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Physicists Pin Down Nuclear Reaction From Moments After the Big Bang

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In a secluded laboratory buried under a mountain in Italy, physicists have re-created a nuclear reaction that happened between two and three minutes after the Big Bang.

Their measurement of the reaction rate, published today in Nature, nails down the most uncertain factor in a sequence of steps known as Big Bang nucleosynthesis that forged the universe’s first atomic nuclei.

Researchers are “over the moon” about the result, according to Ryan Cooke, an astrophysicist at Durham University in the United Kingdom who wasn’t involved in the work. “There’ll be a lot of people who are interested from particle physics, nuclear physics, cosmology and astronomy,” he said.

The reaction involves deuterium, a form of hydrogen consisting of one proton and one neutron that fused within the cosmos’s first three minutes. Most of the deuterium quickly fused into heavier, stabler elements like helium and lithium. But some survived to the present day. “You have a few grams of deuterium in your body, which comes all the way from the Big Bang,” said Brian Fields, an astrophysicist at the University of Illinois, Urbana-Champaign.

The precise amount of deuterium that remains reveals key details about those first minutes, including the density of protons and neutrons and how quickly they became separated by cosmic expansion. Deuterium is “a special super-witness of that epoch,” said Carlo Gustavino, a nuclear astrophysicist at Italy’s National Institute for Nuclear Physics.

But physicists can only deduce those pieces of information if they know the rate at which deuterium fuses with a proton to form the isotope helium-3. It’s this rate that the new measurement by the Laboratory for Underground Nuclear Astrophysics (LUNA) collaboration has pinned down.

The Earliest Probe of the Universe

Deuterium’s creation was the first step in Big Bang nucleosynthesis, a sequence of nuclear reactions that occurred when the cosmos was a super hot but rapidly cooling soup of protons and neutrons.

Starting in the 1940s, nuclear physicists developed a series of interlocking equations describing how various isotopes of hydrogen, helium and lithium assembled as nuclei merged and absorbed protons and neutrons. (Heavier elements were forged much later inside stars.) Researchers have since tested most aspects of the equations by replicating the primordial nuclear reactions in laboratories.

In doing so, they made radical discoveries. The calculations offered some of the first evidence of dark matter in the 1970s. Big Bang nucleosynthesis also enabled physicists to predict the number of different types of neutrinos, which helped drive cosmic expansion.

But for almost a decade now, uncertainty about deuterium’s likelihood of absorbing a proton and turning into helium-3 has fogged up the picture of the universe’s first minutes. Most importantly, the uncertainty has prevented physicists from comparing that picture to what the cosmos looked like 380,000 years later, when the universe cooled enough for electrons to begin orbiting atomic nuclei. This process released radiation called the cosmic microwave background that provides a snapshot of the universe at the time.

Cosmologists want to check whether the density of the cosmos changed from one period to the other as expected based on their models of cosmic evolution. If the two pictures disagree, “that would be a really, really important thing to understand,” Cooke said. Solutions to stubbornly persistent cosmological problems — like the nature of dark matter — could be found in this gap, as could the first signs of exotic new particles. “A lot can happen between a minute or two after the Big Bang and several hundred thousand years after the Big Bang,” Cooke said.

But the all-important deuterium reaction rate that would allow researchers to make these kinds of comparisons is very difficult to measure. “You’re simulating the Big Bang in the lab in a controlled way,” said Fields.

Physicists last attempted a measurement in 1997. Since then, observations of the cosmic microwave background have become increasingly precise, putting pressure on physicists who study Big Bang nucleosynthesis to match that precision — and so allow a comparison of the two epochs.

In 2014, Cooke and co-authors precisely measured the abundance of deuterium in the universe through observations of faraway gas clouds. But to translate this abundance into a precise prediction of the primordial matter density, they needed a much better measure of the deuterium reaction rate.

Confounding the situation further, a purely theoretical estimate for the rate, published in 2016, disagreed with the 1997 laboratory measurement.

“It was a very confused scenario,” said Gustavino, who is a member of the LUNA collaboration. “At this point I became pushy with the collaboration … because LUNA could measure this reaction exactly.”

A Rare Combination

Part of the challenge in measuring how readily deuterium fuses with a proton is that, under laboratory conditions, the reaction doesn’t happen very often. Every second, the LUNA experiment fires 100 trillion protons at a target of deuterium. Only a few a day will fuse.

Adding to the difficulty, cosmic rays that constantly rain down on Earth’s surface can mimic the signal produced by deuterium reactions. “For this reason, we’re in an underground laboratory where, thanks to the rock cover, we can benefit from cosmic silence,” said Francesca Cavanna, who led LUNA’s data collection and analysis along with Sandra Zavatarelli.

