Hexbyte Glen Cove Scientists greatly expand the frequencies generated by a miniature optical ruler

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Spectral translation to create ultra-broadband microcombs. A microring resonator with RW = 1117 nm is pumped by a primary pump at 282 THz and synthesis pump at 192 THz. a Primary comb generation with low primary pump power near threshold. The comb spacing is equal to seven free spectral ranges (FSRs) and reproduced around the synthesis pump and idlers, highlighting the mixing process between the two pumps and the primary portion comb teeth. b Primary comb generation at a higher primary pump power where, as previously, the spectral spacing in the primary portion is matched by that in the synthesis portion, as expected by the FWM-BS theory. c Two-soliton state, where the characteristic 8 FSR modulation in the comb envelope is replicated near the synthesis pump. The inset shows the LLE-calculated two-soliton pulse arrangement that results in the simulated comb envelope shown in red. We highlight the missing comb tooth in the primary portion (Δμ = −4), whose absence is translated onto the synthesized portion of the comb, respecting the FWM-BS phase-matching condition. d Single soliton state, where the impact of the synthesis pump is to expand the comb bandwidth to 1.6 octaves and create new DWs on both ends of the spectrum. The spectrum agrees with the generalized LLE solution using the dual-pump model (red line), and greatly exceeds the expected spectrum if just the primary pump is applied (dashed green line). The phase-coherent nature of the comb is verified through beat note measurements with narrow linewidth lasers throughout the comb spectrum (four left insets). The noise floor for each measurement is shown in dashed lines, and is higher in the O-band due to use of an additional RF amplifier. The rightmost inset shows the LLE simulation of the expected time-domain behavior under dual pumping (red) and if only the primary pump is applied (green). The horizontal bars at the bottom of the graph compare the span achieved here with octave-spanning DKSs from refs. 3, 32. We note that the low frequency portion of the spectrum exhibits OSA artefacts, at 146, 159, and

Like a vocal coach who extends the octave range of an opera singer, researchers at the National Institute of Standards and Technology (NIST) have expanded by nearly two-thirds the frequency range over which a chip-scale device can generate and measure the oscillations of light waves with exquisite accuracy. The expanded range of the system, known as a microring resonator frequency comb, or microcomb, could lead to better sensors of greenhouse gases and may also improve global navigation systems.

Gregory Moille and his colleagues at NIST, including team leader Kartik Srinivasan, along with collaborators at the Joint Quantum Institute (a NIST-University of Maryland research partnership) and the University of Maryland, reported their findings in the December 14, 2021 issue of Nature Communications.

A frequency comb acts like the optical version of a ruler. Just as a ruler, divided into hundreds of tick marks spaced a known distance apart, measures an object of unknown length, a frequency comb features hundreds of different ultrasharp, uniformly spaced frequency spikes to precisely measure light of an unknown frequency (The tool is so named because the frequency spikes resemble the teeth of a comb.)

Over the past two decades, scientists at NIST and other research institutions have shown that microcombs can play an important role in building highly accurate optical clocks, calibrating detectors that analyze starlight to search for planets beyond the solar system, and detecting trace gases in the environment.

One type of microcomb extensively studied at NIST consists of a miniature, rectangular waveguide—a channel that confines light waves—coupled to a ring-shaped resonator about 50 micrometers (millionths of a meter) in diameter. Laser light injected into the waveguide enters the microring resonator and races around the ring.

Ordinarily, the circulating light begins to vary in amplitude and can form different patterns. However, by carefully adjusting the laser, the light within the microring forms a soliton—a solitary wave pulse that preserves its shape as it moves around the ring.

By using two lasers instead of one, NIST researchers have developed a method for nearly doubling the range of the frequency comb generated by a microring resonator. Credit: S. Kelley/NIST

Each time the soliton completes one roundtrip around the microring, a portion of the pulse splits off and enters the waveguide. Soon, an entire train of wave pulses fills the waveguide, with each wave separated in time from its neighbor by the same fixed interval—the time it took for the soliton to complete one lap around the microring. The train of wave pulses in the waveguide corresponds to a single set of evenly spaced frequencies and forms the teeth of the frequency comb. The number and amplitude of the teeth are primarily determined by the size and composition of the ring and the power and frequency of the laser.

