Frequency conversion and entanglement in V configuration atomic ensemble

The frequency conversion and entanglement is studied via polariton technique. We analyze the effect of detuning on the efficiency of frequency conversion (EFC). The results show that by adjusting the second mode frequency, the EFC still can be achieved perfectly when the incoming photon is off-resonant. The effect of the spontaneous emission of the atoms on EFC is included.     

Squeezing by second-harmonic generation in a monolithic resonator

We have measured the intensity fluctuations of the second-harmonic mode generated in a MgO:LiNbO₃ external monolithic cavity pumped by a Nd:YAG laser. The cavity has mirror coatings for both the fundamental and the second-harmonic mode. We scan the cavity using the electro-optic effect of the crystal and observe that the second-harmonic beam of 2 mW exhibits a quantum noise reduction of 40(±5)%. In addition, we report on the active frequency stabilization of the monolithic device used in our squeezing experiments. Several fast tuning parameters such as the electro-optic effect, the photo-elastic effect, and the laser frequency have been investigated. With these tuning parameters the monolithic resonator can be locked on double-resonance at the phase-matching temperature, which is a prerequisit for observing squeezing in a cw-regime.     

Generation of pulsed squeezed light in a mode-locked optical parametric oscillator

The generation of pulsed squeezed light using an optical parametric oscillator (OPO) is discussed. This mode-locked optical parametric oscillator consists of a nonlinear crystal in a cavity which is resonant for both signal and idler waves and which is synchronously pumped by the second-harmonic of an acousto-optically mode-locked cw Nd: YAG laser. The fundamental wavelength of the pump laser provides local oscillator pulses for balanced homodyne detection of squeezed vacuum pulses emitted by the oscillator when operated below oscillation threshold. Photocurrent noise reduction to 30% below the classical shot-noise limit is observed, corresponding to squeezing of the field to a level approximately a factor of two below the mean square vacuum noise.     

Generation of amplitude squeezed green light from a high efficiency PPKTP frequency doubler

We report on a high-efficiency 532 nm green light conversion from an external cavity-enhanced second harmonic generation of a home-made 1064 nm single-frequency Nd:YVO₄ laser with a periodically poled KTP crystal. A stable green power of 60 mW with a conversion efficiency of 75% was measured. Meantime, we investigate the quadrature amplitude noise of the green light at the same experimental setup and 0.6 dB green light squeezing was experimentally observed (taking into account the total detection efficiency of 58%, the squeezing should be 1.1 dB). The squeezing as a function of input power was also studied and we found qualitative agreement with theoretical prediction.     

Generation of a displaced qubit and entangled displaced photon state via conditional measurement and their properties

We study optical schemes for generating both a displaced photon and a displaced qubit via conditional measurement. Combining one mode prepared in different microscopic states (one-mode qubit, single photon, vacuum state) and another mode in macroscopic states (coherent state, single photon added coherent state), a conditional state in the other output mode exhibits properties of a superposition of the displaced vacuum and a single photon. We propose to use the displaced qubit and entangled states composed of the displaced photon as components for quantum information processing. Basic states of such a qubit are distinguishable from each other with high fidelity. We show that the qubit reveals both microscopic and macroscopic properties. Entangled displaced states with a coherent phase as an additional degree of freedom are introduced. We show that additional degree of freedom enables to implement complete Bell state measurement of the entangled displaced photon states.     

Three-Photon Resonant Nondegenerate Six-Wave Mixing in a Dressed Atomic System

We report a three-photon resonant nondegenerate six-wave mixing （NSWM） in a dressed cascade five-level system. It has advantages that phase match condition is not stringent and NSWM signal is enhanced tremendously due to the multiple resonance with the atomic transition frequencies. In the presence of a strong coupling field, the threephoton resonant NSWM spectrum exhibits＂ Autler-Townes splitting. This technique provides a spectroscopic tool for measuring not only the resonant frequency and dephasing rate but also the transition dipole moment between two highly excited atomic states.     

