Hexbyte Glen Cove Researchers develop a new way to control and measure energy levels in a diamond crystal

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Caption:Instrumentation setup in the Quantum Engineering Group at MIT to study dynamical symmetries with qubits in diamond crystals Credit: Guoqing Wang/MIT

Physicists and engineers have long been interested in creating new forms of matter, those not typically found in nature. Such materials might find use someday in, for example, novel computer chips. Beyond applications, they also reveal elusive insights about the fundamental workings of the universe. Recent work at MIT both created and characterized new quantum systems demonstrating dynamical symmetry—particular kinds of behavior that repeat periodically, like a shape folded and reflected through time.

“There are two problems we needed to solve,” says Changhao Li, a graduate student in the lab of Paola Cappellaro, a professor of nuclear science and engineering. Li published the work recently in Physical Review Letters, together with Cappellaro and fellow graduate student Guoqing Wang. “The first problem was that we needed to engineer such a system. And second, how do we characterize it? How do we observe this symmetry?”

Concretely, the quantum system consisted of a diamond crystal about a millimeter across. The crystal contains many imperfections caused by a next to a gap in the lattice—a so-called nitrogen-vacancy center. Just like an electron, each center has a quantum property called a spin, with two discrete . Because the system is a quantum system, the spins can be found not only in one of the levels, but also in a combination of both energy levels, like Schrodinger’s theoretical cat, which can be both alive and dead at the same time.

The energy level of the system is defined by its Hamiltonian, whose periodic time dependence the researchers engineered via microwave control. The system was said to have dynamical symmetry if its Hamiltonian was the same not only after every time period t but also after, for example, every t/2 or t/3, like folding a piece of paper in half or in thirds so that no part sticks out. Georg Engelhardt, a postdoc at the Beijing Computational Science Research, who was not involved in this work but whose own theoretical work served as a foundation, likens the symmetry to guitar harmonics, in which a string might vibrate at both 100 hertz and 50 Hz.

To induce and observe such dynamical symmetry, the MIT team first initialized the system using a laser pulse. Then they directed various selected frequencies of microwave radiation at it and let it evolve, allowing it to absorb and emit the energy. “What’s amazing is that when you add such driving, it can exhibit some very fancy phenomena,” Li says. “It will have some periodic shake.” Finally, they shot another laser pulse at it and measured the visible light that it fluoresced, in order to measure its state. The measurement was only a snapshot, so they repeated the experiment many times to piece together a kind of flip book that characterized its behavior across time.

Dynamical symmetries, which play an essential role in physics, are engineered and characterized by a cutting-edge quantum information processing toolkit. Credit: Image courtesy of the researchers.

“What is very impressive is that they can show that they have this incredible control over the ,” Engelhardt says. “It’s quite easy to solve the equation, but realizing this in an experiment is quite difficult.”

Critically, the researchers observed that the dynamically symmetry of the Hamiltonian—the harmonics of the system’s energy level—dictated which transitions could occur between one state and another. “And the novelty of this work,” Wang says, “is also that we introduce a tool that can be used to characterize any quantum information platform, not just nitrogen-vacancy centers in diamonds. It’s broadly applicable.” Li notes that their technique is simpler than previous methods, those that require constant laser pulses to drive and measure the system’s periodic movement.

One engineering application is in quantum computers, systems that manipulate qubits, bits that can be not only 0 or 1, but a combination of 0 and 1. A diamond’s spin can encode one qubit in its two energy levels.

Qubits are delicate: they easily break down into simple bit, a 1 or a 0. Or the qubit might become the wrong combination of 0 and 1. “These tools for measuring dynamical symmetries,” Engelhardt says, “can be used to as a sanity check that your experiment is tuned correctly—and with a very high precision.” He notes the problem of outside perturbations in quantum computers, which he likens to a de-tuned guitar. By tuning the tension of the strings—adjusting the microwave radiation—such that the harmonics match some theoretical symmetry requirements, one can be sure that the experiment is perfectly calibrated.

The MIT team already has their sights set on extensions to this work. “The next step is to apply our method to more complex systems and study more interesting physics,” Li says. They aim for more than two energy levels—three, or 10, or more. With more energy levels they can represent more qubits. “When you have more qubits, you have more complex symmetries,” Li says. “And you can characterize them using our method here.”

