Mentor
Blog Entries: 28

## 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: 28 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: 28 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: 28 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: 28 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: 28 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: 28 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: 28 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
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)