We address the possibility of using coherent control tools to extract useful information about the interaction of a system with a dissipative environment. To that end we extend previous work, which developed a coherence spectroscopy based on two-pathway excitation phase control, from the isolated molecule limit to dense media. Specifically, we explore the properties of the channel phase, an observable of energy-domain two-pathway excitation experiments that was shown in the isolated molecule limit to carry information about the phase properties of the material system. Our analysis is based on the combination of steady state and time-dependent analytical perturbative approaches within the density matrix formalism, complemented by nonperturbative numerical simulations. We find that the channel phase carries significantly richer information in the presence of decoherence mechanisms than in their absence. In particular, rescattering events in the structured continuum introduce new features in the channel phase spectrum, whose structure conveys information about both the molecular continuum and the system bath interaction.
We address the possibility of using coherent control tools to extract useful information about the interaction of a system with a dissipative environment. To that end we extend previous work, which developed a coherence spectroscopy based on two-pathway excitation phase control, from the isolated molecule limit to dense media. Specifically, we explore the properties of the channel phase, an observable of energy-domain two-pathway excitation experiments that was shown in the isolated molecule limit to carry information about the phase properties of the material system. Our analysis is based on the combination of steady state and time-dependent analytical perturbative approaches within the density matrix formalism, complemented by nonperturbative numerical simulations. We find that the channel phase carries significantly richer information in the presence of decoherence mechanisms than in their absence. In particular, rescattering events in the structured continuum introduce new features in the channel phase spectrum, whose structure conveys information about both the molecular continuum and the system bath interaction.
We extend a recently formulated coherence spectroscopy of dissipative media [J. Chem. Phys. 122, 084502 (2005)] from the stationary excitation limit to the time domain. Our results are based on analytical and numerical solutions of the quantum Liouville equation within the Bloch framework. It is shown that the short pulse introduces a new, controllable time scale that allows better insight into the relation between the coherence signal and the phase properties of the material system. We point to the relation between the time-domain coherence spectroscopy and the method of interferometric two-photon photoemission spectroscopy, and propose a variant of the latter method, where the two time-delayed excitation pathways are distinguishable, rather than identical. In particular, we show that distinguishability of the two excitation pathways introduces the new possibility of disentangling decoherence from population relaxation.
Abstract Two-dimensional (2D) spectroscopy uses multiple electromagnetic pulses to infer the properties of a complex system. A paradigmatic class of target systems are molecular aggregates, for which one can obtain information on the eigenstates, various types of static and dynamic disorder and on relaxation processes. However, two-dimensional spectra can be difficult to interpret without precise knowledge of how the signal components relate to microscopic Hamiltonian parameters and system-bath interactions. Here we show that two-dimensional spectroscopy can be mapped in the microwave domain to highly controllable Rydberg quantum simulators. By porting 2D spectroscopy to Rydberg atoms, we firstly open the possibility of its experimental quantum simulation, in a case where parameters and interactions are very well known. Secondly, the technique may provide additional handles for experimental access to coherences between system states and the ability to discriminate different types of decoherence mechanisms in Rydberg gases. We investigate the requirements for a specific implementation utilizing multiple phase coherent microwave pulses and a phase cycling technique to isolate signal components.
With the model system of a diatomic molecule embedded in a rare gas matrix (Br2/Ar), a method is introduced to enhance buried coherent structures and thus to prepare them out of an overwhelming, but incoherent background. Therefore, phase controlled pulse sequences are superimposed, which are generated with a pulse shaper in a direct way based on spectroscopic information. This requires a precise knowledge on the energetic signatures of the target state. This was accomplished via high resolution excitation spectroscopy covering 630 to 540 nm of the X(v′′ = 0)→B(v′) transition. Narrow zero phonon lines (ZPL) were observed with isotopic resolution from v′ = 2 to v′ = 19. They are accompanied by broad phonon sidebands (PSB), which reflect the matrix phonon density of states. With higher excitation energy, a broadening of ZPL and an increase in PSB due to an increase in matrix influence was observed and quantified. Two energetic positions were identified, where repulsive states cross the B-state. The time resolved pump – probe experiments were carried out with ultrashort and thus energetically very broad laser pulses. The B-state dynamics is completely superimposed by A-state dynamics in single pump - probe experiments with 590 nm. However, with the pulse trains the wave packets were created delayed by the B vibrational roundtrip time, which interfere constructively or destructively and thus select spectrally, depending on the relative phase. The required phase adjusted pulse trains were generated by impressing the molecular spectroscopic signature on the spectral composition of the ultrashort pulse. A phase dependent control scheme requires persisting electronic coherence. This was first determined in experiments with double pulses with a Michelson interferometer to at least 1.5 ps, which corresponds to the length of the experimental pulse sequence. In the case of constructive interference due to ZPL excitation, the B vibrational progression builds up and A-state dynamics is suppressed, successfully, since it adds up incoherently to an unmodulated background. Because of this, a surprisingly long coherence of more than 1 ps - also in the PSB - was determined, due to an inverse experiment, based on destructive interference. Since the efficiency of predissociation depends on the coupling strength of the molecule to the matrix, it was specifically attenuated or enhanced by excitation with wave packets which are predominantly composed of ZPL or PSB. Thereby, the dynamical development of predissociation could be determined. My work thus introduces a combination of spectrally sharp defined excitation and time resolved interrogation, which can only be achieved by the coherent superposition of elongated pulse trains. Further on, simulations on the free molecule in first order perturbation theory were carried out, to investigate the temporal behaviour of the interfering wave packets.
