FOCUS Lunch Talk
(Feb. 28, 2006, 12:00 PM , 340 West Hall )

Title: Control of nonlinear resonant photochemistry of 1,3-cyclohexadiene in solution

Abstract: The photochemistry of small organic molecules such as cyclohexadiene is an important interface of physics, chemistry and biology.  Often photochemical reactions happen within a few hundred femtoseconds.  On this ultrafast timescale, the dynamics of the reaction can be controlled by changing the “shape” of the laser pulse.  We have investigated optical control over the photoisomerization of 1,3-cyclohexadiene to 1,3,5-cis-hexatriene via multiphoton excitation.  Sub-80fs near-infrared pulses were shaped with an acousto-optic modulator.  A closed-loop genetic algorithm (GA) searched for optimal excitation pulses while the effectiveness of each pulse was evaluated using differential absorption of a time-delayed, broadband, ultraviolet probe pulse. Reaction products were subsequently identified in an ultraviolet spectrophotometer. The GA identified pulses that increased the formation of hexatriene by a factor of two over the transform-limited pulse. Detailed analysis of the pulses in the GA search set has identified negative quadratic spectral phase as an important control parameter.  Multiphoton control mechanisms will be discussed.

FOCUS Special Seminar Talk
(Thursday, Jan. 12, 2006, 2:30 PM to 3:30 PM, FishBowl Room (335 West Hall) )
Speaker: Guifre Vidal (Professor)
Title: Entanglement renormalization
Entanglement renormalization has been recently proposed as a novel renormalization group scheme for quantum systems in D=1,2,3... spatial dimensions (cond-mat/0512165). As a numerical technique it has the potential (already confirmed for D = 1 spatial dimensions) of being able
to analyze quantum lattice systems of arbitrary sizes even at a quantum critical point. In this talk, focused on the conceptual aspects of the simulation scheme, I will first introduce the basic notions of
renormalization and entanglement in quantum lattice systems. Then I will briefly describe the main idea behind the new algorithm and illustrate its performance with a system of 16.000 spins

FOCUS Special Seminar Talk
(Thursday Aug 4 2005, 2:00 PM to 3:00 PM, FishBowl Room (4th floor of new Randall) )
Speaker: Pieter Kok (HP)
Title: Scalable quantum computing with matter qubits and linear optics
It is shown how two single-photon sources with an additional internal energy level can be entangled using a "double-heralded" single-photon detection scheme. Using this entangling operation, cluster states with extremely high fidelity can be created efficiently. Adding single-qubit operations and readout, this is sufficient for universal quantum computing. The physical implementation of the photon guns can be as diverse as colour centres in diamond, Pauli-blockade quantum dots with an excess electron, or trapped ions with optical transitions placed in a cavity. This scheme does not rely on strong coupling, photon-number resolving detectors or interferometric stability. In addition, it is robust against the most important errors such as photon loss, spontaneous emission, and mismatch of cavity parameters.

FOCUS Special Seminar Talk
(Thursday Feb. 24 2005, 2:00 PM to 3:00 PM, FishBowl Room (4th floor of new Randall) )
Speaker: Qijin Chen (University of Chicago)
Title: Superfluidity in correlated fermions: From high Tc superconductors to ultracold atomic Fermi gases

Abstract:
Exciting new developments in the field of ultracold atoms make it possible to tune the two-body attractive interactions between fermionic atoms continuously from very weak to very strong; this not only changes the statistics from fermionic to bosonic but also bears on the nature of the superfluidity. In this talk, I discuss recent experiments which elucidate the nature of the superfluid phase as the interaction is continuously tuned via Feshbach resonances. These experiments are interpreted as
displaying a "crossover" between BCS superfluidity and Bose-Einstein condensation. Of particular interest is the intermediate or crossover regime where the s-wave scattering length diverges. This regime provides a prototype for studying both high temperature superconductors and also strongly interacting Fermi gases which are also of interest to nuclear and astro-physicists. I will report recent progress in understanding superfluidity in these two condensed matter systems (cold atomic Fermi gases and high Tc superconductors) with particular emphasis on the former.

