Mentor
Blog Entries: 27

## Recent Noteworthy Physics Papers

G. Kirchmair et al., "State-independent experimental test of quantum contextuality", Nature v.460, p.494 (2009).

Abstract: The question of whether quantum phenomena can be explained by classical models with hidden variables is the subject of a long-lasting debate. In 1964, Bell showed that certain types of classical models cannot explain the quantum mechanical predictions for specific states of distant particles, and some types of hidden variable models have been experimentally ruled out. An intuitive feature of classical models is non-contextuality: the property that any measurement has a value independent of other compatible measurements being carried out at the same time. However, a theorem derived by Kochen, Specker and Bell shows that non-contextuality is in conflict with quantum mechanics. The conflict resides in the structure of the theory and is independent of the properties of special states. It has been debated whether the Kochen–Specker theorem could be experimentally tested at al. First tests of quantum contextuality have been proposed only recently, and undertaken with photons and neutrons. But these tests required the generation of special quantum states and left various loopholes open. Here we perform an experiment with trapped ions that demonstrates a state-independent conflict with non-contextuality. The experiment is not subject to the detection loophole and we show that, despite imperfections and possible measurement disturbances, our results cannot be explained in non-contextual terms.

Zz.
 Mentor Blog Entries: 27 Y. Jompol et al., "Probing Spin-Charge Separation in a Tomonaga-Luttinger Liquid, Science v.325 p.597 (2009). Abstract: In a one-dimensional (1D) system of interacting electrons, excitations of spin and charge travel at different speeds, according to the theory of a Tomonaga-Luttinger liquid (TLL) at low energies. However, the clear observation of this spin-charge separation is an ongoing challenge experimentally. We have fabricated an electrostatically gated 1D system in which we observe spin-charge separation and also the predicted power-law suppression of tunneling into the 1D system. The spin-charge separation persists even beyond the low-energy regime where the TLL approximation should hold. TLL effects should therefore also be important in similar, but shorter, electrostatically gated wires, where interaction effects are being studied extensively worldwide. Just imagine - a charge carrier (say an electron), somehow behaves as if it's spin and its charge have been fractionalized, and thus, move differently. This is what spin-charge separation is. It is one of those fundamental phenomena in condensed matter physics that isn't observed anywhere else, but is something that could potentially be a fundamental principle in the physics of elementary particles. Previous experiments have shown signatures of such spin-charge separation. It has been shown that the charge and thermal currents in 1D organic conductors violate the Wiedemann-Franz law, an indication of a possible spin-charge separation. The charge current had a different dispersion than the thermal currents, something you don't find in a standard Solid State Physics text. In this new experiment, a different type of experiment was done - tunneling into a 1D system. There appears to be clear signatures of the spin-charge separation in the tunneling currents that were observed. Zz.
 Mentor Blog Entries: 27 S. S. Hodgman et al., "Metastable Helium: A New Determination of the Longest Atomic Excited-State Lifetime", Phys. Rev. Lett. v.103, p.053002 (2009). Abstract: Exited atoms may relax to the ground state by radiative decay, a process which is usually very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent such rapid decay, in which case these “metastable” states can have lifetimes of order seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-lived neutral atomic state—the first excited state of helium (the $2 ^3S_1$ metastable state)—to the highest accuracy yet measured. We use laser cooling and magnetic trapping to isolate a cloud of metastable helium (He*) atoms from their surrounding environment, and measure the decay rate to the ground $1 ^1S_0$ state via extreme ultraviolet (XUV) photon emission. This is the first measurement using a virtually unperturbed ensemble of isolated helium atoms, and yields a value of 7870(510) seconds, in excellent agreement with the predictions of quantum electrodynamic theory. Whoa! That's more than 2 hours! Zz.
 Mentor Blog Entries: 27 Z. Bern et al., "Ultraviolet Behavior of N=8 Supergravity at Four Loops", Phys. Rev. Lett. 103, 081301 (2009). Abstract: We describe the construction of the complete four-loop four-particle amplitude of N=8 supergravity. The amplitude is ultraviolet finite, not only in four dimensions, but in five dimensions as well. The observed extra cancellations provide additional nontrivial evidence that N=8 supergravity in four dimensions may be ultraviolet finite to all orders of perturbation theory. Read a review of this work AND get full access to the paper itself at APS Physics. Zz.
 Mentor Blog Entries: 27 L. Maccone "Quantum Solution to the Arrow-of-Time Dilemma", Phys. Rev. Lett. 103, 080401 (2009). Abstract: The arrow-of-time dilemma states that the laws of physics are invariant for time inversion, whereas the familiar phenomena we see everyday are not (i.e., entropy increases). I show that, within a quantum mechanical framework, all phenomena which leave a trail of information behind (and hence can be studied by physics) are those where entropy necessarily increases or remains constant. All phenomena where the entropy decreases must not leave any information of their having happened. This situation is completely indistinguishable from their not having happened at all. In the light of this observation, the second law of thermodynamics is reduced to a mere tautology: physics cannot study those processes where entropy has decreased, even if they were commonplace. Read the Focus article on this paper here: http://focus.aps.org/story/v24/st7 Zz.

