A breakthrough in quantum research

has been achieved with the help of 'magic-wavelength optical tweezers'. Researchers at Durham University in the UK have reported nothing less than a breakthrough in quantum physics. They have succeeded in creating quantum entanglement of molecules and keeping this state stable for a certain period of time.

In the experiment, complex molecules were trapped in a vacuum chamber and processed with "magic tweezers". In addition, various laser beams were used to ensure the most stable environment possible. The experiment was successfully carried out using instruments such as these. © Durham University

Move molecules precisely without touching them

These "magic tweezers" are a tool that scientists call "magic-wavelength optical tweezers". With the help of light beams, molecules or atoms can be moved precisely without having to be physically touched. "The results demonstrate the remarkable control we have over individual molecules. Quantum entanglement is very fragile; yet we were able to entangle 2 molecules through incredibly weak interactions and then maintain the entanglement for a period of almost a second," says lead researcher Simon Cornish.

Entanglement works over distances

Quantum entanglement occurs when 2 parts - atoms or molecules - are entangled with each other so that, viewed as a whole, they assume a definable state. It is not possible to assign a defined state to the individual subsystems. The entanglement is not locally bound and also works over larger distances. Albert Einstein called this "spooky action at a distance". Today this effect is known as the Einstein-Podolsky-Rosen paradox.

Of central importance for quantum computers

The phenomenon of quantum entanglement is of central importance for a large number of quantum mechanical and quantum physical processes - including quantum computers. The entangled states of qubits play a crucial role here. A single qubit has no defined state in this context. However, the entanglement of 2 qubits as a whole has a definable state, which is known as non-local correlation. The research results obtained at Durham University are intended to lay the foundation for further experiments in this direction, according to a press release. The quantum entanglement of molecules represents an important contribution to the further understanding of quantum memories, quantum networks, quantum sensing as well as complex quantum materials.

Recent papers

P. Schulz, G. Wolschin, Relativistic diffuson model for hadron production in p-Pb collisions at the LHC
Phys. Rev. C110, 044910 (2024)
A. Rizzi, G. Wolschin, Spectral eigenfunction decomposition of a Fokker-Planck operator for relativistic heavy-ion collisions
Eur. Phys. J. A 60, 196 (2024)
M. Larsson, G. Wolschin, Time-dependent condensate formation in ultracold atoms with energy-dependent transport coefficients
Phys. Rev. A 110, 023305 (2024)
L. Moehringer, G. Wolschin, Exact solution of the nonlinear boson diffusion equation for gluon scattering
J. Stat. Mech. 073103 (2024)
G. Wolschin, Partial Lyman-alpha thermalization in an analytic nonlinear diffusion model
Scientific Reports (SpringerNature) 14, 4935 (2024)
J. Hoelck, G. Wolschin, Cylindrically symmetric diffusion model for relativistic heavy-ion collisions
Annalen Phys. 536, 2300307 (2024)
J. Hoelck, E. Hiyama, G. Wolschin, Limiting fragmentation in heavy-ion stopping?
Phys. Lett. B 840, 137866 (2023)
G. Wolschin, Nonlinear diffusion of fermions and bosons
EPL 140, 40002 (2022)
A. Kabelac, G. Wolschin, Time-dependent condensation of bosonic potassium
EPJD 76, 178 (2022)
G. Wolschin, Nonlinear diffusion of gluons
Physica A 597, 127299 (2022)
A. Simon, G. Wolschin, Time-dependent condensate fraction in an analytical model
Physica A 573, 125930 (2021)
B. Kellers, G. Wolschin, Centrality dependence of limiting fragmentation
Eur. Phys. J. A57, 47 (2021)
G. Wolschin, Bottomonium spectroscopy in the quark-gluon plasma
Int. J. Mod. Phys. A 35, 2030016 (2020)

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Keywords: gravity and quantum mechanics; black holes; early universe cosmology and cosmic microwave background

Solution found for grandfather paradox?

The well-known paradox leads to logical contradictions. But a new study shows how time loops are possible.
Anyone who travels through time could meet a distant ancestor and thus possibly prevent him from ever being born. The contradiction suggests that time travel in this form is not possible. Experiments have demonstrated that time can be manipulated. Why shouldn't it also be possible to travel through time? Pop culture has devoted a lot of attention to this question, whether in the book The Time Machine by H. G. Wells (published before the development of the theory of relativity) or in the Hollywood blockbusters Back to the Future from the 1980s.

The fate of the grandfather

A popular element of these stories is also the central problem for the idea of time travel: Anyone who can travel into the past could, for example, murder their grandfather there and thus prevent him from ever being born. This "grandfather paradox" is a key argument that time travel cannot actually be possible. And indeed, nature seems to have arranged it in such a way that time distortions caused by the theory of relativity do not allow time travel. But there are exceptions. Perhaps the most fascinating was discovered in 1949 by the Austrian logician Kurt Goedel. At the time, he lived in Princeton as a neighbor and friend of Albert Einstein and presented him with an unusual solution to the field equations of general relativity on his 70th birthday. Goedel showed that closed "timelike" curves are possible in a rotating universe. If you follow such a curve, you will eventually return to the starting point - in space, but also in time. This is possible in the theory of relativity because it sees the universe in space and time as a complete structure - the future is therefore completely predetermined. However, this does not solve the grandfather paradox.

