Multi-Wave Mixing Processes in Multi-level Atomic System

The switch between the enhancement and suppression of the four-wave mixing (FWM) process in an electromagnetically induced transparency (EIT) window via quantum interference has been demonstrated experimentally. The enhancement and suppression of the FWM signal, as well as the EIT window, switch with a change in the probe detuning. Also, by controlling the powers of the dressing and probe fields, the enhancement and suppression of the FWM signal can be switched and an optimized enhancement can be obtained. Such a switch of the enhancement and suppression of the FWM signal intensity can find potential applications in optical switches, optical communication and quantum information processes.

Based on double optical pumping channels, we experimentally study the competition between two coexistent six-wave mixing (SWM) processes falling into two electromagnetically induced transparency windows by scanning the frequency of the probe field in two similar five-level atomic systems of 85Rb. By blocking one optical pumping channel unrelated to the four-wave mixing (FWM) process, one SWM process, together with the FWM process, generated by a conjugated small-angle static grating could be observed in the spectrum. Moreover, the other SWM process obtained by blocking the first SWM channel is also observed together with the FWM process in a lower N-type four-level subsystem. These experimental results agree well with theoretical predictions.

Single photon sources to be used in quantum cryptography must show higher order antibunching (HOA). HOA is reported by us in several many wave mixing processes. In the present work we have investigated the possibility of observing HOA in multiwave mixing processes in general. The generalized Hamiltonian is solved for several particular cases in Heisenberg picture and possibility of observing HOA is investigated with the help of criterion of Pathak and Garcia. The generalized interaction Hamiltonian of a multiwave mixing process is considered as (a†l bm cn + Hermitian conjugate) where 'a', 'b' and 'c' are the annihilation operators corresponding to pump, Stokes and Signal modes respectively. Several particular cases of the generalized Hamiltonian are solved with the help of short time approximation technique and HOA is reported for pump modes of different multiwave mixing processes. It is also found that HOA can not be observed for the signal and stokes modes in of the cases studied here.

We propose a multifunction phase-shifting manipulator with low noise at a single-photon level,by using a threelevel atomic scheme.This three-level system interacts with a strong pumping field and a weak probe field with a large detuning.Due to this large detuning,two lower states can be coherently prepared prior to the injection of the pump and probe fields.In our configuration,the duration of the pumping field is much longer than that of the probe field. By solving the Heisenberg-Langevin equations of our system under the steady state approximation,we calculate the linear susceptibility of the system and examine the quantum noise properties of the probe field in detail.We show that this scheme,which rests on the process of two-wave mixing with initial atomic coherence,exhibits many interesting properties that neither typical electromagnetically induced transparency (EIT) schemes nor active Raman gain (ARG) schemes possess.Although both EIT-and ARG-based schemes have been widely investigated in atomic medium,the direct generalizations of these schemes to the single/few photon limit prove to be more problematic.The low fidelity due to the significant probe-field attenuation in EIT medium and the large quantum noise due to the amplification of the probe field in an active Raman gain medium are the main obstacles that prohibit a high-fidelity,low-noise phase shifter from being realized in the single/few photon limit.Physically,this scheme can be viewed as a hybrid scheme in which two processes of different physical principles are allowed to interfere with each other to achieve many desired functionalities. For instance,it can be used as a lossless two-photon-broadband phase-shifter with suitable system parameters.It can also be used as an attenuator/amplifier and a total transparency with a zero phase shift.In particular,we show that by locking the pump field intensity and the two-photon detuning simultaneously a flat constant π-phase shift can be realized with unit probe fidelity in a broad probe field frequency range.Applying the quantum regression theorem,we calculate the noise spectrum of the outgoing probe field as a large phase shift is achieved,and show that this two-photon-insensitive π-phase shift may significantly reduce the quantum noise fluctuations associated with a Raman gain process,and have a lot of potential applications for quantum information processing and optical telecommunication.The realization of this broadband π-phase-shift with significantly reduced quantum noise fluctuations makes this scheme attractive for the realization of low-noise phase-gate/polarization-gate at single-photon level.

