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In May 2023, Dr Benjamin Lanyon at the University of Innsbruck in Austria took an important step toward creating a new kind of internet: he transferred information along an optical fiber 31 miles (50 kilometers) long using the principles of quantum physics.

Information in quantum physics differs from the units of data binary digits stored and processed by computers that form the core of the current World Wide Web. The quantum physics realm covers the properties and interactions of molecules, atoms and even smaller particles such as electrons and photons.

Quantum bits, or qubits, offer the promise of transmitting information more securely because the particles get changed by the act of observing and measuring them. That means an eavesdropper cant go undetected.

Lanyon said his work makes the quantum internet appear feasible within cities, after which longer intercity distances will be the goal.

You could imagine this being a large-city scale, he said.

His breakthrough was part of an EU research project to bring the goal of a quantum internet closer.

Called the Quantum Internet Alliance, or QIA, the project brings together research institutes and companies across Europe. The initiative is receiving 24 million in EU funding over three and a half years until the end of March 2026.

It is not meant to replace the classical internet, but to work together, said Stephanie Wehner, a German native who coordinates the QIA and is a professor of quantum information at Delft University of Technology in the Netherlands. Were not going to replace Netflix.

A key concept in quantum physics is entanglement. If two particles are entangled, no matter how far apart they are in space, they will possess similar properties for example, both having the same measurement of something called spin, a quantum version of the direction that the particles are spinning.

The spin state of the particles isnt clear until they are observed. Until then, theyre in multiple states called superposition.

But when one is observed, the state of both particles becomes known.

This is useful in secure communications. People hacking a quantum transmission would leave behind an obvious trace of their attempt by causing a change in the state of an observed particle.

We can use the properties of quantum entanglement to achieve a means of secure communication that is provably secure even if the attacker has a quantum computer, said Wehner.

The secure communications afforded by a quantum internet could open up a much broader range of applications that are well beyond the bounds of the classical internet.

In medicine, for example, the physics of entanglement allows for a level of clock synchronization that can improve telesurgery.

If I want to perform surgery on some remote node, I want this to be very precisely timed in order to not make any mistakes, said Wehner.

Astronomy is another potential beneficiary.

Telescopes making distant observations could use a quantum internet to generate entanglement between the sensors to get a much better image of the sky, Wehner said.

A further example might be ATM machines.

At present, were an ATM to crash when a person was withdrawing money, the machine would assume no cash had been delivered while another dispenser would register a money withdrawal. A quantum internet could remove that discrepancy.

Many applications of a quantum internet will likely become apparent only after the technology is created.

It offers a whole range of new possibilities for making precise measurements of space and time and studying how the world and the universe work, said Lanyon.

The trick now is scaling up a quantum internet to use many particles across long distances.

Lanyon and his team have also demonstrated communicating not just between single particles but also trains of particles in this case light particles called photons speeding up the rate of entanglement between quantum nodes.

If you just sent one photon at a time, you have to wait for the travel time, he said. But if you can make trains of many photons at once, this allows you to increase the rate of entanglement between quantum nodes for the distances we want.

The ultimate goal is to extend quantum nodes to much bigger ranges, perhaps 310 miles (500 kilometers), and create a prototype of a quantum internet that can link remote cities much like the classical internet relies on different nodes to create a global internet.

While a quantum internet could exist for specialized applications as soon as 2029, experts are wary of hazarding a guess about when a full version might be available for a wide range of uses.

As the QIA advances the components and systems of the quantum internet, Europe is also working to develop quantum computers themselves.

In June 2023, an EU public-private partnership the European High Performance Computing Joint Undertaking announced that six countries in Europe would host quantum computers. The countries are the Czech Republic, France, Germany, Italy, Poland and Spain.

The aim is to ensure that Europe is at the forefront of the quantum technologies revolution. Quantum computers are expected to have unprecedented calculation power with many uses, including the ability to break the cryptographic algorithms that secure most of the exchanges of the current internet.

With projections that half of the most used cryptographic systems will be broken by the end of the decade, Europe is hardly the only interested party.

China and the US have made advances in quantum computing and the quantum internet in recent years.

Back on the infrastructure front, Europe is taking other steps. Its developing an integrated space and terrestrial infrastructure for secure communications a building block of sorts for the quantum internet.

Im very proud to say we are world-leading in many domains, said Wehner.

