Mentor
Blog Entries: 27

## Recent Noteworthy Physics Papers

T. Jeltes et al., Comparison of the Hanbury Brown–Twiss effect for bosons and fermions, Nature v.445, p.402 (2007).

Abstract: Fifty years ago, Hanbury Brown and Twiss (HBT) discovered photon bunching in light emitted by a chaotic source1, highlighting the importance of two-photon correlations and stimulating the development of modern quantum optics. The quantum interpretation of bunching relies on the constructive interference between amplitudes involving two indistinguishable photons, and its additive character is intimately linked to the Bose nature of photons. Advances in atom cooling and detection have led to the observation and full characterization of the atomic analogue of the HBT effect with bosonic atoms. By contrast, fermions should reveal an antibunching effect (a tendency to avoid each other). Antibunching of fermions is associated with destructive two-particle interference, and is related to the Pauli principle forbidding more than one identical fermion to occupy the same quantum state. Here we report an experimental comparison of the fermionic and bosonic HBT effects in the same apparatus, using two different isotopes of helium: 3He (a fermion) and 4He (a boson). Ordinary attractive or repulsive interactions between atoms are negligible; therefore, the contrasting bunching and antibunching behaviour that we observe can be fully attributed to the different quantum statistics of each atomic species. Our results show how atom–atom correlation measurements can be used to reveal details in the spatial density or momentum correlations in an atomic ensemble. They also enable the direct observation of phase effects linked to the quantum statistics of a many-body system, which may facilitate the study of more exotic situations.

Also read the News and Views of this work in the same issue of Nature, and a review of it in PhysicsWeb.

Zz.

