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News & Events

About April 2013

This page contains all entries posted to Physics Announcements in April 2013. They are listed from oldest to newest.

March 2013 is the previous archive.

May 2013 is the next archive.

Many more can be found on the main index page or by looking through the archives.

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April 2013 Archives

April 10, 2013

Seminar: Wednesday, April 10, 2013 at 3:30 pm

Precise measurement and enhanced imaging with entangled .
and correlated photons observation, physics and consequences'
Sasha Sergienko
Boston University

The flexibility and high information capacity of entangled states has been so far demonstrated in several areas of quantum communication and quantum computing. We have recently concentrated on the development of several new techniques for high efficiency optical imaging and ultra-precise measurement in telecommunication and nanotechnology. The use of high dimensional quantum states of light helps to outperform traditional optical approaches in resolution and in the amount of information obtained about the system under evaluation.

We consider the benefit of using the high-dimensional Hilbert space of correlated orbital angular momentum (OAM) states. A typical imaging procedure requires a significant amount of energy to be registered pixel-by-pixel by a CCD camera before one could start recognizing the type of object that has registered. The new approach [1] enables one to recognize objects much faster and with less required energy (more information is obtained per detected photon).

The nonlinear process of spontaneous parametric down conversion (SPDC) has often been used as an effective source of optical entanglement, and is capable of generating entangled photon pairs that span higher order OAM states. The correlated (joint) detection of two photons in the OAM basis using coincidence counting reveals that the scattering occurs mainly between OAM states of orbital orders that have symmetry elements resonating with geometric structures present in the object.

Such a fast object recognition technology could become useful in situations where the presence or absence of objects with particular symmetry features must be quickly identified in the field of view. For example, some living cells, drug molecules, or viruses have particular rotational symmetries, so that their IEEE and LEOS.

Location: Physics Bldg., Room 401

April 17, 2013

Seminar: Wednesday, April 17, 2013 at 3:30 pm

Why are Pulsars observable
Constantinos Kalapotharakos

Pulsars are extraordinary objects powered by the fast rotation of huge magnetic fields. The resulting electric fields convert continuously part of the Pulsar rotational energy to electromagnetic radiation. At the same time they accelerate particles to energies sufficiently high to produce GeV photons. Their electromagnetic emission is in fact quite complex and ranges from the radio to the multi-GeV regime. It is essential, for the understanding of the Pulsar machine, to know the structure of the magnetosphere around these objects. I will present numerical models of 3D global pulsar magnetospheres covering the entire spectrum between the vacuum retarded dipole and the (ideal) force-free solutions. Finally, I will demonstrate how we exploit these solutions in order to explain the observations and,consequently, to understand the physical mechanisms that take place in pulsar magnetospheres.

Location: Physics Bldg., Room 401

April 18, 2013

PhD Defense - Meimei Lai

Meimei successfully defended her dissertation on April 18, 2013.

Experiments Using Sub-Wavelength Diameter Tapered Optical Fibers in
Rubidium Vapor

In this work, we describe experimental research on a relatively new
nonlinear optics system comprised of a sub-wavelength diameter Tapered
Optical Fiber (TOF) suspended in atomic Rubidium (Rb) vapor. The
compression of the evanescent optical mode propagating along the TOF
enables a dramatic increase in the nonlinear interactions between the
fields and the surrounding Rb atoms, thereby allowing the observation of a
variety of nonlinear optical effects with very low-power fields.
Specifically, we report on the observation of saturated absorption with nW
power levels and, more significantly, the observation of two-photon
absorption using power-levels corresponding to only 10’s to 100’s of
photons interacting with the Rb atoms at a given time.

One significant drawback to this "TOF in Rb" system is that at the
relatively high atomic densities needed for many of these experiments, Rb
atoms accumulating on the TOF surface can cause a significant loss of
overall transmission through the fiber. Here we report direct measurements
of the time-scale associated with this transmission degradation for various
Rb density conditions. We find that transmission is affected almost
immediately after the introduction of Rb vapor into the system, and
declines rapidly as the density is increased.

More significantly, we show how a heating element designed to raise the
TOF temperature can be used to reduce this transmission loss and
dramatically extend the effective TOF transmission lifetime. Our results
indicate that it is possible to achieve relatively high TOF transmission,
even in the presence of the relatively high Rb vapor densities that would
be needed for many low-power nonlinear optics applications. This study
represents a significant step in moving the basic "TOF in Rb" system from a
laboratory setting towards a practical ultra-low-power nonlinear optics

April 24, 2013

Seminar: Wednesday, April 24, 2013 at 3:30 pm

Nature's Accelerators and the Mysterious Origin of Galactic Cosmic Rays
John Hewitt
NASA Goddard

The origin of cosmic rays in our Galaxy is a century-long puzzle. Charged particles with energies up to a few PeV permeate interstellar space. It has long been thought that these cosmic rays are accelerated in the expanding shockwaves of supernovae. Observations with modern gamma-ray telescopes can be used to trace cosmic rays in interstellar space interacting with the ambient gas and low-energy radiation fields, turning our suspicion into certainty.

NASA's Fermi Gamma-ray Space Telescope has detected a variety of both expected and unexpected Galactic GeV gamma-ray sources, including supernova remnants, pulsars and their nebulae, and stellar novae. Importantly, in the two brightest supernova remnants we detect a low-energy pion-decay cutoff, a definitive signature of cosmic ray protons, finally proving that supernova remnants do accelerate Galactic cosmic rays. The growing number of identified supernova remnants by Fermi allows a comparative study of the effects of evolution and environment on acceleration efficiency. These new results promise to resolve the question of origin, and deepen our understanding of the physics of cosmic ray acceleration and transport in the interstellar medium.

Location: Physics Bldg., Room 401