Nonlinear mode coupling in doubly resonant frequency doublers

The concept of a doubly resonant frequency doubler can be used for a variety of experiments concerning both classical phenomena like efficient frequency doubling at low power levels and quantum effects like squeezed states of light or Quantum Non Demolition (QND) measurements. In many of these experiments the strength of the nonlinear coupling of fundamental and second-harmonic modes is of crucial importance. First we treat the general theory for the calculation of the coupling parameter, which depends not only on properties of the nonlinear material but also on resonator geometry and some optical phases. On this basis we discuss in detail the situation for two different monolithic resonator geometries, namely a linear (standing-wave) and a ring (travelling-wave) cavity. Finally we compare theoretical predictions for these resonators to the experimentally achieved results.     

Ultraslow optical solitons in a four-level tripod atomic system

We investigate possible formation and propagation of localized, shape-preserving nonlinear optical pulse in a resonant, lifetime-broadened four-level tripod atomic system via electromagnetically induced transparency (EIT). We prove both analytically and numerically that although in anomalous dispersion regimes near resonance a superluminal optical soliton may appear, such soliton suffers serious absorption. However, by choosing appropriate parameters to make the system work in normal dispersion regimes and within an EIT transparency window, ultraslow optical solitons with very low light intensity can form and propagate stably in the system.     

Quantum correlated twin beams

We discuss recent progress in the study of the non-classical properties of light beams generated by non-degenerate parametric splitting in ⁽²⁾ nonlinear birefringent crystals, with special emphasis on their quantum correlation (twin beams). We describe experimental results using successively pure parametric fluorescence, parametric amplification of a weak signal beam pumped by a pulsed laser, and parametric oscillation in a cavity pumped by a cw laser. In this review, we compare the respective advantages and drawbacks of the different approaches.     

Generation of two-mode quadrature-phase squeezing and intensity-difference squeezing from a cw-NOPO

Received: 17 January 1998     

Effects on squeezing and sub-poissonian of light in fourth harmonic generation up to first-order Hamiltonian interaction

The effects on squeezing and sub-poissonian of light in fourth harmonic generation (FHG) are investigated based on the fully quantum mechanically up to the first order Hamiltonian interaction in gt, where g is the coupling constant between the modes per second and t is the interaction time between the waves during the process in a nonlinear medium. FHG is a process in which an incident laser beam of the fundamental frequency ω interacts with a nonlinear medium to produce the harmonic frequency at 4ω. The coupled Heisenberg equations of motion involving real and imaginary parts of the quadrature operators are established. The occurrence of amplitude squeezing effects in both the quadratures of the radiation field in the fundamental mode is investigated and found to be dependent on the selective phase values of the field amplitude. The photon statistics of the pump mode in this process have also been investigated and found to be sub-poissonian in nature. It is found that there is no possibility to produce squeezed light in the harmonic mode up to first-order interaction in gt. Further, we have found the case up to second-order Hamiltonian interaction in gt that the normal squeezing in the harmonic mode is directly depends upon the fourth-order squeezing of the initial pump field. This gives a method of converting higher-order (fourth-order) squeezing into normal squeezing in the harmonic mode and vice versa.     

Evolution of population in an equispaced three-level ladder-type atomic system

Transient response of nearly equispaced three-level ladder-type atomic system with a broad-band squeezed vacuum (SV) is investigated. We focus our attention in the interplay between the quantum interference and the squeezed field on the population distribution. It is shown that an atomic population inversion can be attained on one of the optical transitions due to the SV. Additionally, we show, with the proper value of the relative phase, the SV can also lead to unexpected population inversion on the transition between two different levels.     

Partial teleportation of entangled atomic states

In this Letter we propose a scheme for partially teleporting entangled atomic states. Our scheme can be implemented using only four two-level atoms interacting either resonantly or off-resonantly with a single cavity-QED. The estimative of losses occurring during this partial teleportation process is accomplished through the phenomenological operator approach technique.     

Generation of broadband squeezed states pumped by CW mode-locked pulses

Broadband high level squeezing was clearly observed from 100 kHz to 80 MHz using crystals Ba₂NaNb₅O₁₅ of 5 mm length, MgO:LiNbO₃ of 19 mm length and KNbO₃ of 5.8 mm length. Maximum noise reductions detected on a spectrum analyzer were –1.2 dB (–24%), –1.25 dB (–25%), and –1.8 dB (–34%) for the three crystals, respectively. The maximum squeezing is limited mostly by optical index damage of the parametric crystals. A detailed analysis of the beam parameters traced along the pump beam, squeezed vacuum, etc. is given. A detailed discussion on the evaluation of the initial squeezed level is given. A preliminary experiment with compressed laser pulses to avoid the optical damage is also described.     