 Mentor Blog Entries: 27 Naomi S. Ginsberg et al., "Coherent control of optical information with matter wave dynamics", Nature v.445, p.623 (2007). Abstract: In recent years, significant progress has been achieved in manipulating matter with light, and light with matter1. Resonant laser fields interacting with cold, dense atom clouds provide a particularly rich system. Such light fields interact strongly with the internal electrons of the atoms, and couple directly to external atomic motion through recoil momenta imparted when photons are absorbed and emitted. Ultraslow light propagation in Bose–Einstein condensates represents an extreme example of resonant light manipulation using cold atoms. Here we demonstrate that a slow light pulse can be stopped and stored in one Bose–Einstein condensate and subsequently revived from a totally different condensate, 160 mum away; information is transferred through conversion of the optical pulse into a travelling matter wave. In the presence of an optical coupling field, a probe laser pulse is first injected into one of the condensates where it is spatially compressed to a length much shorter than the coherent extent of the condensate. The coupling field is then turned off, leaving the atoms in the first condensate in quantum superposition states that comprise a stationary component and a recoiling component in a different internal state. The amplitude and phase of the spatially localized light pulse are imprinted on the recoiling part of the wavefunction, which moves towards the second condensate. When this 'messenger' atom pulse is embedded in the second condensate, the system is re-illuminated with the coupling laser. The probe light is driven back on and the messenger pulse is coherently added to the matter field of the second condensate by way of slow-light-mediated atomic matter-wave amplification. The revived light pulse records the relative amplitude and phase between the recoiling atomic imprint and the revival condensate. Our results provide a dramatic demonstration of coherent optical information processing with matter wave dynamics. Such quantum control may find application in quantum information processing and wavefunction sculpting. This is from Lena Hau group who, a few years ago, demonstrated that light can be stopped and then retransmitted exactly (albeit at a lower intensity). The difference in this experiment is that they used quantum mechanical property (superposition) of the spins to store light in one BEC gas and then retransmit it using a 2nd BEC gas. Very clever! You may read an overview of this work below. Note that the Nature link works only for a short period of time. http://www.nature.com/news/2007/0702.../070205-8.html http://physicsweb.org/articles/news/11/2/7/1 There is also a streaming video from the Nature site: http://www.nature.com/nature/videoar...ght/index.html Zz.
 Mentor Blog Entries: 27 V. Jacques et al., "Experimental Realization of Wheeler's Delayed-Choice Gedanken Experiment", Science v.315, p.966 (2007). Abstract: Wave-particle duality is strikingly illustrated by Wheeler's delayed-choice gedanken experiment, where the configuration of a two-path interferometer is chosen after a single-photon pulse has entered it: Either the interferometer is closed (that is, the two paths are recombined) and the interference is observed, or the interferometer remains open and the path followed by the photon is measured. We report an almost ideal realization of that gedanken experiment with single photons allowing unambiguous which-way measurements. The choice between open and closed configurations, made by a quantum random number generator, is relativistically separated from the entry of the photon into the interferometer. Review can be found also at PhysicsWeb. Zz.
 Mentor Blog Entries: 27 M. Uiberacker et al., "Attosecond real-time observation of electron tunnelling in atoms", Nature v.446, p.627 (2007). Abstract: Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution. Also read the News and Views on this paper, and the PhysicsWeb review. Zz.
 Mentor Blog Entries: 27 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. You may read a couple of reviews of this work here and here. Zz.
 Mentor Blog Entries: 27 S. Groeblacher et al., An experimental test of non-local realism, Nature v.446, p.871 (2007). Abstract: Most working scientists hold fast to the concept of ‘realism’—a viewpoint according to which an external reality exists independent of observation. But quantum physics has shattered some of our cornerstone beliefs. According to Bell’s theorem, any theory that is based on the joint assumption of realism and locality (meaning that local events cannot be affected by actions in space-like separated regions) is at variance with certain quantum predictions. Experiments with entangled pairs of particles have amply confirmed these quantum predictions, thus rendering local realistic theories untenable. Maintaining realism as a fundamental concept would therefore necessitate the introduction of ‘spooky’ actions that defy locality. Here we show by both theory and experiment that a broad and rather reasonable class of such non-local realistic theories is incompatible with experimentally observable quantum correlations. In the experiment, we measure previously untested correlations between two entangled photons, and show that these correlations violate an inequality proposed by Leggett for non-local realistic theories. Our result suggests that giving up the concept of locality is not sufficient to be consistent with quantum experiments, unless certain intuitive features of realism are abandoned. A News and Views article written by Alain Aspect can be found in the same issue of Nature, and a Nature News article can be found here for free for a limited time only. Edit: You can also find a report on this at the PhysicsWeb page. Zz.
 Mentor Blog Entries: 27 M.P. Almeida et al. "Environment-Induced Sudden Death of Entanglement", Science v.316, p.579 (2007) Abstract: We demonstrate the difference between local, single-particle dynamics and global dynamics of entangled quantum systems coupled to independent environments. Using an all-optical experimental setup, we showed that, even when the environment-induced decay of each system is asymptotic, quantum entanglement may suddenly disappear. This "sudden death" constitutes yet another distinct and counterintuitive trait of entanglement. Read also the perspective on this paper in the same issue (p.555). It covers a large "history" of decoherence and this "sudden death" symptom of entanglement. Zz.
 Mentor Blog Entries: 27 A. Pimenov et al., Negative Refraction Observed in a Metallic Ferromagnet in the Gigahertz Frequency Range, Phys. Rev. Lett. 98, 197401 (2007). Abstract: It is generally believed that nature does not provide materials with negative refraction. Here we demonstrate experimentally that such materials do exist at least at GHz frequencies: ferromagnetic metals reveal a negative refraction index close to the frequency of the ferromagnetic resonance. The experimental realization utilizes a colossal magnetoresistance manganite La2/3Ca1/3MnO3 as an example. In this material the negative refractive index can be achieved even at room temperature using external magnetic fields. Commentary: This is quite a find. Previously, all material that exhibit such negative refraction are "metamaterial", i.e. they are constructed out of various elements such as wire arrays and split ring resonators. This is the first such "natural" material that has been shown to exhibit this property. Zz.
 Recognitions: Gold Member Science Advisor How pure is the material? Could this be an echo?
 Recognitions: Gold Member Homework Help Science Advisor Science 25 May 2007: Vol. 316. no. 5828, pp. 1169 - 1172 Reports An On-Demand Coherent Single-Electron Source G. Fève, A. Mahé, J.-M. Berroir, T. Kontos, B. Plaçais, D. C. Glattli, A. Cavanna, B. Etienne, Y. Jin We report on the electron analog of the single-photon gun. On-demand single-electron injection in a quantum conductor was obtained using a quantum dot connected to the conductor via a tunnel barrier. Electron emission was triggered by the application of a potential step that compensated for the dot-charging energy. Depending on the barrier transparency, the quantum emission time ranged from 0.1 to 10 nanoseconds. The single-electron source should prove useful for the use of quantum bits in ballistic conductors. Additionally, periodic sequences of single-electron emission and absorption generate a quantized alternating current.

