We propose an ultrafast femtosecond time scale trichromatic $\ensuremath{\pi}$-pulse illumination scheme for coherent excitation and manipulation of low-lying Rydberg states in rubidium. Selective population of ${nP}_{3/2}$ levels with principal quantum numbers $n\ensuremath{\lesssim}12$ using 75-fs laser pulses is achieved. The density-matrix equations of a four-level ladder system beyond the rotating-wave approximation have to be solved to clarify the balance between the principal quantum numbers, the duration of the laser pulses, and the associated ac-Stark effects for the fastest optimal excitation. The mechanism is robust for femtosecond control using different level configurations for applications in ultrafast quantum information processing and spectroscopy.
We propose an ultrafast femtosecond time scale trichromatic \ensuremathπ-pulse illumination scheme for coherent excitation and manipulation of low-lying Rydberg states in rubidium. Selective population of nP3/2 levels with principal quantum numbers n\ensuremath≲12 using 75-fs laser pulses is achieved. The density-matrix equations of a four-level ladder system beyond the rotating-wave approximation have to be solved to clarify the balance between the principal quantum numbers, the duration of the laser pulses, and the associated ac-Stark effects for the fastest optimal excitation. The mechanism is robust for femtosecond control using different level configurations for applications in ultrafast quantum information processing and spectroscopy.
We present a numerical solution for complete population inversion in a four-level ladder system obtained by using a full π-pulse illumination scheme with resonant ultrashort phase-locked Gaussian laser pulses. We find that a set of pulse areas such as √3π , √2π , and √3π completely inverts the four-level system considering identical effective dipole coupling coefficients. The solution is consistent provided the involved electric fields are not too strong and it is amply accurate also in the case of diverse transition dipole moments. We study the effect of detuning and chirp of the laser pulses on the complete population inversion using the level structure of atomic sodium interacting with ps and fs pulses as an example. Our result opens the door for multiple applications such as efficient ultrashort pulse lasing in the UV or the engineering of quantum states for quantum computing.
We show that multipulse excitation of Rydberg atoms with a sequence of short time-delayed femtosecond pulses results in strong n selection of the final-state amplitudes. In the experiment, a 150 fs laser pulse, which alone populates n levels from 22--32 in Li, is chopped into four time-delayed excitation pulses, which results in a strong modification of the n-level distribution. The mechanism demonstrated in the experiment is derived theoretically: With more pulses, it is shown that the population of a single or a few n levels can be performed on demand and varied as long as the central laser frequency is given.
Control of population inversion and coherence generation between the ground and excited states in Hydrogen–like alkali atoms is presented in this work using multiphoton π-pulse scheme with pulse durations between 100 fs to 10 ps. Density matrix (DM) equations of 4–level Rb and Cs systems are numerically solved beyond the rotating wave approximation (RWA) taking detuning and spectral chirp into account. An extension of the study is done to nP3/2 (n = 12) and nF5/2 (n = 8) of Cs atoms demonstrating the applicability of the current scheme in selective Rydberg excitation and potential extendibility to numerous configurations.