Over three years, the scientists took turns spending weeklong shifts in a lab deep inside Italy’s Gran Sasso mountain. “It’s exciting because you really feel you are inside the science,” Cavanna said. As they gradually collected data, pressure mounted from the wider physics community. “There was a lot of anticipation; there was a lot of expectation,” said Marialuisa Aliotta, a team member.

As it turns out, the team’s newly published measurement may come as a disappointment to cosmologists looking for cracks in their model of how the universe works.

Small Steps

The measured rate — which says how quickly deuterium tends to fuse with a proton to form helium-3 across the range of temperatures found in the epoch of primordial nucleosynthesis — landed between the 2016 theoretical prediction and the 1997 measurement. More importantly, when physicists feed this rate into the equations of Big Bang nucleosynthesis, they predict a primordial matter density and a cosmic expansion rate that closely square with observations of the cosmic microwave background 380,000 years later.

“It essentially tells us that the standard model of cosmology is, so far, quite right,” said Aliotta.

That in itself squeezes the gap that next-generation models of the cosmos must fit into. Experts say some theories of dark matter could even be ruled out by the results.

That’s less exciting than evidence in favor of exotic new cosmic ingredients or effects. But in this era of precision astronomy, Aliotta said, scientists proceed “by making small steps.” Fields agreed: “We are constantly trying to do better on the prediction side, the measurement side and the observation side.”

On the horizon is the next generation of cosmic microwave background measurements. Meanwhile, with deuterium’s behavior now better understood, uncertainties in other primordial nuclear reactions and elemental abundances become more pressing.

A longstanding “fly in the Big Bang nucleosynthesis ointment,” according to Fields, is that the matter density calculated from deuterium and the cosmic microwave background predicts that there should be three times more lithium in the universe than we actually observe.

“There are still lots of unknowns,” said Aliotta. “And what the future will bring is going to be very interesting.”

Source: https://www.quantamagazine.org/physicists-pin-down-nuclear-reaction-from-moments-after-the-big-bang-20201111/

Quantum

Random walks

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A college professor of mine proposed a restaurant venture to our class. He taught statistical mechanics, the physics of many-particle systems. Examples range from airplane fuel to ice cubes to primordial soup. Such systems contain 1024 particles each—so many particles that we couldn’t track them all if we tried. We can gather only a little information about the particles, so their actions look random.

So does a drunkard’s walk. Imagine a college student who (outside of the pandemic) has stayed out an hour too late and accepted one too many red plastic cups. He’s arrived halfway down a sidewalk, where he’s clutching a lamppost, en route home. Each step has a 50% chance of carrying him leftward and a 50% chance of carrying him rightward. This scenario repeats itself every Friday. On average, five minutes after arriving at the lamppost, he’s back at the lamppost. But, if we wait for a time T, we have a decent chance of finding him a distance sqrt{T} away. These characteristic typify a simple random walk.

Random walks crop up across statistical physics. For instance, consider a grain of pollen dropped onto a thin film of water. The water molecules buffet the grain, which random-walks across the film. Robert Brown observed this walk in 1827, so we call it Brownian motion. Or consider a magnet at room temperature. The magnet’s constituents don’t walk across the surface, but they orient themselves according random-walk mathematics. And, in quantum many-particle systems, information can spread via a random walk. 

So, my statistical-mechanics professor said, someone should open a restaurant near MIT. Serve lo mein and Peking duck, and call the restaurant the Random Wok.

This is the professor who, years later, confronted another alumna and me at a snack buffet.

“You know what this is?” he asked, waving a pastry in front of us. We stared for a moment, concluded that the obvious answer wouldn’t suffice, and shook our heads.

“A brownie in motion!”

Not only pollen grains undergo Brownian motion, and not only drunkards undergo random walks. Many people random-walk to their careers, trying out and discarding alternatives en route. We may think that we know our destination, but we collide with a water molecule and change course.

Such is the thrust of Random Walks, a podcast to which I contributed an interview last month. Abhigyan Ray, an undergraduate in Mumbai, created the podcast. Courses, he thought, acquaint us only with the successes in science. Stereotypes cast scientists as lone geniuses working in closed offices and silent labs. He resolved to spotlight the collaborations, the wrong turns, the lessons learned the hard way—the random walks—of science. Interviewees range from a Microsoft researcher to a Harvard computer scientist to a neurobiology professor to a genomicist.

