Correction is more inadvertent than prevention: electronics

The statistics say that, the present rate of error occurrence is around 10^{-27}. This is imposed by the fault-tolerant architecture of logical circuits. These workers now prove that, physical fault tolerant prevention is more efficient than architectural correction in suppressing errors. [PRL 106, 176801 (2011)]

The error rate in complementary transistor circuits is suppressed exponentially in electron number, arising from an intrinsic physical implementation of fault-tolerant error correction. Contrariwise, explicit assembly of gates into the most efficient known fault-tolerant architecture is characterized by a subexponential suppression of error rate with electron number, and incurs significant overhead in wiring
and complexity.We conclude that it is more efficient to prevent logical errors with physical fault tolerance than to correct logical errors with fault-tolerant architecture.

Inaugural Article By C M Will

I really like the nice article by physicist Will. It is titled "On the unreasonable effectiveness of the post-newtonian approximation in gravitational physics". I think, it is accessible to anyone with a fundamental knowledge of general relativity (GR). Personally, I suddenly feel the study of GR so handy. Before I read this article, GR to me is quite like an abstract. This article shows me the experimental aspects in light of GR equations. OK. read it for yourself. [PNAS, 108:5938(2011)].

Suggestions from the editorial board of PR

I find it potentially useful for fresh scholars in writing and preparing their manuscripts. The following are excerpts abridged from the website of APS. Actually, I resonate much with what they say.

Editorial: The Aim of a Good Introduction (October 21, 2005)

A primary goal of Physical Review Letters is to keep a broad spectrum of physicists informed about current research findings in areas outside their specializations. To accomplish this, a Letter needs clearly written introductory paragraphs that are understandable by nonexperts. Communication to a general readership is, however, an ongoing challenge for PRL editors and authors. To aid in this effort, we offer some guidelines to authors for writing the introduction:

1. The introduction should interest people outside the subfield in reading the article. Because it is directed at nonspecialists, it should have a minimum of jargon and acronyms.

2. It should describe the background and history of the problem or research goal addressed in the article. It should explain the importance of this research and of the results being reported, as well as any relevance they have to other areas of physics ("The work described here is motivated by...").

3. A well organized introduction starts with the general discussion described in point (2) and ends with a brief description of the specific results presented ("In this Letter we show..."). Discussions of technical details should be reserved for the main text.

4. In our experience, a good introduction requires a minimum of 1 double-spaced manuscript page, i.e., 32 single-column published lines, and may range up to 2 such pages, or 64 published lines.

Good writing is difficult and requires thought and effort; this is especially true when one attempts to communicate technical results to people outside the field. It would be a useful practice if, before submitting a manuscript to PRL, the authors asked colleagues in other areas to comment on its readability, with particular emphasis on the introduction. Authors who do not feel comfortable writing in English may find it helpful to consult colleagues more experienced in this regard.

[And the following are the criteria for PRL acceptance]

Acceptance Criteria

Physical Review Letters publishes Letters of not more than four journal pages and Comments of not more than one journal page. Both must meet specific standards for substance and presentation, as judged by rigorous review by editors and referees. The Physical Review and Physical Review Letters publish new physics. Thus, prior publication of the same results will preclude consideration of a later Letter. In addition, the findings must not be a marginal extension of previously published work; they must not be a repetition of prior results on a similar system, without additional physical insight.

Substance

Validity.— Work is valid if it is free of detectable error and is presented in sufficient detail that this may be determined. Papers that advance new theoretical views on fundamental principles or theories must contain convincing arguments that the new predictions and interpretations are distinct from existing knowledge and do not contradict experiment.

Importance.— Important results are those that substantially advance a field, open a significant new area of research or solve–or take a crucial step toward solving–a critical outstanding problem, and thus facilitate notable progress in an existing field. A new experimental or theoretical method may be a suitable basis for a Letter, but only if it leads to the significant advances presented above. Mathematical and computational papers that do not have application to physics are generally not suitable. Papers that describe proposed experiments must provide compelling evidence that the proposal is novel and feasible, and that it will lead to valuable new research.

Broad Interest.— Work is of broad interest if it is a major advance in a field of physics or has significant implications across subfield boundaries. A manuscript may also be of broad interest if it is exceptionally pleasing science, aesthetically.

