Category: Physics

Scientists refer to atomic nuclei as “quantum many-body systems” because they are formed by many particles (nucleons, which include neutrons and protons) that interact with each other in complex ways. Nuclei can absorb energy, placing them into excited states. These states then lose energy through decay and may emit different particles. The various processes of decay and particle emission are called decay channels. The interplay between the internal characteristics of the excited states and the different decay channels gives rise to interesting phenomena.

One of these phenomena is superradiance. This occurs when a nucleus reaches a high excitation energy. According to the nuclear shell model, nuclei get excited by promoting nucleons to higher shells. These configurations are called excited states. As the excitation energy available increases, the number of ways the nucleons can be promoted increases, therefore the number of excited states increases. Superradiance can take place when excited states are so close to each other that neighboring excited states overlap with each other. If it happens, instead of observing many states, we see only one “superradiant” state.

To find evidence of superradiance in nuclei, nuclear physicists look for two systems that have the same internal structure but different decay channels. Mirror nuclei have the same total number of protons and neutrons, but the number of protons in one equals the number of neutrons in the other. The internal structure of mirror nuclei is the same since the nuclear force is the same whether between two protons, two neutrons, or a proton and a neutron. This makes the nuclear force “charge independent.” However, the decay channels are different due to the different electric charge repulsion in the two systems because of the difference in each system’s number of protons.

In a new study published in Physical Review C, scientists from Texas A&M University, the CEA research institute in France, the University of Birmingham, UK, and Florida State University have found evidence of the superradiance effect in the differences between the alpha decaying states in Oxygen-18 and Neon-18.

The research team studied the structure of Neon-18 by scattering a radioactively unstable beam of Oxygen-14 on a thick Helium-4 gas target. The gas target allowed the experimentalists to measure the tracks of the incoming and outcoming particles and produce a complete reconstruction of the nuclear events. The structure of Oxygen-18 had been previously studied at Florida State University by scattering Carbon-14 on a Helium-4 target using a particle accelerator. This experiment had very good results, allowing the researchers to use the information about the Oxygen-18 excited states to find the initial parameters for the analysis of the Neon-18 data.

As expected from the charge independence of the nuclear force, the researchers found a correspondence between mirror states in the two nuclei, although some differences emerged when comparing the strength of mirror states. If the internal structure of the nuclei is the same, one would expect mirror levels to have the same strength, but in these cases alignment with slightly different decay channels produces observed differences. The researchers interpreted these differences as evidence of the superradiance effect.

Related research has also been published in Communications Physics.

More information: M. Barbui et al, α-cluster structure of Ne18, Physical Review C (2022). DOI: 10.1103/PhysRevC.106.054310 Alexander

Volya et al, Superradiance in alpha clustered mirror nuclei, Communications Physics (2022). DOI: 10.1038/s42005-022-01105-9

Journal information: Communications Physics 

Provided by US Department of Energy 

A team of physicists at the Physikalisches Institut, Ruprecht-Karls-Universität working with colleagues from Institut für Ionenphysik und Angewandte Physik, Universität Innsbruck, has developed a laser technique to selectively remove the most energetic ions from a sample, cooling those that remain.

In their paper published in the journal Nature Physics, the group describes their method and possible uses for it. The team at Nature has also published a Research Highlights paper in the same issue outlining the work by the team on this new effort.

Cooling molecular ions is useful in a wide variety of applications in both physics and chemistry research, including current work to better understand the chemistry of celestial bodies such as Saturn’s moon Titan. Unfortunately, cooling negatively charged molecules has proven to be more difficult, and their use has been limited. In this new effort, the research team developed a technique that makes the process relatively easy.

The idea stems from prior work involving the use of evaporative cooling, a technology related to evaporative coolers that are used to cool homes in dry areas. In this instance, the technology was used to negatively charge antiprotons by changing the shape of the trap containing them.

In this new work, the team started by using a radio frequency trap to hold a sample of OH^– anions at approximately 370 K. They used a special type of laser known as a single photo-detachment laser to manipulate the anions in the trap. This was done by tuning the laser to a desired threshold, which neutralized ions crossing the laser beam, allowing them to be removed from the trap.

As this process continued, those anions with the highest energies were slowly removed, leaving those with lower energies behind. And that left the entire trap and the remaining sample much cooler. In subsequent trials, the team found that they were able to optimize the process, resulting in cooling the trap by several orders of magnitude.

