Recent Noteworthy Physics Papers

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V. Parigi et al., "Probing Quantum Commutation Rules by Addition and Subtraction of Single Photons to/from a Light Field", Science v.317, p.1890 (2007).

Abstract: The possibility of arbitrarily "adding" and "subtracting" single photons to and from a light field may give access to a complete engineering of quantum states and to fundamental quantum phenomena. We experimentally implemented simple alternated sequences of photon creation and annihilation on a thermal field and used quantum tomography to verify the peculiar character of the resulting light states. In particular, as the final states depend on the order in which the two actions are performed, we directly observed the noncommutativity of the creation and annihilation operators, one of the cardinal concepts of quantum mechanics, at the basis of the quantum behavior of light. These results represent a step toward the full quantum control of a field and may provide new resources for quantum information protocols.

Read also the Perspective on this paper by R. Boyd et al. in the same issue of the journal. In that Perspective, the description of what has been accomplished can be summed up in these 2 paragraphs:

In an intriguing and illustrative report on page 1890 of this issue, Parigi et al. (3) present the results of a laboratory demonstration of what happens in the quantum mechanical operations of photon creation and annihilation, which lacks commutativity. These authors add a single photon to a light beam, which corresponds to the action of the standard quantum mechanical creation operator â. They can also subtract a single photon from the light beam, which corresponds to the annihilation operator a.

Parigi et al. measure the quantum mechanical state of a thermal light field after performing these two operations on it, and they show that the final state depends on the order in which the operations are performed. This result is a striking confirmation of the lack of commutativity of quantum mechanical operators. Moreover, the authors present the strongly counterintuitive result that, under certain conditions, the removal of a photon from a light field can lead to an increase in the mean number of photons in that light field, as predicted earlier.

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A. L. Cavalieri et al., "Attosecond spectroscopy in condensed matter", Nature v.449, p.1029 (2007).

Abstract: Comprehensive knowledge of the dynamic behaviour of electrons in condensed-matter systems is pertinent to the development of many modern technologies, such as semiconductor and molecular electronics, optoelectronics, information processing and photovoltaics. Yet it remains challenging to probe electronic processes, many of which take place in the attosecond (1 as = 10-18 s) regime. In contrast, atomic motion occurs on the femtosecond (1 fs = 10-15 s) timescale and has been mapped in solids in real time using femtosecond X-ray sources. Here we extend the attosecond techniques previously used to study isolated atoms in the gas phase to observe electron motion in condensed-matter systems and on surfaces in real time. We demonstrate our ability to obtain direct time-domain access to charge dynamics with attosecond resolution by probing photoelectron emission from single-crystal tungsten. Our data reveal a delay of approximately 100 attoseconds between the emission of photoelectrons that originate from localized core states of the metal, and those that are freed from delocalized conduction-band states. These results illustrate that attosecond metrology constitutes a powerful tool for exploring not only gas-phase systems, but also fundamental electronic processes occurring on the attosecond timescale in condensed-matter systems and on surfaces.

Please read the News and Views article on this work in the same issue of Nature, and the news report in PhysicsWorld.

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M. König et al, "Quantum Spin Hall Insulator State in HgTe Quantum Wells", Science v.318, p.766 (2007)

Abstract: Recent theory predicted that the quantum spin Hall effect, a fundamentally new quantum state of matter that exists at zero external magnetic field, may be realized in HgTe/(Hg,Cd)Te quantum wells. We fabricated such sample structures with low density and high mobility in which we could tune, through an external gate voltage, the carrier conduction from n-type to p-type, passing through an insulating regime. For thin quantum wells with well width d < 6.3 nanometers, the insulating regime showed the conventional behavior of vanishingly small conductance at low temperature. However, for thicker quantum wells (d > 6.3 nanometers), the nominally insulating regime showed a plateau of residual conductance close to 2e2/h, where e is the electron charge and h is Planck's constant. The residual conductance was independent of the sample width, indicating that it is caused by edge states. Furthermore, the residual conductance was destroyed by a small external magnetic field. The quantum phase transition at the critical thickness, d = 6.3 nanometers, was also independently determined from the magnetic field–induced insulator-to-metal transition. These observations provide experimental evidence of the quantum spin Hall effect.

Read also the Perspective on this paper by Nagaosa in the same issue of Science. This would be a very strong evidence for the quantum spin hall effect.

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D. Akoury et al., "The Simplest Double Slit: Interference and Entanglement in Double Photoionization of H2", Science v.318, p.949 (2007)

Abstract: The wave nature of particles is rarely observed, in part because of their very short de Broglie wavelengths in most situations. However, even with wavelengths close to the size of their surroundings, the particles couple to their environment (for example, by gravity, Coulomb interaction, or thermal radiation). These couplings shift the wave phases, often in an uncontrolled way, and the resulting decoherence, or loss of phase integrity, is thought to be a main cause of the transition from quantum to classical behavior. How much interaction is needed to induce this transition? Here we show that a photoelectron and two protons form a minimum particle/slit system and that a single additional electron constitutes a minimum environment. Interference fringes observed in the angular distribution of a single electron are lost through its Coulomb interaction with a second electron, though the correlated momenta of the entangled electron pair continue to exhibit quantum interference.