Control is important for transferring theoretical scientific knowledge into practical technology for applications in numerous fields. This is why coherent control study is significant on every timescale to have a complete understanding of dynamic processes that occur on the electron, atomic and molecular levels. As a result, numerous schemes have been proposed to carry out effective quantum control of diverse systems and study the dynamics of these systems based on their natural timescales from the picoseconds (10-12 s), femtosecond (10-15 s) to attosecond (10-18 s) regimes. The goals of these various studies depend on the desired application, for instance in Photochemistry a long standing objective is achieving selective population transfer from an initial state to a desired target state with little or no diminution in the energy transferred. In quantum computation, a central issue is the excitation of unoccupied Rydberg states with numerous proposals for its use in the design and implementation of robust fast quantum gates. Also, since the advent of the generation of attosecond XUV pulses, doors have been opened for achieving control of atomic-scale electron dynamics and observing them in real-time. This thesis explores the modelling of dynamical light-matter interaction processes, like effective population inversion and generation of vibrational coherences in atoms and molecules, on their fundamental timescales using the density matrix (DM) theory under and beyond the rotating wave approximation (RWA). The thesis begins by introducing the concept of coherent control of simple quantum systems based on the DM formalism and expands the application to a more complex Oxazine system. Multiphoton p-pulse scheme is demonstrated for the control of population transfer in multilevel systems, for example with a trichromatic p-pulse having a set of areas v3 p, 2p and v3 p, complete population transfer in a four level system can be achieved. The aforementioned scheme is used to achieve effective control of low-lying Rydberg states in rubidium atoms, demonstrating how the effective control can be crucially affected by numerous physical processes. One main advantage of the density matrix approach over other theoretical approaches is that it allows the possibility of easily computing relaxation terms and other physical parameters critical to successful coherent control. The DM formalism is shown to be successful in properly describing the enhancement effects in atoms and complex molecular systems. It is robust in coherent control and quantum control spectroscopy (QCS) schemes and is extendable to numerous systems and geometric configurations. In the last part of the thesis, experiments on laser dressing processes in attosecond transient absorption spectroscopy are compared to numerical simulations using the DM analysis beyond the RWA. The research in this thesis opens a pathway to numerous studies using the DM formalism for applications in diverse fields of femtochemistry, attophysics, high precision spectroscopy and quantum information processing. El control quàntic coherent és important per transferir coneixement científic teòric a la tecnologia, per a aplicacions en nombrosos camps. Per aquest motiu, l'estudi del control coherent és significatiu a totes les escales de temps, per comprendre de manera completa els processos dinàmics que es produeixen en els nivells electrònics, atòmics i moleculars. Com a resultat, s'han proposat nombrosos esquemes per dur a terme un control quàntic eficient de diversos sistemes i estudiar la dinàmica d'aquests sistemes basant-se en les seves escales de temps naturals, des dels picosegons (10-12 s), femtosegons (10-15 s) fins a règims d'attosegons (10-18 s). Els objectius d'aquests estudis depenen de l'aplicació desitjada, per exemple, en fotoquímica, un objectiu buscat extensivament és aconseguir una transferència selectiva de la població des d'un estat inicial fins a un estat final desitjat, amb poca o cap disminució de l'energia transferida. En computació quàntica, un tema central és l'excitació dels estats desocupats de Rydberg, amb nombroses propostes per al seu ús en el disseny i implementació de portes quàntiques robustes i ràpides. A més, des de l'arribada de la generació de polsos XUV d'attosegons, s'han obert camins per aconseguir el control de la dinàmica electrònica a escala atòmica i observar-la en temps real. Aquesta tesi explora la modelització de processos dinàmics d'interacció llum-matèria, com ara la inversió efectiva de població i la generació de coherències vibracionals en àtoms i molècules, en les seves escales de temps fonamentals, utilitzant la teoria de la matriu densitat (MD), en l'aproximació d'ona rotant (RWA) i més enllà. La tesi comença introduint el concepte de control coherent de sistemes quàntics simples basats en el formalisme de MD i amplia l'aplicació a un sistema més complexes, com ara oxazines. Es demostra l'esquema multifotó amb polsos p per al control de la transferència de població en sistemes multinivell, per exemple, amb un pols p-tricromàtic que té un conjunt d'àrees v3 p, 2p i v3 p, amb el qual la transferència completa de població en un sistema de quatre nivells pot ser aconseguit. L'esquema esmentat s'utilitza per aconseguir un control eficient d'estats de Rydberg en àtoms de rubidi, i es mostra com aquest control pot ser afectat crucialment per nombrosos processos físics. Un avantatge principal de l'estudi de la matriu densitat comparat amb altres enfocaments teòrics és que permet la possibilitat de computar fàcilment els termes de relaxació i altres paràmetres físics crítics per a un control coherent efectiu. Es demostra que el formalisme de la MD és exitós per descriure correctament l'amplificació d'efectes tant en àtoms com en sistemes moleculars més complexos. El formalisme de la MD és robust en esquemes de control coherent i d'espectroscòpia quàntica i és extensible a nombrosos sistemes i configuracions. En la darrera part de la tesi, es comparen experiments realitzats sobre processos de "vestiment" amb làsers intensos en espectroscòpia d'absorció transitòria d'attosegon amb simulacions numèriques utilitzant l'anàlisi de MD més enllà de la RWA. La recerca en aquesta tesi obre un camí a nombrosos estudis utilitzant el formalisme de MD per a aplicacions en diversos camps de femtoquímica, attofísica, espectroscòpia d'alta precisió i processament d'informació quàntica.
