Waves pick up information from their environment through which they propagate. A theory of information carried by waves has now been developed at TU Wien – with astonishing results that can be utilised for technical applications.
We use ultrasound to analyse the body, radar systems to study airspace or seismic waves to study the interior of our planet. Many areas of research are dealing with waves that are deflected, scattered or reflected by their surroundings. As a result, these waves carry a certain amount of information about their environment, and this information must then be extracted as comprehensively and precisely as possible.
Searching for the best way to do this has been the subject of research around the world for many years. TU Wien has now succeeded in describing the information carried by a wave about its environment with mathematical precision. This has made it possible to show how waves pick up information about an object and then transport it to a measuring device. This can now be used to generate customised waves to extract the maximum amount of information from the environment - for more precise imaging processes, for example. This theory was confirmed with microwave experiments. The results were published in the journal ‘Nature Physics’.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/TU_Wien_What_waves_know_about_their_surroundings_Teflon_objects_orange_cylinders_were_placed_in_a_waveguide_with_a_rectangular_cross_section_Then_an_electromagnetic_signal_c1c9a7dfc7.jpg" alt="TU Wien: What waves know about their surroundings - Teflon objects (orange cylinders) were placed in a waveguide with a rectangular cross-section. Then, an electromagnetic signal (blue wavefront) was injected from the right to extract information about the metallic cuboid shown in gray. By measuring the wave field in the area indicated in red, the researchers could show how information is generated and transported by an electromagnetic signal. For example, the flow of information about the horizontal position of the cuboid is shown in the inset at the bottom right (blue arrows). One sees that information is generated on the cuboid&#39;s right-hand side and then transported to the right towards the opening of the waveguide."/>
<h4>Where exactly is the information located?</h4>
“The basic idea is quite simple: you send a wave at an object and the part of the wave that is scattered back from the object is measured by a detector,” says Prof Stefan Rotter from the Institute of Theoretical Physics at TU Wien. “The data can then be used to learn something about the object – for example, its precise position, speed or size.” This information about the environment that this wave carries with it is known as ‘Fisher information’.
However, it is often not possible to capture the entire wave. Usually, only part of the wave reaches the detector. This raises the question: Where exactly is this information actually located in the wave? Are there parts of the wave that can be safely ignored? Would a different waveform perhaps provide more information to the detector?
“To get to the bottom of these questions, we took a closer look at the mathematical properties of this Fisher information and came up with some astonishing results,” says Stefan Rotter. “The information fulfils a so-called continuity equation – the information in the wave is preserved as it moves through space, according to laws which are very similar laws to the conservation of energy, for example.”
<h4>A comprehensible path of information</h4>
Using the newly developed formalism, the research team has now been able to calculate exactly at which point in space the wave actually carries how much information about the object. It turns out that the information about different properties of the object (such as position, speed and size) can be hidden in completely different parts of the wave.
As the theoretical calculations show, the information content of the wave depends precisely on how strongly the wave is influenced by certain properties of the object under investigation. “For example, if we want to measure whether an object is a little further to the left or a little further to the right, then the Fisher information is carried precisely by the part of the wave that comes into contact with the right and left edges of the object,” says Jakob Hüpfl, the doctoral student who played a key role in the study. “This information then spreads out, and the more of this information reaches the detector, the more precisely the position of the object can be read from it.”
<h4>Microwave experiments confirm the theory</h4>
In Ulrich Kuhl&#39;s group at the University of Cote d&#39;Azur in Nice, experiments were carried out by Felix Russo as part of his master&#39;s thesis: A disordered environment was created in a microwave chamber using randomly positioned Teflon objects. Between these objects, a metallic rectangle was placed whose position was to be determined. Microwaves were sent through the system and then picked up by a detector. The question now was: How well can the position of the metal rectangle be deduced from the waves caught in the detector in such a complicated physical situation and how does the information flow from the rectangle to the detector?
By precisely measuring the microwave field, it was possible to show exactly how the information about the horizontal and vertical position of the rectangle spreads: it emanates from the respective edges of the rectangle and then moves along with the wave – without any information being lost, just as predicted by the newly developed theory.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/TU_Wien_What_waves_know_about_their_surroundings_Experimental_setup_to_measure_the_information_transport_via_microwaves_at_Universite_Cote_d_Azur_Nice_On_the_table_one_can_f3c6b82fe2.jpg" alt="TU Wien: What waves know about their surroundings - Experimental setup to measure the information transport via microwaves at Université Côte d’Azur. (Nice). On the table, one can see a waveguide into which an electromagnetic signal is injected (blue cables in the background). Inside the waveguide are Teflon objects (white cylinders in the foreground) that scatter the signal. This creates a complex wave field, which was measured at various points to determine the flow of information about the horizontal position of the metallic cuboid in the foreground."/>
<h4>Possible applications in many areas</h4>
“This new mathematical description of Fisher information has the potential to improve the quality of a variety of imaging methods,” says Stefan Rotter. If it is possible to quantify where the desired information is located and how it propagates, then it also becomes possible, for example, to position the detector more appropriately or to calculate customised waves that transport the maximum amount of information to the detector.
“We tested our theory with microwaves, but it is equally valid for a wide variety of waves with different wavelengths,” emphasises Rotter. “We provide simple formulas that can be used to improve microscopy methods as well as quantum sensors.“