Resolving the paradox

Nature does not seem to have built in any "safety measures" against closed time curves at the fundamental level of space and time. However, researchers such as Stephen Hawking were convinced that something like this must exist. A new study, which recently appeared in the journal Classical and Quantum Gravity, now argues that this may not be necessary. Study author Lorenzo Gavassino, a physicist at Vanderbilt University in the USA, is building on a result by the Italian physicist Carlo Rovelli, who researches at the University of Marseille and is known for a form of quantum gravity that he developed. The idea is not to start from the paradox when looking for a solution, but directly from the physical properties of a closed timelike curve. To do this, it is first necessary to question what the difference between the future and the past actually is. Most physical laws do not recognize this difference. The theory of relativity, just like its predecessor, Newton's theory of gravity, is symmetrical in time. The orbit of a planet, for example, can in principle be reversed without violating a physical law. The new study looks at the entropy in the interior of a spaceship that could resemble this artistic impression.

The tyranny of disorder

The fact that we perceive a direction in time has to do with the laws of thermodynamics. While planets could in principle move the wrong way, this does not apply to coffee cups falling off the table. The latter shatter on the floor and cannot be returned to their original state - they are broken. The physical quantity that distinguishes between intact and damaged things is entropy. The fact that we all know for granted that things break over time follows from a more general law, the second law of thermodynamics. It states that entropy cannot decrease in a closed system. "In fact, the law of increasing entropy - a thermodynamic quantity that measures the degree of disorder in a system is the only law of physics that distinguishes between the past and the future," Gavassino told the science portal Livescience.

Memory and chaos

This is the key to answering the question of why we always experience time as flowing in one direction. We humans do not live in isolation from the world described by theormodynamics in which entropy increases, but are a part of it. Countless mechanisms of the human body, such as memory, have a decisive influence on the second law of thermodynamics and the associated increase in Disorder is required - just think of a hormone produced in a gland that is supposed to mix with the blood instead of migrating from the blood back into the gland. According to this logic, the fact that we observe an increase in entropy is because we and our bodies are moving in the flow of increasing entropy. "As far as we know, entropy is the only reason we can remember past events and not predict future ones," says Gavassino. This will be important shortly. If you want to solve the riddle of the grandfather paradox, you have to ask about the flow of entropy in a closed time loop. Entropy cannot increase arbitrarily; after all, when returning to the starting point, the entropy must be the same as at the beginning.

The second law of thermodynamics

allows this in principle if the entropy does not decrease, but also does not increase. Physically, however, not much happens in such a system. Rovelli and Gavassino assume a variable value for entropy. Then there must be a point along the loop with the highest and one with the lowest entropy. This is not without consequences, as Rovelli also writes in his 2019 study: Entropy cannot grow steadily on these curves, and therefore they cannot be uniformly future-oriented in the sense of a phenomenon that distinguishes the past from the future. In other words: In one part of the loop, time runs backwards, viewed at the level of entropy. A person would not observe coffee cups bouncing on the table there. His perception of time would itself be reversed, so that the cup would fall to the floor as usual. Turning point Gavassino works out how this works physically using the example of a closed room in a spaceship. He needs the space because the concept of entropy only makes sense in a closed system. He describes the space in quantum mechanical terms and finds that no paradoxes occur. The story in the loop is consistent. "I applied the framework of quantum mechanics without additional postulates or controversial assumptions - and showed that the self-consistency of history naturally follows from the quantum laws," emphasizes Gavassino. But how would a person in the room experience this situation? If the loop stretches over many decades and several generations of people live in it, couldn't the grandson still kill his grandfather at a certain point? Gavassino's calculation answers this question. At the point at which the increase in entropy in the room stops and reverses, even a person who remembers his past can no longer exist. What is very conceivable from this point on is a person whose time runs backwards, or? This is Gavassino's solution to the grandfather paradox: If people inhabit the room, not every one of them can exist everywhere on the loop with their memories. From a human perspective, it is also no longer possible to say clearly in which direction time in the loop actually runs. In each half of the loop, between the points of highest and lowest entropy, people could live who would describe the time of the people in the other half as running backwards. It is a scenario that is strikingly reminiscent of the Hollywood film Tenet by Oscar winner Christopher Nolan, which is about time reversal. In it, however, the paths of people traveling in different directions in time cross in a visually powerful way. The team behind Christopher Nolan's film Tenet once placed great importance on the fact that it was not a time travel film. But real time travel is strikingly similar to the concepts presented there. This explanation is not without problems. Gavassino addresses some of them in his study. One of the difficulties revolves around the point of lowest entropy, which in a sense represents the beginning of the story for the people living in the spaceship in both directions of time. "Near such a point, our concept of cause and effect breaks down," writes the physicist. The objects at this point have no macroscopic cause, but enter the stage as quantum fluctuations. "If there is a book there, no one has written it," Gavassino gives an example. The act of writing itself sets a state low entropy. But this is not a real paradox, he writes. Complex structures can arise at any time through fluctuations. The waiting time can be very long, however.

Traveling into the future

Whether closed time curves really exist remains an open question. Most experts do not think it is likely. We do not seem to live in a Goedel universe. However, one type of time travel is independent of this: anyone who moves at high speed travels into the future. The crew of the ISS flies around the earth at almost 30,000 kilometers per hour. Anyone who spends a year there will have traveled a few milliseconds into the future upon their return. The writer H. G. Wells could not have known this when he wrote his famous time travel novel about traveling into the future. He published it in 1895, years before the theory of relativity was developed. Later he met Albert Einstein in person, for example in 1929 in the German Reichstag or at a dinner in 1930, where the two sat next to each other. There must have been a lot to talk about.

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