Mentor
Blog Entries: 27
 Quote by laurencn106 D J Kapner et al. " Tests of the Gravitational Inverse-Square Law below the Dark-Energy Length Scale", Phys. Rev. Lett. 98 021101 (2007) Abstract: We conducted three torsion-balance experiments to test the gravitational inverse-square law at separations between 9.53 mm and 55 µm, probing distances less than the dark-energy length scale lambdad=radical(radix(4)[h-bar]c/rho[sub d])[approximate]85 µm. We find with 95% confidence that the inverse-square law holds (|alpha|<=1) down to a length scale lambda=56 µm and that an extra dimension must have a size R<=44 µm.
Thanks for your contribution, but this paper was highlighted already 2 years ago here:

http://www.physicsforums.com/showpos...4&postcount=39

As per the "theme" of this thread, we try to highlight papers within the past year. If you are unsure if a paper has been highlighted here already, do a search on the thread on the first author's name.

Zz.
 Mentor Blog Entries: 27 S. Rao et al. "Measurement of Mechanical Forces Acting on Optically Trapped Dielectric Spheres Induced by Surface-Enhanced Raman Scattering, Phys. Rev. Lett. v.102, p.087401 (2009). Abstract: Surface enhanced Raman scattering (SERS) is studied from optically trapped dielectric spheres partially covered with silver colloids in a solution with SERS active molecules. The Raman scattering and Brownian motion of the sphere are simultaneously measured to reveal correlations between the enhancement of the Raman signal and average position of the sphere. The correlations are due to the momenta transfer of the emitted Raman photons from the probe molecules. The addition of a mechanical force measurement provides a different dimension to the study of Raman processes. You may also read the Physical Review FOCUS review of this work here. Zz.
 Recognitions: Science Advisor A. J. Bennett et al., "Interference of dissimilar photon sources, Nature Physics v.5 p.715-717 (2009). Abstract: If identical photons meet at a semi-transparent mirror they seem to leave in the same direction, an effect called 'two-photon interference'. It has been known for some time that this effect should occur for photons generated by dissimilar sources with no common history, provided the measurement cannot distinguish between the photons. Here, we report a technique for observing such interference with isolated, unsynchronized sources for which the coherence times differ by several orders of magnitude. In an experiment we cause photons generated by different physical processes, with different photon statistics, to interfere. One of the sources is stimulated emission from a tunable laser, which has Poissonian statistics and a nanoelectronvolt bandwidth. The other is spontaneous emission from a quantum dot in a p–i–n diode with a few-microelectronvolt linewidth. We develop a theory to explain the visibility of interference, which is primarily limited by the timing resolution of our detectors. It is well known that there is a close connection between indistinguishability and interference. Therefore recently there have been lots of efforts to test to which extent distinguishable photons can be made indistinguishable in terms of an experiment. This has been shown in several systems before, including single atoms, ions, consecutive single photons from single quantum dots and even different semiconductor nanostructures. Bennett et al. now prove that even photons from completely different light sources can show two-photon interference.
 is there any papers published at the high-school level?
 Quantum Zeno effect explains magnetic-sensitive radical-ion-pair reactions , Phys. Rev. E 80, 056115 (2009) - http://arxiv.org/abs/0806.0739 Abstract:Chemical reactions involving radical-ion pairs are ubiquitous in biology, since not only are they at the basis of the photosynthetic reaction chain, but are also assumed to underlie the biochemical magnetic compass used by avian species for navigation. Recent experiments with magnetic-sensitive radical-ion-pair reactions provided strong evidence for the radical-ion-pair magnetoreception mechanism, verifying the expected magnetic sensitivities and chemical product yield changes. It is here shown that the theoretical description of radical-ion-pair reactions used since the 70s cannot explain the observed data, because it is based on phenomenological equations masking quantum coherence effects. The fundamental density-matrix equation derived here from basic quantum measurement theory considerations naturally incorporates the quantum Zeno effect and readily explains recent experimental observations on low- and high magnetic-field radical-ion-pair reactions.