The team concludes that their technique could be used in lab studies and also with research related to better understanding interstellar clouds or the atmosphere of celestial bodies such as Titan.

More information: Jonas Tauch et al, Laser-induced forced evaporative cooling of molecular anions below 4 K, Nature Physics (2023). DOI: 10.1038/s41567-023-02084-6

Daniel Comparat et al, Anions get cold, Nature Physics (2023). DOI: 10.1038/s41567-023-02085-5

Journal information: Nature Physics  , Nature 

© 2023 Science X Network

The data centers and high-performance computers that run artificial intelligence programs, such as large language models, aren’t limited by the sheer computational power of their individual nodes. It’s another problem—the amount of data they can transfer among the nodes—that underlies the “bandwidth bottleneck” that currently limits the performance and scaling of these systems.

The nodes in these systems can be separated by more than one kilometer. Since metal wires dissipate electrical signals as heat when transferring data at high speeds, these systems transfer data via fiber-optic cables. Unfortunately, a lot of energy is wasted in the process of converting electrical data into optical data (and back again) as signals are sent from one node to another.

In a study published in Nature Photonics, researchers at Columbia Engineering demonstrate an energy-efficient method for transferring larger quantities of data over the fiber-optic cables that connect the nodes. This new technology improves on previous attempts to transmit multiple signals simultaneously over the same fiber-optic cables. Instead of using a different laser to generate each wavelength of light, the new chips require only a single laser to generate hundreds of distinct wavelengths of light that can simultaneously transfer independent streams of data.

A simpler, more energy-efficient method for data transfer

The millimeter-scale system employs a technique called wavelength-division multiplexing (WDM) and devices called Kerr frequency combs that take a single color of light at the input and create many new colors of light at the output. The critical Kerr frequency combs developed by Michal Lipson, Higgins Professor of Electrical Engineering and Professor of Applied Physics, and Alexander Gaeta, David M. Rickey Professor of Applied Physics and Materials Science and Professor of Electrical Engineering, allowed the researchers to send clear signals through separate and precise wavelengths of light, with space in between them.

“We recognized that these devices make ideal sources for optical communications, where one can encode independent information channels on each color of light and propagate them over a single optical fiber,” says senior author Keren Bergman, Charles Batchelor Professor of Electrical Engineering at Columbia Engineering, where she also serves as the faculty director of the Columbia Nano Initiative. This breakthrough could allow systems to transfer exponentially more data without using proportionately more energy.

The team miniaturized all of the optical components onto chips roughly a few millimeters on each edge for generating light, encoded them with electrical data, and then converted the optical data back into an electrical signal at the target node. They devised a novel photonic circuit architecture that allows each channel to be individually encoded with data while having minimal interference with neighboring channels. That means the signals sent in each color of light don’t become muddled and difficult for the receiver to interpret and convert back into electronic data.

“In this way, our approach is much more compact and energy-efficient than comparable approaches,” says the study’s lead author Anthony Rizzo, who conducted this work while a Ph.D. student in the Bergman lab and is now a research scientist at the U.S. Air Force Research Laboratory Information Directorate. “It is also cheaper and easier to scale since the silicon nitride comb generation chips can be fabricated in standard CMOS foundries used to fabricate microelectronics chips rather than in expensive dedicated III-V foundries.”

The compact nature of these chips enables them to directly interface with computer electronics chips, greatly reducing the total energy consumption since the electrical data signals only have to propagate over millimeters of distance rather than tens of centimeters.

Bergman noted, “What this work shows is a viable path towards both dramatically reducing the system energy consumption while simultaneously increasing the computing power by orders of magnitude, allowing artificial intelligence applications to continue to grow at an exponential rate with minimal environmental impact.”

Exciting results pave the way to real-world deployment

In experiments, the researchers managed to transmit 16 gigabits per second per wavelength for 32 distinct wavelengths of light for a total single-fiber bandwidth of 512 Gb/s with less than one bit in error out of one trillion transmitted bits of data. These are incredibly high levels of speed and efficiency. The silicon chip transmitting the data measured just 4mm x 1mm, while the chip that received the optical signal and converted it into an electrical signal measured just 3mm x 1mm—both smaller than a human fingernail.

“While we used 32 wavelength channels in the proof-of-principle demonstration, our architecture can be scaled to accommodate over 100 channels, which is well within the reach of standard Kerr comb designs,” Rizzo adds.

