one photon can be transmitted to another via teleportation. This is a particularly vital step for future quantum communication networks. For example, the scientists used a 270m free-space optical link for their experiments. The results have now been published in the specialist journal Nature Communications.
Long-term European collaboration brings success
A team of doctoral and postdoctoral students at Paderborn University have spent approximately ten years focusing on optical measurements, data evaluation and analysis. As part of this, Professor Klaus Jöns's Paderborn group has been working with the team led by Professor Rinaldo Trotta at the Sapienza University of Rome in Italy. ‘The experiment impressively demonstrates that quantum light sources based on semiconductor quantum dots could serve as a key technology for future quantum communication networks. Successful quantum teleportation between two independent quantum emitters represents a vital step towards scalable quantum relays and thus the practical implementation of a quantum internet’, explained Professor Jöns, head of the ‘Hybrid Photonics Quantum Devices’ research group and a member of the board of the Institute for Photonic Quantum Systems (PhoQS) at Paderborn University.
For background: entangled systems made up of multiple quantum particles offer crucial benefits for quantum communication. Rather than a single state resulting from the conditions of one photon, this produces a whole system made up of multiple states. Systems like these are used in communication, data security or quantum computing. Entanglement means that specific properties of photons are coupled together. A state represents a piece of information being processed. ‘Previously, these photons came from one and the same source, i.e. the same emitter. Although there has been significant process made in recent years, using distinct quantum emitters to implement a quantum relay between independent parties had previously remained out of reach’, Professor Jöns noted.
Some ten years ago, Professor Jöns and Professor Trotta developed a roadmap for how quantum dots could be used as sources of entangled photon pairs for quantum communication and teleportation protocols. ‘This result shows that our long-term strategic planning has paid off’, Professor Jöns said, adding: ‘The combination of excellent materials science, nanofabrication and optical quantum technology was the key to our success.’
Technological excellence across numerous research locations
This success is based on a Europe-wide research collaboration: the quantum dots were developed with the utmost precision at Johannes Kepler University Linz. Nanofabrication of the resonators was completed by partners at the University of Würzburg. Scientists at the Sapienza University of Rome conducted the quantum teleportation experiments, including a 270m free-space optical link connecting two university buildings. The protocol exploits GPS-assisted synchronization, ultra-fast single photon detectors as well as stabilization systems that compensate for atmospheric turbulence. The achieved teleportation state fidelity (i.e. the quality in which quantum states are preserved during teleportation) reaches up to 82 ± 1%, above the classical limit by more than 10 standard deviations.
Looking ahead: first quantum relay with two deterministic sources
This success paves the way for the next major step: demonstrating ‘entanglement swapping’ between two quantum dots. This would be the first quantum relay with two deterministic sources of entangled photon pairs. By way of explanation, deterministic quantum sources produce relatively reliable single photons, almost at the touch of a button. Thus far, this has involved major challenges.
Simultaneous progress
Independently, and virtually at the same time, a research team from Stuttgart and Saarbrücken achieved a similar result using frequency conversion. Together, the two projects represent a vital milestone for European quantum research.
View paper: https://www.nature.com/articles/s41467-025-65911-9
Journal
Nature Communications
A new approach links quantum physics and gravitation
A team at TU Wien combines quantum physics and general relativity theory – and discovers striking deviations from previous results
image:
Large masses – such as a galaxy – curve space-time. Objects move along a geodesic. If we take into account that space-time itself has quantum properties, deviations arise (dashed line vs. solid line).
view moreCredit: Oliver Diekmann, TU Wien
It is something like the “Holy Grail” of physics: unifying particle physics and gravitation. The world of tiny particles is described extremely well by quantum theory, while the world of gravitation is captured by Einstein’s general theory of relativity. But combining the two has not yet worked – the two leading theories of theoretical physics still do not quite fit together.
There are many ideas for such a unification – with names like string theory, loop quantum gravity, canonical quantum gravity or asymptotically safe gravity. Each of them has its strengths and weaknesses. What has been missing so far, however, are observable predictions for measurable quantities and experimental data that could reveal which of these theories actually describes nature best. A new study from TU Wien may now have brought us a small step closer to this ambitious goal.
Cinderella and quantum gravity
“It’s a bit like the Cinderella fairy tale,” says Benjamin Koch from the Institute for Theoretical Physics at TU Wien. “There are several candidates, but only one of them can be the princess we are looking for. Only when the prince finds the slipper can he identify the real Cinderella. In quantum gravity, we have unfortunately not yet found such a slipper – an observable that clearly tells us which theory is the right one.”
