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Laser Cooling Chills Radium Ions For The First Time

Researchers have successfully used laser cooling on radium ions for the first time. Given that lasers are known for heating things up, laser cooling may seem a contradiction in terms.

However, scientists have devised a way to use the technology to achieve unparalleled levels of cold. Radium is the heaviest alkaline earth element, and the only ion in the column that hadn’t been laser cooled.

“I look at this as having the potential for discoveries as well as for constraining theories beyond the Standard Model of particle physics,”

says Andrew Jayich, an assistant professor of physics at the University of California, Santa Barbara. Radium shows promise in diverse fields like quantum information science, optical clocks, and fundamental physics research.

Slowing Atoms Down

To achieve laser cooling, scientists start with a charged atom, an ion, that is trapped with electric fields. Imagine the hot atom bouncing around in an ion trap as a playground swing in motion.

Give it a push at the right time and you’ll send it higher, or, in this analogy, heat up the atom. But a push opposing the swing’s motion, as it’s coming toward you, will actually slow it, which cools the atom.

Each energy state an atom can take has an associated resonance frequency. Scientists can coax the atom to transition into a specific state by hitting them with laser light tuned to that specific frequency.

The atom will absorb the light and transition to the desired state. Then it will drop to a lower energy state, emitting its own photon that carries away some of the atom’s momentum. This literally slows the atom down, and a slow atom translates to a colder atom.

single radium ion contained in the team’s ion trap Mingyu Fan captured with a long exposure.
An image of a single radium ion contained in the team’s ion trap Mingyu Fan captured with a long exposure.
Credit: Mingyu Fan/UC Santa Barbara

Tuning the laser slightly below this resonance frequency makes it more likely you’ll give the atom a push as it’s headed toward the beam, rather than away. It’s a result of the Doppler effect, which shifts the frequency up as the atom moves towards the incoming laser, much like the siren of a fire engine has a higher pitch as it approaches.

Thanks to this method, scientists are never pushing the swing harder, only slowing it down.

Laser cooling and detecting a trapped ion is tricky, and the fact that radium has no stable isotopes made the first laser cooling even more challenging for the researchers.

Because the element is radioactive, they initially had only 10 micrograms to work with, and were concerned they’d use up their sample before finding the correct parameters for trapping and cooling the ions. Finally, after several weeks of searching, the camera caught its first glimpse of a laser-cooled radium ion.

This new paper represents nearly two years the team spent constructing ion-trapping equipment and perfecting their laser-cooling technique.

Symmetry Violation Predictions

For a long time, scientists thought that physics was symmetric — that it worked the same if you flipped the values of the charges or the orientations of all the particles in a system. They also assumed things played out the same going forward in time as backward.

Surprisingly, experiments have shown that these basic assumptions about symmetry are not true. Only the three symmetries — charge, parity, and time — taken together appear to always hold up. Namely, every physical process we know of stays the same if you swap right for left, positive for negative, and future for past.

While physicists have identified processes that violate charge-parity symmetry in fundamental particles, the violations aren’t large enough to explain longstanding questions, like why the universe has more matter than antimatter.

“Theories beyond the Standard Model usually add in new charge-parity symmetry violations to explain these puzzles. The symmetry violations predicted by new physics models would also affect atoms,”

says lead author Mingyu Fan, a doctoral student in the Jayich lab.

Nuclear Amplifier For New Physics

And the heavy nuclei of radium atoms are particularly susceptible to displaying these behaviors. That’s why the team has devoted so much time and energy to learning how to manipulate this element.

The radium nucleus also has a unique shape, like a pear, and a set of two of nuclear states separated by only a small amount of energy. These features make radium more sensitive than other nuclei to time symmetry violation, physics that has not been observed in any atom or molecule, Jayich explains. Some yet undiscovered physics — like a new particle or interaction — might mix these two nuclear states.

Jayich suspects that radium will enhance sensitivity to symmetry violations by 1,000-fold compared to the best systems currently in use.

“Radium is like a nuclear amplifier for new physics,”

he says.

