Search Results (87 total)

The second-order Talbot effect is analyzed for a periodic object illuminated by entangled photon pairs in both the quantum imaging and quantum lithography configurations. The Klyshko picture is applied to describe the quantum imaging scheme, in which self-images of the object that may or may not be magnified can be observed nonlocally in the photon coincidences but not in the singles count rate. In the quantum lithography setup, we find that the second-order Talbot length is half that of the classical first-order case, thus the resolution may be improved by a factor of 2.

We explore the high-density, nonlinear regime of atom-cavity coupling for a gas of three-level atoms at high temperature inside an optical cavity, interacting with the cavity field and a strong driving field. We find a region of parameters where the two side peaks in the transmission spectrum (which, in the linear regime, are associated with “bright polaritons”) split into two pairs of peaks. Experimental results are presented along with a theoretical model that indicates that this phenomenon is unique to the three-level system in the large Doppler-broadening regime.

Spatial displacements of the probe and generated four-wave mixing beams are observed in a three-level V-type, as well as a two-level atomic system near resonance. The observed spatial shift curves reflect the typical enhanced cross-Kerr nonlinear dispersion properties in the electromagnetically induced transparency (EIT) systems. The spatial beam displacements are controlled by the strong control laser beam and the atomic density. Studying such controlled spatial beam shifts can be important in image storage and in generating spatially correlated (entangled) laser beams in multilevel EIT systems.

Tunneling of individual electrons into and out of a GaAs quantum dot is measured in real time by an adjacent charge detector. By controllably increasing the tunneling rate at thermal equilibrium, the full-counting statistics of these tunneling events shows a sub- to super-Poissonian transition, accompanied by a sign reversal of its third statistical moment. These anomalies are believed to be caused by electron tunneling through the singlet-triplet states of an elongated double dot, confirmed by a self-consistent Poisson-Schrödinger wave-function calculation.

We theoretically investigate the dually dressed electromagnetically induced transparency, and the multidressed four-wave mixing (FWM) and six-wave mixing (SWM) processes in an inverted-Y-type atomic system with Zeeman sublevels. The results show that the Zeeman degeneracy of the dark states can be lifted by the dressing field as its intensity is increased. Moreover, the derived analytical expressions indicate that one can, for example, selectively create secondary dark states on the multi-Zeeman-sublevel dark states (by tuning the coupling field), distinguish two different types of dark states generated in two FWM processes (by properly controlling the coupling field intensity), and selectively enhance multi-FWM signals coming from various paths consisting of split Zeeman sublevels (by tuning the dressing field). The SWM signals can be either enhanced or suppressed by controlling the dressing field.

Multi-normal-mode splitting peaks are experimentally observed in a system with Doppler-broadened two-level atoms inside a relatively long optical cavity. In this system, the atom-cavity interaction can reach the “superstrong-coupling” condition with atom-cavity coupling strength gsqrt[N] to be near or larger than the cavity free-spectral range Δ_FSR. In such case, normal-mode splitting can occur in many cavity longitudinal modes to generate the multi-normal-mode splitting peaks, which can be well explained by the linear-dispersion enhancement due to the largely increased atomic density in the cavity. Many new interesting phenomena might come out of this superstrong atom-cavity coupling regime.

Using resonance fluorescence, we investigate how acoustic phonons mediate couplings between multiple electronic levels in a semiconductor quantum dot. We show the first direct evidence of population up-conversion in a single quantum dot, which we attribute to absorption of single acoustic phonons. Moreover, through a rate equation analysis, we find that below 20 K such single-phonon mediated couplings between the exciton ground state and the first excited state are primarily responsible for decoherence of the exciton quantum state.

Effects of multiplicative noise on optical bistability in a three-level atomic medium in a ring cavity are investigated experimentally and theoretically. Steady-state intensity distributions as well as switching and dwell times have been measured as functions of noise strength. System response to an external periodic signal is also explored. It is found that the probability for the system to stay in the upper state increases with increasing noise strength. When a small periodic signal is added to the input field, the probability for the upper state has a maximum as a function of multiplicative noise strength. A simple theoretical model based on nonlinear atomic susceptibility is presented to interpret the experimental observations.