These chips can be fabricated using the same facilities used to make the microelectronics chips found in a standard consumer laptop or cellphone, providing a straightforward path to volume scaling and real-world deployment.

The next step in this research is to integrate the photonics with chip-scale driving and control electronics to further miniaturize the system.

More information: Massively scalable Kerr comb-driven silicon photonic link, Nature Photonics (2023). DOI: 10.1038/s41566-023-01244-7 , www.nature.com/articles/s41566-023-01244-7

Journal information: Nature Photonics 

Provided by Columbia University School of Engineering and Applied Science 

Researchers at North Carolina State University have discovered a new distinct form of silicon called Q-silicon which, among other interesting properties, is ferromagnetic at room temperature. The findings could lead to advances in quantum computing, including the creation of a spin qubit quantum computer that is based on controlling the spin of an electron.

“The discovery of Q-silicon having robust room temperature ferromagnetism will open a new frontier in atomic-scale, spin-based devices and functional integration with nanoelectronics,” said Jay Narayan, the John C. Fan Family Distinguished Chair in Materials Science and corresponding author of a paper describing the work published in Materials Research Letters.

Ferromagnetism in materials outside of transition metals and rare earths has excited scientists worldwide for a long time. This is because spin-polarized electrons can be used to process and store information with atomic resolution. However, materials with even numbers of electrons, such as carbon and silicon, without unpaired spins were not considered seriously in terms of bulk ferromagnetism. The dangling bonds in bulk carbon and silicon materials usually reconstruct and eliminate sources of unpaired electrons.

The NC State researchers showed that laser melting and quenching silicon can result in the formation of Q-silicon. The entire process is completed in less than a fraction of a microsecond, or millionth of a second. Narayan pioneered the use of lasers to create new materials with novel properties in work spanning more than four decades.

Besides ferromagnetism, other Q-silicon properties of interest include enhanced hardness and superconductivity, Narayan says.

“This discovery of Q-silicon stands to revolutionize modern microelectronics by adding new functionalities, such as spintronics, or spin-based quantum computing,” Narayan said. “Modern microelectronics is based upon the charge of an electron, making them relatively slow with limited mobility. By using Q-silicon, we make use of the spin of the electron, making computers much faster with negligible power consumption.

“In short, Q-silicon provides an ideal platform for integration of spintronics with microelectronics on a chip,” said Narayan.

More information: Jagdish Narayan et al, Synthesis and novel properties of Q-silicon (January 2023), Materials Research Letters (2023). DOI: 10.1080/21663831.2023.2224396

Provided by North Carolina State University 

An optical isolator developed at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) could drastically improve optical systems for many practical applications.

All optical systems—used for telecommunications, microscopy, imaging, quantum photonics, and more—rely on a laser to generate photons and beams of light. To prevent those lasers from damage and instability, these systems also require isolators, components that prevent light from traveling in undesired directions. Isolators also help cut down on signal noise by preventing light from bouncing around unfettered. But conventional isolators have been relatively bulky in size and require more than one type of material to be joined together, creating a roadblock to achieving enhanced performance.

Now, a team of researchers led by electrical engineer Marko Lončar at SEAS has developed a method for building a highly-efficient integrated isolator that’s seamlessly incorporated into an optical chip made of lithium niobate. Their findings are reported in Nature Photonics.

“We constructed a device that lets light emitted by the laser propagate unaltered, while the reflected light that travels back towards the laser changes its color and gets re-routed away from the laser,” said Lončar, Tiantsai Lin Professor of Electrical Engineering at SEAS. “This is accomplished by sending electrical signals in the direction of the reflected optical signals, thus taking advantage of the excellent electro-optic properties of lithium niobate,” in which voltage can be applied to change the properties of optical signals, including speed and color.

“We wanted to create a safer environment for a laser to operate in, and by designing this one-way street for light, we can protect the device from the laser’s reflection,” said Mengjie Yu, co-first author on the paper and a former postdoctoral researcher in Lončar’s lab. “To our knowledge, when compared to all other demonstrations of integrated isolators, this device performs the best optical isolation in the world. In addition to isolation, it offers the most competitive performance across all metrics including loss, power efficiency, and tunability.”

“What’s exceptional about this device is that at its core it’s incredibly simple—it’s really just one single modulator,” says Rebecca Cheng, co-first author on the paper and a current Ph.D. student in Lončar’s lab. “All previous attempts at engineering something like this required multiple resonators and modulators. The reason we can do this with such remarkable performance is because of lithium niobate’s properties.”