What waves know about their surroundings

For the first time, thorium nuclei have now been successfully transferred from one state to another using a laser – but there has already been speculation for decades about the technical possibilities that would arise if this feat were one day possible. Expectations are high: probably the best-known idea is the nuclear clock, but precision measurements based on the thorium transition could open up new possibilities in completely different areas of research.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/TU_Wien_Nuclear_Clocks_and_the_Fundamental_Constants_of_Nature_The_nuclear_clock_a_spectacular_application_made_possible_by_the_discovery_of_the_thorium_transition_cbfb93c6fb.jpg" alt="TU Wien: The nuclear clock: a spectacular application, made possible by the discovery of the thorium transition."/>
<h4>Time measurement</h4>
The most accurate time measuring devices today are atomic clocks – they have even changed the definition of our units of time. Today, the second is defined as the length of time corresponding to a very specific number of oscillations of the light that must be shone on a caesium atom in order to switch its electrons from one state to another.
Similar to the pendulum of a pendulum clock, the light oscillation of the caesium atoms plays the role of the timer that provides the regular ticking for the most accurate time measurement possible. Today, atomic clocks are used, for example, for the coordination of satellites, they enable the high accuracy of GPS signals and they also play a role in telecommunications.
Instead of cesium atoms, however, thorium atomic nuclei can now also be used for keeping time. Only when the thorium nuclei are irradiated with a laser beam of exactly the right frequency do they change their state. If the laser changes its frequency slightly (e.g. due to external interference), this immediately results in a change of the response of the thorium nuclei. The combination of laser and thorium nuclei therefore makes it possible to keep the laser frequency extremely stable and ensures that the laser frequency does not drift away.
Because atomic nuclei are much smaller than atoms and much less sensitive to electromagnetic perturbations, atomic nuclei can achieve much higher precision than atoms when it comes to measuring time.
<h4>Everything gets better with better clocks</h4>
With better time measurement, other physical quantities can also be measured more accurately. This is a well-known phenomenon: in nautical navigation, for example, people have struggled for a long time with the problem that although it is easy to determine the latitude based on the position of the sun, an accurate clock is required to calculate the longitude correctly. Better time measurement often makes other measurements possible.
For example, Einstein&#39;s theory of relativity states that time does not pass in the same way everywhere: the flow of time depends on the gravitational field. In this way, an extremely precise clock could therefore be used to accurately measure the Earth&#39;s gravitational field – with many possible applications ranging from the search for mineral resources to research into plate tectonics and earthquake prediction.
Great hopes are also pinned on the possibility that new, better clocks could clarify previously unsolved fundamental questions of physics: If it is possible to measure the constants of nature much more accurately than before, then it would also be possible to test the theory that these natural constants may not be perfectly constant at all. Perhaps they change over time? Research into dark matter is also hoping to gain new insights through even more precise measurements.
It is not always possible to get closer to the fundamental laws of nature just by looking at the smallest particles in a particle accelerator or by looking at the most distant regions of space. Sometimes you just need greater precision – and this is where the newly discovered thorium transition and all its potential applications should help in the coming years and decades.
<h4>Back to the main article:</h4>
<ul>
<li><a href="https://gcms.labrulez.com/article/4273">Atomic Nucleus Excited with Laser: A Breakthrough after Decades</a></li>
</ul>

