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One of the most important tools in the theoretical physicists toolkit is the thought experiment. If you study relativity, quantum mechanics, or any area of physics applying to environments or situations in which you cannot (or should not) place yourself, youll find that you spend a lot more time working through imaginary scenarios than setting up instruments or taking measurements.

Unlike physical experiments, thought experiments are not about collecting data, but rather about posing an imaginary question and working through an if/then logical sequence to explore what the theory really means.

Asking what has to happen if the theory is true? is invaluable for developing intuition and anticipating new applications. In some cases, a thought experiment can reveal the deep philosophical implications of a theory, or even present what appears to be an unsolvable paradox.

Probably the most famous of all physics thought experiments is that of Schrdingers Cat both because it involves (purely hypothetical!) carnage, and because its implications for the nature of reality in a quantum world continue to challenge students and theorists everywhere.

The basic again, purely hypothetical experimental setup is this. Imagine you have a radioactive material in which there is a 50 per cent chance of a nuclear decay in some specified amount of time (lets say, one hour).

You put this material in a box along with a small glass vial of poison and a device that will break the vial if a radioactive decay is detected. Then, you put a live cat in the box, close the lid, wait an hour, and then open the box once again.

Based on this setup, its straightforward to deduce that since the chance the atom decays and triggers the poison is 50 per cent, half the time you do the experiment, you should find a living cat, and half the time, you should find a dead one, assuming youre not re-using the same cat each time.

But when Erwin Schrdinger described the thought experiment to Albert Einstein in 1935, he did so to highlight an apparent consequence of quantum theory that seemed to both scientists to be complete nonsense: the idea that before you open the box, the cat is both alive and dead at the same time.

Ultimately, it comes down to the principle of uncertainty in quantum mechanics. Unlike classical mechanics (the kind of physics that applies to our everyday experiences), in quantum mechanics, there seems to be a fundamental uncertainty built into the nature of reality.

When you flip a coin (a classical event), its only random because youre not keeping careful enough track of all the motions and forces involved. If you could measure absolutely everything, you could predict the outcome every time its deterministic.

But in the quantum mechanical version of a coin flip, the radioactive decay, nothing you measure can possibly tell you the outcome before it occurs. As far as an outside observer is concerned, until the measurement of the quantum coin flip occurs, the system will act like its in both states at once: the atom is both decayed and not decayed, in what we call a superposition.

Superposition is a real phenomenon in quantum mechanics, and sometimes we can even use it to our advantage. Quantum computing is built on the idea that a quantum computer bit (or qubit), instead of being just one or zero, can be in a superposition of one and zero, massively increasing the computers ability to do many complex calculations at once.

In the case of Schrdingers Cat, the apparently absurd conclusion that the cat is both alive and dead comes from considering the whole apparatus the atom, the trigger device, and the poison vial, and the cat to be a single quantum system, each element of which exists in a superposition.

The atom is decayed and not, the device is triggered and dormant, the vial is broken and intact, and the cat is therefore simultaneously dead and alive, until the moment the box is opened.

Whether this conclusion is actually absurd is an open question. What both Schrdinger and Einstein concluded was that true, fundamental uncertainty simply cannot apply to the real, macroscopic, world. These days, most physicists accept that uncertainty is real, at least for subatomic particles, but how that uncertainty 'collapses' when a measurement is made remains up for debate.

In one interpretation, any measurement thats performed fundamentally alters reality though it is usually argued that the trigger device, or, at least, the cat itself, provides a measurement for that purpose. In another interpretation, called Many Worlds, the entire Universe duplicates itself every time a quantum coin is flipped, and the measurement simply tells you whether youre in the dead-cat or alive-cat universe from now on.

While we cant say how long it will take before we fully understand whats really going on in the black box of quantum superposition, applications of quantum theory are already bringing us incredible technological advances, like quantum computers. And in the meantime, clever thought experiments allow us to follow our curiosity, without running the risk of killing any cats.

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A newly discovered phenomenon dubbed "collectively induced transparency" (CIT) causes groups of atoms to abruptly stop reflecting light at specific frequencies.

CIT was discovered by confining ytterbium atoms inside an optical cavityessentially, a tiny box for lightand blasting them with a laser. Although the laser's light will bounce off the atoms up to a point, as the frequency of the light is adjusted, a transparency window appears in which the light simply passes through the cavity unimpeded.