Presentation

The diversity of the readership of Physical Review Letters places special demands on style. A Letter must begin with an introduction that states the issues it addresses and its primary achievements in language understandable across physics subfields. Each Letter should present a complete discussion within the constraints of a short communication. Letters must be clearly written, devoid of unnecessary jargon, with symbols defined, figures well drawn, and tables and figures thoroughly captioned. When appropriate, a Letter should be followed by a more extensive report in the Physical Review or elsewhere.

Importance of Introductory Paragraphs

Physical Review Letters is unique in its commitment to keep broadly interested readers well informed on vital current research in all fields of physics. This is achieved with introductory paragraphs that state, for each article, the issues addressed and the primary achievements. It is essential that these paragraphs be clearly written and comprehensible to nonexperts. To assure compliance, the referees are instructed to pay particular attention to the introductory section. In addition, the editors will make an independent evaluation of the adequacy and clarity of the introduction.

Physics and Physicists: "What Is The Most Important Thing You Learned In Becoming A Scientist?"

Physics and Physicists: "What Is The Most Important Thing You Learned In Becoming A Scientist?"

Graphene is really quantum

Some 'non-locality' is claimed with the electrons in graphene [Science 15 April 2011: 328-330]. The experimenters measure the voltages between two electrodes that are far away from the region where the currents are suppsed to flow. And the voltage does not vanish at even room temperature, which is not possible for classical electrons. They associate this with the strongly coherent motion of the graphene electrons. They are so quantum !

In their experiments, an electrical current is passed between two electrodes in close proximity to each other (see the figure, upper inset), and they detect a voltage far away from those electrodes. In an ordinary material under the same conditions (at room temperature and small magnetic fields), this would not happen because the electrons are incoherent and behave classically. After a few collisions with impurities in the system, the electrons would lose their phase information and undergo random walk. To reach nonlocal contacts, this would be a walk too far, and no voltage would be generated. This indicates that graphene is behaving in a quantum mechanical way and creating a nonlocal voltage under unexpected conditions. How is this possible? [http://www.sciencemag.org/content/332/6027/315.full]

A big jump

I heard of this jump weeks ago. But I did not play dice on it, anyway I feel it may not hold up, but it might. What will be the result ? NO body knows for the moment. There is indeed a churr in the community, as described in this news [http://www.sciencemag.org/content/332/6027/296.full]:

Particle physicists haven't discovered anything truly surprising in 35 years, so a mere hint of something odd works them up in a hurry. So it was last week, when, aided by press reports, news spread that scientists in the United States may have spotted a bit of matter unlike any seen before. But even as they contemplate the implications, physicists are taking the result with a grain of salt. The supposed signal could be an experimental artifact, caution the researchers who found it. And if a new particle is there, physicists may have to perform theoretical contortions to explain why they didn't spot it before. “I think the result is rather inconclusive,” says Christopher Hill, an experimenter at Ohio State University in Columbus, who was not involved in the work.

The finding comes from the 700-member team working with the CDF particle detector at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois. The team analyzed the billions of collisions of protons and antiprotons produced by Fermilab's atom smasher, the 25-year-old Tevatron, which will shut down this year. Those high-energy collisions can blast into fleeting existence massive subatomic particles not seen in the everyday world. Physicists try to identify those particles by studying the combinations of familiar particles into which they decay.

In this case, experimenters searched for collisions that produced a particle called a W boson, which weighs about 86 times as much as a proton, along with some other particle that disintegrates into two sprays of particles called “jets.” A jet arises when a collision or decay kicks out a particle called a quark. A quark cannot exist on its own but must be bound to other quarks or an antiquark. So the energetic quark quickly rips more quarks and antiquarks out of the vacuum of empty space, and they instantaneously form particles called mesons, each containing a quark and an antiquark. From the energies and momenta of the two jets, researchers can infer the mass of the particle that produced them.

CDF researchers see about 250 events in which the jets seem to come from a particle weighing about 155 times as much as proton. Those events show up as an unexpected peak in a data plot (see diagram). The chances that random jets or jet pairs from other sources would produce a fake signal that strong are 1 in 1300, the physicists estimate. “We've been struggling for 6 months to make this peak go away, and we haven't been able to do it,” says Robert Roser, a physicist at Fermilab and co-spokesperson for the CDF team. Still, he says, the signal is “not even close” to strong enough to claim a discovery.