Based on color-locking noisy field correlation, the subtle Markovian field correlation effects in three stochastic models have been investigated in studying the Raman- and Rayleigh-enhanced four-wave mixing (FWM). Homodyne and heterodyne detections of the Raman attosecond sum-frequency polarization beats (ASPB), the Rayleigh ASPB, and the coexisting Raman and Rayleigh ASPB have also been investigated, respectively. Raman- and Rayleigh-enhanced FWM processes strongly compete with each other in ASPB. The heterodyne detected signal of ASPB potentially offers rich dynamic information about the homogeneous broadening material phase of the third-order nonlinear susceptibility.

Using phase control between four-wave mixing (FWM) and six-wave mixing (SWM) channels in a four-level atomic system, we demonstrate temporal and spatial interferences between these two nonlinear optical processes. Efficient and coexisting FWM and SWM signals are produced in the same electromagnetically induced transparency window via atomic coherence. The temporal interference has a femtosecond time scale corresponding to the optical transition frequency. Such studies of intermixing between different order nonlinear optical processes with a controllable phase delay can have important applications in high-precision measurements, coherence quantum control, and quantum information processing.

We show that by placing a single CdSe/ZnS quantum dot (QD) near a high-reflective optical mirror, both the photoluminescence intensity and the blinking statistics can be modified significantly and controlled deterministically by changing the dot-mirror distance. The modified local optical mode density in proximity to the QD alters the internal dynamics of the QD, which controls its blinking behavior. Such external controls of single-QD blinking can help us to better understand the underlying mechanism of the blinking process and lead to interesting applications of such QDs.

A squeezed vacuum field can be amplified or deamplified when it is injected, as the signal beam, into a phase-sensitive optical parametric amplifier (OPA) inside an optical cavity. The spectral features of the reflected quantized signal field are controlled by the relative phase between the injected squeezed vacuum field and the pump field for the OPA. The experimental results demonstrate coherent phenomena of OPA in the quantum regime and show phase-sensitive manipulations of quantum fluctuations for quantum information processing.

We report a likely observation of cw lasing without inversion in a gas of hot rubidium atoms in an optical cavity under conditions of electromagnetically induced transparency (EIT). The medium is pumped coherently and resonantly by a single “coupling” beam which also produces EIT in the lasing transition. The steady-state intensity exhibits thresholds as a function of the atomic density and the strength of the coupling beam. A theoretical model for an effective three-level lambda system indicates that gain without inversion is possible in this system if the two ground states are coupled by depolarizing collisions, and if the decay branching ratios meet certain conditions.

We present the theory of “normal mode splitting” (sometimes also called, in the low-intensity limit, “vacuum Rabi splitting”) for a Doppler-broadened two-level medium in an optical cavity, and illustrate it with experimental results. We note that, in the extreme Doppler limit, the absorptive and dispersive properties of the medium are due to almost completely different groups of atoms, and hence it is possible, in principle, to partially saturate the absorbing atoms without changing the dispersion very much; as a result of this, for a sufficiently high probe intensity, transmission peaks well within the Doppler absorption profile are visible, even though the medium is optically dense. Some interesting features exhibited by this system in various regimes are transmission peaks that are unrelated to the “zero-phase” condition, and three-peaked transmission spectra for high atomic density and high cavity input power.

We study three (nested, parallel, and sequential cascade) types of schemes for doubly dressed four-wave-mixing processes in an open five-level atomic system. The interaction between two dressing fields of the nested-cascade scheme is strongest and weakest for the parallel-cascade scheme, with the sequential scheme intermediate between them. Mutual-dressing processes and constructive or destructive interference between two coexisting dressed multiwave mixing channels in such a system are also considered. Investigations of these multidressing mechanisms and interactions are very useful to understand and control the generated high-order nonlinear optical signals.

We demonstrate efficient energy exchange during propagation between four-wave-mixing (FWM) and six-wave-mixing (SWM) signals generated in a four-level inverted-Y atomic system, which fall in the electromagnetically induced transparency window. After an initial growth distance for both FWM and SWM fields in the atomic medium, these two nonlinear optical processes compete and exchange energy between them, and eventually reach their respective steady-state values at long interaction distance. This energy exchange phenomenon can be explained by considering established atomic coherences among various atomic states and quantum interferences between three-photon and five-photon excitation pathways. Understanding these high-order nonlinear optical processes and interplays between them can be very important for correlated FWM (or SWM) photon-pair generations and quantum information processing.

The transmission spectrum of three-level atoms in a vapor cell inside an optical cavity shows distinct peaks associated with atom-cavity polaritons in the system. We develop the theory of these resonances in a Doppler-broadened medium and present the results of experimental observations of these spectra in three-level Λ-type rubidium atoms inside an optical ring cavity.