Recently, the NIST scientists wondered what would happen if they produced a microcomb using two lasers, each generating a different frequency of light, instead of just one. They found that through a complex series of interactions with the light circulating in the microring resonator, the second laser induced two new sets of teeth, or evenly spaced frequencies, that are replicas of the original set of teeth but shifted to higher and lower frequencies. The lower frequency set lies in the infrared part of the spectrum, while the other is at much high frequencies, close to visible light. The comb also retains its original teeth, at near-infrared frequencies.

The extended range of the microcomb enables a host of applications at different frequencies. The system is the first time that researchers have produced a stable microcomb that ties together such a wide range of frequencies of light, Srinivasan said.

In addition, the team discovered that by varying the frequency of the second laser, the new sets of teeth could be easily shifted to higher or lower frequencies independent of the shape or composition of the microring resonator. This makes the system extremely versatile.

The feat may enable a single microcomb to measure the characteristic vibrations of atoms and molecules, including pollutants, that both emit and absorb light over a broad range of frequencies, thus enhancing the sensitivity of detectors.

The broader coverage could also help subsequent efforts to stabilize the , so that its tick marks remain fixed rather than wander slightly from their original set of colors. The enhanced stability may spur the development of portable optical accurate enough to be employed outside the laboratory, leading to more accurate and precise navigation systems, said Moille.

More information:
Gregory Moille et al, Ultra-broadband Kerr microcomb through soliton spectral translation, Nature Communications (2021). DOI: 10.1038/s41467-021-27469-0

Scientists greatly expand the frequencies generated by a miniature optical ruler (2022, February 23)
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Hexbyte Glen Cove Carbon payments play a pivotal role in forest protection program

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The Sacred Forest Wingwi Mapofu in Pemba, Tanzania. Credit: Monique Borgerhoff Mulder

When pay-to-conserve programs don’t come through with payments, they don’t conserve, indicates a case study by the University of California, Davis, of a REDD+ Readiness program on the island of Pemba, off the coast of Tanzania.

REDD+ is a United Nations program that stands for Reducing Emissions from Deforestation and Forest Degradation. It aims to incentivize developing economies to conserve forests by paying them for added carbon storage through the carbon market. The “plus” refers to social benefits, such as empowering women, providing tenure security and enhancing biodiversity, that can come from conserving local forests.

The study, published today in the journal Environmental Research Letters, found that alone, in the absence of payments, were not enough to slow deforestation in a REDD+ project in Pemba. Using satellite imagery and statistical matching methods, the authors found no quantitative difference in forest cover change between areas of Pemba with and without a REDD+ program.

Prioritizing payment

“We’re not saying REDD+ doesn’t work at all,” said lead author Amy Collins, a Ph.D. student at UC Davis at the time of the study and currently a postdoctoral researcher at Colorado State University. “REDD+ has the potential to be useful, but it needs to be implemented properly. Certainly, payments for ecosystem services have to be in place.”

A REDD-protected forest in Pemba, Tanzania. Credit: Amy Collins, UC Davis

Payments through REDD+ are mostly associated with carbon sequestration. In Pemba, the process to measure, report and verify carbon sequestration was left incomplete by the carbon agent due to technical shortcomings and lack of follow-through.

Pemba is not alone. A previous study found that two-thirds of REDD+ programs as of 2018 had not received payments, for various reasons.

To help ensure compensation, the authors suggest that REDD+ programs should confirm they have a viable strategy for monitoring and verifying outcomes before a project begins on the ground.

Other benefits

Carbon storage is the main criteria for REDD+ payments, but community forestry projects often have positive, noncarbon effects as well. The study suggests that those other benefits be better monitored and integrated into payment criteria.

Beehives sit in the treetops in a forest in Pemba, Tanzania. The hives are part of the locally-run business Pemba Honey. Credit: Amy Collins, UC Davis

For example, despite little change in , the authors saw firsthand social and environmental benefits that the program produced, including community empowerment, especially for local women.