One of the reasons for the recent rising interest in electromagnetically induced transparency (EIT) in multilevel atomic systems [1] is its potential application in enhancing nonlinear optical processes. Several experiments were carried out recently to show that the atomic coherence effect in multi-level atomic systems could reduce absorption and enhance frequency conversion and four-wave mixing processes [2, 3]. On the other hand, when two level atoms are put into an optical cavity, many nonlinear processes, such as optical bistability and instability, occur. Recently, several theoretical papers proposed switching effects in three-level and four-level atomic systems with and without an optical cavity [4, 5]. These nonlinear optical effects are caused by enhanced Kerr-nonlinearity and dramatic changes in the dispersion slope due to atomic coherence in such multi-level atomic systems. Recently, we have measured the enhanced Kerr-nonlinear index of refraction n 2 of three-level A-type atoms by using an optical ring cavity and found a large modification of n 2 as functions of the frequency detunings of the probe and coupling lasers. We also observed self-pulsation of cavity transmission when the coupling frequency is tuned to atomic resonance and the probe frequency is detuned from resonance by a few MHz. In this paper, we report such observation and discuss its physical mechanism.

We theoretically investigate the influence of a dressing field on parametric amplification multi-wave mixing (PA-MWM) processes in an atomic ensemble for the first time. The quantum spatial properties of PA-MWM signals are demonstrated by studying the cone emissions. Meanwhile, we investigate the intensities, the intensity difference squeezing, and the intensity noise correlation of bright twin PA-MWM beams, which are generated by injecting the MWM signals into the input ports of a spontaneous parametric four-wave mixing process. Both the spatial properties and the quantum correlation can be enhanced or suppressed via a bright state or a dark state, which is induced by the single or double dressing effect. This study has potential applications in quantum security, quantum imaging and long distance quantum communications.

Magnet-free optical nonreciprocity has significant applications in quantum communication, quantum networks, and optical information processing. In this research, considering a degenerate two-level thermal atomic system with the Doppler effect of thermal atoms, the nonreciprocal amplification (NRA) of dual-path degenerate four-wave mixing (FWM) signals is achieved under the action of a co-propagating pumping field. On this basis, spatially multiplexed multiple FWM processes are formed by introducing another counter-propagating pumping field, thereby achieving the reciprocal amplification (RA) of the dual-channel FWM signals. Furthermore, by using multiple sets of spiral phase plates to load spiral phases on the signal light and the pumping light respectively, higher-order Laguerre-Gaussian vortex beams carrying different optical orbital angular momentum (OAM) are generated and participate in the FWM process, achieving the transfer of the OAM of the pumping light to the amplified FWM fields. Simultaneously, using the Mach-Zehnder interferometer, the conservation characteristics of the OAM of each FWM signal in the NRA-RA conversion are further analyzed. Furthermore, experimental results demonstrate that in the multiple FWM process induced by a pair of counter-propagating pump fields, the OAM of the amplified FWM signal in each channel varies with that of the pump field. However, the overall process maintains the OAM conservation. This study provides a feasible solution for expanding the channel capacity using OAM based on NRA-RA system, showing that the OAM has potential application prospects in achieving high-capacity optical communication and multi-channel signal processing.

Our research is focused on the application of quantum coherence induced by the interaction of laser light with atoms. These effects have practical applications in quantum information processing in quantum memories, which are necessary components of quantum optical computers and quantum repeaters. We address complex quantum coherence effects in multilevel atomic systems, and prove that many dark states in multi‐level systems exist which are dependent on the polarization of the interacting light and quantization direction of magnetic fields. Such dark states can be used for multichannel storage of quantum information. We need to know how to suppress the noise of the quantum states for faithful storage of quantum states in atoms. We theoretically analyze the complicated quantum noise spectrum of the delayed light passing through the atomic system.

A three-level lambda system driven by multicolor control, pump, and probe fields is investigated. The pump and probe fields are derived from the same laser with opposite propagating directions. Due to the Doppler effect, the zero group-velocity atoms face bichromatic fields, while other atoms face trichromatic fields. The atomic medium shows distinct characteristics and exhibits simultaneous electromagnetically induced transparency (EIT) and electromagnetically induced absorption (EIA) at two frequencies. EIT and EIA peaks have a fixed relationship with frequency, which is determined by the Doppler shifts.