FOCUS Research Lunch
(Tuesday Feb. 22 2005, 11:30 AM to 1:00 PM, 340 West Hall (Lunch in 337 West hall))

Speaker: Cynthia Aku-Leh
Title: Observation of Coherent Superconducting Gap Excitations in 2H-NbSe2 and MgB2

Abstract:
I will discuss the first observation of superconducting gap oscillations in MgB2, a superconductor with a critical temperature (the temperature at which resistivity goes to zero) of 39 K, and in 2H-NbSe2, with a critical temperature of 7 K. These oscillations are observed using femtosecond time-resolved pump-probe spectroscopy. In MgB2, multiple superconducting gap frequencies are observed which are in reasonable agreement with other reported experimental results and published theoretical models. Spontaneous Raman measurements on MgB2 show a single strong resonance at 107 cm^-1 near the superconducting gap. Coherent superconducting gap frequencies in 2H-NbSe2 are observed at 18 cm^-1 (E symmetry) and 14 cm^-1 (A symmetry), consistent with spontaneous Raman measurements. The analysis suggests that the impulsive-stimulated Raman scattering mechanism can account for the superconducting gap oscillations in 2H-NbSe2, but another mechanism or impurities are responsible for the oscillations in MgB2.

FOCUS Research Lunch
(Tuesday Feb. 15, 2005, 2:00 PM to 3:00 PM in 335 West Hall)

Title: Single carbon nanotube electronics and photonics

Abstract:
Single-walled carbon nanotubes (SWNTs) are truly novel materials that possess exotic electronic and mechanical properties. The recent discovery that SWNTs exhibit fluorescence in the near-infrared has open up new possibilities for fundamental research and potential applications. We will focus on novel insights into to their unique electronic structure achieved through single molecule photoluminescence spectroscopic and ultrafast optical spectroscopic measurements of individual and isolated SWNTs, respectively. Fluorescence spectra from individual SWNTs can be used to easily and directly identify their structure. However, fluorescence from SWNTs with identical structures can have different fluorescence energies and line widths, which has not been observed previously, and indicates the strong effect of localized electronic perturbations on SWNT electronic properties. Unlike for most single molecules, for SWNTs the photoluminescence unexpectedly does not show any intensity or spectral fluctuations at room temperature, an phenomenon still not understood. The decay of photoexcited electrons in isolated SWNTs is multi-exponential with decay times from 200 fs to over 120 ps. The magnitude of the longest-lived component in the ultrafast signal specifically depends on resonant excitation, thus suggesting that this lifetime (> 120 ps) corresponds to the band-edge electron recombination time. The implications of these findings with regards to quantized recombination of discrete excitons in SWNTs will be discussed.

FOCUS Research Lunch
( Tuesday Jan. 25, 2004, from 11:30 AM to 1:00 PM in 340 West Hall (Lunch in 337 West hall))

Speaker: Martin John Madsen
Title: Hot topics in Cool ions and Fast Lasers

Abstract:
I will talk about a violation of the Bell inequality with a single photon prepared in a probabilistic entangled state. We will use these entangled states to generate remotely entangled atom pairs that are the building blocks of a quantum communication network. Fast unitary excitation of the atoms will dramatically increase entanglement rates. We are investigating the interaction of ps laser pulses with cold, trapped ions, including fast Rabi oscillations on the S->P transition and broadband laser cooling. With these fast laser pulses, we used a new method for measuring excited state lifetimes to precisely measure the Cd+ P(1/2) and P(3/2) lifetimes.

FOCUS Contact Information Professor Chris Monroe, Director,Randall Laboratory,University of Michigan,500 East University Ave.,Ann Arbor, MI 48109-1120 Phone: (734) 764-8459, Fax: (734) 764-5153 email:mamurn@umich.edu