While in all interested countries much work remains, the potential benefits signal further advances and breakthroughs before too long.

People are developing new applications of quantum networks at quite a high rate, Lanyon said.

Research in this article was funded by the EU. This article was originally published in Horizon, the EU Research and Innovation magazine.

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In a groundbreaking announcement, physicists from University College London (UCL) have presented a radical theory that unifies the realms of gravity and quantum mechanics while preserving the classical concept of spacetime, as outlined by Einstein.

This innovative approach, detailed in two simultaneously published papers, challenges over a century of scientific consensus and proposes a revolutionary perspective on the fundamental nature of our universe.

Modern physics rests on two contradictory pillars: quantum theory, which rules the microscopic world, and Einsteins theory of general relativity, explaining gravity through spacetime curvature. These theories, despite their individual successes, have remained irreconcilable, creating a significant rift in our understanding of the universe.

Traditionally, scientists have believed that a quantum version of Einsteins theory of gravity was necessary. This belief fueled the development of string theory and loop quantum gravity. However, the new theory from UCL takes a divergent path.

Professor Jonathan Oppenheim of UCL Physics & Astronomy, the lead proponent of this theory, argues for a postquantum theory of classical gravity. This radical idea, as elaborated in his paper in Physical Review X (PRX), suggests that spacetime may remain classical and not subject to quantum mechanics.

Instead of altering spacetime, this theory revises quantum theory itself, predicting unpredictable and significant fluctuations in spacetime. These fluctuations, larger than those anticipated by quantum theory, could render the weight of objects uncertain at precise measurements.

A second paper in Nature Communications, led by Professor Oppenheims former PhD students, proposes an experiment to validate this theory. The experiment involves measuring a mass (like the 1kg standard previously used by the International Bureau of Weights and Measures in France) with extreme precision to detect potential weight fluctuations over time.

Professor Oppenheim, Professor Carlo Rovelli, and Dr. Geoff Penington leading proponents of quantum loop gravity and string theory, respectively have placed a bet with 5000:1 odds on the outcome of the experiment, or any other evidence that might emerge, which would confirm the quantum versus classical nature of spacetime.

For the past five years, the UCL research team has been rigorously examining this theory and its implications. Professor Oppenheim notes the importance of resolving the contradiction between quantum theory and general relativity.

Oppenheim stated, Quantum theory and Einsteins theory of general relativity are mathematically incompatible with each other, so its important to understand how this contradiction is resolved. Should spacetime be quantised, or should we modify quantum theory, or is it something else entirely? Now that we have a consistent fundamental theory in which spacetime does not get quantised, its anybodys guess.

Zach Weller-Davies, a co-author and key contributor to the theory, highlights that this discovery not only challenges our understanding of gravity but also provides a method to probe its potential quantum nature. If spacetime doesnt have a quantum nature, then there must be random fluctuations in the curvature of spacetime with a particular signature that can be verified experimentally, he explains.

We have shown that if spacetime doesnt have a quantum nature, then there must be random fluctuations in the curvature of spacetime which have a particular signature that can be verified experimentally, Weller-Davies continued. In both quantum gravity and classical gravity, spacetime must be undergoing violent and random fluctuations all around us, but on a scale which we havent yet been able to detect.But if spacetime is classical, the fluctuations have to be larger than a certain scale, and this scale can be determined by another experiment where we test how long we can put a heavy atom in superposition* of being in two different locations.

Co-authors Dr. Carlo Sparaciari and Dr. Barbara oda emphasize the significance of these experiments in determining the correct approach to understanding gravity.

Dr oda said, Because gravity is made manifest through the bending of space and time, we can think of the question in terms of whether the rate at which time flows has a quantum nature, or classical nature.And testing this is almost as simple as testing whether the weight of a mass is constant, or appears to fluctuate in a particular way.

Dr Sparaciari elucidated, While the experimental concept is simple, the weighing of the object needs to be carried out with extreme precision.But what I find exciting is that starting from very general assumptions, we can prove a clear relationship between two measurable quantities the scale of the spacetime fluctuations, and how long objects like atoms or apples can be put in quantum superposition of two different locations. We can then determine these two quantities experimentally.

This postquantum theory extends its influence beyond understanding gravity. It negates the need for the problematic measurement postulate in quantum theory. Quantum superpositions would naturally localize due to their interactions with classical spacetime.