Also see review of this work at the PhysicsWorld website (free registration required) and at PhysOrg.

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L. Ozyuzer et al., "Emission of Coherent THz Radiation from Superconductors", Science v.318, p.1291 (2007).

Abstract: Compact solid-state sources of terahertz (THz) radiation are being sought for sensing, imaging, and spectroscopy applications across the physical and biological sciences. We demonstrate that coherent continuous-wave THz radiation of sizable power can be extracted from intrinsic Josephson junctions in the layered high-temperature superconductor Bi2Sr2CaCu2O8. In analogy to a laser cavity, the excitation of an electromagnetic cavity resonance inside the sample generates a macroscopic coherent state in which a large number of junctions are synchronized to oscillate in phase. The emission power is found to increase as the square of the number of junctions reaching values of 0.5 microwatt at frequencies up to 0.85 THz, and persists up to ~50 kelvin. These results should stimulate the development of superconducting compact sources of THz radiation.

Also read the Perspective of this work in the same issue of Science. A PhysicsWorld review of this work can be found here.

I know quite a bit regarding this work, since I've done tunneling spectroscopy on superconductors and that I've worked with this Bi compound. Not only that, I personally know 2 of the authors in this paper, including the lead author. In fact, I believe I was in the lab when they were doing this work (yes, I'm nosy and tend to stick my nose into people's lab, if they let me). So I'm terribly happy that they've managed to publish in Science and get quite a bit of publicity regarding this work.

Secondly, note that this is simply another one in a long line of examples where something that appears to be esoteric and purely "physics" such as quantum tunneling, Josephson current, and the physics of superconductivity can produce a clear useful application. This is still physics, not engineering. Yet, there is a clear application of a physics principle at work here.

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T. Paterek et al. "Experimental Test of Non-Local Realistic Theories Without The Rotational Symmetry Assumption", Phys. Rev. Lett. 99, 210406 (2007).

Abstract: We analyze the class of nonlocal realistic theories that was originally considered by Leggett [Found. Phys. 33, 1469 (2003)] and tested by us in a recent experiment [Nature (London) 446, 871 (2007)]. We derive an incompatibility theorem that works for finite numbers of polarizer settings and that does not require the previously assumed rotational symmetry of the two-particle correlation functions. The experimentally measured case involves seven different measurement settings. Using polarization-entangled photon pairs, we exclude this broader class of nonlocal realistic models by experimentally violating a new Leggett-type inequality by 80 standard deviations.

This appears to a be a follow up to their earlier Nature paper which I had highlighted here in this thread. They claim to have excluded even a larger class of non-local realistic model.

Interestingly enough, there is ANOTHER paper right after this that presents a similar report of the violation of the Leggett inequality.

Cyril Branciard et al. " Experimental Falsification of Leggett's Nonlocal Variable Model", Phys. Rev. Lett. 99, 210407 (2007).

Abstract: Bell's theorem guarantees that no model based on local variables can reproduce quantum correlations. Also, some models based on nonlocal variables, if subject to apparently “reasonable” constraints, may fail to reproduce quantum physics. In this Letter, we introduce a family of inequalities, which use a finite number of measurement settings, and which therefore allow testing Leggett's nonlocal model versus quantum physics. Our experimental data falsify Leggett's model and are in agreement with quantum predictions.

When you have two different experiments done by two independent group coming up with the same conclusion, it makes for a very convincing argument for the validity of such a conclusion.

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Johannes Kofler and Časlav Brukner, " Classical World Arising out of Quantum Physics under the Restriction of Coarse-Grained Measurements", Phys. Rev. Lett. 99, 180403 (2007).

Abstract: Conceptually different from the decoherence program, we present a novel theoretical approach to macroscopic realism and classical physics within quantum theory. It focuses on the limits of observability of quantum effects of macroscopic objects, i.e., on the required precision of our measurement apparatuses such that quantum phenomena can still be observed. First, we demonstrate that for unrestricted measurement accuracy, no classical description is possible for arbitrarily large systems. Then we show for a certain time evolution that under coarse-grained measurements, not only macrorealism but even classical Newtonian laws emerge out of the Schrödinger equation and the projection postulate.

A review of this work can be found in Nature's Daily Science news (link may be restricted or open only for a limited time). This is an interesting and important work because they are trying to show the quantum to classical "transition" via a different approach then the standard decoherence scenario. Essentially, the coarse-grained measurement that we make causes the classical world to emerge. If we make our measurement more precise for a system that has a large number of particles, then we should start detecting random "jumps" in the system that signify the emergence of the quantum world.