New Advanced Experiments

The team’s initial success in laser-cooling radium ions opens the door to an impressive set of experiments and applications that Jayich intends to pursue in the coming years. In terms of technology, a heavy atom like radium is less susceptible to getting bumped around, thereby minimizing noise in a trapped ion system.

Now the team has to actually measure these states.

“Very basic properties of the radium ion, such as lifetimes and branching ratios, things that an atomic physicist would take for granted, have not been measured because it hadn’t been laser cooled. We need to measure the radium ion’s basic properties so we can build advanced experiments. We hope to finish these fundamental measurements within the next year,”

says Jayich.

The team is planning a new experiment using a different form of radium with a nuclear spin that will open new physics opportunities. Although the radium-225 has a half-life of only 15 days, the team will use a clever method to continuously replenish their supplies using an isotope of thorium that radioactively decays to radium-225.

The thorium isotope has a 7,300-year half-life, so the team isn’t concerned about running out. As a bonus, the two elements are very easy to separate, ensuring that no thorium contaminates the sample.

Laser cooling radium also opens up the possibility of putting the atom inside molecules.

The group plans to make and study radium hydroxide in their ion trap, which is trickier than simply synthesizing a compound in a test tube. This would increase sensitivity to new physics by another factor of 1,000 compared to just an atomic system, Jayich says.

Time will tell. The team’s planned experiments in this line of research could take several years to yield results once they are up and running, he adds.

“It’s really the sum of many things that come together to make radium special, and they all come together to give you a really nice platform for both fundamental physics and photonic quantum technologies,”

Jayich says.

Original Study: Laser Cooling of Radium Ions

Top Image: Scott Eckersley/Unsplash

Quantum Control With Light Paves Way For Ultra-Fast Computers

Terahertz light can control some of the essential quantum properties of superconducting states, report researchers.

Jigang Wang patiently explains his latest discovery in quantum control that could lead to superfast computing based on quantum mechanics: He mentions light-induced superconductivity without energy gap. He brings up forbidden supercurrent quantum beats. And he mentions terahertz-speed symmetry breaking.

Then he backs up and clarified all that. After all, the quantum world of matter and energy at terahertz and nanometer scales — trillions of cycles per second and billionths of meters — is still a mystery to most of us.

“I like to study quantum control of superconductivity exceeding the gigahertz, or billions of cycles per second, bottleneck in current state-of-the-art quantum computation applications. We’re using terahertz light as a control knob to accelerate supercurrents,”

says Wang, a professor of physics and astronomy at Iowa State University.

Terahertz Light

Superconductivity is the movement of electricity through certain materials without resistance.

It typically occurs at very, very cold temperatures. Think -400 Fahrenheit for “high-temperature” superconductors.

Terahertz light is light at very, very high frequencies. Think trillions of cycles per second. It’s essentially extremely strong and powerful microwave bursts firing at very short time frames.

It all sounds esoteric and strange. But the new method could have very practical applications.

“Light-induced supercurrents chart a path forward for electromagnetic design of emergent materials properties and collective coherent oscillations for quantum engineering applications,”

Wang and his coauthors write in a paper in Nature Photonics.

In other words, the discovery could help physicists “create crazy-fast quantum computers by nudging supercurrents,”

Wang writes in a summary of the research team’s findings.

Quantum World Control

Finding ways to control, access, and manipulate the special characteristics of the quantum world and connect them to real-world problems is a major scientific push these days. The National Science Foundation has included the “Quantum Leap” in its “10 big ideas” for future research and development.

“By exploiting interactions of these quantum systems, next-generation technologies for sensing, computing, modeling, and communicating will be more accurate and efficient. To reach these capabilities, researchers need understanding of quantum mechanics to observe, manipulate, and control the behavior of particles and energy at dimensions at least a million times smaller than the width of a human hair,”

says a summary of the science foundation’s support of quantum studies.

A Universal Tool

The researchers are advancing the quantum frontier by finding new macroscopic supercurrent flowing states and developing quantum controls for switching and modulating them.

A summary of the research team’s study says experimental data they obtained from a terahertz spectroscopy instrument indicates terahertz light-wave tuning of supercurrents is a universal tool “and is key for pushing quantum functionalities to reach their ultimate limits in many cross-cutting disciplines” such as those mentioned by the science foundation.