Originating from Professor Oppenheims efforts to solve the black hole information problem, this theory allows for the possibility of information destruction, contradicting standard quantum theory but aligning with general relativitys predictions about black holes.

This announcement marks a potential paradigm shift in physics. As Professor Sougato Bose of UCL Physics & Astronomy, not involved in this specific announcement but a pioneer in related research, remarks, Experiments to test the nature of spacetime will take a large-scale effort, but theyre of huge importance from the perspective of understanding the fundamental laws of nature.

Indeed, these efforts could lead to a unified understanding of gravity and quantum mechanics, resolving one of the most profound dilemmas in modern physics. The implications of this theory, if proven correct, are vast, potentially reshaping our understanding of the universe at its most fundamental level.

As mentioned above, Einsteins theory of relativity, a cornerstone of modern physics, revolutionized our understanding of space, time, and gravity. This theory comes in two parts: Special Relativity and General Relativity.

Albert Einstein introduced Special Relativity in 1905. This theory fundamentally changed our perception of space and time. It asserts two key principles:

The Laws of Physics are the Same for All Non-accelerating Observers: No matter how fast an observer is moving, they will measure the same speed of light and observe the same laws of physics.

The Speed of Light is Constant: The speed of light in a vacuum is the same for all observers, regardless of their relative motion or the motion of the light source.

Special Relativity led to several groundbreaking conclusions:

Time Dilation: Time passes slower for objects moving at high speeds compared to those at rest. This effect becomes significant only at speeds close to the speed of light.

Length Contraction: Objects contract in length along the direction of motion as they approach the speed of light.

E=mc: This famous equation relates energy (E) to mass (m) with the speed of light (c) as the constant of proportionality. It implies that energy and mass are interchangeable, laying the groundwork for nuclear energy and weapons.

Ten years later, Einstein expanded on his theory with General Relativity, which addresses gravity and acceleration:

Einstein proposed that gravity is not a force between masses but rather a result of the curvature of spacetime caused by mass and energy. Massive objects like stars and planets warp the space around them, and other objects move along these curves, which we perceive as gravitational attraction.

This principle states that the effects of gravity are indistinguishable from the effects of acceleration. For instance, being in a closed room on Earths surface (where gravity pulls you down) feels the same as being in a room in a spaceship that accelerates upwards.

Light Bending: It predicts that light bends in a gravitational field. Observations during solar eclipses have confirmed this, where stars positions near the sun appear shifted due to the suns gravity bending the light.

Time Dilation Due to Gravity: Clocks run slower in stronger gravitational fields. This effect, tested using precise atomic clocks at different altitudes, is integral for the accuracy of GPS systems.

Gravitational Waves: Predicted by Einstein, these ripples in spacetime, caused by massive accelerating objects (like merging black holes), were directly detected in 2015, confirming a major prediction of General Relativity.

In summary, Einsteins theory of relativity redefined our understanding of the universe. Special Relativity showed that space and time are relative and interconnected, leading to phenomena like time dilation and mass-energy equivalence. General Relativity further advanced this by describing gravity as the curvature of spacetime, profoundly influencing cosmology and astrophysics.

As also discussed above, quantum mechanics, a fundamental theory in physics, describes the behavior of matter and energy at the atomic and subatomic levels. It emerged in the early 20th century as scientists explored phenomena that classical physics couldnt explain. Here are some key aspects of quantum mechanics:

Quantum mechanics introduces the concept of wave-particle duality. Particles, such as electrons and photons, exhibit both particle-like and wave-like properties. For example, electrons can produce interference patterns (a wave property) in a double-slit experiment, while also showing particle characteristics in other contexts.

Werner Heisenberg formulated the Uncertainty Principle, a cornerstone of quantum mechanics. It states that it is impossible to simultaneously know the exact position and momentum of a particle. The more precisely you measure one, the less precise the measurement of the other becomes. This principle challenges the classical notion of determinism.

Quantum Superposition: Quantum particles can exist in multiple states simultaneously, as illustrated by Schrdingers cat thought experiment. A particle in a superposition state doesnt have a specific position, energy, or other physical property until its measured.

Quantum Entanglement: Particles can become entangled, meaning the state of one particle instantly influences the state of another, regardless of the distance separating them. This phenomenon, famously described by Einstein as spooky action at a distance, defies classical ideas of spatial separation and information transfer.

