Showing papers on "Atomic coherence published in 2001"

Enhanced Kerr nonlinearity via atomic coherence in a three-level atomic system.

Abstract: We measure the Kerr-nonlinear index of refraction of a three-level Lambda-type atomic system inside an optical ring cavity. The Kerr nonlinearity is modified and greatly enhanced near atomic resonant conditions for both probe and coupling beams. The Kerr nonlinear coefficient n(2) changes sign when the coupling beam frequency detuning switches sign, which can lead to interesting applications in optical devices such as all-optical switches.

Bistability and instability of three-level atoms inside an optical cavity

Abstract: Optical bistability and dynamic instability are experimentally observed and studied in a system consisting of three-level A-type rubidium atoms in an optical ring cavity. The bistable behavior and self-pulsing frequency are experimentally manipulated by changing the controlling and cavity field parameters (power and frequency). These nonlinear effects are influenced by the enhanced Kerr nonlinearity due to atomic coherence in such a system.

Nonadiabatic approach to quantum optical information storage

Abstract: We show that there is no need for adiabatic passage in the storage and retrieval of information in the optically thick vapor of Lambda-type atoms. This information can be mapped into and retrieved out of long-lived atomic coherence with nearly perfect efficiency by strong writing and reading pulses with steep rising and falling edges. We elucidate similarities and differences between the ``adiabatic'' and ``instant'' light storage techniques, and conclude that for any switching time, an almost perfect information storage is possible if the group velocity of the signal pulse is much less than the speed of light in the vacuum c and the bandwidth of the signal pulse is much less then the width of the two-photon resonance. The maximum loss of the information appears in the case of instantaneous switching of the writing and reading fields compared with adiabatic switching, and is determined by the ratio of the initial group velocity of the signal pulse in the medium and speed of light in the vacuum c, which can be very small. Quantum restrictions to the storage efficiency are also discussed.

Quantum interferences and the question of thermodynamic equilibrium

Abstract: We derive from first principles the dynamical equations for the interaction between a heat bath and a multilevel atom with some near degenerate states. Such dynamical equations exhibit atomic coherence terms which arise from the interference of transition amplitudes. We address the question whether such equations lead to a steady state that is consistent with the thermodynamic equilibrium. We show that coherence affects the dynamics of the system, but the equilibrium conditions are still characterized by Boltzmann factors. We also show how an asymmetric treatment of spontaneous and stimulated processes could lead to a steady state which is at variance with the principles of thermodynamic equilibrium. We show that such a steady state can be realized by pumping with broadband laser fields. Finally, we show that coherences in the dynamical equations can be probed via the spectrum of fluorescence.

Large polarization rotation via atomic coherence.

Abstract: We report significant enhancement of the nonlinear Faraday rotation in optically thick Rb vapor. Polarization rotation angles as large as 10 rad were observed for what is believed to be the first time for sub-Gauss magnetic fields. The use of this effect for high-precision magnetometry is also discussed.

Absorption with inversion and amplification without inversion in a coherently prepared V system: A dressed-state approach

Abstract: Light-induced absorption with population inversion and amplification without population inversion in a coherently prepared closed three level V-type system are investigated. This study is performed from the point of view of a two color dressed-state basis. Both of these processes are possible due to atomic coherence and quantum interference contrary to simple intuitive predictions. Merely on a physical basis, one would expect a complementary process to the amplification without inversion. We believe that absorption in the presence of population inversion found in the dressed-state picture utilized in this study, constitutes such a process. We derive approximate analytic time-dependent solutions, for coherences and populations that are compared with full numerical solutions exhibiting good agreement. Steady-state quantities are also calculated, and the conditions under which the system exhibits absorption and gain with and without inversion, in the dressed-state representation are derived. It is found that for weak input probe-laser field absorption with inversion and amplification without inversion may occur, for a range of system parameters. These take place at resonance and the generalized Rabi ac Stark-shifted frequencies, in agreement with exprimental investigations, and at beat frequencies, depending on the relevant parameters.

Spatial evolution of short laser pulses under coherent population trapping

Abstract: The spatial and temporal evolution is studied of two powerful short laser pulses having different wavelengths and interacting with a dense three-level \ensuremath{\Lambda}-type optical medium under coherent population trapping. The general case of unequal oscillator strengths of the transitions is considered. The durations of the probe pulse and the coupling pulse ${T}_{1,2} {(T}_{2}g{T}_{1})$ are assumed to be shorter than any of the relevant atomic relaxation times. We propose analytical and numerical solutions of a self-consistent set of coupled Schr\"odinger equations and reduced wave equations in the adiabatic limit taking account of the first nonadiabatic correction. The adiabaticity criterion is also discussed taking account of pulse propagation. The dynamics of propagation is found to be strongly dependent on the ratio of the transition oscillator strengths. It is shown that the envelopes of the pulses slightly change throughout the medium length in the initial stage of propagation. This distance can be large compared to the one-photon resonant absorption length. Eventually, the probe pulse is completely reemitted into the coupling pulse during propagation. An effect of localization of the atomic coherence was observed similar to the one predicted by Fleischhauer and Lukin [Phys. Rev. Lett. 84, 5094 (2000)].