Nuclear Clocks and the Fundamental Constants of Nature

Physicists have been hoping for this moment for a long time: for many years, scientists all around the world have been searching for a very specific state of thorium atomic nuclei that promises revolutionary technological applications. It could be used, for example, to build an nuclear clock that could measure time more precisely than the best atomic clocks available today. It could also be used to answer completely new fundamental questions in physics - for example, the question of whether the constants of nature are actually constant or whether they change in space and time.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/TU_Wien_Oliver_Diekmann_Atomic_Nucleus_Excited_with_Laser_A_Breakthrough_after_Decades_A_laser_beam_changes_the_state_of_thorium_nucleui_which_are_embedded_in_a_crystal_940b6404dc.jpg" alt="TU Wien / Oliver Diekmann: A laser beam changes the state of thorium nuclei, which are embedded in a crystal."/>
Now this hope has come true: the long-sought thorium transition has been found, its energy is now known exactly. For the first time, it has been possible to use a laser to transfer an atomic nucleus into a state of higher energy and then precisely track its return to its original state. This makes it possible to combine two areas of physics that previously had little to do with each other: classical quantum physics and nuclear physics. A crucial prerequisite for this success was the development of special thorium-containing crystals. A research team led by Prof. Thorsten Schumm from TU Wien (Vienna) has now published this success together with a team from the National Metrology Institute Braunschweig (PTB) in the journal &quot;Physical Review Letters&quot;.
<blockquote>
The &quot;thorium transition&quot;, which physicists have been looking for for decades, has now been excited for the first time with lasers. This paves the way for revolutionary high precision technologies, including nuclear clocks.
</blockquote>
<h4>Switching quantum states</h4>
Manipulating atoms or molecules with lasers is commonplace today: if the wavelength of the laser is chosen exactly right, atoms or molecules can be switched from one state to another. In this way, the energies of atoms or molecules can be measured very precisely. Many precision measurement techniques are based on this, such as today&#39;s atomic clocks, but also chemical analysis methods. Lasers are also often used in quantum computers to store information in atoms or molecules.
For a long time, however, it seemed impossible to apply these techniques to atomic nuclei. &quot;Atomic nuclei can also switch between different quantum states. However, it usually takes much more energy to change an atomic nucleus from one state to another – at least a thousand times the energy of electrons in an atom or a molecule,&quot; says Thorsten Schumm. &quot;This is why normally atomic nuclei cannot be manipulated with lasers. The energy of the photons is simply not enough.&quot;
This is unfortunate, because atomic nuclei are actually the perfect quantum objects for precision measurements: They are much smaller than atoms and molecules and are therefore much less susceptible to external disturbances, such as electromagnetic fields. In principle, they would therefore allow measurements with unprecedented accuracy.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/TU_Wien_Foto_Wilke_Atomic_Nucleus_Excited_with_Laser_A_Breakthrough_after_Decades_Thorsten_Schumm_with_a_crystal_containing_thorium_03f3d62c0f.jpg" alt="TU Wien / Foto Wilke: Thorsten Schumm with a crystal containing thorium."/>
<h4>The needle in the haystack</h4>
Since the 1970s, there has been speculation that there might be a special atomic nucleus which, unlike other nuclei, could perhaps be manipulated with a laser, namely thorium-229. This nucleus has two very closely adjacent energy states – so closely adjacent that a laser should in principle be sufficient to change the state of the atomic nucleus.
For a long time, however, there was only indirect evidence of the existence of this transition. &quot;The problem is that you have to know the energy of the transition extremely precisely in order to be able to induce the transition with a laser beam,&quot; says Thorsten Schumm. &quot;Knowing the energy of this transition to within one electron volt is of little use, if you have to hit the right energy with a precision of one millionth of an electron volt in order to detect the transition.” It is like looking for a needle in a haystack – or trying to find a small treasure chest buried on a kilometer-long island.
<h4>The thorium crystal trick</h4>
Some research groups have tried to study thorium nuclei by holding them individually in place in electromagnetic traps. However, Thorsten Schumm and his team chose a completely different technique. &quot;We developed crystals in which large numbers of thorium atoms are incorporated,&quot; explains Fabian Schaden, who developed the crystals in Vienna and measured them together with the PTB team. &quot;Although this is technically quite complex, it has the advantage that we can not only study individual thorium nuclei in this way but can hit approximately ten to the power of seventeen thorium nuclei simultaneously with the laser – about a million times more than there are stars in our galaxy.&quot; The large number of thorium nuclei amplifies the effect, shortens the required measurement time and increases the probability of actually finding the energy transition.
On November 21, 2023, the team was finally successful: the correct energy of the thorium transition was hit exactly, the thorium nuclei delivered a clear signal for the first time. The laser beam had actually switched their state. After careful examination and evaluation of the data, the result has now been published.
&quot;For us, this is a dream coming true,&quot; says Thorsten Schumm. Since 2009, Schumm had focused his research entirely on the search for the thorium transition. His group as well as competing teams from all over the world have repeatedly achieved important partial successes in recent years. &quot;Of course we are delighted that we are now the ones who can present the crucial breakthrough: The first targeted laser excitation of an atomic nucleus,&quot; says Schumm.
<img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/TU_Wien_PTB_Atomic_Nucleus_Excited_with_Laser_A_Breakthrough_after_Decades_PTB_researcher_Johannes_Tiedau_in_the_laser_lab_e7fa06332a.jpg" alt="TU Wien / PTB: PTB-researcher Johannes Tiedau in the laser lab."/>
<h4>The dream of the atomic nucleus clock</h4>
This marks the start of a new exciting era of research: now that the team knows how to excite the thorium state, this technology can be used for precision measurements. &quot;From the very beginning, building an atomic clock was an important long-term goal,&quot; says Thorsten Schumm. &quot;Similar to how a pendulum clock uses the swinging of the pendulum as a timer, the oscillation of the light that excites the thorium transition could be used as a timer for a new type of clock that would be significantly more accurate than the best atomic clocks available today.&quot;
But it is not just time that could be measured much more precisely in this way than before. For example, the Earth&#39;s gravitational field could be analyzed so precisely that it could provide indications of mineral resources or earthquakes. The measurement method could also be used to get to the bottom of fundamental mysteries of physics: Are the constants of nature really constant? Or can tiny changes perhaps be measured over time? &quot;Our measuring method is just the beginning,&quot; says Thorsten Schumm. &quot;We cannot yet predict what results we will achieve with it. It will certainly be very exciting.&quot;