"We never knew this transparency window existed," says Caltech's Andrei Faraon (BS '04), William L. Valentine Professor of Applied Physics and Electrical Engineering, and co-corresponding author of a paper on the discovery that was published on April 26 in the journal Nature. "Our research has primarily become a journey to find out why."

An analysis of the transparency window points to it being the result of interactions in the cavity between groups of atoms and light. This phenomenon is akin to destructive interference, in which waves from two or more sources can cancel one another out. The groups of atoms continually absorb and re-emit light, which generally results in the reflection of the laser's light. However, at the CIT frequency, there is a balance created by the re-emitted light from each of the atoms in a group, resulting in a drop in reflection.

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"An ensemble of atoms strongly coupled to the same optical field can lead to unexpected results," says co-lead author Mi Lei, a graduate student at Caltech.

The optical resonator, which measures just 20 microns in length and includes features smaller than 1 micron, was fabricated at the Kavli Nanoscience Institute at Caltech.

"Through conventional quantum optics measurement techniques, we found that our system had reached an unexplored regime, revealing new physics," says graduate student Rikuto Fukumori, co-lead author of the paper.

Besides the transparency phenomenon, the researchers also observed that the collection of atoms can absorb and emit light from the laser either much faster or much slower compared to a single atom depending on the intensity of the laser. These processes, called superradiance and subradiance, and their underlying physics are still poorly understood because of the large number of interacting quantum particles.

"We were able to monitor and control quantum mechanical lightmatter interactions at nanoscale," says co-corresponding author Joonhee Choi, a former postdoctoral scholar at Caltech who is now an assistant professor at Stanford University.

Though the research is primarily fundamental and expands our understanding of the mysterious world of quantum effects, this discovery has the potential to one day help pave the way to more efficient quantum memories in which information is stored in an ensemble of strongly coupled atoms. Faraon has also worked on creating quantum storage by manipulating the interactions of multiple vanadium atoms.

"Besides memories, these experimental systems provide important insight about developing future connections between quantum computers," says Manuel Endres, professor of physics and Rosenberg Scholar, who is a co-author of the study.

Reference:Lei M, Fukumori R, Rochman J, et al. Many-body cavity quantum electrodynamics with driven inhomogeneous emitters. Nature. Published online April 26, 2023:1-6. doi:10.1038/s41586-023-05884-1

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Scientists at Forschungszentrum Jlich have fabricated a new type of transistor from a germaniumtin alloy that has several advantages over conventional switching elements. Charge carriers can move faster in the material than in silicon or germanium, which enables lower voltages in operation. The transistor thus appears to be a promising candidate for future low-power, high-performance chips, and possibly also for the development of future of quantum computers.

Over the past 70 years, the number of transistors on a chip has doubled approximately every two years according to Moores Law, which is still valid today. The circuits have become correspondingly smaller, but an end to this development appears to be in sight. We have now reached a stage where structures are only 2 to 3 nanometers in size. This is approximately equal to the diameter of 10 atoms, which takes us to the limits of what is feasible. It doesnt get much smaller than this, says Qing-Tai Zhao of the Peter Grnberg Institute (PGI-9) at Forschungszentrum Jlich.

For some time now, researchers have been looking for a substitute for silicon, the primary material used in the semiconductor industry. The idea is to find a material that has more favourable electronic properties and can be used to achieve the same performance with larger structures, the professor explains.

The research is in part focused on germanium, which was already being used in the early days of the computer era. Electrons can move much faster in germanium than in silicon, at least in theory. However, Qing-Tai Zhao and his colleagues have now gone one step further. To optimize the electronic properties even further, they incorporated tin atoms into the germanium crystal lattice. The method was developed several years ago at the Peter Grnberg Institute (PGI-9) of Forschungszentrum Jlich.

The germaniumtin system we have been testing makes it possible to overcome the physical limitations of silicon technology, says Qing-Tai Zhao. In experiments, the germaniumtin transistor exhibits an electron mobility that is 2.5 times higher than a comparable transistor made of pure germanium.

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Another advantage of the new material alloy is that it is compatible with the existing CMOS process for chip fabrication. Germanium and tin come from the same main group in the periodic table as silicon. The germanium-tin transistors could therefore be integrated directly into conventional silicon chips with existing production lines.