We numerically investigate the dynamics of inward circular finite-energy Airy beams carrying different orbital angular momentum (OAM) numbers in a close-Λ three-level atomic vapor with the electromagnetically induced transparency (EIT) window. We report that due to the EIT induced by the microwave field, the transverse intensity distribution properties of Airy beam can be feasibly manipulated and modulated through adjusting OAM numbers l and the frequency detuning, as well as the propagation distance, in the multi-level atomic systems. What’s more, the rotation of the beam also can be observed with different positions in atomic ensembles. The investigation may provide a useful tool for studying particle manipulation, signal processing and propagation in graded-index (GRIN) fibers.

We experimentally demonstrate spatial interference between simultaneously generated four-wave mixing (FWM) and six-wave mixing (SWM) signals in a multilevel atomic system. By tuning the relative phase between the FWM and SWM processes the relative strengths between these two high-order nonlinear optical processes can be investigated.

Resonant intracavity four-wave mixing in a composite atom–cavity system

The behavior of multilevel atomic ensembles (e.g., alkali-metal atoms) can be influenced significantly by the intensity of a driving field (or controlling/coupling field). The phase coherence between two transition pathways driven by a probe light and a driving field can lead to the effect known as electromagnetically induced transparency (EIT). In EIT, the probe light can pass through a three-level alkali-metal atomic vapor without absorption or reflection when two coherent resonances (transition pathways driven by the driving and probe fields) are present and the linewidths of the transparency windows are sufficiently narrow. The optical characteristics of atomic systems can also be affected by the Doppler broadening of the absorption profile in a spectroscope. Our analysis indicates that both broadenings (related to the transitions excited by the driving and probe fields) can be expanded, leading to an increase in the transmittance and reflectance broadenings when a coupling field with adaptive strength is applied; the corresponding temperature would, thus, be implemented and readable. We show that the most suitable preparation for temperature sensing via an EIT vapor is to provide 80 times the spontaneous decay rate (SDR) of the excited atomic levels. This configuration results in reflectance and transmittance values that range between zero and one and cover a temperature range of 0 K to 600 K. As an example, we demonstrate the integration of specialized coating technologies with EIT ensembles for temperature sensing in the range of dozens of kelvins at and above room temperature. A key advantage of this temperature-sensing system is its ability to use adaptive resonant visible light as the probe field. This novel approach may find applications in providing unprecedented levels of precision and control in temperature sensing for coating processes and in the design of other photonic or optical devices. It can also be used to determine the temperature-dependent behavior of the specific heat of alkali-metal solids and gases (including the latent heats of vaporization or sublimation of alkali-metal solids) through the reflection and transmission spectra of the vaporized EIT atomic vapors.

We present an investigation of electromagnetically induced transparency (EIT) without Doppler background due to a locked probe laser. The EIT is theoretically studied based on a multilevel ladder-type cesium atomic system 6${S}_{1/2}$--6${P}_{3/2}$--8${S}_{1/2}$ in a room-temperature vapor cell. The experimental results agree with the theoretical calculations. Compared with the traditional EIT spectra with a Doppler profile limiting the spectral resolution for keeping the coupling laser locked and scanning the probe laser, these EIT spectra with the probe laser locked and the coupling laser scanned have a flat background, which seem be of great benefit for applications such as the measurement of hyperfine intervals between excited states, the study of highly excited Rydberg states, laser-frequency stabilization, etc.

We propose a scheme to construct a two-dimensional Aubry-Andr\'e (AA) model and realize two-dimensional AA localization of light waves via electromagnetically induced transparency (EIT). The system we suggest is a cold, resonant atomic gas with an $N$-type level configuration and interacting with probe, control, assisted, and far-detuned laser fields. We show that under EIT conditions the probe-field envelope obeys a modified nonlinear Schr\"odinger equation with a quasiperiodic potential, which becomes a two-dimensional nonlinear AA model when the system parameters are suitably chosen. The quasiperiodic potential is obtained by the cross-phase modulation of the assisted field and the Stark shift of the far-detuned laser field. In addition, the cubic nonlinearity term appearing in the model is contributed by the self-phase modulation of the probe field. We demonstrate that the system can be used to not only realize various two-dimensional AA localizations of light waves, but also to display nonlinearity and dimensionality effects on the AA localizations.