Experimenters have several reasons to be cautious. The analysis depends critically on physicists' understanding of jets. CDF does not measure every particle in a jet, so researchers must make a 25% upward correction to a jet's measured energy. If the uncertainty in that fudge factor is bigger than they estimate, “then maybe the excess isn't so significant,” says Shahram Rahatlou of Sapienza University of Rome.

CDF physicists must also take care that they haven't mistaken random pairs of jets for new particles. To see the peak, they must subtract out a huge “background” produced by events containing a W and random jets. If that subtraction isn't just right, it could produce a fake signal. “The real question is how well do we understand that [background],” says Joseph Lykken, a theorist at Fermilab.

But those caveats have not stopped theorists from trying to explain the curious bump in terms of new particles. Felix Yu, a theorist at the University of California, Irvine, suggests that the new particle could be one known as a Z′ (pronounced Z-prime), which would convey a new force much like a very short-range electromagnetic force. Estia Eichten, a theorist at Fermilab, and colleagues say the particle could be a “technipion,” a particle predicted by a type of theory called “technicolor,” which posits a new kind of strong nuclear force.

To have escaped notice until now, however, a particle would have to have some weird properties. Generally, a Z′ ought to decay into an electron and an antielectron. In fact, experimenters have already searched for and failed to find that decay. So Yu's Z′ must not decay that way for some reason. The technipion may face similar problems. CDF researchers are searching for the long-sought Higgs boson, the key to physicists' understanding of mass, by looking for events in which it is produced with a W boson and in which the Higgs decays into two jets specifically triggered by particles called bottom quarks. The hypothetical new particle hasn't shown up in those Higgs searches, so it must not often decay into bottom quarks, as one would expect a technipion to do.

For those reasons, some physicists say such explanations of the bump seem contrived or “unnatural.” “Yesterday, these models weren't popular,” Hill says. Yu counters that “having a theory that looks pretty but doesn't fit the data isn't natural.”

The supposed signal should be confirmed or ruled out in short order. The CDF team has analyzed only half of the data it has already collected. And the Tevatron's other large particle detector, D0, has a data set as big as CDF's. If the particle is there, D0 should see it, too. “We hope that within a few weeks you'll be hearing from us,” says Dmitri Denisov, a physicist at Fermilab and co-spokesperson for the D0 team. In the meantime, physicists will enjoy the buzz.

Spin diffuses through interacting fermi gas

This is definitely a typical non-equilibrium problem. In their experiment, "A spin current is induced by spatially separating two spin components and observing their evolution in an external trapping potential." [Nature, 472:401(2011)] They found that, "interactions can be strong enough to reverse spin currents, with components of opposite spin reflecting off each other. Near equilibrium, we obtain the spin drag coefficient, the spin diffusivity and the spin susceptibility as a function of temperature on resonance and show that they obey universal laws at high temperatures. In the degenerate regime, the spin diffusivity approaches a value set by /m, the quantum limit of diffusion, where /m is Planck’s constant divided by 2π and m the atomic mass. For repulsive interactions, our measurements seem to exclude a metastable ferromagnetic state^{9, 10, 11}." For a review, click here.

Metastable states are important in reality

Metastable states are common in nature: supercooled or superheated liquid are daily examples. These states are not the lowest-energy state, and, by thermodynamic principles, one should not expect them to be long-lived in nature. Indeed, they exist only under very stringent conditions, and very little disturbance can push the system off to a stable state around. Thermodynamically stable states dwell on one of the global minima of the free energy, around which, however, local minima might exist that are separated from the global minima by energy barriers. When a system has not reached the stable states, it will be constantly kicked by its surroundings and eventually transits from where it is in to a stable state after some period (the life of the metastable state). The interesting point is that, the life time can sometimes be very long and real transitions can hardly be seen, a situation similar to ergodicity breaking. For example, diamond has a higher energy than graphite, but it can exist for ever. The reason is because the time to make the transition is cosmologically long, due to the hugely high barrier. Another material is graphene, which should not be stable according to Wagner-Mermin-Honberg theorem. Yet, it was produced in 2004. Glasses are the third examples, in which case, transition has been frustrated by its structure. In the case of supercooled water, the transition is suppressed by distilling process.