Another reason for the high performance and efficiency has to do with the size of the device—the team built it at the Harvard Center for Nanoscale Systems, fabricating a chip measuring 600 nanometers thick with etchings (to guide the light using prescribed nanostructures) up to 320 nanometers deep.

“With a smaller device, you can control light more easily and also put that light in closer proximity to the electrical signals, thus achieving a stronger electrical field with the same voltage,” enabling more powerful control of light, Yu said.

The scaled-down dimensions and ultralow loss property of this platform also boost optical power. “Since the light doesn’t have to travel so far, there is less decay and loss of power,” Cheng said.

Finally, the teams show the device can successfully protect an on-chip laser from external reflection. “We are the first team to show the laser’s phase-stable operation under the protection of our optical isolator,” said Yu.

Altogether, the advance represents a significant leap forward for practical, high-performance optical chips. The team reports that it can be used with a range of laser wavelengths, only requiring a counter-propagating electrical signal to achieve the desired effects.

The team hopes the breakthrough—part of a larger effort to integrate lasers and photonics components on a chip at extremely small scales—will unlock new capabilities in a range of applications, spanning the telecommunications industry to time-frequency transfer, a way of precisely measuring time down to the atomic and sub-atomic scale that could have implications for quantum research and computing.

“Integrating all aspects of an optical system onto a single chip could replace many larger, more costly, and less efficient systems,” Yu said. “Combining all these things could revolutionize many fields of work.”

More information: Mengjie Yu et al, Integrated electro-optic isolator on thin-film lithium niobate, Nature Photonics (2023). DOI: 10.1038/s41566-023-01227-8

Journal information: Nature Photonics 

Provided by Harvard John A. Paulson School of Engineering and Applied Sciences 

How are plasma eruptions in near-Earth space formed? Vlasiator, a model designed at the University of Helsinki for simulating near-Earth space, demonstrated that the two central theories on the occurrence of eruptions are simultaneously valid: eruptions are explained by both magnetic reconnection and kinetic instabilities.

Rapid plasma eruptions known as plasmoids take place on the nightside of the magnetosphere. Plasmoids are also associated with the sudden brightening of the aurora. The space physics research group at the University of Helsinki investigates and simulates these difficult-to-predict eruptions in near-Earth space using the Vlasiator model.

“The phenomena associated with plasmoids cause the most intense but the least predictable magnetic disturbances, which can cause, for example, disturbances in electrical grids,” says Professor of Computational Space Physics Minna Palmroth from the University of Helsinki. “These eruptions occur on a daily basis, in varying sizes, in the ‘tail’ of the magnetosphere.”

Palmroth is also the director of the Center of Excellence in Research of Sustainable Space, and the principal investigator for the Vlasiator simulation.

“The chain of events leading to plasmoids is one of the longest-standing unresolved questions in space physics: solutions have been sought for it since the 1960s,” Palmroth says.

Near-Earth space is a unique place for understanding plasma eruptions

Two competing lines of thinking have been proposed to explain the course of events, the first asserting that magnetic reconnection severs a part of the magnetotail into a plasmoid. According to the other explanation, kinetic instabilities disrupt the current sheet (a wide, thin distribution of electric current) maintaining the tail, which eventually results in the ejection of a plasmoid. Arguments about the primacy of these two phenomena have been ongoing for decades.

“It now appears that the causalities are in fact more complex than previously understood,” Palmroth says.

The Vlasiator simulation, which requires the processing power of a supercomputer, modeled near-Earth space for the first time in six dimensions and on a scale corresponding to the size of the magnetosphere. The 6D modeling was successful in describing the physics phenomena underlying both paradigms.

“It was a difficult technical challenge that no one else has been able to model,” Palmroth says. Behind the achievement is more than 10 years of software development.

Consequently, the study was able to demonstrate that both magnetic reconnection and kinetic instabilities explain the functioning of the magnetotail. The phenomena associated with these seemingly contradictory theories actually both take place, and simultaneously.

The finding helps to understand how plasma eruptions can occur. This helps in designing spacecraft and equipment, observing these events for further research, and improving the predictability of space weather by improving the understanding of near-Earth space.

The paper is published in the journal Nature Geoscience.