The four-wave mixing produced with two ultrashort phase-locked !-3! laser pulses propagating coherently in a two-level system in the infrared spectral region is shown to depend on the pulses relative phase. The Maxwell-Bloch equations are solved beyond the rotating-wave approximation to account for field frequencies which are largely detuned from the atomic resonance. The relative phase dominating the efficiency of the coupling to the 5! anti-Stokes Raman component is determined by sign of the total ac Stark shift induced in the system, in such a way that the phase influence disappears precisely where the ac Stark effect due to both pulses is compensated. This fundamental quantum interference effect can be the basis for nonlinear ultrafast optical spectroscopy techniques. Phenomena arising from the coherent control of nonlinear interactions are of importance in fields as diverse as optoelectronics and materials research, in high harmonic generation, in photoionization or molecular dissociation, and in biological applications such as spectroscopy and imaging, among others [1, 2, 3, 4, 5, 6, 7]. Optical quantum coherent control is based on the fact that the phases of interfering transition amplitudes in light-matter interactions can be controlled through the optical phase of coherent light sources that drive the interaction, in such a way that the transition rates to final states and the dynamics at various stages of the process can be modified [1]. In a recent paper [8], a theoretical investigation on the quantum coherent control of the optical transient four-wave mixing of two intense phase-locked femtosecond laser pulses of central angular frequencies ω and 3ω propagating in a two-level atom (TLA) was reported. It was shown how the nonlinear (χ (3) ) coupling to the antiStokes Raman field at frequency 5ω depends critically on the initial relative phase φ of the propagating pulses. In Ref. [8], the study was centered to intense pulses in the visible and ultraviolet spectral regions, with frequencies at resonance or lower than the atomic transition. The phenomena observed in [8], however, can be scaled to various laser and material parameters. In the infrared spectral range, for instance, experiments on ultrafast molecular dynamics are frequently performed by help of twocolor pump probe nonlinear spectroscopy techniques. In this type of studies, due to the high intensities inherent to ultrashort (subpicosecond) pulses, nonlinear effects such as stimulated Raman processes may become important and are often utilized as a complementary tool to gain information [9]. In this Letter, we address the conditions for quantum coherent control of transient four-wave mixing interactions with two phase-locked ω-3ω femtosecond laser pulses in the mid-IR spectral region. Our purpose is to reveal the basic physics for a strict two-level medium and to this end the Maxwell-Bloch TLA will be considered. We will study pulses with a duration of 300 fs (with a w 5w
We investigate the time dynamics of a four-level rubidium atomic system excited by a train of $0\ensuremath{\pi}$ pulses. Resonant $0\ensuremath{\pi}$ pulse shaping is achieved by propagation of weak femtosecond pulses through rubidium vapor and characterized using frequency-resolved optical gating and within the linear dispersion theory. The excitation of the Rb atoms by $0\ensuremath{\pi}$ pulse train is studied experimentally by modified direct frequency comb spectroscopy and theoretically within the density-matrix approach. We show that accumulation of populations and coherences, as typical pulse train excitation effects, strongly depend on the strength of the resonant $0\ensuremath{\pi}$ pulse shaping. Stronger pulse shaping reduces the coherent accumulation effects, eventually leading to the disappearance of the velocity selective optical pumping observed in the frequency domain.