Recognitions:
Gold Member
Staff Emeritus
V. V. Flambaum and M. G Kozlov, "Limit on the Cosmological Variation of $m_p/m_e$ from the Inversion Spectrum of Ammonia", Phys. Rev. Lett. 98, 240801 (2007)

 Fundamental constant is pretty much constant The idea that fundamental constants do not actually stay constant over space and time has long played on the mind of physicists. But by looking at how a distant galaxy has absorbed the light from a quasar, researchers in Australia have obtained a new limit on how much one fundamental constant -- the ratio of the electron and proton masses -- is changing with time. Their result, which is 10 times more accurate than previous measurements, gives the thumbs up to our current understanding of physics (Phys. Rev. Lett. 98 240801).
http://physicsweb.org/articles/news/11/6/11/1

 Mentor Blog Entries: 27 I. Neder et al. "Interference between two indistinguishable electrons from independent sources", Nature v.448, p.333 (2007). Abstract: Very much like the ubiquitous quantum interference of a single particle with itself, quantum interference of two independent, but indistinguishable, particles is also possible. For a single particle, the interference is between the amplitudes of the particle's wavefunctions, whereas the interference between two particles is a direct result of quantum exchange statistics. Such interference is observed only in the joint probability of finding the particles in two separated detectors, after they were injected from two spatially separated and independent sources. Experimental realizations of two-particle interferometers have been proposed; in these proposals it was shown that such correlations are a direct signature of quantum entanglement between the spatial degrees of freedom of the two particles ('orbital entanglement'), even though they do not interact with each other. In optics, experiments using indistinguishable pairs of photons encountered difficulties in generating pairs of independent photons and synchronizing their arrival times; thus they have concentrated on detecting bunching of photons (bosons) by coincidence measurements. Similar experiments with electrons are rather scarce. Cross-correlation measurements between partitioned currents, emanating from one source, yielded similar information to that obtained from auto-correlation (shot noise) measurements. The proposal of ref. 3 is an electronic analogue to the historical Hanbury Brown and Twiss experiment with classical light. It is based on the electronic Mach–Zehnder interferometer that uses edge channels in the quantum Hall effect regime. Here we implement such an interferometer. We partitioned two independent and mutually incoherent electron beams into two trajectories, so that the combined four trajectories enclosed an Aharonov–Bohm flux. Although individual currents and their fluctuations (shot noise measured by auto-correlation) were found to be independent of the Aharonov–Bohm flux, the cross-correlation between current fluctuations at two opposite points across the device exhibited strong Aharonov–Bohm oscillations, suggesting orbital entanglement between the two electron beams. In other words, the interference from different, independent sources that is taken for granted for classical electromagnetic waves, is now demonstrated for electrons as well for the very first time. Also read the News and Views about this paper in the same issue of Nature. Zz.
 Mentor Blog Entries: 27 T. Wilk et al., "Single-Atom Single-Photon Quantum Interface", Science, v.317, p.488 (2007). Abstract: A major challenge for a scalable quantum computing architecture is the faithful transfer of information from one node to another. We report on the realization of an atom-photon quantum interface based on an optical cavity, using it to entangle a single atom with a single photon and then to map the quantum state of the atom onto a second single photon. The latter step disentangles the atom from the light and produces an entangled photon pair. Our scheme is intrinsically deterministic and establishes the basic element required to realize a distributed quantum network with individual atoms at rest as quantum memories and single flying photons as quantum messengers. Zz.
 Mentor Blog Entries: 27 H. Müller et al., "Tests of Relativity by Complementary Rotating Michelson-Morley Experiments", Phys. Rev. Lett. v.99, p.050401 (2007). Abstract: We report relativity tests based on data from two simultaneous Michelson-Morley experiments, spanning a period of more than 1 yr. Both were actively rotated on turntables. One (in Berlin, Germany) uses optical Fabry-Perot resonators made of fused silica; the other (in Perth, Australia) uses microwave whispering-gallery sapphire resonators. Within the standard model extension, we obtain simultaneous limits on Lorentz violation for electrons (5 coefficients) and photons (8) at levels down to 10^-16, improved by factors between 3 and 50 compared to previous work. The more they test it, the more they verify it. Zz.