“We believe that it is fair to say that the present study opens a new arena of light-wave superconducting electronics via terahertz quantum control for many years to come,”

the researchers conclude.

Original Study: Lightwave-driven gapless superconductivity and forbidden quantum beats by terahertz symmetry breaking

Image: Denisse Leon/Unsplash

Hybrid Nano-probe Can Detect Live Cancer Cells

A new hybrid nano-probe that could lead to noninvasive detection and treatment of cancer at the level of a single cell has been developed by a University of Southern California scientist.

Fabien Pinaud, assistant professor of biological sciences, chemistry and physics and astronomy at USC Dornsife, created a method for amplifying a biochemical signal on the surface of cancer cells.

The new technique binds and assembles gold nanoparticles in living cells using two fragments of a fluorescent protein as “molecular glue.” These tiny probes act as amplifiers, enhancing researchers’ ability to detect distinct biomarkers — things like overexpressed or mutated proteins — found in cancer cells.

The boosted signal allows the scientists to distinguish cancer cells from healthy cells through the use of Raman spectroscopy — a specialized laser imaging technique.

Molecular Glue Assemblies

Using “molecular glue” assemblies to design novel nano-probes is common practice in biomedical research today, but most scientists build these with DNA rather than protein. While promising optical probes are being generated using DNA assemblies in test tubes, DNA is not a practical adhesive in live cells.

Proteins are often better.

“Our approach takes advantage of the fact that we have two different nanoparticles which, on their own, are not active, but which become active when they assemble on cancer cells,”

said Pinaud, principle investigator of the Single Molecule Biophotonics Group and co-author of a related study.

Pinaud and his team start with a fluorescent protein, one that glows when ultraviolet-blue light shines on it. The fluorescent protein is split into two fragments and each piece is attached to a set of gold nanoparticles.

hybrid nanoprobe
Credit: Fabien Pinaud

Both sets of nanoparticles zero-in on cells and bind specifically to biomarkers at the cell surface. As the nanoparticles collide on a cancer cell, the protein fragments naturally reassemble into the whole fluorescent protein.

Restructuring Advantages

The restructuring process provides two advantages. First, the activation of a new biochemical signal in the fluorescent protein is massively amplified by the nanoparticles, which allows for detection by Raman imaging.

Second, heat and ultrasounds are produced when the laser hits the nanoparticles, and that can be measured with ultrasound detectors. This dual effect provides high confidence that a detected cell is actually cancerous and not a false-positive signal from a healthy cell.

Photoacoustic imaging of in situ assembled split-FP AuNP clusters on cells.
Photoacoustic imaging of in situ assembled split-FP AuNP clusters on cells.
Credit: Tuğba Köker, et al. CC-BY

Scientists will next explore the possibility of destroying individual cancer cells, while leaving healthy cells unharmed, by using the laser to heat up the nanoparticles.

“Going from imaging to killing cells is just about turning the knob on the laser that you use,”

Pinaud said.

The work was supported by the National Science Foundation, Division of Material Research, the Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Research Chair, and the Ministère du Québec de l’Économie, Science et Innovation.

Original Paper: Cellular imaging by targeted assembly of hot-spot SERS and photoacoustic nanoprobes using split-fluorescent protein scaffolds

Top Image: Matthew Savino

Photonic Crystals Self-Assembled From Colloidal Particles

Scientists have worked for decades to get colloidal spheres to arrange themselves in sparser lattices, which would unleash potentially valuable optical properties. The structures, called photonic crystals, could increase the efficiency of lasers, make optical components even smaller, and increase engineers’ ability to control the flow of light.

Now, New York University researchers report a pathway toward the self-assembly of these elusive photonic crystal structures never assembled before on the sub-micrometer scale.

Their research introduces a new design principle based on preassembled components of the desired superstructure, much as a prefabricated house begins as a collection of pre-built sections.

The researchers report they were able to assemble the colloidal spheres into diamond and pyrochlore crystal structures — a particularly difficult challenge because so much space is left unoccupied.