Quantum tunneling occurs when particles pass through barriers that they shouldnt be able to, according to classical physics. This effect is crucial in many modern technologies, such as semiconductors and superconducting devices.

Upon measurement, a quantum system collapses from a superposition of states to a single state. This collapse is instantaneous and is at the heart of many interpretations of quantum mechanics, including the famous Copenhagen interpretation.

Quantum mechanics has led to numerous technological advancements:

Semiconductors: The foundation of modern electronics, including computers and smartphones, relies on quantum mechanics.

Quantum Computing: Quantum computers use quantum bits or qubits, which can be in superpositions of states, offering potentially exponential increases in computing power for certain problems.

Medical Imaging: Techniques like MRI and PET scans depend on principles of quantum mechanics.

In summary, quantum mechanics reveals a strange, counterintuitive world at the smallest scales, fundamentally different from our everyday experiences. Its principles have not only deepened our understanding of the universe but also driven significant technological progress.

The full study was published in the journal Nature Communications.

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The ongoing challenge in physics has been to harmonise these two theories, each governing different aspects of the universe.

Quantitative theory deals with the microscopic realm and general relativity, explaining gravity through the curvature of spacetime.

Previous approaches focused on modifying Einsteins theory to align with quantum mechanics, but UCLs new theory challenges this consensus.

Led by Professor Jonathan Oppenheim, the theory proposes an alternative perspective, suggesting that spacetime may remain classical, untouched by quantum principles.

Named a postquantum theory of classical gravity, it argues that the breakdown in predictability lies within quantum theory itself, leading to random and violent fluctuations in spacetime.

These fluctuations, larger than those predicted by quantum theory, make the precise measurement of object weight unpredictable.

To put their theory to the test, UCL researchers proposed an experiment. The experiment involves measuring a mass with extreme precision to observe potential fluctuations in its weight over time.

The outcome of this experiment, or any evidence confirming the nature of spacetime, is the subject of a ongoing debate between Professor Oppenheim and proponents of other quantum gravity theories, such as string theory and loop quantum gravity.

The UCL research group has tested the theorys implications over the past five years.

Professor Oppenheim emphasizes the significance of resolving the mathematical incompatibility between quantum theory and general relativity, stating, Now that we have a consistent fundamental theory in which spacetime does not get quantized, its anybodys guess.

The proposed experiment aims to determine whether spacetime has classical attributes by detecting random fluctuations in mass, offering an alternative approach to experiments aiming to verify the quantum nature of spacetime through gravitationally mediated entanglement.

The theory also extends beyond gravity, challenging the conventional measurement postulate of quantum theory.

While acknowledging the experimental challenges, the researchers express optimism about the potential to unveil the fundamental nature of spacetime within the next two decades.

The UCL teams postquantum theory opens new avenues for exploring the interplay between quantum particles and classical spacetime, pushing the boundaries of our understanding of the laws governing the universe.

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Advancements in quantum networking have been made by stretching diamond films, enabling quantum bits to function more effectively and with less expense, marking a significant step towards practical quantum networks.

Breakthrough by Argonne, UChicago researchers could help pave way for quantum infrastructure.

In work supported by the Q-NEXT quantum research center, scientists stretch thin films of diamond to create more cost-effective and controllable qubits.

A future quantum network may become less of a stretch thanks to researchers at the University of Chicago, Argonne National Laboratory and Cambridge University.

A team of researchers announced a breakthrough in quantum network engineering: By stretching thin films of diamond, they created quantum bits that can operate with significantly reduced equipment and expense. The change also makes the bits easier to control.

The researchers hope the findings, published on November 29 in the journal Physical Review X, can make future quantum networks more feasible.

This technique lets you dramatically raise the operating temperature of these systems, to the point where its much less resource-intensive to operate them, said Alex High, assistant professor with the Pritzker School of Molecular Engineering, whose lab led the study.

By stretching thin films of diamond, researchers have created quantum bits that can operate with significantly reduced equipment and expense. Credit: Peter Allen

Quantum bits, or qubits, have unique properties that make them of interest to scientists searching for the future of computing networksfor example, they could be made virtually impervious to hacking attempts. However, there are significant challenges to work out before it could become a widespread, everyday technology.

One of the chief issues lies within the nodes that would relay information along a quantum network. The qubits that make up these nodes are very sensitive to heat and vibrations, so scientists must cool them down to extremely low temperatures to work.