“Our broader research team is finding women’s engagement with objectives has grown as a result of different levels of exposure to REDD+, and also that the community forest model supported by REDD+ is spreading across the island despite an absence of payments,” said project leader Monique Borgerhoff Mulder, professor emerita of anthropology at UC Davis, and now at the Max Planck Institute for Evolutionary Anthropology, Leipzig.

For example, the program allowed for multiple uses in the forest, not just strictly preservation. One group composed largely of women began a beekeeping business called Pemba Honey, which produced honey with the distinctive taste of cloves, the main crop in Pemba.

“When we were conducting our research on Pemba island, we saw a lot of women driving this program,” Collins said. “Enabling women to have prominent roles in business and livelihoods was recognized as a potential co-benefit to this REDD+ project from the start.”

Additional co-authors include Mark Grote and James Thorne of UC Davis, Tim Caro of UC Davis and University of Bristol, Aniruddha Ghosh of UC Davis and International Center for Tropical Agriculture in Nairobi, and Jonathan Salerno of Colorado State University.

More information:
Amy Collins et al, How community forest management performs when REDD+ payments fail, Environmental Research Letters (2022). DOI: 10.1088/1748-9326/ac4b54

Carbon payments play a pivotal role in forest protection program (2022, February 23)
retrieved 24 February 2022
from https://phys.org/news/2022-02-carbon-payments-pivotal-role-forest.html

This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no
part may be reproduced without the written permission. The content is provided for information purposes only.

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Hexbyte Glen Cove Scientists visualize electron crystals in a quantum superposition

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Illustration of two sites of graphene lattice.  Credit: Image courtesy of the researchers

Princeton scientists are using innovative techniques to visualize electrons in graphene, a single atomic layer of carbon atoms. They are finding that strong interactions between electrons in high magnetic fields drive them to form unusual crystal-like structures similar to those first recognized for benzene molecules in the 1860s by chemist August Kekulé. These crystals exhibit a spatial periodicity that corresponds to electrons being in a quantum superposition. The experiments also show the Kekulé quantum crystals have defects that have no analog to those of ordinary crystals made up of atoms. These findings shed light on the complex quantum phases electrons can form because of their interaction, which underlies a wide range of phenomena in many materials.

Physicists learned to control how electrons interact with one another through the application of a strong magnetic field and, most recently, by stacking multiple layers of graphene on top of each other. In fact, the discovery of graphene in the first decade of the 21st century—a discovery that led to a Nobel Prize in physics in 2010—opened a new arena for exploring the physics of electrons, especially for examining how electrons behave collectively.

Now, Princeton researchers led by Ali Yazdani, the Class of 1909 Professor of Physics and director of the Center for Complex Materials at Princeton University, have discovered that the strong interaction between electrons in graphene drives them to form crystal structures with complex patterns determined by quantum superposition—electrons residing at multiple atomic sites at the same time. The experiment, recently published in Science, also shows that this novel quantum crystal hosts exotic deformations that correspond to the twisting and winding of the electrons’ wavefunction.

Graphene consists of a single layer of carbon atoms arranged in a two-dimensional hexagonal, or honeycomb-like, lattice. It is produced in a deceptively simple but painstaking manner. Graphite, the same material found in pencils, is progressively exfoliated strip by strip until this single-atom-thin layer of carbon is reached.

“Previous studies have shown that graphene demonstrates novel electrical properties,” Yazdani said. “But never before have researchers been able to peer so deeply and with such spatial resolution into the nature of quantum states.”

To achieve this unparalleled level of resolution, Yazdani’s group used a device called a (STM). This device relies on a phenomenon called “quantum tunneling,” where voltage is used to funnel electrons between the sharp metallic tip of the microscope and the sample only a few ångströms away. The microscope uses this tunneling current rather than light to view the world of electrons on the atomic scale. Yazdani’s microscopes operate in a very high vacuum to keep the sample surface clean and at very low temperatures to allow for high resolution measurements, unperturbed by thermal agitation.