You can find my episode on Instagram, Apple Podcasts, Google Podcasts, and Spotify. We discuss the bridging of disciplines; the usefulness of a liberal-arts education in physics; Quantum Frontiers; and the delights of poking fun at my PhD advisor, fellow blogger and Institute for Quantum Information and Matter director John Preskill

Source: https://quantumfrontiers.com/2021/01/17/random-walks/

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How Dynamical Quantum Memories Forget

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Lukasz Fidkowski1, Jeongwan Haah2, and Matthew B. Hastings3,2

1Department of Physics, University of Washington, Seattle, WA 98195-1560, USA
2Microsoft Quantum and Microsoft Research, Redmond, WA 98052, USA
3Station Q, Microsoft Research, Santa Barbara, CA 93106-6105, USA

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Abstract

Motivated by recent work showing that a quantum error correcting code can be generated by hybrid dynamics of unitaries and measurements, we study the long time behavior of such systems. We demonstrate that even in the “mixed” phase, a maximally mixed initial density matrix is purified on a time scale equal to the Hilbert space dimension (i.e., exponential in system size), albeit with noisy dynamics at intermediate times which we connect to Dyson Brownian motion. In contrast, we show that free fermion systems $—$ i.e., ones where the unitaries are generated by quadratic Hamiltonians and the measurements are of fermion bilinears $—$ purify in a time quadratic in the system size. In particular, a volume law phase for the entanglement entropy cannot be sustained in a free fermion system.

► BibTeX data

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Cited by

[1] Matteo Ippoliti, Michael J. Gullans, Sarang Gopalakrishnan, David A. Huse, and Vedika Khemani, “Entanglement phase transitions in measurement-only dynamics”, arXiv:2004.09560.

[2] Adam Nahum, Sthitadhi Roy, Brian Skinner, and Jonathan Ruhman, “Measurement and entanglement phase transitions in all-to-all quantum circuits, on quantum trees, and in Landau-Ginsburg theory”, arXiv:2009.11311.

[3] Michael J. Gullans, Stefan Krastanov, David A. Huse, Liang Jiang, and Steven T. Flammia, “Quantum coding with low-depth random circuits”, arXiv:2010.09775.

[4] Jason Iaconis, Andrew Lucas, and Xiao Chen, “Measurement-induced phase transitions in quantum automaton circuits”, arXiv:2010.02196.

[5] Ali Lavasani, Yahya Alavirad, and Maissam Barkeshli, “Topological order and criticality in (2+1)D monitored random quantum circuits”, arXiv:2011.06595.

[6] Sarang Gopalakrishnan and Michael J. Gullans, “Entanglement and purification transitions in non-Hermitian quantum mechanics”, arXiv:2012.01435.

[7] Matteo Ippoliti and Vedika Khemani, “Postselection-free entanglement dynamics via spacetime duality”, arXiv:2010.15840.

[8] Oliver Lunt, Marcin Szyniszewski, and Arijeet Pal, “Dimensional hybridity in measurement-induced criticality”, arXiv:2012.03857.

[9] Chao-Ming Jian, Bela Bauer, Anna Keselman, and Andreas W. W. Ludwig, “Criticality and entanglement in non-unitary quantum circuits and tensor networks of non-interacting fermions”, arXiv:2012.04666.

[10] Shengqi Sang, Yaodong Li, Tianci Zhou, Xiao Chen, Timothy H. Hsieh, and Matthew P. A. Fisher, “Entanglement Negativity at Measurement-Induced Criticality”, arXiv:2012.00031.

The above citations are from SAO/NASA ADS (last updated successfully 2021-01-18 07:19:20). The list may be incomplete as not all publishers provide suitable and complete citation data.

On Crossref’s cited-by service no data on citing works was found (last attempt 2021-01-18 07:19:18).

Source: https://quantum-journal.org/papers/q-2021-01-17-382/

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Autonomous Ticking Clocks from Axiomatic Principles

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Mischa P. Woods

Institute for Theoretical Physics, ETH Zurich, Switzerland

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Abstract

There are many different types of time keeping devices. We use the phrase $textit{ticking clock}$ to describe those which – simply put – “tick” at approximately regular intervals. Various important results have been derived for ticking clocks, and more are in the pipeline. It is thus important to understand the underlying models on which these results are founded. The aim of this paper is to introduce a new ticking clock model from axiomatic principles that overcomes concerns in the community about the physicality of the assumptions made in previous models. The ticking clock model in [1] achieves high accuracy, yet lacks the autonomy of the less accurate model in [2]. Importantly, the model we introduce here achieves the best of both models: it retains the autonomy of [2] while allowing for the high accuracies of [1]. What is more, [2] is revealed to be a special case of the new ticking clock model.