 Mentor Blog Entries: 27 A. Ourjoumtsev et al., "Generation of optical 'Schrödinger cats' from photon number states", Nature v.448, p.784 (2007). Abstract: Schrödinger's cat is a Gedankenexperiment in quantum physics, in which an atomic decay triggers the death of the cat. Because quantum physics allow atoms to remain in superpositions of states, the classical cat would then be simultaneously dead and alive. By analogy, a 'cat' state of freely propagating light can be defined as a quantum superposition of well separated quasi-classical states—it is a classical light wave that simultaneously possesses two opposite phases. Such states play an important role in fundamental tests of quantum theory and in many quantum information processing tasks, including quantum computation, quantum teleportation, 10 and precision measurements. Recently, optical Schrödinger 'kittens' were prepared; however, they are too small for most of the aforementioned applications and increasing their size is experimentally challenging. Here we demonstrate, theoretically and experimentally, a protocol that allows the generation of arbitrarily large squeezed Schrödinger cat states, using homodyne detection and photon number states as resources. We implemented this protocol with light pulses containing two photons, producing a squeezed Schrödinger cat state with a negative Wigner function. This state clearly exhibits several quantum phase-space interference fringes between the 'dead' and 'alive' components, and is large enough to become useful for quantum information processing and experimental tests of quantum theory. Zz.
 Mentor Blog Entries: 27 C. Guerlin et al., "Progressive field-state collapse and quantum non-demolition photon counting", Nature v.448, p.889 (2007). Abstract: The irreversible evolution of a microscopic system under measurement is a central feature of quantum theory. From an initial state generally exhibiting quantum uncertainty in the measured observable, the system is projected into a state in which this observable becomes precisely known. Its value is random, with a probability determined by the initial system's state. The evolution induced by measurement (known as 'state collapse') can be progressive, accumulating the effects of elementary state changes. Here we report the observation of such a step-by-step collapse by non-destructively measuring the photon number of a field stored in a cavity. Atoms behaving as microscopic clocks cross the cavity successively. By measuring the light-induced alterations of the clock rate, information is progressively extracted, until the initially uncertain photon number converges to an integer. The suppression of the photon number spread is demonstrated by correlations between repeated measurements. The procedure illustrates all the postulates of quantum measurement (state collapse, statistical results and repeatability) and should facilitate studies of non-classical fields trapped in cavities. Also See the News and Views article in the same issue. A summary of this paper can also be found at the PhysicsWorld webpage. Zz.
 Mentor Blog Entries: 27 S. Fölling et al., "Direct observation of second-order atom tunnelling", Nature v.448, p.1029 (2007). Abstract: Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing dynamical behaviour can arise because the particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report direct, time-resolved observations of the correlated tunnelling of two interacting ultracold atoms through a barrier in a double-well potential. For the regime in which the interactions between the atoms are weak and tunnel coupling dominates, individual atoms can tunnel independently, similar to the case of a normal Josephson junction. However, when strong repulsive interactions are present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second-order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime in which a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Such second-order tunnelling events, which are the dominating dynamical effect in the strongly interacting regime, have not been previously observed with ultracold atoms. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of proposals for realizing quantum magnetism. I am highlighting this paper to show how difficult it is to get whole atoms to tunnel through a barrier. We constantly get questions (and hypothesis) about things like tennis balls or even a person tunneling through walls, under the pretense that since tunneling phenomena is real for single particles, then in principle, whole macroscopic objects can as well. This is a fallacy. The requirement and the complications for whole objects to undergo quantum tunneling are astounding. Requiring that each part of the atom or each part of the object be in total coherence with each other for the whole thing to tunneling through is one almost-impossible barrier (no pun intended). As can be seen just from this experiment, other effects that are not present or not significant in single-particle tunneling will start to creep up. The nature of the barrier and what is embedded in it will play a larger role in such tunneling process. It isn't easy nor obvious that such macro object tunneling can be done. It is already this difficult for simple atoms that, in the scale of things, can be easily made to be in coherent with all of the parts within it. The same cannot be said with a tennis ball. Zz.