The microscope is also able to view electrons as they reach their dominated by their quantum properties.

In the presence of a magnetic field, the microscope can be used to determine the spatial structure of the quantized energy level.

“One of the special properties of graphene is its behavior in a magnetic field, when electrons are forced to orbit around the magnetic field in a circle,” said Yazdani. “This quantizes their energies, resulting in quantization of graphene’s electrical properties.”

Quantization of energy refers to the creation of discrete values of energy, without any intermediate values, which is a characteristic of quantum physics, as opposed to classical physics, where continuous energy values is permitted.

The researchers focused their attention on the quantized level with the lowest energy in graphene, for which previous research first reported by Phuan Ong, Eugene Higgins Professor of Physics at Princeton, had revealed some unusual electrical properties. This level dominates the electrical properties when there are no excess charges added or removed from graphene—in other words, when the charge is neutral. Ong had shown that electrons “freeze” when the charge is neutral, and the graphene layer acts as an insulator with the application of a magnetic field. The nature of this frozen state of electrons in graphene has been a mystery for almost a decade, since Ong’s initial discovery.

A vortex of Kekule pattern. The left panel shows the change of Kekule pattern in space. The bottom right panel illustrates the texture of the vortex extracted from the left panel that resembles a hurricane. Credit: Image courtesy of the researchers

“The insulating state that we found puzzled everyone and strongly challenged the prevailing theories at that time,” said Ong, who was not involved in the current research. “It remained an enduring puzzle for 13 years until the beautiful results obtained by Yazdani. The new results resolve the puzzle in a very exciting fashion.”

Yazdani and his team used the microscope to map the wavefunction of the lowest quantized energy level in the presence of a magnetic field. The researchers found of electron waves when graphene was tuned to a neutral state with a nearby electrical gate.

In metals, electrons’ wavefunction are spread throughout the crystal, while in a normal insulator, electrons are frozen without any particular preference to the crystal structure of the atomic sites. At very low fields, STM images showed electron wavefunctions of graphene choosing one of the sub-lattice sites over the other. More importantly, by increasing the magnetic field, a remarkable bond-like pattern is observed, which corresponds to electrons’ wavefunction residing in a quantum superposition. This means that an electron occupies the two inequivalent sites at the same time.

In particular, the image corresponded to the bond-like structure first recognized by Kekulé for benzene. This consists of alternating single and double bonds. In a single bond, one electron from each atom binds with its neighbor electron; in a double bond, two electrons from each atom participate.

People have speculated that electrons may form such Kekulé patterns,” said Yazdani, “but now we’re seeing it for the first time. One couldn’t distinguish this state of electrons any other way unless it is imaged.”

The researchers then used the microscope to map the uniformity of the Kekulé crystal and its properties near imperfections, or defects in the graphene. One remarkable finding they made was near charge defects where they found the Kekulé pattern to evolve continuously in its patterns around the defect on the surface.

Teaming up with Michael Zaletel of the University of California, Berkeley, the team developed a method for extracting from the STM data the mathematical properties of the quantum wavefunction of electrons, so-called phase angles describing their . The analysis revealed remarkable winding of one of these phase angles around the defect and correlated changes in the other angle.

“When the group applied their technique to measure the phase-angle above a defect in the substrate, they found a ‘vortex’ in the Kekulé pattern, which is like a hurricane around which the phase-angle winds around by 12 hours [as on a clock],” said Zaletel. “When making predictions about such quantum, nanoscale, objects, you rarely think you’ll have the pleasure to really ‘see’ a picture of them, but the group has been able to do just that.”

The team believes that the techniques they have developed to uncover this unusual quantum crystal of electrons in a strong magnetic field can have applications elsewhere in the field. Other two-dimensional materials and their stack can exhibit similar quantum crystals with novel defects. The team aims to apply their methodology to a wider class of such materials.

In addition to Yazdani and Zaletel, contributors to the study included authors Xiaomeng Liu, Gelareh Farahi and Cheng-Li Chiu, all at the Joseph Henry Laboratories and Department of Physics, Princeton University; Zlatko Papic, School of Physics and Astronomy, University of Leeds, United Kingdom; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Material Science in Japan.