 Mentor Blog Entries: 27 R. Gerritsma et al. "Quantum simulation of the Dirac equation", Nature v.463, p.68 (2010) . Abstract: The Dirac equation successfully merges quantum mechanics with special relativity. It provides a natural description of the electron spin, predicts the existence of antimatter and is able to reproduce accurately the spectrum of the hydrogen atom. The realm of the Dirac equation—relativistic quantum mechanics—is considered to be the natural transition to quantum field theory. However, the Dirac equation also predicts some peculiar effects, such as Klein’s paradox and ‘Zitterbewegung’, an unexpected quivering motion of a free relativistic quantum particle. These and other predicted phenomena are key fundamental examples for understanding relativistic quantum effects, but are difficult to observe in real particles. In recent years, there has been increased interest in simulations of relativistic quantum effects using different physical set-ups in which parameter tunability allows access to different physical regimes. Here we perform a proof-of-principle quantum simulation of the one-dimensional Dirac equation using a single trapped ion set to behave as a free relativistic quantum particle. We measure the particle position as a function of time and study Zitterbewegung for different initial superpositions of positive- and negative-energy spinor states, as well as the crossover from relativistic to non-relativistic dynamics. The high level of control of trapped-ion experimental parameters makes it possible to simulate textbook examples of relativistic quantum physics. Zz.
 Mentor Blog Entries: 27 R.B. Lanyon et al., "Towards quantum chemistry on a quantum computer" Nature Chemistry v.2, p.106 (2009). Abstract: Exact first-principles calculations of molecular properties are currently intractable because their computational cost grows exponentially with both the number of atoms and basis set size. A solution is to move to a radically different model of computing by building a quantum computer, which is a device that uses quantum systems themselves to store and process data. Here we report the application of the latest photonic quantum computer technology to calculate properties of the smallest molecular system: the hydrogen molecule in a minimal basis. We calculate the complete energy spectrum to 20 bits of precision and discuss how the technique can be expanded to solve large-scale chemical problems that lie beyond the reach of modern supercomputers. These results represent an early practical step toward a powerful tool with a broad range of quantum-chemical applications. You can read a review of this work at Wired. Zz.
 Recognitions: Science Advisor D. W. Berry, et al., "Fair-sampling assumption is not necessary for testing local realism" Phys. Rev. A 81, 012109 (2010). Abstract: Almost all Bell inequality experiments to date have used postselection and therefore relied on the fair sampling assumption for their interpretation. The standard form of the fair sampling assumption is that the loss is independent of the measurement settings, so the ensemble of detected systems provides a fair statistical sample of the total ensemble. This is often assumed to be needed to interpret Bell inequality experiments as ruling out hidden-variable theories. Here we show that it is not necessary; the loss can depend on measurement settings, provided the detection efficiency factorizes as a function of the measurement settings and any hidden variable. This condition implies that Tsirelson’s bound must be satisfied for entangled states. On the other hand, we show that it is possible for Tsirelson’s bound to be violated while the Clauser-Horne-Shimony-Holt (CHSH)-Bell inequality still holds for unentangled states, and present an experimentally feasible example. Although I do not care much about interpretational issues and all that nonlocality vs. local realism stuff, a lot of people around here do. Therefore some people on these forums might be interested in this formal treatment on the meaning of fair sampling.
 Mentor Blog Entries: 27 Holger Müller, Achim Peters, & Steven Chu "A precision measurement of the gravitational redshift by the interference of matter waves", Nature v.463, p.926 (2010). Abstract: One of the central predictions of metric theories of gravity, such as general relativity, is that a clock in a gravitational potential U will run more slowly by a factor of 1 + U/c^2, where c is the velocity of light, as compared to a similar clock outside the potential. This effect, known as gravitational redshift, is important to the operation of the global positioning system, timekeeping and future experiments with ultra-precise, space-based clocks (such as searches for variations in fundamental constants). The gravitational redshift has been measured using clocks on a tower, an aircraft and a rocket, currently reaching an accuracy of 7 × 10^-5. Here we show that laboratory experiments based on quantum interference of atoms enable a much more precise measurement, yielding an accuracy of 7 × 10^-9. Our result supports the view that gravity is a manifestation of space-time curvature, an underlying principle of general relativity that has come under scrutiny in connection with the search for a theory of quantum gravity. Improving the redshift measurement is particularly important because this test has been the least accurate among the experiments that are required to support curved space-time theories. You may read a report on this work at the PhysicsWorld website. Also, note the name of one of the authors of this paper. There is a "Steven Chu", who is currently the Secretary of the US Dept. of Energy! :) Zz.