Entropy and entanglement of an effective two-level atom interacting with two quantized field modes in squeezed displaced fock states

Abstract: We have studied the influences of ac-Stark shifts on the field quantum entropy, with “squeezed displaced Fock states” (SDFSs) basis. By a unitary transformation we derive a Raman-coupled Hamiltonian perturbatively in coupling constants. The exact results are employed to perform a careful investigation of the temporal evolution of entropy. A factorization of the initial density operator is assumed, with the privileged field mode being in the SDFS. We invoke the mathematical notion of maximum variation of a function to construct a measure for entropy fluctuations. The results show that the effect of the SDFS changes the quasiperiod of the field entropy evolution and entanglement between the atom and the field. The Rabi oscillation frequency, the collapse and revival times of the atomic coherence are found to have strikingly different photon-intensity dependent than those found previously. The general conclusions reached are illustrated by numerical results.

Atomic Coherence in the Micromaser Injected with Slow V-type Three-State Atoms: Emission Probability

Abstract: The effects of atomic coherence on the single-mode two-photon micromaser injected with slow V-type three-state atoms are studied for the first time. It is shown that the atomic coherence can modify the atomic emission probability. The effects of the atomic centre-of-mass momentum, the cavity length and other parameters are also studied.

Spectral linewidth of inversionless and Raman lasers in V-type three-level systems

Abstract: A theoretical analysis of the spectral linewidth of V-type inversionless and Raman lasers is presented. First, we examine the effects of the atomic coherence between dressed states and the Autler-Townes splitting on the linewidth. It is demonstrated that near above threshold, the V inversionless laser has a narrower linewidth than that of the two-level laser. Instead of the dressed coherence, it is the Autler-Townes splitting that is responsible for the linewidth reduction though the dressed coherence determines the laser gain. Next, we explore the effects of the generated laser intensity on the linewidth. It is shown that the linewidths of the V inversionless and Raman lasers follow the usual 1/I decrease for smaller laser intensity I, but a slower decrease than 1/I for larger laser intensity. For the V Raman laser, even more surprisingly, with the laser intensity increasing, the linewidth appreciably increases as well. As a result, well above threshold, the V inversionless and Raman lasers may have a larger linewidth than that of the two-level laser. Finally, a comparison is made between the V lasers and the Λ lasers. It is found that the linewidth of the Λ inversionless laser shows a fast 1/I2 decay under optimum conditions.

Four-wave mixing with short pulses and optimized atomic coherence

Abstract: We present a time-dependent calculation for four-wave mixing using a combination of long, short, and time delayed laser pulses in the context of electromagnetically induced transparency. Two transform limited nanosecond lasers are used to create a highly coherent mixture of the ground state and an excited state via a two-photon process. Once the induced transparency is established, a laser with short pulse length is injected after a suitable delay to generate four-wave mixing. We show that the wave mixing process is phase matched for all detunings, and with appropriately selected atomic coherence and populations, near 100% photon flux conversion efficiency can be obtained, independent of the intensity of the short pulse laser. In addition, we show that for small detunings for the short pulse laser, the four-wave mixing field travels with the speed of light in vacuum and suffers no pulse distortion even though the medium is highly dispersive at the frequency of the generated wave. These advantages open a door for future applications of the scheme for highly efficient, very stable UV generation.

Coherent two-photon pulse propagation through a coherently prepared medium

Abstract: Propagation of pulses through a medium comprising of coherently prepared two-level atoms undergoing two-photon transition has been studied using Maxwell–Bloch equations. The solution of the electromagnetic field envelopes has been obtained under various conditions and the effect of initial atomic coherence is clearly brought out in these solutions.

Laser Induced Atomic Coherence and Interference in Cold Atoms

Abstract: We report experimental studies of electro magnetically induced transparency and enhancement of third-order optical nonlinearities in A-type atomic systems realized in 87Rb atoms cooled and confined in a magneto-optical trap. We observed complete transparency of a weak probe laser in the A type Rb system with a moderate coupling field. With a third pump laser tuned to a fourth atomic state, the atomic system exhibits the interference-enhanced third-order susceptibility and only absorbs two photons, but not one photon. Our experiment demonstrates large nonlinear absorption of the weak probe laser, from which greatly enhanced Kerr nonlinearity in the Rb system is inferred.