The study, “Visualizing broken symmetry and topological defects in a quantum Hall ferromagnet,” by Xiaomeng Liu, Gelareh Farahi, Cheng-Li Chiu, Zlatko Papic, Kenji Watanabe, Takashi Taniguchi, Michael Zaletel and Ali Yazdani, was published Dec. 2, 2021 in the journal Science.

More information:
Xiaomeng Liu et al, Visualizing broken symmetry and topological defects in a quantum Hall ferromagnet, Science (2022). DOI: 10.1126/science.abm3770

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Hexbyte Glen Cove The algebra of neurons: Study deciphers how a single nerve cell can multiply

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Neurons in the fruit fly brain multiply by dividing by the reciprocal. Credit: MPI for Biological Intelligence, i.f./ Kuhl

Neurons are constantly performing complex calculations to process sensory information and infer the state of the environment. For example, to localize a sound or to recognize the direction of visual motion, individual neurons are thought to multiply two signals. However, how such a computation is carried out has been a mystery for decades. Researchers at the Max Planck Institute for Biological Intelligence have now discovered in fruit flies the biophysical basis that enables a specific type of neuron to multiply two incoming signals. This provides fundamental insights into the algebra of neurons—the computations that may underlie countless processes in the brain.

We easily recognize objects and the direction in which they move. The calculates this information based on local changes in light intensity detected by our retina. The calculations occur at the level of individual . But what does it mean when neurons calculate? In a network of communicating nerve cells, each cell must calculate its outgoing signal based on a multitude of incoming signals. Certain types of signals will increase and others will reduce the outgoing signal—processes that neuroscientists refer to as “” and “.”

Theoretical models assume that seeing motion requires the multiplication of two signals, but how such arithmetic operations are performed at the level of single neurons was previously unknown. Researchers from Alexander Borst’s department at the Max Planck Institute for Biological Intelligence have now solved this puzzle in a specific type of neuron.

Recording from T4 cells

The scientists focused on so-called T4 cells in the visual system of the fruit fly. These neurons only respond to visual motion in one specific direction. The lead authors Jonatan Malis and Lukas Groschner succeeded for the first time in measuring both the incoming and the outgoing signals of T4 cells. To do so, the neurobiologists placed the animal in a miniature cinema and used minuscule electrodes to record the neurons’ electrical activities. Since T4 cells are among the smallest of all neurons, the successful measurements were a methodological milestone.

Together with computer simulations, the data revealed that the activity of a T4 cell is constantly inhibited. However, if a visual stimulus moves in a certain direction, the inhibition is briefly lifted. Within this short time window, an incoming excitatory signal is amplified: Mathematically, constant inhibition is equivalent to a division; removing the inhibition results in a multiplication. “We have discovered a simple basis for a complex calculation in a single neuron,” explains Lukas Groschner. “The inverse operation of a division is a multiplication. Neurons seem to be able to exploit this relationship,” adds Jonatan Malis.

Relevance for behavior

The T4 cell’s ability to multiply is linked to a certain on its surface. “Animals lacking this receptor misperceive visual motion and fail to maintain a stable course in behavioral experiments,” explains co-author Birte Zuidinga, who analyzed the walking trajectories of fruit flies in a virtual reality setup. This illustrates the importance of this type of computation for the animals’ behavior.

“So far, our understanding of the basic algebra of neurons was rather incomplete,” says Alexander Borst. “However, the comparatively simple brain of the fruit fly has allowed us to gain insight into this seemingly intractable puzzle.” The researchers assume that similar neuronal computations underlie, for example, our abilities to localize sounds, to focus our attention, or to orient ourselves in space.

More information:
Lukas N. Groschner et al, A biophysical account of multiplication by a single neuron, Nature (2022). DOI: 10.1038/s41586-022-04428-3

The algebra of neurons: Study deciphers how a single nerve cell can multiply (2022, February 23)
retrieved 24 February 2022
from https://phys.org/news/2022-02-algebra-neurons-deciphers-nerve-cell.html

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