Apart from classical digital computers, quantum computers could also benefit from the germaniumtin transistor. For some time, there have been efforts to integrate parts of the control electronics directly on the quantum chip, which is operated inside a quantum computer at temperatures close to absolute zero. Measurements suggest that a transistor made of germanium-tin will perform significantly better under these conditions than those made of silicon.

The challenge is to find a semiconductor whose switching can still be very fast with low voltages at very low temperatures, explains Qing-Tai Zhao. For silicon, this switching curve flattens out below 50 Kelvin. Then, the transistors need a high voltage and thus a high power, which ultimately leads to failures of the sensitive quantum bits because of the heating. Germaniumtin performs better at these temperatures in measurements down to 12 Kelvin, and there are hopes to use the material at even lower temperatures, says Qing-Tai Zhao.

In addition, the germaniumtin transistor is a further step towards optical on-chip data transmission. The transmission of information with light signals is already standard in many data networks because it is considerably faster and more energy-efficient than data transfer via electrical conductors. In the field of micro- and nanoelectronics, however, data is usually still sent electrically. Colleagues from the Jlich working group of Dr. Dan Buca have already developed a germanium-tin laser in the past that opens up the possibility to transmit data optically directly on a silicon chip. The germanium-tin transistor, along these lasers, provides a promising solution for the monolithic integration of nanoelectronics and photonics on a single chip.

Reference:Liu M, Junk Y, Han Y, et al. Vertical GeSn nanowire MOSFETs for CMOS beyond silicon. Commun Eng. 2023;2(1):1-9. doi:10.1038/s44172-023-00059-2

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Eight Yale seniors and a recent graduate have been awarded fellowships for graduate study at the universities of Oxford or Cambridge in the United Kingdom.

These fellowship recipients are in addition to the students previously announced in Yale News who have won Rhodes and Marshall Scholarships.

The fellowship winners and their awards are:

Danielle Castro has received a Paul Mellon Fellowship to pursue an M.Phil. in population health sciences at the University of Cambridge. Next month, she will graduate from Yale with a certificate in global health and a joint B.S./M.S. degree in molecular biochemistry. Her thesis is on the development of novel drug candidates for chordoma spine cancers in the laboratory of Craig Crews, the John C. MaloneProfessor of Molecular, Cellular and Developmental Biology. She has a strong connection to her Peruvian and Indigenous heritage, and is passionate about social justice and reducing health inequities. She has worked toward this goal while interning in the New Haven Public Schools, serving on the board of the HAVEN Free Clinic, and conducting public health research in Connecticut and the Peruvian Amazon. She enjoys meeting and mentoring other first-generation immigrant and low-income students, especially in her role as co-president of Latina Women at Yale.

Aidan Evans has been awarded a Huawei Hisilicon Scholarship to earn a Ph.D. in computer science at the University of Cambridge. He is majoring in computer science and philosophy at Yale and additionally is completing a B.S./M.S. in computer science. During his time at Yale he published research on quantum computing at the premier conference on software engineering. He has also served as a teaching assistant for seven courses, ranging from those on systems programming and computer organization to graduate courses on the interplay of computer science with law. Most recently he has taken on the project of writing a book on the history of Yales computer science department. At Cambridge, he will study the logic and the foundations of computer science under the supervision of Professor Anuj Dawar.

Beasie Goddu was awarded a Paul Mellon Fellowship for graduate study at the University of Cambridge, where she will pursue an M.Phil. in English literature. She will examine the portrayal of womens rights in early 20th-century British fiction. She is majoring in English at Yale with a concentration in creative writing. Her academic thesis explored womens agency over physical space in the works of Virginia Woolf and E.M. Forster. Her creative writing thesis is a collection of essays about vision. She serves as a writing partner at Yales Poorvu Center for Teaching and Learning, is a senior editor of The New Journal, a student-run magazine that features creative nonfiction, and is an undergraduate editorial fellow at The Yale Review. She is also president of St. Anthony Hall, an arts and literary society. She aspires to a career in editing, highlighting marginalized female voices.

