# Recent Noteworthy Physics Papers

by ZapperZ
Tags: noteworthy, papers, physics
 Mentor P: 28,836 C.G. Camara et al. "Correlation between nanosecond X-ray flashes and stick–slip friction in peeling tape", Nature v.455, p.1089 (2008). Abstract: Relative motion between two contacting surfaces can produce visible light, called triboluminescence. This concentration of diffuse mechanical energy into electromagnetic radiation has previously been observed to extend even to X-ray energies. Here we report that peeling common adhesive tape in a moderate vacuum produces radio and visible emission along with nanosecond, 100-mW X-ray pulses that are correlated with stick–slip peeling events. For the observed 15-keV peak in X-ray energy, various models give a competing picture of the discharge process, with the length of the gap between the separating faces of the tape being 30 or 300 mum at the moment of emission. The intensity of X-ray triboluminescence allowed us to use it as a source for X-ray imaging. The limits on energies and flash widths that can be achieved are beyond current theories of tribology. This thing has been getting a lot of popular media coverage because the simple act of peeling an ordinary scotch tape in moderate vacuum can actually generate a small amount of short x-ray burst. Zz.
 P: 189 Event-by-Event Simulation of Einstein-Podolsky-Rosen-Bohm Experiments: http://www.springerlink.com/content/p28v88867w7213mu/ Open Access http://arxiv.org/pdf/0712.3693 Abstract We construct an event-based computer simulation model of the Einstein-Podolsky-Rosen-Bohm experiments with photons. The algorithm is a one-to-one copy of the data gathering and analysis procedures used in real laboratory experiments. We consider two types of experiments, those with a source emitting photons with opposite but otherwise unpredictable polarization and those with a source emitting photons with fixed polarization. In the simulation, the choice of the direction of polarization measurement for each detection event is arbitrary. We use three different procedures to identify pairs of photons and compute the frequency of coincidences by analyzing experimental data and simulation data. The model strictly satisfies Einstein’s criteria of local causality, does not rely on any concept of quantum theory and reproduces the results of quantum theory for both types of experiments. We give a rigorous proof that the probabilistic description of the simulation model yields the quantum theoretical expressions for the single- and two-particle expectation values.
 Mentor P: 28,836 A. Fragner et al. "Resolving Vacuum Fluctuations in an Electrical Circuit by Measuring the Lamb Shift", Science v.322, p.1357 (2008). Abstract: Quantum theory predicts that empty space is not truly empty. Even in the absence of any particles or radiation, in pure vacuum, virtual particles are constantly created and annihilated. In an electromagnetic field, the presence of virtual photons manifests itself as a small renormalization of the energy of a quantum system, known as the Lamb shift. We present an experimental observation of the Lamb shift in a solid-state system. The strong dispersive coupling of a superconducting electronic circuit acting as a quantum bit (qubit) to the vacuum field in a transmission-line resonator leads to measurable Lamb shifts of up to 1.4% of the qubit transition frequency. The qubit is also observed to couple more strongly to the vacuum field than to a single photon inside the cavity, an effect that is explained by taking into account the limited anharmonicity of the higher excited qubit states. An amazing feat to detect a Lamb shift in a many-body system such an a superconducting electronic circuit. Zz.
 Mentor P: 28,836 J. S. Lundeen and A. M. Steinberg, "Experimental Joint Weak Measurement on a Photon Pair as a Probe of Hardy's Paradox", Phys. Rev. Lett. 102, 020404 (2009). Abstract: It has been proposed that the ability to perform joint weak measurements on postselected systems would allow us to study quantum paradoxes. These measurements can investigate the history of those particles that contribute to the paradoxical outcome. Here we experimentally perform weak measurements of joint (i.e., nonlocal) observables. In an implementation of Hardy's paradox, we weakly measure the locations of two photons, the subject of the conflicting statements behind the paradox. Remarkably, the resulting weak probabilities verify all of these statements but, at the same time, resolve the paradox. This experiment appears to be the confirmation and resolution of the Hardy's paradox. A news article on this can be found here. Zz.
