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Physics News Update 639

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  • Alejandro Dubrovsky
    PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of Physics News Number 639 May 30, 2003 by Phillip F. Schewe, Ben Stein, and James Riordon
    Message 1 of 1 , Jun 1, 2003
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      PHYSICS NEWS UPDATE
      The American Institute of Physics Bulletin of Physics News
      Number 639 May 30, 2003 by Phillip F. Schewe, Ben Stein, and James
      Riordon

      OPTICAL PERISTALSIS. Part of the digestion process consists of the
      massaging movement of powerful esophageal muscles urging food
      particles along the alimentary track. The same sort of
      "peristalsis" can also be carried out at the nanoscopic level with
      small objects in the grip of cleverly crafted light pulses. David
      Grier and Brian Koss at the University of Chicago use the optical
      tweezer method of controlling particles with multiple laser beams,
      but instead of a static array of beams, they use computer-generated
      holograms to convert a single beam of light into large numbers of
      optical traps. Each hologram may be considered to be a specialized
      diffraction grating, producing intricately articulated networks of
      hundreds of optical traps. Objects can fall into these light traps
      and then the traps can be moved, thus transporting the objects. The
      aim is to move and position sub-micron things in 3D space.
      Applications include inserting the object into a microscopic
      reservoir and pulling it back (parallelism is one of the technique's
      strengths), or centering or rotating a biological cell in a
      microscope's field of view. Grier's work has led to a commercial
      version of this holographic optical tweezers, one in which a pattern
      of 200 optical traps can be refreshed or modified at a rate of 100
      times per second. (By the way, how forefront research is turned
      into saleable products is an interesting story by itself. For
      example, the company Grier started, Arryx,
      Inc.---http://arryx.com---has a scientific advisory board (SAB) with
      notable scientists from Princeton, NIH, the Whitehead Institute,
      Harvard, and Northwestern.) In the "peristalsis" mode of
      operation, particles are deliberately handed off from
      one optical trap to another, as in a bucket brigade. In a separate
      "thermal ratchet" mode of operation, the transfer from trap to trap
      might involve intervals of free diffusion; this mode should be
      useful for fractionating DNA molecules (see previous Update story at
      http://www.aip.org/enews/physnews/2003/split/627-1.html ) as part of
      the process of sequencing a gene.
      Speaking as a physicist, Grier says the most important aspect of his
      group's holographically generated tweezer patterns is the ability to
      implement time-varying potential energy landscapes for moving tiny
      objects in a "force-free" way. Speaking as a biophysicist, Grier
      points to the ability to reach into a microscopic environment and to
      position samples just where you want them. (Koss and Grier, Applied
      Physics Letters, 2 June 2003; d- grier@..., 773-702-9176,
      lab website at http://griergroup.uchicago.edu/~grier/hot/ )


      A NEW OPTICAL GEOMETRIC PHASE has been measured for the first time,
      by a group of physicists at Colgate University. The new geometrical
      phase is associated with light beams carrying orbital angular
      momentum. This development can be considered yet another step toward
      understanding and exploiting the weirdness of quantum reality for
      performing novel feats of computation. To see the meaning behind
      the new effect, we shall break the explanation into parts,
      considering in turn the issues of phase, orbital angular momentum in light, and then geometrical
      phase in light. First, phase. Many common periodic things have phase. The
      orientation or phase of a minute hand on a clock is the amount by
      which the hand has swept around the clock face: a quarter past the
      hour, half past the hour, etc. Except when going into a new time
      zone the phase of the clock regularly returns to its original
      position every sixty minutes. The phase of a water wave specifies
      where along the wave's crest-to-trough cycle it might be at any
      moment. Now consider a different kind of phase. Picture a sign with
      an arrow on it, oriented north. Starting at the equator, and
      without changing its orientation, push the sign along the ground one
      fourth of the way around the world. Next push the sign due north
      until you reach the north pole, where, without changing the sign's
      orientation, you move directly south again to return to your
      starting point. Even though you will have traced a closed loop the
      sign will now have a westerly orientation. In other words, because
      of the intrinsic curved geometry of the path, a change in phase will
      have occurred. This kind of phase change can occur in a quantum
      system.
      Second, orbital angular momentum. The ordinary forward momentum of
      a particle of light is equal to Planck's constant divided by the
      wavelength of the equivalent light wave. Furthermore, the light is
      said to possess an intrinsic angular momentum, or "spin." The spin
      angular momentum can be oriented by polarizers so that the electric
      field of the light wave is oscillating vertically up and down, or
      horizontally back and forth. Equivalently, if the light wave is
      circularly polarized (the electric field precesses in corkscrew
      fashion as the wave moves along) the two contrary states of the spin
      would then correspond to the light wave's electric field precessing
      clockwise (in a "right-handed" way) or anticlockwise (in a"left
      handed" way). For the purposes of data processing a 0 or 1 bit can
      be associated respectively with vertical and horizontal polarizations or, equivalently, with
      clockwise or anticlockwise polarizations. But what does it mean for
      light to have "orbital" angular momentum? What is it that orbits?
      To ponder this issue, picture the electric field values for a vertical planar slice
      of the light beam. For vertically-polarized light, the electric
      field at all the points on the slice are vertically oriented. Look
      at the sameslice at a later time and the fields are still vertically oriented.
      For circularly polarized light, the fields in the slice will, at a
      certain moment, also be oriented in the same way. A moment later,
      however, the electric field will have precessed a bit (from the one o'clock
      position, say, to the three o'clock position; another way of saying
      this is that the phase of the electric field will have advanced a
      bit) but the orientation of the field at each point on the vertical
      slice will be the same. With the use of special gratings one can
      produce an entirely different mode of light, one in which the
      electric field phase coils around the beam axis, and the light is
      said to possess an orbital angular momentum, or OAM. This condition
      is visualized at the following website prepared by physicists at
      Colgate University:
      departments.colgate.edu/physics/research/optics/oamgp/gp.htm. This
      extra property of "coiled light" might be exploitable for future
      quantum computing. For instance, recently a group at the University
      of Vienna used OAM in light to create a three-dimensional entangled
      state, or "qutrit" (Vaziri et al., Physical Review Letters, 9 Dec
      2002). Third issue: geometrical phase. When a light pulse is made
      to follow a closed loop path in real space, the phase of the
      returning beam might be slightly off from the phase of light
      starting off at that point. This disparity (which can result in an
      interference effect) can be modified by changing the path length.
      It can also be modified by changing the path geometry. In addition,
      the space does not need to be real space. When the "mode" (set of
      standing waves in the beam) is changed, it can also produce a phase
      when changing the geometry of the path in "mode space," and it is
      this that the Colgate physicists have measured. (see a schematic of
      the setup at this website:
      departments.colgate.edu/physics/research/optics/oamgp/geomph.htm ).
      The change in phase that a quantum system undergoes in going around
      a closed path in a space of states or parameters is called a
      "geometrical phase," and can be measured when the light emerges from
      the path to form a spiral shaped interference pattern at an external
      detector (Galvez et al., Physical Review Letters, 23 May 2003;
      contact Kiko Galvez, egalvez@..., 315-228-7205). (For
      further background, see Physical Review Focus item at
      focus.aps.org/story/v9/st29 and an article on geometric phase in
      Physics Today, Dec 1990.)

      ***********
      PHYSICS NEWS UPDATE is a digest of physics news items arising
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