Most qubits today require a special fridge the size of a room and a team of highly trained people to run it, so if youre picturing an industrial quantum network where youd have to build one every five or 10 kilometers, now youre talking about quite a bit of infrastructure and labor, explained High.

Highs lab worked with researchers from Argonne National Laboratory, a U.S. Department of Energy national lab affiliated with UChicago, to experiment with the materials these qubits are made from to see if they could improve the technology.

One of the most promising types of qubits is made from diamonds. Known as Group IV color centers, these qubits are known for their ability to maintain quantum entanglement for relatively long periods, but to do so they must be cooled down to just a smidge above absolute zero.

The team wanted to tinker with the structure of the material to see what improvements they could makea difficult task given how hard diamonds are. However, the scientists found that they could stretch out the diamond at a molecular level if they laid a thin film of diamond over hot glass. As the glass cools, it shrinks at a slower rate than the diamond, slightly stretching the diamonds atomic structurelike pavement expands or contracts as the earth cools or warms beneath it, High explained.

This stretching, though it only moves the atoms apart an infinitesimal amount, has a dramatic effect on how the material behaves.

First, the qubits could now hold their coherence at temperatures up to 4 Kelvin (or -452F). Thats still very cold, but it can be achieved with less specialized equipment. Its an order of magnitude difference in infrastructure and operating cost, High said.

Secondly, the change also makes it possible to control the qubits with microwaves. Previous versions had to use light in the optical wavelength to enter information and manipulate the system, which introduced noise and meant the reliability wasnt perfect. By using the new system and the microwaves, however, the fidelity went up to 99%.

Its unusual to see improvements in both these areas simultaneously, explained Xinghan Guo, a Ph.D. student in physics in Highs lab and first author on the paper.

Usually if a system has a longer coherence lifetime, its because its good at ignoring outside interferencewhich means it is harder to control, because its resisting that interference, he said. Its very exciting that by making a very fundamental innovation with materials science, we were able to bridge this dilemma.

This technique lets you dramatically raise the operating temperature of these systems, to the point where its much less resource-intensive to operate them. Alex High

By understanding the physics at play for Group IV color centers in diamond, we successfully tailored their properties to the needs of quantum applications, said Argonne National Laboratory scientist Benjamin Pingault, also a co-author on the study. With the combination of prolonged coherent time and feasible quantum control via microwaves, the path to developing diamond-based devices for quantum networks is clear for tin vacancy centres, added Mete Atature, a professor of physics with Cambridge University and a co-author on the study.

Reference: Microwave-Based Quantum Control and Coherence Protection of Tin-Vacancy Spin Qubits in a Strain-Tuned Diamond-Membrane Heterostructure by Xinghan Guo, Alexander M. Stramma, Zixi Li, William G. Roth, Benchen Huang, Yu Jin, Ryan A. Parker, Jess Arjona Martnez, Noah Shofer, Cathryn P. Michaels, Carola P. Purser, Martin H. Appel, Evgeny M. Alexeev, Tianle Liu, Andrea C. Ferrari, David D. Awschalom, Nazar Delegan, Benjamin Pingault, Giulia Galli, F. Joseph Heremans, Mete Atatre and Alexander A. High, 29 November 2023, Physical Review X.DOI: 10.1103/PhysRevX.13.041037

The researchers used the Pritzker Nanofabrication Facility and Materials Research Science and Engineering Center at UChicago.

Other study authors included Zixi Li, Benchen Huang, Yu Jin, Tianle Lu, Prof. Giulia Galli and Prof. David Awschalom with the University of Chicago; Nazar Delegan and Benjamin Pingault with Argonne National Laboratory; and Alexander Stramma (co-first author), William Roth, Ryan Parker, Jesus Arjona Martinez, Noah Shofer, Cathryn Michales, Carola Purser, Martin Appel, Evgeny Alexeev, and Andrea Ferrari with the University of Cambridge.

Funding: Air Force Office of Scientific Research, U.S. Department of Energy Q-NEXT National Quantum Information Science Research Center, ERC Advanced Grant PEDASTAL, EU Quantum Flagship, National Science Foundation, EPSRC/NQIT, General Sir John Monash Foundation and G-research, Winton Programme and EPSRC DTP, EU Horizon 2020 Marie Sklodowska-Curie Grant.

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