More information: Minna Palmroth et al, Magnetotail plasma eruptions driven by magnetic reconnection and kinetic instabilities Nature Geoscience (2023). DOI: 10.1038/s41561-023-01206-2. www.nature.com/articles/s41561-023-01206-2

Journal information: Nature Geoscience 

Provided by University of Helsinki 

Researchers report a new single-photon Raman lidar system that operates underwater and can remotely distinguish various substances. They also show that the new system can detect the thickness of the oil underwater up to 12 m away, which could be useful for detecting oil spills.

“Differentiating substances in water and detecting their distribution characteristics in the ocean are of great significance for marine monitoring and scientific research,” said research team leader Mingjia Shangguan from Xiamen University in China. “For instance, the remote sensing of underwater oil that we demonstrated could be useful for monitoring leaks in underwater oil pipelines.”

Although lidar approaches based on Raman signals have been previously used for detection of underwater substances, existing systems are impractical because they are bulky and require large amounts of power.

In the journal Applied Optics, the researchers describe their new lidar system, which uses just 1 μJ of pulse energy and 22.4 mm of receiver aperture. The entire lidar system is 40 cm long with a diameter of 20 cm and can be operated up to 1 km underwater. To boost sensitivity, the researchers incorporated single-photon detection into their compact underwater Raman lidar system.

“Mounting an underwater Raman lidar system on an autonomous underwater vehicle or remotely operated vehicle could enable monitoring for leaks in underwater oil pipelines,” said Shangguan. “It could potentially also be used to explore oceanic resources or be applied in detecting seafloor sediment types, such as coral reefs.”

Single-photon sensitivity in underwater lidar

Traditional lidar systems designed to operate above water on ships, aircraft or satellites can achieve large-scale ocean profiling, but their detection depth is limited, especially during rough sea conditions. Raman lidar systems, however, can be used for analysis underwater at different depths without being affected by sea conditions.

Raman lidar works by emitting a pulse of green laser light into the water that interacts with substances such as oil. This excites inelastic Raman signals that can be used to identify substances. By measuring the intensity of Raman signals at specific wavelengths, lidar can provide information about the oil content in the water.

“Traditional Raman lidar systems rely on increasing laser power and telescope aperture to achieve remote sensing detection, which leads to a large system size and high-power consumption that make it difficult to integrate lidar systems onto underwater vehicles,” said Shangguan. “The use of single-photon detection technology made this work possible by improving detection sensitivity to the level of single photons.”

The researchers demonstrated their new lidar system by using it to detect varying thicknesses of gasoline oil in a quartz cell that was 12 m away from the system. Both the lidar system and the quartz cell were submerged at a depth of 0.6 m underwater in a large pool. The lidar system was able to detect and distinguish all thicknesses of gasoline, which ranged from 1 mm to 15 mm.

The researchers are now working to increase the number of detection channels and the Raman spectral resolution of the single-photon lidar system to enhance its ability to distinguish different substances in water. This would allow it to be used to analyze underwater bubble types and to detect corals and manganese nodules.

More information: Mingjia Shangguan et al, Remote sensing oil in water with an all-fiber underwater single-photon Raman lidar, Applied Optics (2023). DOI: 10.1364/AO.488872

Provided by Optica 

If you imagine yourself peering through a microscope, you probably picture looking at a glass slide with an amoeba, or maybe a human cell, or perhaps even a small insect of some kind.

But microscopes can see much more than these small living things, and a new type of microscopy developed at Caltech is making it easier to see the very molecules that make up living things.

In a paper appearing in the journal Nature Photonics, researchers from the lab of Lu Wei, assistant professor of chemistry and investigator with the Heritage Medical Research Institute, demonstrate what they are calling bond-selective fluorescence-detected infrared-excited spectro-microscopy, or BonFIRE.

BonFIRE combines two microscopy techniques into one process with greater selectivity and sensitivity, enabling researchers to visualize biological processes at the unprecedented single-molecule level and understand biological mechanisms from a molecular point of view.

“With our new microscope, we can now visualize single molecules with vibrational contrast, which is challenging to do with existing technologies.” says Dongkwan Lee, study co-author and chemical engineering graduate student.

One technique involved in BonFIRE is fluorescence microscopy, which images molecules and other microscopic structures by tagging them with fluorescent chemical markers, causing them to glow when imaged.

