RESUMO
Unitary Fourier transform lies at the core of the multitudinous computational and metrological algorithms. Here we show experimentally how the unitary Fourier transform-based phase estimation protocol, used namely in quantum metrology, can be translated into the classical linear optical framework. The developed setup made of beam splitters, mirrors and phase shifters demonstrates how the classical coherence, similarly to the quantum coherence, poses a resource for obtaining information about the measurable physical quantities. Our study opens route to the reliable implementation of the small-scale unitary algorithms on path-encoded qudits, thus establishing an easily accessible platform for unitary computation.
RESUMO
Ultrafast electron-phonon coupling dynamics in ZnO are investigated by degenerate pump-probe reflectivity measurements using near resonant 400 nm light. Because of the below-gap excitation, photocarriers are excited primarily from deep levels of intrinsic point defects. At least six coherent phonon modes, including two E(2) modes and their combination, longitudinal optical (LO) mode and its overtone (2LO), are observed. Unlike the previous resonant Raman studies using 325 nm light, the intensity of the polar LO phonons is only moderately enhanced by the Fröhlich interaction. The non-polar E(2) phonons decay considerably faster than under visible excitation, indicative of strong deformation potential interaction with photocarriers. The overdamped oscillation centered at around 20 THz is tentatively attributed to the upper branch of the plasmon-phonon coupled mode. The pump-polarization dependence of the phonon amplitudes is consistent with the impulsive Raman generation.
RESUMO
We tailor the shape and phase of the pump-pulse spectrum in order to study coherent lattice dynamics in tellurium. Employing coherent control by splitting the pump pulse into a two-pulse sequence, we show that the oscillations due to A(1) coherent phonons can be cancelled but not enhanced as compared to single-pulse excitation of the same energy. We further demonstrate that a crucial factor for coherent phonon generation seems to be the bandwidth of the pulse spectrum. We observe that the coherent amplitude for long pump pulses decreases exponentially with pulse duration for both Gaussian and rectangular pulses. Finally, by varying the pulse chirp, we show that the coherent amplitude is independent of the chirp sign while the oscillation lifetime is slightly dependent on the chirp sign.
RESUMO
We have studied the A(1g) coherent phonons in bismuth generated by high fluence ultrashort laser pulses. We observed that the nonlinear regime, where the phonons' oscillation parameters depend on fluence, consists of subregimes with distinct dynamics. Just after entering the nonlinear regime, the phonons become chirped. Increasing the fluence further leads to the emergence of a collapse and revival, which next turns into multiple collapses and revivals. This is explained by the dynamics of a wave packet in an anharmonic potential, where the packet periodically breaks up and reconstitutes in its original form, giving convincing evidence that the phonons are in a quantum state, with no classical analog.
RESUMO
We have investigated the temperature dependence of the optical reflectivity on a femtosecond scale in a near optimally doped YBa 2Cu 3O (7-delta) superconductor. The combined study of the lattice and carrier dynamics at temperatures above T(c) allows us to identify two crossover temperatures in the normal state, giving evidence for an inhomogeneity of the pseudogap regime. These crossovers exhibit a clear hysteresis behavior depending on the direction of temperature change. The carrier and lattice dynamics within the crossover regimes show distinct differences from and similarities to the superconducting state, which may help in choosing between the competing theories for the pseudogap state.