High energy neutrinos undetected in IceCube

High energy neutrinos are expected to come out of gamma ray bursts. On earth there is a IceCube, which is designed to catch these neutrinos. However, after 13 months of run, they find null results. [http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.106.141101]

GRBs result from a giant star explosion or the collision of star remnants. These cosmic cataclysms produce—in addition to gamma rays—high-speed protons that are thought to account for the highest energy cosmic rays observed on Earth. Near the source, these protons may run into photons and end up generating neutrinos with energies far above 1 TeV. In the past, neutrino detectors on Earth have not been large enough to capture one of these high-energy neutrinos with any likelihood.

IceCube, which was completed in December 2010, is a kilometer-cubed array of photodetectors that have been drilled down into the Antarctic ice cap. Neutrinos typically fly through the array without leaving a trace, but occasionally one will collide with a nucleus and create a charged particle that emits light as it moves through the ice. The IceCube team compared 13 months of their data (collected when the array was half finished) to observations of 117 GRBs measured independently over the same time period. Contrary to expectations, no high-energy neutrinos were detected within a half-hour of each GRB. Theorists may need to rethink their models of GRBs, as well as look for other possible sources for the highest energy cosmic rays. – Michael Schirber

1D is special

1D means enough space but little mobility. One always blocks another even if a bit of interaction gets in the way. And this makes the boundary between bosons and fermions fuzzy. Fermions naturally set hurdles to their compatriots for exclusion principle. While bosons, although without that famous principle, will also demobilize their partners in the presence of strong repulsions. If so, fermions and bosons will resemble each other. This is indeed what happens in this setup proposed in this work [Phys. Rev. Lett. 106, 153601 (2011) ]!

In this work we show that light-matter excitations (polaritons) generated inside a hollow-core onedimensional fiber filled with two types of atoms, can exhibit Luttinger liquid behavior. We first explain how to prepare and drive this quantum-optical system to a strongly interacting regime, described by a bosonic two-component Lieb-Liniger model. Utilizing the connection between strongly interacting bosonic and fermionic systems, we then show how spin-charge separation could be observed by probing the correlations in the polaritons. This is performed by first mapping the polaritons to propagating photon pulses and then measuring the effective photonic spin and charge densities and velocities by analyzing the correlations in the emitted photon spectrum. The necessary regime of interactions is achievable with
current quantum-optical technology.

Q&A with Philip Philips

You cannot fail to recognize him ! He has a good head of hairs ! [10.1126/science.caredit.a1100031]

Phillips, whose Ph.D. is in theoretical chemistry, says he probably would not have taken such an unusual approach if his background had been more conventional. Science Careers talks to Phillips about how his circuitous route from chemistry to physics prepared him to go down this new research avenue.

The following highlights from the interview were edited for brevity and clarity.

Q: How did you first become interested in science?

P.P.: My parents were in the humanities. I was born in Tobago, and we moved to the U.S. when I was 10. I was very interested in math as a kid. Science was interesting, but I had very bad teachers in high school and so I had no real way of knowing what science would be about.

At the university, I took a chemistry class, and that's when I really became interested in science. So I started taking many more science classes, and I realized too late that my real interest was in physics. I needed one more class for a physics major, and so I had degrees in math and chemistry.

Q: How did you go about choosing your Ph.D. program?

P.P.: I wanted to be a theoretician because math was always my thing, and if I was going to do science, I wanted to apply math to understanding physical problems. [But] I realized, given my limited undergraduate background, that somehow being able to chart a course that was intellectually what would be my focus for the rest of my life was just not possible. So I viewed a Ph.D. as a degree in which I learned how to do research, and the particular problem wasn't something I was deeply interested in at all. My project was on explaining phosphorescence lifetimes in small molecules.

Q: Did you get what you needed out of your Ph.D.?

P.P.: I had an adviser who was an incredibly brilliant person, and he really taught me how to get something done. He gave me this sense of just being able to take on a challenge even if you have no experience with the field.

Q: What was your next step?

P.P.: I got a Miller Fellowship at [the University of California,] Berkeley. The key thing I got interested in was disordered systems, and I started really thinking about many-body systems and phenomena that arise from collective physics, the sorts of things that would define my career.

I just started reading all the papers, and then I defined a new problem that others had not solved that I thought would advance the field. So the problem I was working on at Berkeley was an electron moving in a random array of scatterers. I learned the necessary math tricks to be able to solve this problem, and then I started doing it. That's what I'd learned from my adviser: how to chop something down that is completely new and make progress on it.