The other technique is vibrational microscopy, which makes use of natural vibrations in the bonds that hold together the atoms of a molecule. A sample to be imaged is bombarded with light, in this case infrared light. That bombardment causes the bonds of the material’s molecules to vibrate in such a way that their type can be identified. Vibrations of a triple bond will “sound” different than the vibrations of a single bond, and the vibrations of a carbon atom bonded to another carbon atom will sound different than the vibrations of a carbon atom bonded to a nitrogen atom, for example. It’s not unlike how a trained guitarist would be able to tell which string on a guitar was plucked and what material it is made from just by listening to the tone it makes.

Wei says that fluorescence microscopy allows researchers to observe single molecules, but it does not provide rich chemical information. On the other hand, vibrational microscopy provides that rich chemical information but only works when the molecule being imaged is present in large amounts.

BonFIRE gets around these limitations by coupling vibrations to fluorescence, effectively combining the strengths of the two techniques. The process works like this: The sample is first stained with a fluorescent dye that bonds to the molecules intended to be imaged. The sample is then bombarded by a pulse of infrared light, whose frequency is tuned to excite a specific bond found in that dye. Once the bond is excited by just a single photon of that light, a second higher-energy pulse of light shines on it and excites it to fluoresce with a glow that can be detected by the microscope. In this way, the microscope can image entire cells or single molecules.

“We are fascinated by this spectroscopy process and are excited to turn it into a novel tool for modern bioimaging,” says Haomin Wang, study co-author and postdoctoral scholar research associate in chemistry. “Over the past three years, we have been on an adventure to build our custom BonFIRE microscope and gain deeper understanding on this spectroscopic process, which further helped us to optimize each component in our setup to reach the performance we have now.”

In their paper, the researchers also demonstrate the ability to tag biomolecules with “colors,” allowing them to be differentiated from each other. This is done by using several isotopes of the atoms that make up the dye molecule. (Isotopes are forms of an element with different atomic weights because their nuclei have greater or fewer neutrons). The frequency at which their bonds vibrate changes with the increased or decreased mass of the atoms.

“Unlike conventional fluorescence microscopy, which can only distinguish a handful of colors at a time, BonFIRE uses infrared light to excite different chemical bonds and produces a rainbow of vibrational colors,” Wei says. “You can label and image many different targets from the same sample at a time and reveal the molecular diversity of life in stunning detail. We hope to be able to demonstrate the imaging capability with tens of colors in live cells in the near future.”

Additional co-authors are chemistry graduate students Yulu Cao, Xiaotian Bi, Jiajun Du, and Kun Miao.

More information: Haomin Wang et al, Bond-selective fluorescence imaging with single-molecule sensitivity, Nature Photonics (2023). DOI: 10.1038/s41566-023-01243-8

Journal information: Nature Photonics 

Provided by California Institute of Technology 

Students at the Faculty of Physics of the University of Warsaw (UW) and researchers from the QOT Center for Quantum Optical Technologies have developed an innovative method that allows the fractional Fourier Transform of optical pulses to be performed using quantum memory. This achievement is unique on the global scale, as the team was the first to present an experimental implementation of the said transformation in this type of system.

The results of the research were published in the journal Physical Review Letters. In their work, the students tested the implementation of the fractional Fourier Transform using a double optical pulse, also known as a “Schrödinger’s cat” state.

The spectrum of the pulse and temporal distribution

Waves, such as light, have their own characteristic properties—pulse duration and frequency (corresponding, in the case of light, to its color). It turns out that these characteristics are related to each other through an operation called the Fourier Transform, which makes it possible to switch from describing a wave in time to describing its spectrum in frequencies.

The fractional Fourier Transform is a generalization of the Fourier Transform that allows a partial transition from a description of a wave in time to a description in frequency. Intuitively, it can be understood as a rotation of a distribution (for example, the chronocyclic Wigner function) of the considered signal by a certain angle in the time-frequency domain.

It turns out that transforms of this type are exceptionally useful in the design of special spectral-temporal filters to eliminate noise and enable the creation of algorithms that make it possible to use the quantum nature of light to distinguish pulses of different frequencies more precisely than traditional methods. This is especially important in spectroscopy, which helps study the chemical properties of matter, and telecommunications, which requires the transmission and processing of information with high precision and speed.