► BibTeX data

► References

[1] M. P. Woods, R. Silva, G. Pütz, S. Stupar, and R. Renner, “Quantum clocks are more accurate than classical ones,” (2018a), arXiv:1806.00491v2 [quant-ph].
arXiv:1806.00491v2

[2] P. Erker, M. T. Mitchison, R. Silva, M. P. Woods, N. Brunner, and M. Huber, Phys. Rev. X 7, 031022 (2017).
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[6] P. Erker, “The Quantum Hourglass,” (2014), ETH Zürich.
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[7] S. Ranković, Y.-C. Liang, and R. Renner, “Quantum clocks and their synchronisation | the Alternate Ticks Game,” (2015), arXiv:1506.01373v1 [quant-ph].
arXiv:1506.01373v1

[8] M. P. Woods, R. Silva, and J. Oppenheim, Ann. Henri Poincaré (2018b), 10.1007/​s00023-018-0736-9.
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[9] S. Khandelwal, M. P. Lock, and M. P. Woods, Quantum 4, 309 (2020).
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[10] Y. Yang, L. Baumgärtner, R. Silva, and R. Renner, “Accuracy enhancing protocols for quantum clocks,” (2019), arXiv:1905.09707 [quant-ph].
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[13] W. Pauli, in Handbuch der Physik, Vol. 24 (Springer, 1933) pp. 83–272.
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[20] H.-P. Breuer and F. Petruccione, The Theory of Open Quantum Systems (Oxford University Press, 2007) . See section 3.3 Microscopic derivations. In particular, 3.3.1 Weak-coupling limit and 3.3.3 Singular-coupling limit.
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[23] S. Stupar, C. Klumpp, N. Gisin, and R. Renner, “Performance of Stochastic Clocks in the Alternate Ticks Game,” (2018), arXiv:1806.08812 [quant-ph].
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Cited by

[1] Antoine Rignon-Bret, Giacomo Guarnieri, John Goold, and Mark T. Mitchison, “Thermodynamics of precision in quantum nano-machines”, arXiv:2009.11303.

[2] G. J. Milburn, “The thermodynamics of clocks”, Contemporary Physics 61 2, 69 (2020).

[3] Emanuel Schwarzhans, Maximilian P. E. Lock, Paul Erker, Nicolai Friis, and Marcus Huber, “Autonomous Temporal Probability Concentration: Clockworks and the Second Law of Thermodynamics”, arXiv:2007.01307.

The above citations are from SAO/NASA ADS (last updated successfully 2021-01-18 06:59:53). The list may be incomplete as not all publishers provide suitable and complete citation data.

On Crossref’s cited-by service no data on citing works was found (last attempt 2021-01-18 06:59:51).

Source: https://quantum-journal.org/papers/q-2021-01-17-381/

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‘Ultrasound drill’ and nanodroplets break apart blood clots

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Ultrasound drill
On target: artist’s impression of how an ultrasound drill and injection tube for nanodroplets could be used to break up blood clots in the body. (Courtesy: North Carolina State University)

A precision “ultrasound drill” combined with specially engineered nanodroplets could soon be used inside the body to break up stubborn, impenetrable blood clots – according to Leela Goel, Xiaoning Jiang  and colleagues at North Carolina State University. The team has done in vitro experiments demonstrating the technique, which if approved for clinical trials, could lead to promising new treatments for dangerous forms of thrombosis.

If blood clots do not break down quickly enough, they can retract over periods of several days, forming dense, non-porous clumps of cells. Each year, up to 600,000 people in the US alone can be affected by these clots – known as deep vein thromboses. In the past, their treatment has largely involved drugs that activate certain enzymes in the blood to break down the structures of the clots. However, the high drug doses and long treatment times required in this approach can cause significant damage in surrounding tissues.

More recently, a technique called sonothrombolysis has emerged. This uses ultrasound waves to cause microbubbles surrounding a clot to oscillate – enhancing both mechanical erosion and drug diffusion in the clot. However, this technique relies on large external ultrasound transducers, and cannot be used to treat veins that are obscured by ultrasound-blocking organs like the lungs or ribs.

Low boiling point

In their study, the North Carolina State team delivered nanodroplets to a clot created in an experimental apparatus. Because of their small size, the nandroplets easily penetrate retracted clots. Alongside the tube that delivers the nanodroplets is a catheter-based ultrasound “drill”, which produces precisely-aimed acoustic waves via a tiny, forward-viewing transducer.

The nanodroplets are specially engineered to have a low boiling point. The small amount of energy delivered by the drill is enough to vaporize the nanodroplets forming gas-filled microbubbles that rapidly expand and contract. These oscillations break down the clot through the process of cavitation – the creation of microscale streams and jets that weaken the clot’s mechanical structure. At the same time, the vibrations open up holes in the clot that enable enzyme-enhancing drugs to penetrate more easily. This enhances breakdown even further, while avoiding the need for high drug doses and long treatment times.

Over 30 min timescales, they found that clot masses could be reduced by around 40% – much more than the 17% for treatments that combine ultrasound, microbubbles, and enzyme-activating drugs. Although the team’s approach is still a long way from entering clinical practice, their results suggest that a breakthrough in the treatment of deep vein thromboses could be just over the horizon.

The technique is described in Microsystems & Nanoengineering.

Source: https://physicsworld.com/a/ultrasound-drill-and-nanodroplets-break-apart-blood-clots/

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