 Mentor P: 28,836 A. Cabello et al., "Proposed Bell Experiment with Genuine Energy-Time Entanglement", Phys. Rev. Lett. v.102, p.040401 (2009). Abstract: Franson's Bell experiment with energy-time entanglement [Phys. Rev. Lett. 62, 2205 (1989)] does not rule out all local hidden variable models. This defect can be exploited to compromise the security of Bell inequality-based quantum cryptography. We introduce a novel Bell experiment using genuine energy-time entanglement, based on a novel interferometer, which rules out all local hidden variable models. The scheme is feasible with actual technology. Zz.
 P: 92 this is a recent paper addressing the origin of spin glass in hole-doped cuprate superconductors. The author attempts a new mechanism for spin glass that can live with Zhang-Rice singlet states. The paper is located at J. Phys.: Condens. Matter 21 (2009) 075702 Abstract: To address the incompatibility of Zhang–Rice singlet formation and the observed spin glass behavior, an effective model is proposed for the electronic behavior of cuprate materials. The model includes an antiferromagnetic interaction between the spin of the hole in a Zhang–Rice orbital and the spin of the hole on the corresponding copper site. While in the large interaction limit this recovers the t–J model, in the low energy limit the Zhang–Rice singlets are deformed. It is also shown that such deformation can induce random defect ferromagnetic (FM) bonds between adjacent local spins, an effect herein referred to as unusual double exchange, and then spin glass behavior shall result in the case of localized holes. A derivation of the model is also presented.
 Mentor P: 28,836 V. Moshchalkov et al., "Type-1.5 Superconductivity" Phys. Rev. Lett. 102, 117001 (2009) Abstract: We demonstrate the existence of a novel superconducting state in high quality two-component MgB2 single crystalline superconductors where a unique combination of both type-1 (lambda1/xi1<1/sqrt(2)) and type-2 (lambda2/xi2>1/sqrt(2)) superconductor conditions is realized for the two components of the order parameter. This condition leads to a vortex-vortex interaction attractive at long distances and repulsive at short distances, which stabilizes unconventional stripe- and gossamerlike vortex patterns that we have visualized in this type-1.5 superconductor using Bitter decoration and also reproduced in numerical simulations. If this is true, they have found a new phase of superconductivity where both Type I and Type II properties resides in the same material (but in different bands). You may also read a review of this work at Physics, AND, get a free copy of the exact paper. Zz.
 Sci Advisor P: 1,563 D. Gross, S.T. Flammia and J. Eisert, "Most Quantum States Are Too Entangled To Be Useful As Computational Resources" Phys. Rev. Lett. 102, 190501 (2009) Abstract: It is often argued that entanglement is at the root of the speedup for quantum compared to classical computation, and that one needs a sufficient amount of entanglement for this speedup to be manifest. In measurement-based quantum computing, the need for a highly entangled initial state is particularly obvious. Defying this intuition, we show that quantum states can be too entangled to be useful for the purpose of computation, in that high values of the geometric measure of entanglement preclude states from offering a universal quantum computational speedup. We prove that this phenomenon occurs for a dramatic majority of all states: the fraction of useful n-qubit pure states is less than exp(-n2). This work highlights a new aspect of the role entanglement plays for quantum computational speedups. Quantum computers are still far from realization. Usually fast decoherence and the problem of producing high degrees of entanglement between large numbers of qubits are mentioned as the first big problems, which one thinks of. Now Gross et al. show that most highly entangled quantum states will not provide a significant increase in computational speed compared to classical computers. So it might be necessary in future to identify and understand the few remaining entangled states, which are indeed useful for computation. There is also an accompanying viewpoint to this article: http://link.aip.org/link/?&l_creator...2FPhysics.2.38
 Mentor P: 28,836 M. Gu et al. "More really is different", Physica D: Nonlinear Phenomena, v.238, p.835 (2009). Abstract: In 1972, P.W. Anderson suggested that ‘More is Different’, meaning that complex physical systems may exhibit behavior that cannot be understood only in terms of the laws governing their microscopic constituents. We strengthen this claim by proving that many macroscopic observable properties of a simple class of physical systems (the infinite periodic Ising lattice) cannot in general be derived from a microscopic description. This provides evidence that emergent behavior occurs in such systems, and indicates that even if a ‘theory of everything’ governing all microscopic interactions were discovered, the understanding of macroscopic order is likely to require additional insights. Read the News and Views article on this paper in Nature 459, 332-334 (21 May 2009). Edit: read the arXiv version here. Zz.