More information:
Guoqing Wang et al, Observation of Symmetry-Protected Selection Rules in Periodically Driven Quantum Systems, Physical Review Letters (2021). DOI: 10.1103/PhysRevLett.127.140604

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

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Hexbyte Glen Cove Higher levels of organic pollutants found in homes located near natural gas wells, study finds thumbnail

Hexbyte Glen Cove Higher levels of organic pollutants found in homes located near natural gas wells, study finds

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Credit: Pixabay/CC0 Public Domain

A University of Toronto study has found that those living close to natural gas wells are exposed to higher levels of certain organic pollutants in their homes.

The study looked at levels of volatile organic compounds (VOCs) found in the air and drinking water in homes of pregnant women living in a region of northeastern British Columbia.

“There’s very little research about indoor air quality in regions with a lot of unconventional natural gas exploitation,” says Élyse Caron-Beaudoin, an assistant professor in the department of health and society at U of T Scarborough and lead author of the study.

For the study, 85 pregnant women from the Peace River region were recruited and passive air samplers were placed in their homes. Water samples were also taken from their kitchen taps. Researchers found that 40 out of the 47 VOCs tested for were detected in air samples, while three out of 40 VOCs tested for were detected in water samples.

VOCs are organic chemicals, some of which have negative short- and long-term health effects. They are released by a variety of products and industrial processes.

The researchers also looked at how many natural gas wells were located near homes as well as the distances. They found that the amount and proximity of natural gas wells to a home were linked to higher levels of certain VOCs. They also accounted for other factors related to exposure levels, including whether a home had an attached garage, the tap water source and whether the study participant smokes or is exposed to second-hand smoke.

They also included each participant’s Indigenous status. A previous pilot study done in the same region of B.C. by Caron-Beaudoin found higher levels of VOC metabolites in the urine samples of pregnant Indigenous women compared to pregnant non-Indigenous women.

In the current study, the levels of VOCs associated with the amount and proximity of natural gas wells were similarly higher in the homes of Indigenous participants. While the researchers are unsure why higher levels were found in the homes of Indigenous participants, they point to research that shows ethnicity, Indigeneity and socioeconomic status all being linked to heightened health risks from industrial activities.

The study, published in the journal Science of the Total Environment, also compared levels to the Canadian average. For a few of the VOCs—in particular acetone and in air samples, and trihalomethanes (THMs) in water samples—some participants recorded levels that placed them in the top 95th percentile in Canada. In other words, they had the highest exposure levels compared to the general Canadian population.

THMs in particular stood out. More than 60 percent of study participants were found to be above the 95th percentile of exposure levels compared to the Canadian average.

“These levels are really high,” says Caron-Beaudoin. “For some of the participants, it was even over the guidelines for safe drinking water—so we had to contact them to let them know.”

She adds that and chloroform are used as solvents in fracking fluid, while THMs occur when used to disinfect water reacts with natural organic matter. THM levels tend to be higher in areas close to natural gas exploitation because greater amounts of wastewater are generated during the extraction process, she said.

Pregnant women were recruited for the study because of the potential negative birth outcomes linked to living close to natural gas operations. Caron-Beaudoin points to research finding higher rates of pre-term births, low birth weight and heart malformations, among others. There’s also a link to higher cancer rates in children and increased levels of chronic respiratory disease in adults—such as asthma and chronic obstructive pulmonary disease—and cancer in adults.

Caron-Beaudoin leads the only research group that is actively looking at the potential health impacts linked to natural gas exploitation in Canada. As one of the largest global producers of natural gas, she says more research needs to be done in Canada on its potential health effects.

The area of northeastern British Columbia where the research participants are located will also be home to a massive new gas plant that could increase the number of wells in the area to more than 100,000.

“This is happening with very little data on exposure levels—including air and water quality,” Caron-Beaudoin says. “There’s currently no monitoring program, and as a result, no way to check the health status of people living near these wells.”