As a Miller fellow, I was doing this on my own. It was a big jump from single-particle stuff to, essentially, statistical mechanics, and the mindset was very different. It was painful and a lot of stuff I had to learn, but it was what I knew I wanted to do.

This Science Careers article is a tie-in to Science magazine’s feature on superconductivity.

Q: You then obtained a faculty position at the Massachusetts Institute of Technology (MIT), in the chemistry department. What did you work on there?

P.P.: This problem led me to look at Anderson localization, which is the problem of electrons moving in a random lattice. Solids have a regular array of atoms, and what Anderson showed is that if enough of the atoms are different, the electrons change from being able to move freely to being completely stuck. In one and two dimensions, it's generally been thought that any amount of disorder would lead to localization. But we found the general exceptions. And then I showed that certain classes of conducting polymers could be explained by the examples that lead to exceptions to localization. That application was motivated entirely because I was in a chemistry department.

Q: You then decided to move to Illinois after 9 years at MIT. Why?

P.P.: I was forced out. What I was doing had nothing to do with their definition of chemistry. Some supported me and some didn't. But even if I had gotten tenure, I would have had to move to a physics department.

Q: Were you aware at the time that you were risking tenure?

P.P.: Yes, I was very much aware of this. You have to be honest with yourself. There were no problems in chemistry that interested me. I have always just thought that you should do what you think is important regardless of whether it might work out or regardless of whether or not your colleagues think it will work out.

Q: You obtained tenure at the University of Illinois right away. Do you feel as though you've found your place now?

P.P.: Illinois has been absolutely a gold mine for me, yes. Personality-wise, in terms of the research I'm able to pursue here, in terms of a supportive environment, in terms of my research being central to the condensed matter effort of the department.

Credit: Rick Kubetz, University of Illinois

Philip Phillips collaborated with theoretical particle physicist Robert Leigh, also of the University of Illinois, Urbana-Champaign. Not shown in the picture and also contributors to the work are postdoctoral fellow Mohammad Edalati and graduate student Ka Wai Lo.

Q: So what has been your focus at Illinois?

P.P.: The big problem I have been trying to solve since 1995 is the physics of strongly coupled electron systems. I have been attempting to figure out what it is that the electrons are doing as they interact with one another [within cuprate superconductors]. Our work shows that they form composites, and so once you understand what the composites are, then you can begin to describe the macroscopic properties of the system.

[In 1998, Argentinean physicist Juan] Maldacena made a conjecture in which he argued that there is a relationship between a strongly coupled quantum mechanical system and a gravitational system [that] is entirely classical Einsteinian gravity. So in fact, strongly coupled quantum mechanical systems that are charged are equivalent to a curved space-time with a black hole in it. We showed that if you just introduce some probe fermions and these probe fermions are coupled to the space-time in a particular way, that system looks identical to the normal [nonsuperconducting] state of high-temperature superconductors.

Others have used this mapping before. What we did that was new is that we used a particular interaction between the probe fermions and the black hole that is really irrelevant to the physics of the black hole but changes the physics at the boundary of the space-time [which is where the quantum mechanical theory lives]. No one suspected it.

With such a model, you can just forget about trying to figure out what the basic building blocks are, just go and solve this geometry problem and extrapolate it to what's going on at the surface of this geometry, and you'll see what the quantum mechanical system is doing.

Q: And so have all these years of getting closer to your true scientific interests finally paid off?

P.P.: It takes a lot of daring to invest the time to go and learn this machinery [for geometrizing quantum mechanics] because it's fairly nontrivial stuff, and to think that it has answers for a real-life system is another risk.

I certainly thought that, God, if I were more traditionally trained, maybe I could have known some of the pitfalls of some of the things I've tried in the past. But now it's turning out that it was a good thing. The most important thing about my roundabout way is that I don't have any biases. I'm very open to new approaches and new problems and I don't mind just going and rolling up my sleeves and trying something new. This research is the culmination of what I thought I wanted to do when I was a graduate student.