An ordinary glass lens is capable of focusing a monochromatic beam of light falling on it to almost a single point (focus). Changing the angle of incidence of light on the lens results in a change in the position of the focus. This allows us to convert angles of incidence into positions, obtaining the analogy of the Fourier Transform, in the space of directions and positions. A classical spectrometer based on a diffraction grating uses this effect to convert the wavelength information of light into positions, allowing us to distinguish between spectral lines.

Time and frequency lenses

Similarly to the glass lens, time and frequency lenses allow the conversion of a pulse’s duration into its spectral distribution, or effectively, perform a Fourier transform in time and frequency space. The right selection of powers of such lenses makes it possible to perform a fractional Fourier Transform. In the case of optical pulses, the action of time and frequency lenses corresponds to applying quadratic phases to the signal.

To process the signal, the researchers used a quantum memory—or more precisely a memory equipped with quantum light processing capabilities—based on a cloud of rubidium atoms placed in a magneto-optical trap. The atoms were cooled to a temperature of tens of millions of degrees above absolute zero. The memory was placed in a changing magnetic field, allowing components of different frequencies to be stored in different parts of the cloud. The pulse was subjected to a time lens during writing and reading, and a frequency lens acted on it during storage.

The device developed at the UW allows the implementation of such lenses over a very wide range of parameters and in a programmable way. A double pulses is very prone to decoherence, hence it is often compared to the famous Schrödinger cat—a macroscopic superposition of being dead and alive, almost impossible to achieve experimentally. Still, the team was able to implement faithful operations on those fragile dual-pulse states.

Before direct application in telecommunications, the method must first be mapped to other wavelengths and parameter ranges. Fractional Fourier transform, however, could prove crucial for optical receivers in state-of-the-art networks, including optical satellite links. A quantum light processor developed at the UW makes it possible to find and test such new protocols in an efficient way.

More information: Bartosz Niewelt et al, Experimental Implementation of the Optical Fractional Fourier Transform in the Time-Frequency Domain, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.240801

Journal information: Physical Review Letters 

Provided by University of Warsaw 

Understanding the behavior of nuclear matter—including the quarks and gluons that make up the protons and neutrons of atomic nuclei—is extremely complicated. This is particularly true in our world, which is three dimensional. Mathematical techniques from condensed matter physics that consider interactions in just one spatial dimension (plus time) greatly simplify the challenge.

Using this two-dimensional approach, scientists solved the complex equations that describe how low-energy excitations ripple through a system of dense nuclear matter. This work indicates that the center of neutron stars, where such dense nuclear matter exists in nature, may be described by an unexpected form.

Being able to understand the quark interactions in two dimensions opens a new window into understanding neutron stars, the densest form of matter in the universe. The approach could help advance the current “golden age” for studying these exotic stars. This surge in research success was triggered by recent discoveries of gravitational waves and electromagnetic emissions in the cosmos.

This work shows that for low-energy excitations, all of the complications of the three-dimensional quark interactions fall away. These low-energy excitations are slight disturbances triggered as a neutron star emits radiation or by its own spinning magnetic fields. This approach might also enable new comparisons with quark interactions in less dense but much hotter nuclear matter generated in heavy-ion collisions.

The modern theory of nuclei, known as quantum chromodynamics, involves quarks bound by the strong nuclear force. This force, carried by gluons, confines quarks into nucleons (protons and neutrons).

When the density of nuclear matter increases, as it does inside neutron stars, the dense system behaves more like a mass of quarks, without sharp boundaries between individual nucleons. In this state, quarks at the edge of the system are still confined by the strong force, as quarks on one side of the spherical system interact strongly with quarks on the opposite side.

This work by researchers at Brookhaven National Laboratory uses the one-dimensional nature of this strong interaction, plus the dimension of time, to solve for the behavior of excitations with low energy near the edge of the system. These low energy modes are just like those of a free, massless boson—which is known in condensed matter as a “Luttinger liquid.”

This method allows scientists to compute the parameters of a Luttinger liquid at any given density. It will advance their ability to explore qualitatively new phenomena expected to occur at the extreme densities within neutron stars, where nuclear matter behaves quite differently than it does in ordinary nuclei, and compare it with much hotter (trillion-degree) dense nuclear matter generated in heavy-ion collisions.

The work is published in the journal Physical Review D.

More information: Marton Lajer et al, When cold, dense quarks in 1+1 and 3+1 dimensions are not a Fermi liquid, Physical Review D (2022). DOI: 10.1103/PhysRevD.105.054035

Journal information: Physical Review D 

Provided by US Department of Energy