So let's see some clever experimentalist design an experiment to verify this. :)

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This is not entirely new research as such, but today's review article in Nature is still noteworthy as a review.

P. Monthoux, D. Pines and G. G. Lonzarich, Nature 450, 1177 (2007)

Abstract: The idea of superconductivity without the mediating role of lattice vibrations (phonons) has a long history. It was realized soon after the publication of the Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity 50 years ago that a full treatment of both the charge and spin degrees of freedom of the electron predicts the existence of attractive components of the effective interaction between electrons even in the absence of lattice vibrations—a particular example is the effective interaction that depends on the relative spins of the electrons. Such attraction without phonons can lead to electronic pairing and to unconventional forms of superconductivity that can be much more sensitive than traditional (BCS) superconductivity to the precise details of the crystal structure and to the electronic and magnetic properties of a material.

The paper discusses superconductivity may be found near the onset of a magnetically (or the analogously with charge density) ordered state. The phase space for an effectively attractive e-e interaction is shown to be much wider than that traditionally assumed from phonon mediated interactions.

http://www.nature.com/nature/journal...ature06480.pdf

http://www.eurekalert.org/pub_releas...-tqf122007.php

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C. Hertlein et al, "Direct measurement of critical Casimir forces", Nature v.451, p.172 (2008).

Abstract: When fluctuating fields are confined between two surfaces, long-range forces arise. A famous example is the quantum-electrodynamical Casimir force that results from zero-point vacuum fluctuations confined between two conducting metal plates. A thermodynamic analogue is the critical Casimir force: it acts between surfaces immersed in a binary liquid mixture close to its critical point and arises from the confinement of concentration fluctuations within the thin film of fluid separating the surfaces. So far, all experimental evidence for the existence of this effect has been indirect. Here we report the direct measurement of critical Casimir force between a single colloidal sphere and a flat silica surface immersed in a mixture of water and 2,6-lutidine near its critical point. We use total internal reflection microscopy to determine in situ the forces between the sphere and the surface, with femtonewton resolution6. Depending on whether the adsorption preferences of the sphere and the surface for water and 2,6-lutidine are identical or opposite, we measure attractive and repulsive forces, respectively, that agree quantitatively with theoretical predictions and exhibit exquisite dependence on the temperature of the system. We expect that these features of critical Casimir forces may result in novel uses of colloids as model systems.

Also read the News and Views article on this work in the same issue of Nature, and a review of this work at PhysicsWorld.

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I. Ferreras et al., "Necessity of Dark Matter in Modified Newtonian Dynamics within Galactic Scales", Phys. Rev. Lett. v.100, p.031302 (2008).

Abstract: To test modified Newtonian dynamics (MOND) on galactic scales, we study six strong gravitational lensing early-type galaxies from the CASTLES sample. Comparing the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis), we conclude that strong gravitational lensing on galactic scales requires a significant amount of dark matter, even within MOND. On such scales a 2 eV neutrino cannot explain the excess of matter in contrast with recent claims to explain the lensing data of the bullet cluster. The presence of dark matter is detected in regions with a higher acceleration than the characteristic MOND scale of ~10^-10 m/s^2. This is a serious challenge to MOND unless lensing is qualitatively different [possibly to be developed within a covariant, such as Tensor-Vector-Scalar (TeVeS), theory]

With the Bullet cluster evidence, and now this, could MOND be in serious trouble now?

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A.J. Leggett, "Realism and the physical world", Rep. Prog. Phys. v.71, p.022001 (2008)

Abstract: I consider the extent to which the applicability of the concept of classical realism is constrained, irrespective of the validity or not of the quantum formalism, by existing experiments both in the EPR–Bell setup, including recent experiments testing 'nonlocal realistic' theories, and in the area of 'macroscopic quantum coherence'. Unless we are willing to sacrifice one or more other intuitively plausible notions such as that of the conventional 'arrow of time', it appears impossible, in either context, to maintain the classical notion of realism.

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A. Caprez et al., "A macroscopic test of the Aharonov-Bohm effect", Phys. Rev. Lett., v99, p.210401 (2007).

Abstract: The Aharonov-Bohm (AB) effect is a purely quantum mechanical effect. The original (classified as Type-I) AB-phase shift exists in experimental conditions where the electromagnetic fields and forces are zero. It is the absence of forces that makes the AB-effect entirely quantum mechanical. Although the AB-phase shift has been demonstrated unambiguously, the absence of forces in Type-I AB-effects has never been shown. Here, we report the observation of the absence of time delays associated with forces of the magnitude needed to explain the AB-phase shift for a macroscopic system.

Also see the ArXiv preprint version.

A Perspective on this work can also be found in March 20, 2008 issue of Nature (Nature, v.452, p.298 (2008)).