The third-rank susceptibility tensor that describes the second-harmonic generation changes sign after an inversion operation indicating its sensitivity to twinning structures. Here, we show the far-field scattering patterns of second-harmonic (SH) radiation generated by single twinned and twin-free ZnO rods. The patterns have been successfully correlated with the twinning structures. A finite dipole-wire model has been effectively employed to model the pattern. It indicates that the dark or bright fringes at the 0° scattering angle originate as a result of the destructive or constructive interference of SH waves emitted from each halves of the twinned or twin-free ZnO rod.

We have theoretically analyzed nonclassical paired photons generated spontaneously from a four-level inverted-Y atomic system. We discuss the feasibility of biphoton generation due to different pumping arrangements. Two types of correlated photon pair emissions have been carefully examined: dressed cascade and dressed double-Λ emissions. In the dressed cascade two-photon emission, the coincidence counting rate may exhibit a damped Rabi oscillation, in contrast with a simple exponential decay feature in the literatures. In the dressed double-Λ configuration, we show that not only is the temporal correlation of entangled Stokes–anti-Stokes photons in agreement with discussions presented by Wen [Phys. Rev. A 76, 013825 (2007)], but also the oscillation period may be manipulated by altering either the control Rabi frequency or the dressing Rabi frequency or both. All the observed damped Rabi oscillations in the two-photon coincidences result from the destructive interference among the possible four-wave mixing processes occurring in the atom-field interaction system. This feature of the two-photon amplitude is governed by the convolution between the phase matching and third-order nonlinear susceptibility. The methodology adopted here can be applied to other atomic-level configurations and provides a useful tool to study the spectroscopy of the system. The generated narrow-band photon pairs may have potential applications in long-distance quantum communication and quantum information processing.

We experimentally demonstrate that broad, tunable bandwidth of an optical ring cavity can be achieved by making use of the negative dispersion of the Kerr nonlinear refractive index in the three-level electromagnetically-induced transparency medium. For a given cavity field intensity, the white-light cavity condition can always be satisfied by choosing the appropriate coupling beam parameters. This white-light cavity scheme can be used effectively to achieve large buildup intensity in the signal-recycling cavity with a broad bandwidth in future advanced gravitational wave detectors.

We present an experimental system to generate large cross-phase modulation (XPM) in cold rubidium atoms. By using an efficient state-preparation technique in the ⁸⁷Rb D1 line, an ideal four-level tripod-type atomic system is formed, which generates large cross-Kerr nonlinearity via interacting dark states in this system. The induced phase shift due to XPM for the probe beam is measured for different trigger beam intensities, which is the key to achieving conditional quantum phase gates and many other applications in quantum information processing.

The bistable behavior and field induced transparency are demonstrated in a quantum-mechanical treatment of coherently driven superconducting quantum interference devices having cubic nonlinearity in the polarizability.

We show that resonance fluorescence, i.e., the resonant emission of a coherently driven two-level system, can be realized with a semiconductor quantum dot. The dot is embedded in a planar optical microcavity and excited in a waveguide mode so as to discriminate its emission from residual laser scattering. The transition from the weak to the strong excitation regime is characterized by the emergence of oscillations in the first-order correlation function of the fluorescence, g(τ), as measured by interferometry. The measurements correspond to a Mollow triplet with a Rabi splitting of up to 13.3  μeV. Second-order correlation measurements further confirm nonclassical light emission.

Highly efficient four-wave mixing (FWM) and six-wave mixing (SWM) processes can coexist in a four-level Y-type atomic system due to atomic coherence. The simultaneously opened dual electromagnetically induced transparency windows in this four-level atomic system allow observation of these two nonlinear optical processes at the same time, which enables detailed studies of the interplay between the FWM and SWM processes. Three-photon and five-photon destructive interferences are also observed.

We demonstrate a simple, all-optical technique to prepare and determine the desired internal quantum states in multi-Zeeman-sublevel atoms. By choosing appropriate coupling and pumping laser beams, atoms can be easily prepared in a desired Zeeman sublevel with high purity or in any chosen ground-state population distributions (spin-polarized quantum-state engineering). The population distributions or state purities of such prepared atomic states can be determined by using a weak, circularly polarized probe beam due to differences in transition strengths among different Zeeman sublevels. This technique will have potential impact on quantum-information processing in multilevel atomic systems.