Philip Phillips C.V. Highlights

1979: Receives bachelor's degree in math and chemistry from Walla Walla Community College in Washington state

1979–81: Graduate research assistant in theoretical chemistry at the University of Washington, Seattle

1981–84: Miller Postdoctoral Fellowship, University of California, Berkeley

1984–90: Assistant professor of chemistry, Massachusetts Institute of Technology, Cambridge

1990–93: Associate professor, Department of Chemistry, Massachusetts Institute of Technology, Cambridge

1993–99: Associate professor, Department of Physics, University of Illinois, Urbana-Champaign

2000–Present: Professor of physics, Department of Physics, University of Illinois, Urbana-Champaign

Superconductivity at its centenery

In a previous entry, I have mentioned that, this year witnesses the 100th anniversary. Many scientific journals and magazines are piling special sections on this event. Here are what come from Science:

• Superconductivity's Smorgasbord of Insights: A Movable Feast

  • Adrian Cho

  Science 8 April 2011: 190-192.
  • Summary
  • Full Text
  • Full Text (PDF)
• News Search for Majorana Fermions Nearing Success at Last?

  • Robert F. Service

  Science 8 April 2011: 193-195.
  • Summary
  • Full Text
  • Full Text (PDF)
• The Challenge of Unconventional Superconductivity

  • Michael R. Norman

  Science 8 April 2011: 196-200.
  • Abstract
  • Full Text
  • Full Text (PDF)
• The Electron-Pairing Mechanism of Iron-Based Superconductors

  • Fa Wang and
  • Dung-Hai Lee

  Science 8 April 2011: 200-204.

A TED talk by Janna Levin

I would like to embed the link to her talk. In this film, she talks of black holes , dynamics of space-time in the form of gravitational waves and the big bang. Especially, she discusses how such things can be heard. She presented to audience some astounding examples. I'm really impressed in the wonders of the pictures she brought there. They were actually made by numerical simulations.
That is marvelous !

Schrodinger's cat fattened up

The famous Schrodinger cat now gets fattened up to 430 atoms [http://www.nature.com/news/2011/110405/full/news.2011.210.html]!

In the famous thought experiment conceived by Erwin Schrödinger in 1935 to illustrate the apparent paradoxes of quantum theory, a cat would be poisoned or not depending on the state of an atom — the atom's state being governed by quantum rules. Because quantum theory required that these rules allowed superpositions, it seemed that Schrödinger's cat could itself exist in a superposition of 'live' and 'dead' states.

The paradox highlights the question of how and when the rules of the quantum world – in which objects such as atoms can exist in several positions at once – give way to the 'classical' mechanics that governs the macroscopic world of our everyday experience, where things must be one way or the other but not both at the same time. This is called the quantum-to-classical transition.

It is now generally thought that 'quantumness' is lost in a process called decoherence, in which disturbances in the immediate environment make the quantum wavefunction describing many-state superpositions appear to collapse into a well-defined, unique classical state. This decoherence tends to become more pronounced the bigger the object, as the opportunities for interacting with the environment increase.

One manifestation of quantum superposition is the interference that can occur between quantum particles passing through two or more narrow slits. In the classical world the particles pass through with their trajectories unchanged, like footballs rolling through a doorway.

But quantum particles can behave like waves, which interfere with one another as they pass through the slits, either enhancing or cancelling each other out to produce a series of bright and dark bands. This interference of quantum particles, first seen for electrons in 1927, is effectively the result of each particle passing through more than one slit: a quantum superposition.

As the experiment is scaled up in size, at some point quantum behaviour (interference) should give way to classical behaviour (no interference). But how big can the particles be before that happens?

Scaling up

In 1999, a team at the University of Vienna demonstrated interference in a many-slit experiment using beams of 60-atom carbon molecules (C₆₀), which are shaped like hollow spheres². Now Markus Arndt, one of the researchers involved in that experiment, and his colleagues in Austria, Germany, the United States and Switzerland have shown much the same effect for considerably larger molecules tailor-made for the purpose — up to 6 nanometres (millionths of a millimetre) across and composed of up to 430 atoms. These are bigger than some small protein molecules, such as insulin.

In the team's experiment, the beams of molecules are passed through three sets of slits. The first slit, made from a slice of silicon nitride patterned with a grating consisting of slits 90 nanometres wide, forces the molecular beam into a coherent state, in which the matter waves are all in step. The second, a 'virtual grating' made from laser light formed by mirrors into a standing wave of light and dark, causes the interference pattern. The third grating, also of silicon nitride, acts as a mask to admit parts of the interference pattern to a quadrupole mass spectrometer, which counts the number of molecules that pass through.