V-type three-level-atom mazer with atomic coherence: velocity selection

Abstract: The velocity selection of the V-type three-level-atom mazer with atomic coherence is studied. It is shown that the atomic coherence can strongly affect transmission probability, and in turn, the velocity selection of atoms through the micromaser cavity. When the incident atoms are initially prepared in the lower state of the two upper ones (i.e., no injected atomic coherence), velocity selectivity is at a maximum. This shows that atomic coherence does not have a favourable effect on velocity selectivity in this scheme. The effects of other parameters are also studied.

Micromaser injected with ultra-cold v-type three-level atoms:effects of atomic coherence on photon statistics

Abstract: The effects of the atomic coherence on the photon statistics of the micromaser injected with ultra-cold V-type three-level atoms are studied.In the plot of the atomic emission probability versus the cavity length, there are resonance peaks and non-resonance platforms, which can be adjusted by the atomic coherence parameters.These features of emission probability affect the properties of photon statistics directly. In the plot of the mean number of photons versus the cavity length, there are resonance peaks and non-resonance platforms,too. Given a cavity length, with the change of the atomic coherence parameters, the distribution of photon numbers can move towards the large or small photon numbers, and the mean number of photons increases or decreases accordingly.We also find, with appropriate atomic coherence parameters, that the photon statistics are sub-Poissonian in large region of the cavity length.

Quantum state manipulation via Raman coherence

Abstract: Summary form only given. The study on the interaction between electromagnetic fields and multi-level atoms or molecules has discovered many interesting effects including electromagnetically induced transparency (EIT) enhanced dispersion with reduced absorption, and lasing without inversion. Recent interests include ultraslow group velocity of light, and superluminal light propagation. Atomic coherence created between different atomic levels plays an essential rule in these experiments. On the other hand, quantum features in the interaction between electromagnetic fields and three-level systems based on atomic coherence have attracted extensive theoretical interests. These include generation of amplitude-squeezed and quantum noise-correlated states of light with EIT, entanglement of atomic states with light fields, and quantum memory for light. According to these theoretical works, manipulation of quantum states of either light fields or atoms via atomic coherence is possible. We study quantum state manipulation of light via coherently driven Raman coherence in a /spl Lambda/-type three-level system of rubidium vapor under far off one-photon resonance condition both theoretically and experimentally.

Reflection of a Slow Atom by a Cavity

Abstract: We study the center of mass movement of a cold two-level atom crossing through an electromagnetic cavity This cavity contains both a quantized field mode and a set of A fixed two-level atoms resonant with the field We find that, even in the absence of fixed atoms, the atomic coherence allows us to switch the interaction from an exclusive barrier potential to an attractive potential In such a way that the initial atomic phase behaves as a quantum switch The set of fixed atoms, initially prepared in semiclassical eigenstate, gives origin to an effective growing of the coupling constant of the moving atom and the field, allowing an important increase of the threshold value of kinetic energy of moving atom where a total reflection can be experimentally observed

Atomic Coherence Effects in Doppler-Broadened Three-Level Systems with Standing-Wave Drive

Abstract: We study atomic coherence effects (e.g., electromagnetically induced transparency, EIT, and amplification without inversion, AWI) for a probe travelling-wave (TW) laser field in closed Doppler-broadened three-level systems driven by a standingwave (SW) laser field of moderate intensity (its Rabi frequencies are smaller than the Doppler width of the driven transition). We show that probe windows of transparency occur only for values of the probe to drive field frequency ratio R close to half integer values. For optical transitions and typical values of Doppler broadening for atoms in a vapor cell, we show that for R > 1 a SW drive field is appreciably more efficient than a TW drive in inducing probe transparency. We show that folded (cascade) schemes driven by a sufficiently detuned SW field can exhibit AWI for odd (even) values of R. Results for AWI with frequency up-conversion ratios R = 3 and 4 are presented and compared to those obtained with a TW drive.

Coherently pumped micromaser as a maser without inversion

Abstract: In this paper, we use microscopic maser theory developed by Filipovicz, Javanaien and Meystre [Phys. Rev. A 34, 3077 (1986)] to investigate the properties of the coherently pumped two-level micromasers. We flnd that under right conditions, the initial atomic coherence locks the fleld phase and unlike ordinary lasers and masers there is no longer threshold. Consequently, the population inversion is not necessary.

Optical Ramsey-type magnetometer using coherent population trapping

Abstract: Summary form only given. By now there have been proposed and partly realized in experiments several optical schemes for the precise measurement of a magnetic field. Among them the most promising methods are based on atomic coherence (first proposed by Scully and Fleischhauer in 1992). In one of optical schemes the interaction with spatially separated fields was used, following to the Ramsey method. We propose a new variant of magnetometer, where the advantages of the Ramsey method are combined with the high-contrast detection provided by the coherent population trapping effect. In our scheme for the measurement of arbitrary directed magnetic field B a cell with atomic vapor is used.