More information:
Élyse Caron-Beaudoin et al, Volatile organic compounds (VOCs) in indoor air and tap water samples in residences of pregnant women living in an area of unconventional natural gas operations: Findings from the EXPERIVA study, Science of The Total Environment (2021). DOI: 10.1016/j.scitotenv.2021.150242

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Hexbyte Glen Cove Trophic levels are an 'insufficient' measure of sustainability for today's aquaculture policy thumbnail

Hexbyte Glen Cove Trophic levels are an ‘insufficient’ measure of sustainability for today’s aquaculture policy

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Credit: CC0 Public Domain

Born in food web ecology, the concept of trophic levels—the hierarchy of who eats who in the natural world—is an elegant way to understand how biomass and energy move through a natural system. It’s only natural that the idea found its way into the realm of aquaculture, where marine and freshwater farmers try to maximize their product with efficient inputs.

“It’s often used as a measure of how sustainable it is to harvest or consume that species,” said Rich Cottrell, a postdoctoral researcher at UC Santa Barbara’s National Center for Ecological Analysis & Synthesis (NCEAS). As plants (level 1) become food to plant eaters (level 2), who in turn are consumed by carnivores (level 3) and so on, the amount of energy required to support the same weight of organisms increases, he explained. As a result, species at levels 4 or 5, such as tuna, require far more energy per pound than would species in the lower trophic levels. It’s the same reason are often considered to be more sustainable than meat-eating ones.

“In the same manner, trophic level measures are now being recommended in policy settings for use as an indicator of the of fish farming, or ,” Cottrell said. The lower the trophic level, the more sustainable the species is considered to be, and so policy often calls for more farming of low-trophic species.

However, argue Cottrell and fellow aquaculture experts in a paper published in the journal Reviews in Aquaculture, at this time of increasing complexity and sophistication in the aquaculture sector, the same simplicity that makes trophic levels attractive as a measure of sustainability also makes them insufficient.

The causes for that have largely to do with how today’s farmed fish are fed.

“Most of the fish and invertebrates that we farm for food are produced using human-made feeds,” Cottrell explained. “But these feeds are constantly changing, and so the meaning of farmed trophic levels is changing through time.” For instance, he pointed out, salmon are considered to be at a higher trophic level because their naturally carnivorous diets would require large amounts of fishmeal and oil, but advances in feed and manufacturing have reduced the proportion of fish-based ingredients to 10-15% in modern salmon diets. Meanwhile, herbivorous species such as carp and tilapia have been found to respond favorably to small amounts of fishmeal in their feed.

“In reality, they’re now farmed at similar trophic levels,” Cottrell said. “The line between ‘low’ and ‘high’ trophic levels will continue to blur with innovation.”

The trophic level concept misses still another important aspect of aquaculture sustainability in the realm of feed and resource efficiency, or how efficiently the farmed animals convert what they are fed into edible food.

“This is not well explained by trophic level,” Cottrell said, adding that despite their high trophic placement, many carnivorous farmed fish could be more feed-efficient than their naturally carnivorous counterparts. And because aquaculture is increasingly turning to agriculture to provide replacements for fishmeal and oil, the promise of sustainability might be an empty one.

“Replacing fish-based ingredients with crops has led to a dramatic reduction in the trophic level of fed aquaculture species, but we know very little about how sustainable it is to increase pressure on global agricultural systems,” he said.

As the global aquaculture sector strives to meet the growing demand for farmed seafood, the researchers say it’s time to rethink the use of as a rule for and measure of sustainability. Stipulating low trophic level aquaculture recommendations may not be successful in promoting greater sustainability, Cottrell said. Boosting the supply of mussels, for instance, may not fulfill increasing demand for shrimp or salmon.

“It behooves us to find a way to ensure that for high-demand products, we produce these in the most environmentally efficient and socially responsible way possible,” he said. “Trophic levels will not get us there.”

Fortunately, there are efforts at more nuanced sustainability assessments, such as voluntary certifications through the Aquaculture Stewardship Council or Best Aquaculture Practices, which examine the impacts of aquaculture at the farm level and through supply chains.

“Greater support for these programs and incentives for producers from various regions and production systems to join them would be a far more robust way to strengthen the sustainability of the aquaculture sector going forward,” Cottrell said.

More information:
Richard S. Cottrell et al, Time to rethink trophic levels in aquaculture policy, Reviews in Aquaculture (2021). DOI: 10.1111/raq.12535


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