Non-destructive detection of the photon
1 December 2013
A method of the so-called “weak quantum measurements” was implemented in a number of experiments; the quantum state of the system in the configuration under study was not destroyed because only a part of quantum information was extracted. Thus, S. Haroche et al. detection in the 1990s by using “weak measurements”, photons of microwave radiation in a resonator. Researchers from the Institute of quantum optics of the Max Planck Gesellschaft (Germany) A. Reiserer, S. Ritter and G. Rempe were the first to implement such a non-destructive method of detection relevant to photons of the optical range. The experiment worked with the 87Rb atom placed in an optical resonator between
two semi-transparent mirrors. The atom was in the state of superposition of two states, one of which was connected to a resonator and the other was not. In the former of these states, photons could not
penetrate the resonator, as they were immediately reflected. In the latter state of the atom, photons
penetrated the resonator and were reflected by the second mirror. The two indicated atomic states being changed by 180 degrees. The phase difference of the atom, placed into a preliminarily prepared superposition of photons was measured from the properties of the emitted fluorescent photons, and it was concluded if the atom did undergo a phase shift after transmitting a photon or was not. It is important that the establishment of the fact of a photon flying through the resonator failed to destroy the quantum state of the qubit (quantum bit) for which this photon could code, for instance, in terms of its polarization state.
Source: Science 342 1349 (2013)
1 December 2013
D.C. Glattli (The Institute of Matter and Radiation, Saclay, France) and his colleagues were able to generate in their experiment the quasiparticles which were predicted nearly 20 years ago by L.S. Levitov, D.A. Ivanov, G.B. Lesovik and H.W. Lee (the L.D. Landau Institute of Theoretical Physics of the Russian Academy of Sciences (RAS), the Institute of Solid State Physics of the RAS and the Massachusetts Institute of Technology (USA)). The quasiparticles that the authors of the experiment gave the name “leviton” (Levitov’s solitons) transfer the electron density wave; it is important that no hole (electron vacancy) is created simultaneously with the electron excitation. In the experiment above, levitons moved through a conducting heterostructure between two electrodes, being born owing to a time-dependant potential. As predicted by the theory, if the potential varies in the form of the Laurentz distribution, then levitons are born at maximum efficiency because the Lawrentz curve corresponds to an integral valued quantity of charge transferred by the excitation. Checking demonstrated that levitons were born less efficiently in the case of a different shape of the potential momentum (rectangular or sinusoidal), and the contribution due to holes was high. Levitons were recorded using two techniques. In the first of them, electron noise in
the sample was measured. In the case of levitons, the noise was considerably weaker when the electron and whole pairs were born. The second technique was based on generating two levitons shifted in time and on measuring their anti-correlation (Hong-U-Mandel effect) at the center of the conductor. The measurement data demonstrated that levitons are indeed quantum quasiparticles obeying the Fermi statistics. Levitons may find applications as agents for translation of quantum information in solid-state quantum computers.
Source: Nature 502 659 (2013)
Dissociation of hydrogen molecules
1 December 2013
J. Robert (the University of Paris-Sud, France) and his colleagues in France and Brazil observed for the first time the channel of dissociation of H2 molecules into a pair of hydrogen atoms in metastable states 22S. Even though the dissociation of H2 into H(22S) has already been studied in detail,
two atoms of H(22S) have not been recorded in experiments when originating from the same molecule. The reason for this is the small cross-section of dissociation using this channel. In the new experiment, a beam of H2 molecules intercepted a beam of electrons, and collisions between them lead to dissociation
of molecules. At distances of about several centimeters from the intersection point, two photodetectors were installed underneath and above the cross-over plane in which the two beams lie. These detectors recorded the Lyα photons emitted when atoms transferred to the ground energy level. The study using the coincidence technique made it possible to detect several thousands of H(22S)-H(22S) pairs of atoms.
When detectors were placed at different distances from the collision point, simultaneous time shift was observed between the signals; it was explainable by the atoms flying an additional distance until the metastable level decayed.
Source: Phys. Rev. Lett. 111 183203 (2013)
Brownian motion of non-symmetrical particles
1 December 2013
Chakrabarti (University of Kent, USA) and others studied the Brownian motion of colloidal macroparticles shaped as a boomerang, and established that at the beginning of the observation, the particles move predominantly along the symmetry line, in the direction of expansion of the boomerang shoulders, while later the motion of particles changes to chaotic. Boomerang-shaped particles with shoulder lengths of 2.1 µm placed at right angles, were manufactured by a photolithographic technique from a polymer material. They form suspension between two sheets of glass, and their quasi-two-dimensional motion allow to observe
that this behavior differed from the behavior of round or ellipsoidal particles which move chaotically from the very beginning. The properties of the boomerang-shaped particles suspension, and its quasi-two-dimensional motion could be observed through a glass using a microscope and a video camera. If one starts tracing a particle, then at the beginning, as predicted theoretically, it was moving along the symmetry axis with none-zero mean displacement in the direction of the total force exerted on it by molecules. However, the motion looks chaotic over large time intervals, as in the case of the Brownian motion of symmetric particles. This is explained by the fact that with time, the particle rotates and it’s predominant direction of motion changes. Clarification of the properties of Brownian motion of non-symmetric particles may prove useful for sorting and ordering large organic molecules.
Source: Phys. Rev. Lett. 111 160603 (2013)
A remote galaxy
1 December 2013
A galaxy characterized by a high rate of star formation at red shift z = 7.5078 ± 0.0004 has been discovered
using the MOSFIRE infrared spectrograph of the 10 m Keck I telescope. The galaxy is observable during the epoch when the age of the universe was 700,000,000 years, and is the record farthest of the galaxies for which spectroscopic confirmation of the value of z has at the moment been obtained. S.L. Finklestein (The Texas University of Austin, USA) et al. studied 43 galaxies at z≥6.5 among the candidate galaxies selected earlier with the Hubble telescope. The spectral line which can be interpreted as an Lyα with high reliability, was detectable only in the case of this one galaxy mentioned above. It appears that some effects at z≥6.5 make
the registration of Lyα difficult. This could be a high fraction of neutral hydrogen in the intergalactic medium, or a large amount of gas in galaxies themselves. The color of the above galaxy corresponds to a relatively high content of heavy elements in it. Owing to high z, the enrichment in metals should have completed very fast:
the estimate of the star formation rate should ≈ 330M☉ year-1, which exceeds the current rate of
star formation in our Galaxy by 2 orders of magnitude.
Source: arXiv:1310.6031 [astro-ph.CO]
The Extracts from the Internet is a section of Uspekhi Fizicheskih Nauk (Physics Uspekhi) the monthly rewiew journal of the current state of the most topical problems in physics and in associated fields. The presented News is devoted to the fundamental discoveries of physics and astrophysics.
Permanent editor is Yu.N. Eroshenko.
It is compiled from a multitude of Internet sources.