The researchers report in Nature Communications today that this number rises and falls periodically as the outgoing beam is scanned from left to right, showing that interference, and therefore superposition, is present.

Although this might not sound like a Schrödinger cat experiment, it probes the same quantum effects. It is essentially like firing the cats themselves at the interference grating, rather than making a single cat's fate contingent on an atomic-scale event.

Quantum physicist Martin Plenio of the University of Ulm in Germany calls the study part of an important line of research. "We have perhaps not gained deep new insights into the nature of quantum superposition from this specific experiment," he admits, "but there is hope that with increasing refinement of the experimental technique we will eventually discover something new."

Arndt says that such experiments might eventually allow tests of fundamental aspects of quantum theory, such as how wavefunctions collapse under observation. "Predictions, such as that gravity might induce wavefunction collapse beyond a certain mass limit, should become testable at significantly higher masses in far-future experiments," he says.

Can living organisms – perhaps not cats, but microorganisms such as bacteria – be placed in superpositions? That has been proposed for viruses³, the smallest of which are just a few nanometres across – although there is no consensus about whether viruses should be considered truly alive. "Tailored molecules are much easier than viruses to handle in such experiments," says Arndt. But he adds that if various technical issues can be addressed, "I don't see why it should not work".

This cat has been at the center of quantum community since its first time being proposed by Schrodinger. Arguably, it contains the main piece of every mystery of quantum physics: it is stated that, the cat can be in a superposition state of death and alive. Somebody think there might exist a kind of 'quantum-to-classical' transition, say, due to gravity. Although it is quite fair to claim that quantum mechanics applies to cats of any mass, not only microscopic ones but also macroscopic, it is still a special interest to experimentally confirm it. If quantum mechanics really captures something true, it would be crazy to think that, it is applicable only to small things. The success of this theory in all kinds of practices has only demonstrated its universality.

Spins coupled to a mechanical resonator

Employing the spin-phonon coupling, they argued that, a tiny magnet welded with a torsional mechanical oscillator can be described by a spin-boson model [PRL, 106:147203(2011)]. What interests me is actually this, is it possible to filter spins using magnetoptical coupling schemes ? Especially, what is the implication for the DNA filtering effects that were reported earlier in this blog. See also the review [Phyics, 4:28(2011)].

Remarks on phase transitions

It is usually said that, phase transitions are associated with singular, non-analytic and discontinuous behaviors of physical functions such as the thermodynamic potential or other non-equilibrium ones. But this is so only for infinite systems, in which any small difference in energy density can be infinitely magnified due the infinity of volumn. In infinite systems, phase transitions are sharp and abrupt, and ergodicity is completely lost when certain state is selected under symmetry breaking. This means infinite life time of the selected state, and thus very sharp transition. For finite systems, which represent the reality, phase transitions are never as sharp as that in infinite systems, since in this case the life time, though long, but finite. Also, no genuine singularities exist. Only strong crossovers can be observed, provided sufficient resolution. A crucial feature of finite systems might be that, configurations with small energy density differences could have very strong mixing and fluctuations and may not be distinguishable for certain resolutions.

Manifesto for higher Tc

A perspective by A.V.Chubukov, who is an enthusiast of spin-fluctuation type gluon. He compared the iron-based and the copper-based superconductors and surmise a higher Tc, observing
(1)Kexp/KLDA for existing high Tc SCs is around 0.5;
(2)superfluid density ~ dc conductivity x Tc.
Point (1) indicates that, high Tc is likely with a mixture of mobility and localization [Nature Physics, 7:272(2011)].

More on the pseudogap in cuprate

This article [http://www.nature.com/nphys/journal/v7/n4/full/nphys1921.html?WT.ec_id=NPHYS-201104#/affil-auth] definitely refreshes one's mind in thinking about the nature of the pseudogap of cuprates. The review is here [http://www.nature.com/nphys/journal/v7/n4/full/nphys1973.html?WT.ec_id=NPHYS-201104]. The authors measured the specific heat of UD YBCO under magnetic field. Oscillations were found above H* ~27T. But not only that, there is a background scaling as the square root of H.
What does this imply ?

SC fluctuations not so strong as previously thought

Here is a wonderful Letter [Nature Physics, 7:298(2011)] which, in a sense, denies the pseudogap as precursor of SC in cuprate. These authors probes LSCO, a very typical p-type high Tc, by Terahertz spectroscopy. They found that, such SC fluctuations persist up to 16 K above Tc, much weaker than former speculations. This is actually quite consistent with a latest work with ARPES [see my yesterday's entry].