Generation of a Non-Classical Correlated Photon Pair via Spontaneous Four-Wave Mixing in a Cold Atomic Ensemble

Counter propagated write and read lasers can be used to generate non-classical correlated photon pairs in an atomic ensemble. We experimentally investigate how the detuning of the write laser affects the non-classical correlation function between the Stokes photon and the anti-Stokes photon, which are generated via a spontaneous four-wave mixing process using an off-axis configuration in a cold 85 Rb atomic ensemble. The change of the time-resolved second-order correlated function between the Stokes and anti-Stokes photons is presented. The experimental result suggests that a suitable choice of detuning should be considered in such an experiment.     

Effect of spontaneously generated coherence on Kerr nonlinearity in a four-level atomic system

We propose a scheme for giant enhancement of the Kerr nonlinearity in a four-level atomic system in which spontaneously generated coherence is present. The physics mechanism of the enhancement of Kerr nonlinearity is mainly based on the presence of an extra atomic coherence induced by the spontaneously generated coherence. Numerical values obtained by solving the density matrix equations agree well with these exact analytical values.     

Observation of Quantum Beat in Rb by Parametric Four-Wave Mixing

Two coupled paralnetric four-wave-mixing processes in Rb atoms are studied using perturbation theory, which reveals clear evidence of tile appearance of quantum beat at 608cm^-1, corresponding to the energy difference of the 7s - 5d states of Rb atoms, in the parametric four-wave-mixing signals. A pump-probe technique is utilized to observe the quantum beat. Tinle-varying characteristics of the quantum beat are investigated using time-dependellt Fourier transform. The results show that the time-varying characteistics of the quantum beat not oldy offers a sensitive detecting method for observing the decay of atomic wave packets, but also provides a potential tool for monitoriHg the dissociation of molecules.     

Efficient four-wave mixing of a coupled double quantum-well nanostructure

We propose an efficient four-wave mixing (FWM) scheme in an asymmetric semiconductor double quantum-well (SDQW) structure based on intersubband transitions, and obtain the corresponding explicit analytical expressions for the input probe and generated FWM pulsed fields by use of the coupled Schrödinger-Maxwell approach. Under the resonant and phase-matched conditions, the FWM efficiency versus several variables is also discussed in details and the maximum FWM efficiency of the system under study is greater than 25%. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the controllable interference strength. This nonlinear optical process in the SDQW solid-state material can be used for efficiently generating coherent short-wavelength radiation.     

Mid-infrared efficient generation by resonant four-wave mixing in a three-coupled-quantum-well nanostructure

We present a cascade configuration for the realization of highly efficient four-wave mixing (FWM) process in an asymmetric semiconductor three-coupled-quantum-well (TCQW) structure based on intersubband transitions (ISBTs). In the proposed TCQW scheme, the efficiency of the generated FWM mid-infrared (MIR) signal is significantly enhanced and the obtained maximum efficiency is greater than 50%. The corresponding explicit analytical expressions for the input probe and generated FWM pulsed fields are derived by use of the coupled Schrödinger-Maxwell approach and the FWM efficiency versus several variables is also discussed in details. Such a semiconductor system is much more practical than its atomic counterpart because of its flexible design and the wide adjustable parameters. This nonlinear optical process in the TCQW solid-state material can be used for efficiently generating coherent short-wavelength radiation.     

Frequency-filtered parametric fluorescence interacting with an atomic ensemble

The frequency spectrum of the fluorescence must be reduced when studying interactions between atoms and parametric fluorescence using the photon counting method since photon counting does not distinguish the light frequency. An interference filter and etalons successfully reduced the frequency spectrum of the parametric fluorescence from 6.6 THz to 1.7 GHz. The parametric fluorescence after frequency filtering showed the non-classical feature violating a Cauchy-Schwartz inequality for the intensity correlation function. We used slow light propagation with Rb gas to demonstrate that the obtained light source interacts with the atoms.