 Mentor Blog Entries: 27 H. Shishido et al., "Tuning the Dimensionality of the Heavy Fermion Compound CeIn3" Science v.327, p.980 (2010). Abstract: Condensed-matter systems that are both low-dimensional and strongly interacting often exhibit unusual electronic properties. Strongly correlated electrons with greatly enhanced effective mass are present in heavy fermion compounds, whose electronic structure is essentially three-dimensional. We realized experimentally a two-dimensional heavy fermion system, adjusting the dimensionality in a controllable fashion. Artificial superlattices of the antiferromagnetic heavy fermion compound CeIn3 and the conventional metal LaIn3 were grown epitaxially. By reducing the thickness of the CeIn3 layers, the magnetic order was suppressed and the effective electron mass was further enhanced. Heavy fermions confined to two dimensions display striking deviations from the standard Fermi liquid low-temperature electronic properties, and these are associated with the dimensional tuning of quantum criticality. Also see the Perspective article by Piers Coleman in the same issue. This is a very interesting work since now, the "parameter" that is controlling the quantum phase transition is the dimensionality: 3D to 2D. Zz.
 Mentor Blog Entries: 27 R. Reyes et al., "Confirmation of general relativity on large scales from weak lensing and galaxy velocities", Nature v.464, p.256 (2010). Abstract: Although general relativity underlies modern cosmology, its applicability on cosmological length scales has yet to be stringently tested. Such a test has recently been proposed, using a quantity, E G, that combines measures of large-scale gravitational lensing, galaxy clustering and structure growth rate. The combination is insensitive to ‘galaxy bias’ (the difference between the clustering of visible galaxies and invisible dark matter) and is thus robust to the uncertainty in this parameter. Modified theories of gravity generally predict values of E G different from the general relativistic prediction because, in these theories, the ‘gravitational slip’ (the difference between the two potentials that describe perturbations in the gravitational metric) is non-zero, which leads to changes in the growth of structure and the strength of the gravitational lensing effect. Here we report that E G = 0.39 ± 0.06 on length scales of tens of megaparsecs, in agreement with the general relativistic prediction of E G ≈ 0.4. The measured value excludes a model1 within the tensor–vector–scalar gravity theory which modifies both Newtonian and Einstein gravity. However, the relatively large uncertainty still permits models within f(R) theory, which is an extension of general relativity. A fivefold decrease in uncertainty is needed to rule out these models. Edit: See PhysicsWorld coverage of this: http://physicsworld.com/cws/article/news/41948 Zz.
 Mentor Blog Entries: 27 Y. Kajiwara et al., "Transmission of electrical signals by spin-wave interconversion in a magnetic insulator" Nature v.464, p.262 (2010). Abstract: The energy bandgap of an insulator is large enough to prevent electron excitation and electrical conduction. But in addition to charge, an electron also has spin, and the collective motion of spin can propagate—and so transfer a signal—in some insulators. This motion is called a spin wave and is usually excited using magnetic fields. Here we show that a spin wave in an insulator can be generated and detected using spin-Hall effects, which enable the direct conversion of an electric signal into a spin wave, and its subsequent transmission through (and recovery from) an insulator over macroscopic distances. First, we show evidence for the transfer of spin angular momentum between an insulator magnet Y3Fe5O12 and a platinum film. This transfer allows direct conversion of an electric current in the platinum film to a spin wave in the Y3Fe5O12 via spin-Hall effects. Second, making use of the transfer in a Pt/Y3Fe5O12/Pt system, we demonstrate that an electric current in one metal film induces voltage in the other, far distant, metal film. Specifically, the applied electric current is converted into spin angular momentum owing to the spin-Hall effect in the first platinum film; the angular momentum is then carried by a spin wave in the insulating Y3Fe5O12 layer; at the distant platinum film, the spin angular momentum of the spin wave is converted back to an electric voltage. This effect can be switched on and off using a magnetic field. Weak spin damping3 in Y3Fe5O12 is responsible for its transparency for the transmission of spin angular momentum. This hybrid electrical transmission method potentially offers a means of innovative signal delivery in electrical circuits and devices. This appears to be the first instance of electrical signal being transmitted via spin waves. This should bring the possiblity of spintronics a step closer to reality. Zz.