Tyler Jager was awarded a Kings-Yale Fellowship to pursue an M.Phil. in political thought and intellectual history at the University of Cambridge. He will focus on early 20th-century history and efforts to restrict migration and the freedom of movement, particularly in the British Empire. He will graduate from Yale with a joint B.A./M.A. degree in political science and a certificate in human rights. He was the 2022 winner of the Elie Wiesel Prize in Ethics for an essay he wrote on aid workers in the Mediterranean. He has also written about that topic, tenant organization, and lead poisoning in New Haven for a number of campus and national publications, and currently serves as co-editor of BRINK, Yales undergraduate book review. His senior thesis, an ethnographic study in Greece, explored how aid workers presence in host communities affects anti-refugee prejudice in European Union external border zones. Jager is a tour guide at the Yale University Art Gallery and was the coordinator of the Yale Hunger and Homelessness Action Project Fast, the universitys largest student fundraiser. He has interned at the U.S. Holocaust Memorial Museum and at the journal Foreign Affairs.

Hamzah Jhaveri has received a Keasbey Scholarship to pursue an M.Phil. in social anthropology at the University of Cambridge. At Yale, he majored in anthropology, with a particular interest in the study of moral economics and the corporate form. He has been researching gun culture and commerce in America, and his senior thesis investigates the transformation of the gun-making trade in an early American settlement in Pennsylvania known for its pacifist religious values and socialist economy. Jhaveri served as the editor-in-chief of the Yale Herald, wrote and performed with sketch company groups including the Fifth Humor and Playspace, and has been an organizer with the Yale Endowment Justice Coalition, Sunrise New Haven (the local chapter of a national movement to stop the climate crisis and create millions of new jobs), and New Haven Rising (a community organization dedicated to achieving economic, racial, and social justice through collective action). He has spent summers teaching fifth-graders about climate organizing, interning at a First Amendment law firm, researching petrochemical companies, and harvesting micro greens at an urban hydroponics farm.

Elizabeth Hopkinson was awarded a Paul Mellon Fellowship to pursue an M.Phil. in health, medicine, and society at Clare College, Cambridge. She graduated from Yale in December 2022 with a B.A. in environmental studies. Her senior thesis explored end-of-life care using geographic concepts of place and place-making. At Cambridge, she will continue to study how places affect experiences of aging, dying, and disability. She was a leader of FOOT (First-year Orientation Trips), was a first-year counselor in Jonathan Edwards College, a Yale Daily News editor, and a research assistant at the Yale School of Nursing and in the Human Nature Lab. During the height of the COVID pandemic, she worked as an EMT near her home in Westborough, Massachusetts.

Shaezmina Khan has been awarded the Rotary Global Grant Scholarship to pursue an M.Sc. in global governance and diplomacy from the University of Oxford. She is majoring in global affairs at Yale and will obtain a certificate in human rights from Yale Law School. For her senior capstone, Khan worked for the Afghanistan War Commission and assessed U.S. diplomatic efforts to achieve political settlement in Afghanistan between 2002 and 2021. At Oxford, she hopes to focus her research on regional security dilemmas and conflict mediation in the Afghanistan-Pakistan-India region. She is passionate about American foreign policy, national security, diplomacy, and peacebuilding in the Middle East and North Africa region. She served as a policy trainee at the European Commission in Brussels and as a legislative intern for U.S. Congresswoman Rosa DeLauro in Washington, D.C. She served as the executive director of the Yale International Relations Association and president of the Muslim Students Association, and was a research assistant at both the Yale Law School and Jackson School for Global Affairs.

Ethan Pesikoff received a Henry Fellowship to earn a Master of Advanced Studies (MASt) degree in pure mathematics at the University of Cambridge. At Yale, he is majoring in both mathematics and Near Eastern Languages and Civilizations (NELC). He served on the board of the Yale Undergraduate Math Society, which organizes academic support and social activities for students, and he conducted original mathematical research at Williams College and the University of Minnesota during summer breaks. His senior thesis for NELC seeks to understand previously untranslated Akkadian texts from the early second millennium BCE. After completing his MASt at Cambridge, Pesikoff plans to pursue a Ph.D. in mathematics.

Melissa Wang was awarded a Paul Mellon Fellowship to pursue an M.Phil. in U.S. history at the University of Cambridge, where she will study the consolidation of correctional officer power in late 20th-century America and its effect on mass incarceration policy and prisoners lives. Her research is intended to place correctional officers within a broader history of American law enforcement, militarism, and race. At Yale, she is majoring in history, and ethnicity, race, and migration, and is a scholar in the Multidisciplinary Academic Program in Human Rights. She has served on the board of the Yale Undergraduate Prison Project (YUPP) and Yale Womens Center, and captains the Yale club Wushu team. Her research interests were inspired by work with the Stop Solitary Connecticuts legislative campaign as a project leader at YUPP and as a research assistant at the Yale Law School Lowenstein Clinic. A painter, she is also a volunteer with Justice Arts Coalition, a national network and resource for those creating art in and around the criminal legal system.