Any real system, no matter how well isolated, unavoidably interacts with the environment. This interaction makes the evolution of a quantum system nonunitary and destroys the coherence of quantum superpositions. This process, known as decoherence, is widely regarded as the most important and fundamental obstacle to the practical realization of quantum information processing. On the other hand, the objectives of quantum control are predicated on the ability to manipulate the system coherently over a period of time. Quantum control has been applied successfully to several experimental systems and coupling such a controlled system to an environment makes the study of coherence loss possible. The standard model of decoherence is formulated in terms of a system, defined by the observables that can be measured, and a reservoir, de¯ned by those degrees of freedom that are not accessed by observation. The vibrational and rotational degree of freedom of diatomic molecules form an experimental model of such a system and bath 1. The nature of the ro-vibrational coupling does not allow a Decoherence Free Subspace, instead we try to identify a Decoherence Resistant Subspace using closed loop control. We set up a learning control loop experiment to monitor the coherence in a vibrational wavepacket, consisting of about 6 vibrational states of the A(1Σ+g ) electronic state, excited by a shaped, ultrashort optical pulse, and then to determine the pulse shape that yielded the longest coherence lifetime. The coherence of the vibrational wavepacket was monitored by measuring the fluorescence quantum beats from the outer turning point of the wave packet's trajectory. The closed loop controller altered the shape of the driving optical pulse to maximize the the depth of modulation of the quantum beats at a prescribed time after excitation, within the constraints of the laser system. The outcome of this closed loop control experiment shows an improvement of the beat decay constant of about 60%, as shown in Fig. (1), a result which occurs for several di®erent pulse shapes. Our experiments show that a number of interesting decoherence-resistant excitations exist. The nature of the dephasing was examined theoretically. Using measured optical pulse shapes, we simulated numerically the molecular wavepacket in the limit of perturbation theory. The experiment and simulations show that there are di®erent mechanisms contributing to the decoherence resistant excitation. The shaped pulse reduces the °uctuations in the wave packet spatial extent, which in turn changes the system reservoir coupling. Other components of the optimal pulse are a sequence of pulses that creates constructive interference of the vibrational wavepacket at the desired time, and chirp which compensates for the anharmonicity of the adiabatic potential. The experimental results show general agreement with numerical simulations. We conclude that wavepackets resistant to dephasing do exist,and that learning algorithms provide an effcient means for finding them.
Optical multidimensional coherent spectroscopy is a powerful tool for studying structure and dynamics in complex systems, such as semiconductors. In optical two-dimensional coherent spectroscopy (2DCS), where the spectrum is presented in a two-dimensional (2D) plane with two frequency axes, an important advantage is the ability to isolate quantum pathways by unfolding a one-dimensional spectrum onto a 2D plane. For many systems, however, the quantum pathways are only partially separated in a 2D spectrum. In order to completely isolate the quantum pathways, we extend 2DCS into a third dimension to generate three-dimensional (3D) spectra in which the spectrum is further unfolded. A 3D spectrum provides complete and well-isolated information of the third-order optical response of the system. The information can be used to fully characterize the quantum pathways and to determine the system’s Hamiltonian. Quantitative knowledge of the Hamiltonian enables prediction and control of quantum processes. For instance, such information is essential for deterministic control and improved performance of coherent control schemes.
Diagrammatic perturbation theory and the spherical tensor formalism are used to model the signal observed in time-resolved, two-colour resonant four-wave mixing (TC-RFWM). An analysis using the Liouville formalism illustrates how TC-RFWM can be used to perform state-selective quantum beat spectroscopy in three-level systems by suitably designing three experimental features: the excitation scheme for the matter-field interaction, the time ordering of the laser pulses and the polarization of the incident laser beams. In contrast to traditional fluorescence quantum beat spectroscopy, time-resolved TC-RFWM results in a background-free, coherent signal that allows for high-resolution detection of the spectrally unresolved energy structure within a selected material state of the three-level system. The beats are shown to result from the quantum interference between different Liouville-space pathways in the laser-induced evolution of the material density operator and appear as a temporal modulation in the amplitude of the light beam emerging as the output of the mixing process. This type of control of a coherent optical field, based on the selective preparation of superposition states, is reminiscent of other recently developed optical techniques such as coherent population trapping and lasing without inversion.