Yanhong Xiao

Surface Plasmon Resonance Enhanced Magneto-Optics (SuPREMO): Faraday Rotation Enhancement in Gold-Coated Iron Oxide Nanocrystals

We report enhanced optical Faraday rotation in gold-coated maghemite (gamma-Fe(2)O(3)) nanoparticles. The Faraday rotation spectrum measured from 480-690 nm shows a peak at about 530 nm, not present in either uncoated maghemite nanoparticles or solid gold nanoparticles. This peak corresponds to an intrinsic electronic transition in the maghemite nanoparticles and is consistent with a near-field enhancement of Faraday rotation resulting from the spectral overlap of the surface plasmon resonance in the gold with the electronic transition in maghemite. This demonstration of surface plasmon resonance-enhanced magneto-optics (SuPREMO) in a composite magnetic/plasmonic nanosystem may enable design of nanostructures for remote sensing and imaging of magnetic fields and for miniaturized magneto-optical devices.

Anti-parity–time symmetry with flying atoms

Parity–time symmetry in optics is studied in a warm atomic vapour, where its counterpart, anti-parity–time symmetry, as well as refractionless propagation, can also be observed.

Repeated interaction model for diffusion-induced Ramsey narrowing.

In a recent paper [Y. Xiao et al., Phys. Rev. Lett. 96, 043601 (2006)] we characterized diffusion-induced Ramsey narrowing as a general phenomenon, in which diffusion of coherence in-and-out of an interaction region such as a laser beam induces spectral narrowing of the associated resonance lineshape. Here we provide a detailed presentation of the repeated interaction model of diffusion-induced Ramsey narrowing, with particular focus on its application to Electromagnetically Induced Transparency (EIT) of atomic vapor in a buffer gas cell. We compare this model both to experimental data and numerical calculations.

EIT and diffusion of atomic coherence

We study experimentally the effect of diffusion of Rb atoms on electromagnetically induced transparency (EIT) in a buffer gas vapour cell. In particular, we find that diffusion of atomic coherence in and out of the laser beam plays a crucial role in determining the EIT resonance lineshape and the stored light lifetime.

SPECTRAL LINE NARROWING IN ELECTROMAGNETICALLY INDUCED TRANSPARENCY

Electromagnetically induced transparency (EIT) can make an otherwise opaque medium transparent by utilizing quantum coherence. EIT has attracted great interest since the 1990s for its wide applications in metrology, nonlinear and quantum optics, and quantum information science. We present a review on spectral line narrowing mechanisms in EIT-related systems, accounting for linewidths much narrower than expected from the standard mechanisms of Doppler broadening, transit time broadening, magnetic field inhomogeneity and power broadening. Topics covered include Dicke narrowing, atomic-motion-induced Ramsey narrowing, motional narrowing, density narrowing, laser induced narrowing and power-broadening-free resonance. Intuitive pictures are provided to describe the essential physics of each mechanism.

Tuning the phase sensitivity of a double-lambda system with a static magnetic field.

We study the effect of a DC magnetic field on the phase sensitivity of a double-lambda system coupled by two laser fields, a probe and a pump. It is demonstrated that the gain and the refractive index of the probe can be controlled by either the magnetic field or the relative phase between the two laser fields. More interestingly, when the system reduces to a single-lambda system, turning on the magnetic field transforms the system from a phase-insensitive process to a phase-sensitive one. In the pulsed-probe regime, we observed switching between slow and fast light when the magnetic field or the relative phase was adjusted. Experiments using a coated 87Rb vapor cell produced results in good agreement with our numerical simulation. This work provides a novel and simple means to manipulate phase sensitive electromagnetically-induced-transparency or four-wave mixing, and could be useful for applications in quantum optics, nonlinear optics and magnetometery based on such systems.

Slow light in narrow paraffin-coated vapor cells

Alkali vapor cells with antirelaxation coated walls can have long atomic coherence times. However, using such coated cells in the hyperfine configuration for electromagnetically induced transparency (EIT) requires longitudinal atomic motion to be confined to less than the hyperfine wavelength. We employed a narrow (1 mm) coated cell geometry to study hyperfine EIT and slow and stored light in warm R87b vapor, with results comparable to those in buffer gas cells and showing the promise of such cells for several applications.

Coherence-Assisted Resonance with Sub-Transit-Limited Linewidth

We demonstrate a novel approach to obtain a resonance linewidth below the transit limit. The cross correlation between the induced intensity modulation of two lasers coupling the target resonance exhibits a narrow spectrum. 1/30 of the transit-limited width is achieved in a proof-of-principle experiment where two ground states are the target resonance levels. Attainable linewidth is only limited by laser shot noise in principle. The experimental results qualitatively agree with an intuitive analytical model and numerical calculations. This technique can be easily implemented and should be applicable to many atomic, molecular, and solid state spin systems for spectroscopy, metrology, and resonance-based sensing and imaging.

Two-axis-twisting spin squeezing by multipass quantum erasure

Many-body entangled states are key elements in quantum information science and quantum metrology. One important problem in establishing a high degree of many-body entanglement using optical techniques is the leakage of the system information via the light that creates such entanglement. We propose an all-optical interference-based approach to erase this information. Unwanted atom-light entanglement can be removed by destructive interference of three or more successive atom-light interactions, with only the desired effective atom-atom interaction left. This quantum erasure protocol allows implementation of Heisenberg-limited spin squeezing using coherent light and a cold or warm atomic ensemble. Calculations show that significant improvement in the squeezing exceeding 10 dB is obtained compared to previous methods, and substantial spin squeezing is attainable even under moderate experimental conditions. Our method enables the efficient creation of many-body entangled states with simple setups, and thus is promising for advancing technologies in quantum metrology and quantum information processing.

Optimizing slow and stored light for multidisciplinary applications

We present a preliminary experimental study of the dependence on optical depth of slow and stored light pulses in Rb vapor. In particular, we characterize the efficiency of slow and stored light as a function of Rb density; pulse duration, delay and storage time; and control field intensity. Experimental results are in good qualitative agreement with theoretical calculations based on a simplified three-level model at moderate densities.