Category: Physics

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

Testing Hyper-complex Quantum Theories With An Exotic Metamaterial

Physicists have looked for deviations from standard quantum mechanics, testing whether quantum mechanics requires a more complex set of mathematical rules.

A research team led by Philip Walther at the University of Vienna designed a photonic experiment using exotic metamaterials, which were fabricated at the University of California Berkeley. Their experiment supports standard quantum mechanics and allows the scientists to place bounds on alternative quantum theories.

The results could help to guide theoretical work in a search for a more general version of quantum mechanics.

Quantum mechanics is based on a set of mathematical rules, describing how the quantum world works. These rules predict, for example, how electrons orbit a nucleus in an atom, and how an atom can absorb photons, particles of light.

The standard rules of quantum mechanics work extremely well, but, given that there are still open questions regarding the interpretation of quantum mechanics, scientists are not sure whether the current rules are the final story.

This has motivated some scientists to develop alternative versions of the mathematical rules, which are able to properly explain the results of past experiments, but provide new insight into the underlying structure of quantum mechanics. Some of these alternative mathematical rules even predict new effects, which require new experimental tests.

Mathematical Rules In Daily Life

In everyday life, if we walk all the way around a park we end up back at the same place regardless of whether we choose to walk clockwise or counter-clockwise. Physicists would say that these two actions commute.

Not every action needs to commute, though.

If, on our walk around the park, we walk clockwise, and first find money lying on the ground and then encounter an ice cream man, we will exit the park feeling refreshed. However, if we instead travel counter-clockwise, we will see the ice cream man before finding the money needed to buy the ice cream. In that case, we may exit park feeling disappointed.

In order to determine which actions commute or do not commute physicists provide a mathematical description of the physical world.

In standard quantum mechanics, these mathematical rules use complex numbers. However, recently an alternative version of quantum mechanics was proposed which uses more complex, so-called “hyper-complex” numbers. These are a generalization of complex numbers.

With the new rules, physicists can replicate most of the predictions of standard quantum mechanics. However, hyper-complex rules predict that some operations that commute in standard quantum mechanics do not actually commute in the real world.

Hyper-complex Numbers

The research team has now tested for deviations from standard quantum mechanics predicted by the alternative hyper-complex quantum theory. In their experiment the scientists replaced the park with an interferometer, a device which allows a single photon to travel two paths at the same time.

They replaced the money and ice cream with a normal optical material and a specially designed metamaterial. The normal optical material slightly slowed down light as it passed through, whereas the metamaterial slightly sped the light up.

The rules of standard quantum mechanics dictate that light behaves the same no matter whether it first passes through a normal material and then through a metamaterial or vice versa. In other words, the action of the two materials on the light commutes.

In hyper-complex quantum mechanics, however, that might not be the case. From the behavior of the measured photons the physicists verified that hyper-complex rules were not needed to describe the experiment.

“We were able to place very precise bounds on the need for hyper-complex numbers to describe our experiment,” says Lorenzo Procopio, a lead author of the study.

However, the authors say that it is always very difficult to unambiguously rule something out. Lee Rozema, another author of the paper, says

“we still are very interested in performing experiments under different conditions and with even higher precision, to gather more evidence supporting standard quantum mechanics.”

This work has placed tight limits on the need for a hyper-complex quantum theory, but there are many other alternatives which need to be tested, and the newly-developed tools provide the perfect avenue for this.

Funding for the work was provided by the European Commission, the Austrian Science Fund, the Vienna Science and Technology Fund, and the United States Air Force Office of Scientific Research.

Original Paper: Single-photon test of hyper-complex quantum theories using a metamaterial
Image: MSc. Jonas Schmöle, Faculty of Physics, University of Vienna

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

Diamond Nitrogen-vacancy Centers Power World’s Smallest Radio Receiver

A tiny radio whose building blocks are the size of two atoms has been developed by Researchers from the Harvard John A. Paulson School of Engineering and Applied Sciences.

Built out of an assembly of atomic-scale defects in pink diamonds, it is the world’s smallest radio receiver. The device can withstand extremely harsh environments and is biocompatible, meaning it could work anywhere from a probe on Venus to a pacemaker in a human heart.

The radio takes advantage of miniscule imperfections in diamonds called nitrogen-vacancy (NV) centers. To make NV centers, researchers replace one carbon atom in a diamond crystal with a nitrogen atom and remove a neighboring atom — creating a system that is essentially a nitrogen atom with a hole next to it.

Nitrogen-vacancy centers can be used to emit single photons or detect very weak magnetic fields. They have photoluminescent properties, meaning they can convert information into light, making them powerful and promising systems for quantum computing, phontonics and sensing.

Nitrogen-vacancy Centers

Virtually all radios consist of five basic components: a power source, a receiver, a transducer to convert the high-frequency electromagnetic signal in the air to a low-frequency current, speaker or headphones to convert the current to sound and a tuner.

For the new device, electrons in diamond NV centers are powered, or pumped, by green light emitted from a laser. These electrons are sensitive to electromagnetic fields, including the waves used in FM radio, for example.

When a nitrogen-vacancy center receives radio waves it converts them and emits the audio signal as red light. A common photodiode converts that light into a current, which is then converted to sound through a simple speaker or headphone.

Extremely Resilient

An electromagnet creates a strong magnetic field around the diamond, which can be used to change the radio station, tuning the receiving frequency of the NV centers.

Research leader Marko Loncar and his graduate student Linbo Shao used billions of NV centers in order to boost the signal, but the radio works with a single NV center, emitting one photon at a time, rather than a stream of light.

The radio is extremely resilient, thanks to the inherent strength of diamond. The team successfully played music at 350 degrees Celsius—about 660 Fahrenheit.

“Diamonds have these unique properties,” said Loncar. “This radio would be able to operate in space, in harsh environments and even the human body, as diamonds are biocompatible.”

Image: Eliza Grinnell/Harvard SEAS

Original Study: Diamond Radio Receiver: Nitrogen-Vacancy Centers as Fluorescent Transducers of Microwave Signals