Diamond Lattice

The team took inspiration from a metal alloy of magnesium and copper that occurs naturally in diamond and pyrochlore structures as sub-lattices. They saw that these complex structures could be decomposed into single spheres and tetrahedral clusters (four spheres permanently bound).

superlattice
The superlattice is made up of two interpenetrating sublattices, one diamond, shown in green, and the other pyrochlore, shown in red. Preformed red tetrahedral clusters and green spheres self-assemble into a MgCu2 superlattice. (Credit: David Pine, Etienne Ducrot, Gi-Ra Yi)

To realize this in the lab, they prepared sub-micron plastic colloidal clusters and spheres, and employed DNA segments bound to their surface to direct the self-assembly into the desired superstructure.

“We are able to build those complex structures because we are not starting with single spheres as building blocks, but with pre-assembled parts already ‘glued’ together. We fill the structural voids of the diamond lattice with an interpenetrated structure, the pyrochlore, that happens to be as valuable as the diamond lattice for future photonic applications,”

says Etienne Ducrot, a postdoctoral researcher at the New York University Center for Soft Matter Research.

Open Colloidal Crystals

Ducrot says open colloidal crystals, such as those with diamond and pyrochlore configurations, are desirable because, when composed of the right material, they may possess photonic band gaps—ranges of light frequency that cannot propagate through the structure—meaning that they could be for light what semiconductors are for electrons.

“This story has been a long time in the making as those material properties have been predicted 26 years ago but until now, there was no practical pathway to build them,” he says. “To achieve a band gap in the visible part of the electromagnetic spectrum, the particles need to be on the order of 150 nanometers, which is in the colloidal range. In such a material, light should travel with no dissipation along a defect, making possible the construction of chips based on light.”

Coauthor David J. Pine, chair of the chemical and biomolecular engineering department at NYU Tandon School of Engineering, adds that self-assembly technology is critical to making production of these crystals economically feasible because creating bulk quantities of crystals with lithography techniques at the correct scale would be extremely costly and very challenging.

Original Study: Colloidal alloys with preassembled clusters and spheres

Top Image: Leon Hupperichs CC BY-SA 3.0

Time Scale Stretching For Improving Prediction Of Extreme Events

A technique currently applied in the field of photonics could help predict rogue wave events on the ocean surface, along with other extreme natural phenomena.

Stretching time scales to explore extreme events in nature is possible, according to a team from the Institut FEMTO-ST, which used an innovative measurement technique enabling the capture of such events in real time. This research was conducted in collaboration with teams from Finland, Ireland, and Canada,

Instability and chaos in physical systems are random natural phenomena that are generally highly sensitive to fluctuations in initial conditions, however small they may be.

To understand these complex and omnipresent phenomena in nature, researchers recently conducted experiments involving the propagation of light waves, and leading to the formation of ultrafast pulses on a picosecond timescale (a millionth of a millionth of a second).

Overcoming Limitations

The study of such phenomena in optics offers the advantage of taking place on very short timescales, thus making it possible to measure a representative sample of events and to reliably characterize its statistical properties.

Although they have helped improve the understanding of the dynamics connected to extreme events, until now these studies have nevertheless been conducted indirectly, due to the response time of current detectors, which are too slow to capture these rare events.

Recent experiments carried out at the Institut Femto-ST in Besançon have made it possible to overcome this limitation.

Based on the principle of a time lens, which stretches the timescale by a factor of 100 while increasing resolution, this new method has enabled researchers to observe giant light pulses in real time, with an intensity 1,000 times greater than that of the initial fluctuations from the light source, a laser.

To do so, they used a butterfly effect known in optics as modulation instability, which magnifies the microscopic noise intrinsically present in a laser beam traveling along telecommunication fiber optics.

The scope of these results goes well beyond the field of photonics, since this type of background noise is generally considered to be one of the possible mechanisms behind the destructive rogue waves that suddenly appear on the surface of oceans, and is also believed to be present in other systems such as plasma dynamics in the early Universe.

The ability to stretch timescales in optics opens a new path for the exploration and understanding of numerous natural systems for which it remains quite difficult to directly study instabilities, and thereby obtain reliable statistical samples.

Study: Real-time measurements of spontaneous breathers and rogue wave events in optical fibre modulation instability

Image: John Dudley, Institut Femto-ST/CNRS