To determine the correct “shoe size” – in other words, to find measurable criteria for testing different theories – the team took a closer look at the concept of geodesics. “Practically everything we know about general relativity relies on the interpretation of geodesics,” explains Benjamin Koch.
“A geodesic is the shortest connection between two points – on a flat plane that’s simply a straight line, whereas on curved surfaces things become more complicated.” For example, if you want to move from the North Pole to the South Pole on the surface of a sphere, the shortest path is a semicircle.
In relativity theory, space and time are inseparably linked. Together, they form a four-dimensional spacetime, which is curved by masses such as stars or planets. According to general relativity, the Earth orbits the Sun because the Sun’s mass bends space and time, thereby curving the geodesic along which the Earth moves into an approximately circular path.
The quantum version of geodesics
The course of these geodesics is determined by the so-called metric – a measure of how strongly spacetime is curved. “We can now try to apply the rules of quantum physics to this metric,” says Benjamin Koch. “In quantum physics, particles have neither a precisely defined position nor a precisely defined momentum. Instead, both are described by probability distributions. The more precisely you know one of them, the more fuzzy and uncertain the other becomes.”
In a similar way to how particle positions and momenta are replaced in quantum physics by a more complicated mathematical object – a quantized wave function – one can now also try to replace the metric of general relativity with a quantized version. In that case, spacetime curvature is no longer exactly defined at every point; it is replaced by a quantum-mechanically fuzzy version of this quantity.
This approach leads to major mathematical challenges.
But together with his PhD student Ali Riahinia and Angel Rincón (Czech Republic), Benjamin Koch has now succeeded in quantizing the metric in a novel way for an important special case – that of a spherically symmetric gravitational field that does not change over time.
Such a field can be used, for example, to describe the gravity of the Sun. “Next, we wanted to calculate how a small object behaves in this gravitational field – but using the quantum version of this metric,” says Koch. “In doing so, we realized that one has to be very careful – for instance, whether one is allowed to replace the metric operator by its expectation value, a kind of quantum average of the spacetime curvature. We were able to answer this question mathematically.”
The result was an equation which the team calls the q-desic equation, in analogy to the classical concept of geodesics. “This equation shows that in a quantum spacetime, particles do not always move exactly along the shortest path between two points, as the classical geodesic equation would predict.” This means that by observing the trajectories of freely moving particles in spacetime (such as an apple falling toward Earth in outer space), one can infer the quantum properties of the metric.
Shoe size 10^(-35) or rather 10^(+21)?
So how large are the differences between a q-desic and a classical geodesic? If we consider only ordinary gravitation, the weakest of the known fundamental forces, it turns out that the difference is minimal. “In this case, we end up with deviations of only about 10^(-35) meters – far too small to ever be observed in any experiment,” says Benjamin Koch.
However, general relativity includes another important quantity – the cosmological constant, which is also known as “dark energy”. It is responsible for the accelerated expansion of the universe on the largest scales. This cosmological constant can also be included in the q-desic equation. “And when we did that, we were in for a surprise,” reports Benjamin Koch. “The q-desics now differ significantly from the geodesics one would obtain in the usual way without quantum physics.”
Interestingly, there are deviations both at very small distances and at very large distances. While the deviations at small distances will probably remain unobservable, at length scales of around 10^(21) meters there can be substantial differences: “In between, for example when it comes to the Earth’s orbit around the Sun, there is practically no difference. But on very large cosmological scales – precisely where major puzzles of general relativity remain unsolved – there is a clear difference between the particle trajectories predicted by the q-desic equation and those obtained from unquantized general relativity,” says Benjamin Koch.
A new perspective on observational data
The work, which has been published in the journal Physical Review D, is not only a novel mathematical approach to linking quantum theory and gravitation – above all, it opens up new ways of comparing the theory with observations. “At first I would not have expected quantum corrections on large scales to produce such dramatic changes,” says Benjamin Koch. “We now need to analyze this in more detail, of course, but it gives us hope that by further developing this approach we can gain a new, and observationally well testable, insight into important cosmic phenomena – such as the still unsolved puzzle of the rotation speeds of spiral galaxies.”
Or, to return to the Cinderella story: we may finally have identified an observable that allows us to distinguish between viable and incorrect approaches to quantum gravity. A slipper has been found – now we have to find out which theory it truly fits.
Journal
Physical Review D
Method of Research
Computational simulation/modeling
Subject of Research
Not applicable
Article Title
Geodesics in quantum gravity
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