The nature of the underdoped pseudogap regime of the high-temperature copper oxide superconductors has been a matter of long-term debate^{1, 2, 3}. On quite general grounds, we expect that, owing to their low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties in this part of the phase diagram^{4, 5}. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap and their significance for understanding high-temperature superconductivity^{6, 7, 8, 9, 10}. Here we use THz time-domain spectroscopy to probe the temporal fluctuations of superconductivity above the critical temperature (T_c) in La_2−xSr_xCuO₄ (LSCO) thin films over a doping range that spans almost the entire superconducting dome (x=0.09–0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, at most 16 K above T_c. Our measurements show that superconducting correlations do not make an appreciable contribution to the charge-transport anomalies of the pseudogap in LSCO at temperatures well above T_c.

A way to sense the change in natural constants

Physical laws bear some constants, which are deemed natural and for the moment lacks an explanation. It is believed that, such constants might be derivable from more fundamental laws, which remain to be discovered yet. If so, it is expected that, the value of such constants should be determined by the initial conditions of the universe and might evolve in time. Given this, how do we measure such change ?

A paper in PRL [Phys. Rev. Lett. 106, 100801 (2011)] suggests one method. It looks at the e/p mass ratio. See the synopsis on this [http://www.nature.com/nphys/journal/v7/n4/full/nphys1982.html?WT.ec_id=NPHYS-201104]: "Methanol is among the simplest molecules that undergo internal rotation. It is also one of the most abundant molecules and is responsible for prominent radio emission lines generated by astrophysical masers. Paul Jansen and colleagues have found that transitions that convert energy from internal rotations of the methyl (CH₃) and hydroxyl (OH) groups of methanol to gross rotation of the molecule as a whole depend strongly on the value of the proton-to-electron mass ratio. If this ratio were to change by a fraction, their calculations indicate that the emission frequency of this transition should change by 50 times the fraction."

A century after Onne's discovery

This year witnesses the centenary of superconductivity, which has spawned incredibly rich physics not limited to cryogenic field. Look at this review [http://www.nature.com/nphys/journal/v7/n4/full/nphys1981.html?WT.ec_id=NPHYS-201104]:

But continuing work on fundamental superconductivity is not the only legacy of the original discovery. Nor is the research concentrated on the high-temperature superconductors. On the contrary, organic superconductors, heavy fermions and ruthenates all continue to hold secrets. Over the past century, many ideas spawned by superconductivity have influenced or directly led to whole new fields of research. These include the study of helium-3, both for its cryogenic applications and multiple superfluid phases — Landau's Fermi liquid theory was originally proposed to explain the properties of helium-3. The study of non-Fermi liquids, with several examples of quantum criticality, is another active field.

In fact, these are all examples of strongly correlated electron systems, in which the whole is greater than the sum of its parts. Or, to quote Philip Anderson: “more is different”. Such materials exhibit all kinds of unexpected behaviour such as geometric frustration, glassy dynamics and metal–insulator transitions, to name but a few. Work in low-dimensional systems, including the study and manipulation of heterointerfaces within a superlattice structure, is also ongoing; and superconductivity even has an important role to play in the search for Majorana fermions in topological insulators.

Two Consecutive Thermal phase transitons make a High Tc supercoductor !

This is claimed in this wonderful article [Science, 331:1579(2011)], which I have already mentioned in my yesterday's entry.

In cuprate superconductors, people observe not only one d-wave SC gap but an additional gap, which opens up at the zone boundary and at a temperature T* far above Tc. A natural question is how these two gaps are connected. Two proposed scenarios are common in the market: one takes both originating from the same source while the other associates them with respective orders. Clarifying this question paves the way to the ultimate theory of high Tc copper oxides.

Now this article provides convincing evidence, combining ARPES, Polor Ker Effect and Time Resolved Reflectivity, that, T* signals a true but somewhat rounded thermal phase transition into a non-Sc phase. Actually, their work revealed three temperatures: Tc, T* and Tg. Here the Tg is a bit higher than Tc but far below T*, indicating the pairing fluctuations. Their mean-field calculations some candidate orders suggest that the paring energy and the pseudogap energy are of the same order, raising the question if they are connected in a deeper manner.