 Mentor P: 28,836 A. V. Ponomarev et al., "ac-Driven Atomic Quantum Motor", Phys. Rev. Lett. v.102, p.230601 (2009) . Abstract: We propose an ac-driven quantum motor consisting of two different, interacting ultracold atoms placed into a ring-shaped optical lattice and submerged in a pulsating magnetic field. While the first atom carries a current, the second one serves as a quantum starter. For fixed zero-momentum initial conditions the asymptotic carrier velocity converges to a unique nonzero value. We also demonstrate that this quantum motor performs work against a constant load. A review of this paper can also be found at the ScienceNews website. Zz.
 Mentor P: 28,836 R. Horodecki et al., "Quantum Entanglement", Rev. Mod. Phys. v.81, p865 (2009). Abstract: From the point of view of quantum information science, entanglement is a resource that can be used to perform tasks that are impossible in a classical world. In a certain sense, the more entanglement we have, the better we can perform those tasks. Thus, one of the main goals in this field has been to identify under which conditions two or more systems are entangled, and how entangled they are. This paper reviews the main criteria to detect entanglement as well as entanglement measures and also discusses the role of entanglement in quantum communication and cryptography. This is a HUGE, 78-page review of quantum entanglement. We get a lot of frequent questions on this topic, so it is appropriate to post a source that has a wealth of information and references. The Arxiv version of this paper can be found here. Zz.
 Mentor P: 28,836 M. Karski et al., "Quantum Walk in Position Space with Single Optically Trapped Atoms", Science v.325, p. 174 (2009). Abstract: The quantum walk is the quantum analog of the well-known random walk, which forms the basis for models and applications in many realms of science. Its properties are markedly different from the classical counterpart and might lead to extensive applications in quantum information science. In our experiment, we implemented a quantum walk on the line with single neutral atoms by deterministically delocalizing them over the sites of a one-dimensional spin-dependent optical lattice. With the use of site-resolved fluorescence imaging, the final wave function is characterized by local quantum state tomography, and its spatial coherence is demonstrated. Our system allows the observation of the quantum-to-classical transition and paves the way for applications, such as quantum cellular automata. Read the ScienceNow review of this work here. Zz.
 Mentor P: 28,836 M. Aßmann et. al., "Higher-Order Photon Bunching in a Semiconductor Microcavity", Science v.325, p.297 (2009). Abstract: Quantum mechanically indistinguishable particles such as photons may show collective behavior. Therefore, an appropriate description of a light field must consider the properties of an assembly of photons instead of independent particles. We have studied multiphoton correlations up to fourth order in the single-mode emission of a semiconductor microcavity in the weak and strong coupling regimes. The counting statistics of single photons were recorded with picosecond time resolution, allowing quantitative measurement of the few-photon bunching inside light pulses. Our results show bunching behavior in the strong coupling case, which vanishes in the weak coupling regime as the cavity starts lasing. In particular, we verify the n factorial prediction for the zero-delay correlation function of n thermal light photons. The bunching and anti-bunching phenomena are considered to be THE strongest evidence for photons. These have no classical equivalence. Zz.
 Mentor P: 28,836 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 P: 28,836 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 P: 28,836 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 P: 28,836 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 P: 28,836 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.

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