Looks like the AB effect is non-local after all!

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A.N. Pasupathy et al., "Electronic Origin of the Inhomogeneous Pairing Interaction in the High-Tc Superconductor Bi2Sr2CaCu2O8+{delta}", Science v.320, p.196 (2008).

Abstract: Identifying the mechanism of superconductivity in the high-temperature cuprate superconductors is one of the major outstanding problems in physics. We report local measurements of the onset of superconducting pairing in the high–transition temperature (Tc) superconductor Bi2Sr2CaCu2O8+{delta} using a lattice-tracking spectroscopy technique with a scanning tunneling microscope. We can determine the temperature dependence of the pairing energy gaps, the electronic excitations in the absence of pairing, and the effect of the local coupling of electrons to bosonic excitations. Our measurements reveal that the strength of pairing is determined by the unusual electronic excitations of the normal state, suggesting that strong electron-electron interactions rather than low-energy (<0.1 volts) electron-boson interactions are responsible for superconductivity in the cuprates.

A review of this work can be found on ScienceDaily. So the cuprates may not have a "glue" that is responsible for the superconducting mechanism? Oh my! Phil Anderson might be right after all! :)

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E.V. Linder, "Mapping the cosmological expansion", Rep. Prog. Phys. v.71, p.056901 (2008).

Abstract: The ability to map the cosmological expansion has developed enormously, spurred by the turning point one decade ago of the discovery of cosmic acceleration. The standard model of cosmology has shifted from a matter dominated, standard gravity, decelerating expansion to the present search for the origin of acceleration in the cosmic expansion. We present a wide ranging review of the tools, challenges and physical interpretations. The tools include direct measures of cosmic scales through Type Ia supernova luminosity distances, and angular distance scales of baryon acoustic oscillation and cosmic microwave background density perturbations, as well as indirect probes such as the effect of cosmic expansion on the growth of matter density fluctuations. Accurate mapping of the expansion requires understanding of systematic uncertainties in both the measurements and the theoretical framework, but the result will give important clues to the nature of the physics behind accelerating expansion and to the fate of the universe.

And excellent review source, especially if you're interested in how various results in cosmology are obtained.

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D. N. Matsukevich et al., "Bell Inequality Violation with Two Remote Atomic Qubits", Phys. Rev. Lett. v.100, p.150404 (2008).

Abstract: We observe violation of a Bell inequality between the quantum states of two remote Yb+ ions separated by a distance of about 1 m with the detection loophole closed. The heralded entanglement of two ions is established via interference and joint detection of two emitted photons, whose polarization is entangled with each ion. The entanglement of remote qubits is also characterized by full quantum state tomography.

Could we be on a clear path for a loophole-free Bell-type experiment? This report certainly is providing a convincing evidence that we are well on our way!

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M.T. Murphy et al. "Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe", Science v. 320, p. 1611 (2008).

Abstract: The Standard Model of particle physics assumes that the so-called fundamental constants are universal and unchanging. Absorption lines arising in molecular clouds along quasar sightlines offer a precise test for variations in the proton-to-electron mass ratio, µ, over cosmological time and distance scales. The inversion transitions of ammonia are particularly sensitive to µ as compared to molecular rotational transitions. Comparing the available ammonia spectra observed toward the quasar B0218+357 with new, high-quality rotational spectra, we present the first detailed measurement of µ with this technique, limiting relative deviations from the laboratory value to |{Delta}µ/µ| < 1.8 x 10–6 (95% confidence level) at approximately half the universe's current age—the strongest astrophysical constraint to date. Higher-quality ammonia observations will reduce both the statistical and systematic uncertainties in these observations.

In other words, even as far back as half of the universe's age, this ratio of the mass of the proton to the mass of electron, hasn't changed up to the accuracy limit of this measurement.

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L. Li et al., "Phase Transitions of Dirac Electrons in Bismuth", Science v.321, p.547 (2008).

Abstract: The Dirac Hamiltonian, which successfully describes relativistic fermions, applies equally well to electrons in solids with linear energy dispersion, for example, in bismuth and graphene. A characteristic of these materials is that a magnetic field less than 10 tesla suffices to force the Dirac electrons into the lowest Landau level, with resultant strong enhancement of the Coulomb interaction energy. Moreover, the Dirac electrons usually come with multiple flavors or valley degeneracy. These ingredients favor transitions to a collective state with novel quantum properties in large field. By using torque magnetometry, we have investigated the magnetization of bismuth to fields of 31 tesla. We report the observation of sharp field-induced phase transitions into a state with striking magnetic anisotropy, consistent with the breaking of the threefold valley degeneracy.

Read a report of this work at Science Daily.

This is another example where relativistic equations need not require some esoteric conditions to be applicable. Some of them can be found in the very material that we use in our electronics.

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