Atomic Nucleus Excited with Laser: A Breakthrough after Decades

Electrons can behave very differently depending on how much energy they have. Whether you shoot an electron with high or low energy into a solid body determines which effects can be triggered. Electrons with low energy can be responsible for the development of cancer, for example, but conversely they can also be used to destroy tumors. They are also important in technology, for example for the production of tiny structures in microelectronics.These slow electrons, however, are extremely difficult to measure. Knowledge about their behaviour in solid materials is limited, and often scientists can only rely on trial and error. However, TU Wien has now succeeded in obtaining valuable new information about the behaviour of these electrons: Fast electrons are used to generate slow electrons directly in the material. This allows details to be deciphered that were previously inaccessible experimentally. The method has now been presented in the journal ‘Physical Review Letters’.<h4>Two types of electrons at the same time</h4>“We are interested in what the slow electrons do inside a material, for example inside a crystal or inside a living cell,” says Prof Wolfgang Werner from the Institute of Applied Physics at TU Wien. “To find out, you would actually have to build a mini-laboratory directly in the material to be able to measure directly on site. But that's not possible, of course."You can only measure electrons that come out of the material, but that doesn't tell you where in the material they were released and what has happened to them since then. The team at TU Wien solved this problem with the help of fast electrons that penetrate the material and stimulate various processes there. For example, these fast electrons can disturb the balance between the positive and negative electrical charges of the material, which can then lead to another electron detaching itself from its place, travelling at a relatively low speed and in some cases escaping from the material.The crucial step now is to measure these different electrons simultaneously: “On the one hand, we shoot an electron into the material and measure its energy when it leaves again. On the other hand, we also measure which slow electrons come out of the material at the same time.” And by combining this data, it is possible to obtain information that was previously inaccessible.<h4>Not a wild cascade, but a series of collisions</h4>The amount of energy that the fast electron has lost on its journey through the material provides information on how deeply it has penetrated the material. This in turn provides information about the depth at which the slower electrons were released from their place.This data can now be used to calculate to what extent and in what way the slow electrons in the material release their energy. Numerical theories on this can be reliably validated for the first time using the data.This led to a surprise: it was previously thought that the release of electrons in the material took place in a cascade: A fast electron enters the material and hits another electron, which is then ripped away from its place, causing two electrons to move. These two electrons would then remove two more electrons from their place, and so on. The new data show that this is not true: instead, the fast electron undergoes a series of collisions, but always retains a large part of its energy and only one comparatively slow electron is detached from its place in each of these interactions.“Our new method offers opportunities in very different areas,” says Wolfgang Werner. “We can now finally investigate how the electrons release energy in their interaction with the material. It is precisely this energy that determines whether tumor cells can be destroyed in cancer therapy, for example, or whether the finest details of a semiconductor structure can be correctly formed in electron beam lithography.”

New method unravels the mystery of slow electrons

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The TU Wien is Austria's largest research and educational institution in the field of technology and natural sciences. More than 4,000 scientists are researching "technology for people" in five main research areas at eight faculties. The content of the studies offered is derived from the excellent research. More than 26,000 students in 62 degree programmes benefit from this. As a driver of innovation, TU Wien strengthens the business location, facilitates cooperation and contributes to the prosperity of society.

New method unravels the mystery of slow electrons

Nuclear Clocks and the Fundamental Constants of Nature

Atomic Nucleus Excited with Laser: A Breakthrough after Decades

What waves know about their surroundings

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