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Fellowship winners will continue their studies in England - Yale News

New research from a multidisciplinary team helps to illuminate the mechanisms behind circadian rhythms, offering new hope for dealing with jet lag, insomnia and other sleep disorders.

Using innovative cryo-electron microscopy techniques, the researchers have identified the structure of the circadian rhythm photosensor and its target in fruit flies (Drosophila melanogaster), one of the major organisms used to study circadian rhythms. The research,Cryptochrome-Timeless Structure Reveals Circadian Clock Timing Mechanismspublished April 26 in Nature.

The research focused on fruit fly cryptochromes, key components of the circadian clocks of plants and animals, including humans. In flies and other insects, cryptochromes, activated by blue light, serve as the primary light sensors for setting circadian rhythms. The target of the cryptochrome photosensor, known as Timeless (TIM), is a large, complex protein that could not previously be imaged and thus its interactions with the cryptochrome are not well understood.

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Circadian rhythms work via what are basically genetic feedback loops. The researchers found that the TIM protein, along with its partner, the Period (PER) protein, act together to inhibit the genes that are responsible for their own production. With suitable delays between the events of gene expression and repression, an oscillation in protein levels is established.

This oscillation represents the the ticking of the clock and seems to be fairly unique to the circadian rhythm, said senior authorBrian Crane, the George W. and Grace L. Todd Professor and chair of chemistry and chemical biology in the College of Arts and Sciences.

Blue light, Crane said, changes the chemistry and structure of cryptochromes flavin cofactor, which allows the protein to bind the TIM protein and inhibit TIMs ability to repress gene expression and thereby reset the oscillation.

Much of the hard work of the study went into figuring out how to produce the complex of cryptochrome-TIM so it could be studied, because TIM is such a large, unwieldy protein, Crane said. To achieve their results, first author Changfan Lin, M.S. 17, Ph.D. 21, modified the cryptochrome protein to improve the stability of the cryptochrome-TIM complex and used innovative techniques to purify the samples, making them suitable for high-resolution imaging.

These new methods allowed us to obtain detailed images of the protein structures and gain valuable insights into their function, said Lin, a Friedrich's Ataxia Research Alliance Postdoctoral Fellow at the California Institute of Technology. This research not only deepens our understanding of circadian rhythm regulation but also opens up new possibilities for developing therapies targeting related processes.

Co-author Shi Feng, a doctoral student in the field of biophysics, did much of the cryo-electron microscopy work. Cristina C. DeOliveira, a doctoral student in the field of biochemistry and molecular and cell biology, was also a co-author.

One unexpected result from the study sheds light on how DNA damage is repaired in a cell. Cryptochromes are closely related to a family of enzymes involved in repairing damage to DNA, called photolyases. Crane said the research explains why these families of proteins are closely related to each other, even though they're doing quite different things theyre making use of the same molecular recognition in different contexts.

The study also offers an explanation for the genetic variation of flies that allows them to adapt to higher latitudes, where days are shorter in the winter and it's cooler. These flies have more of a certain genetic variant that involves a change in the TIM protein, and it wasnt clear why the variation could help them. The researchers found that because of how the cryptochrome binds TIM, the variation reduces the affinity of TIM for the cryptochrome. The interaction between the proteins is then modulated and the ability of light to reset the oscillation is changed, thus altering the circadian clock and extending the period of the flys dormancy, which helps it survive the winter.

Some of the interactions that we see here in the fruit fly can be mapped onto human proteins, Crane said. This study may help us understand key interactions between components that regulate sleep behavior in people, such as how the critical delays in the basic timing mechanism get built into the system.

Another exciting finding, said Lin, was the discovery of an important structural area in TIM, called the groove, which helps explain how TIM enters the cell nucleus. Previous studies had identified some factors involved in this process, but the exact mechanism remained unclear. Our research provided a clearer understanding of this phenomenon, Lin said.

Reference:Lin C, Feng S, DeOliveira CC, Crane BR. CryptochromeTimeless structure reveals circadian clock timing mechanisms. Nature. 2023:1-6. doi:10.1038/s41586-023-06009-4

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