90 research outputs found
Squeezed Phonon States: Modulating Quantum Fluctuations of Atomic Displacements
We study squeezed quantum states of phonons, which allow the possibility of
modulating the quantum fluctuations of atomic displacements below the
zero-point quantum noise level of coherent phonon states. We calculate the
corresponding expectation values and fluctuations of both the atomic
displacement and the lattice amplitude operators, and also investigate the
possibility of generating squeezed phonon states using a three-phonon
parametric amplification process based on phonon-phonon interactions.
Furthermore, we also propose a detection scheme based on reflectivity
measurements.Comment: 4 pages, RevTeX. The previous entry had a wrong page number in the
Journal-ref fiel
Retinal photoisomerization versus counterion protonation in light and dark-adapted bacteriorhodopsin and its primary photoproduct
Discovered over 50 years ago, bacteriorhodopsin is the first recognized and most widely studied microbial retinal protein. Serving as a light-activated proton pump, it represents the archetypal ion-pumping system. Here we compare the photochemical dynamics of bacteriorhodopsin light and dark-adapted forms with that of the first metastable photocycle intermediate known as “K”. We observe that following thermal double isomerization of retinal in the dark from bio-active all-trans 15-anti to 13-cis, 15-syn, photochemistry proceeds even faster than the ~0.5 ps decay of the former, exhibiting ballistic wave packet curve crossing to the ground state. In contrast, photoexcitation of K containing a 13-cis, 15-anti chromophore leads to markedly multi-exponential excited state decay including much slower stages. QM/MM calculations, aimed to interpret these results, highlight the crucial role of protonation, showing that the classic quadrupole counterion model poorly reproduces spectral data and dynamics. Single protonation of ASP212 rectifies discrepancies and predicts triple ground state structural heterogeneity aligning with experimental observations. These findings prompt a reevaluation of counter ion protonation in bacteriorhodopsin and contribute to the broader understanding of its photochemical dynamics
Hyperspectral Probing of Exciton dynamics and Multiplication in PbSe Nanocrystals
Height time hyperspectral near IR probing providing broad-band coverage is employed on PbSe nanocrystals, uncovering spectral evolution following high energy photo-excitation due to hot exciton relaxation and recombination. Separation of single, double and triple exciton state contributions to these spectra is demonstrated, and the mechanisms underlying the course of spectral evolution are investigated. In addition no sign of MEG was detected in this sample up to a photon energy 3.7 times that of the band gap
Quantum Phonon Optics: Coherent and Squeezed Atomic Displacements
In this paper we investigate coherent and squeezed quantum states of phonons.
The latter allow the possibility of modulating the quantum fluctuations of
atomic displacements below the zero-point quantum noise level of coherent
states. The expectation values and quantum fluctuations of both the atomic
displacement and the lattice amplitude operators are calculated in these
states---in some cases analytically. We also study the possibility of squeezing
quantum noise in the atomic displacement using a polariton-based approach.Comment: 6 pages, RevTe
Phonon Squeezed States Generated by Second Order Raman Scattering
We study squeezed states of phonons, which allow a reduction in the quantum
fluctuations of the atomic displacements to below the zero-point quantum noise
level of coherent phonon states. We investigate the generation of squeezed
phonon states using a second order Raman scattering process. We calculate the
expectation values and fluctuations of both the atomic displacement and the
lattice amplitude operators, as well as the effects of the phonon squeezed
states on macroscopically measurable quantities, such as changes in the
dielectric constant. These results are compared with recent experiments.Comment: 4 pages, REVTE
A Gate-tunable Polarized Phase of Two-Dimensional Electrons at the LaAlO3/SrTiO3 Interface
Controlling the coupling between localized spins and itinerant electrons can
lead to exotic magnetic states. A novel system featuring local magnetic moments
and extended 2D electrons is the interface between LaAlO3 and SrTiO3. The
magnetism of the interface, however, was observed to be insensitive to the
presence of these electrons and is believed to arise solely from extrinsic
sources like oxygen vacancies and strain. Here we show the existence of
unconventional electronic phases in the LaAlO3/SrTiO3 system pointing to an
underlying tunable coupling between itinerant electrons and localized moments.
Using anisotropic magnetoresistance and anomalous Hall effect measurements in a
unique in-plane configuration, we identify two distinct phases in the space of
carrier density and magnetic field. At high densities and fields, the
electronic system is strongly polarized and shows a response, which is highly
anisotropic along the crystalline directions. Surprisingly, below a
density-dependent critical field, the polarization and anisotropy vanish
whereas the resistivity sharply rises. The unprecedented vanishing of the easy
axes below a critical field is in sharp contrast with other coupled magnetic
systems and indicates strong coupling with the moments that depends on the
symmetry of the itinerant electrons. The observed interplay between the two
phases indicates the nature of magnetism at the LaAlO3/SrTiO3 interface as both
having an intrinsic origin and being tunable.Comment: Finalized version containing modifications introduced after
peer-review. The results are completely unchange
Quantum entanglement growth under random unitary dynamics
Characterizing how entanglement grows with time in a many-body system, for example, after a quantum quench, is a key problem in nonequilibrium quantum physics. We study this problem for the case of random unitary dynamics, representing either Hamiltonian evolution with time-dependent noise or evolution by a random quantum circuit. Our results reveal a universal structure behind noisy entanglement growth, and also provide simple new heuristics for the “entanglement tsunami” in Hamiltonian systems without noise. In 1D, we show that noise causes the entanglement entropy across a cut to grow according to the celebrated Kardar-Parisi-Zhang (KPZ) equation. The mean entanglement grows linearly in time, while fluctuations grow like ðtimeÞ 1 = 3 and are spatially correlated over a distance ∝ ðtimeÞ 2 = 3 . We derive KPZ universal behavior in three complementary ways, by mapping random entanglement growth to (i) a stochastic model of a growing surface, (ii) a “minimal cut” picture, reminiscent of the Ryu-Takayanagi formula in holography, and (iii) a hydrodynamic problem involving the dynamical spreading of operators. We demonstrate KPZ universality in 1D numerically using simulations of random unitary circuits. Importantly, the leading-order time dependence of the entropy is deterministic even in the presence of noise, allowing us to propose a simple coarse grained minimal cut picture for the entanglement growth of generic Hamiltonians, even without noise, in arbitrary dimensionality. We clarify the meaning of the “velocity” of entanglement growth in the 1D entanglement tsunami. We show that in higher dimensions, noisy entanglement evolution maps to the well-studied problem of pinning of a membrane or domain wall by disorder
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