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Nonstationary molecular states which contain electronic coherences can be impulsively created and manipulated by using recently developed ultrashort optical and x-ray pulses via photoexcitation, photoionization, and Auger processes. We propose several stimulated-Raman detection schemes that can monitor the subsequent phase-sensitive electronic and nuclear dynamics. Three detection protocols of an x-ray broadband probe are compared: frequency-dispersed transmission, integrated photon number change, and total pulse energy change. In addition, each can be either linear or quadratic in the x-ray probe intensity. These various signals offer different gating windows into the molecular response, which is described by correlation functions of electronic polarizabilities. Off-resonant and resonant signals are compared. [ABSTRACT FROM AUTHOR]

Quantum optical fields offer numerous control knobs which are not available with classical light and may be used for monitoring the properties of matter by novel types of spectroscopy. It has been recently argued that such quantum spectroscopy signals can be obtained by a simple averaging of their classical spectroscopy counterparts over the Glauber-Sudarshan quasiprobability distribution of the quantum field; the quantum light thus merely provides a novel gating window for the classical response functions. We show that this argument only applies to the linear response and breaks down in the nonlinear regime. The quantum response carries additional valuable information about response and spontaneous fluctuations of matter that may not be retrieved from the classical response by simple data processing. This is connected to the lack of a nonlinear fluctuation-dissipation relation. [ABSTRACT FROM AUTHOR]

We predict several effects associated with the optical response of systems prepared in a nonequilibrium state by impulsive optical excitations. The linear response depends on the phase of the electric field even if the initial nonequilibrium state has only populations, no coherences. Initial quantum coherences induce additional phase dependence, which also shows new resonances in nonlinear wave mixing. In systems strongly driven by an external optical field, the field frequency generates a phase-dependent probe absorption. This gives further control to manipulate the relative contribution to the linear signal due to initial populations and coherences. [ABSTRACT FROM AUTHOR]

We calculate the third-order susceptibility x(3) of a pair of two-level atoms that interact via the exchange of photons. QED corrections to second order in coupling to vacuum field modes yield collective two-photon absorption resonances, which can be observed in transmission spectroscopy of shaped broadband pulses. While some collective effects can be obtained by introducing an effective interatomic coupling using a quantum master equation, the predicted signals contain distinct features that are missed by that level of theory and require a full diagrammatic QED treatment. [ABSTRACT FROM AUTHOR]

The time- and frequency-resolved nonlinear light scattering (NLS) signals from a time-evolving charge distribution of valence electrons prepared by impulsive x-ray pulses are calculated using a superoperator Green's function formalism. The signal consists of a coherent ~N²-scaling difference-frequency-generation and an incoherent fluorescence ~N-scaling component, where N is the number of active molecules. The former is given by the classical Larmor formula based on the time-dependent charge density. The latter carries additional information about the electronic structure and may be recast in terms of transition amplitudes representing quantum matter pathways. [ABSTRACT FROM AUTHOR]

The nonclassical spectral and temporal features of entangled photons offer new possibilities to investigate the interactions of excitons in photosynthetic complexes and to target the excitation of specific states. Simulations of fluorescence in the bacterial reaction center induced by entangled light demonstrate a degree of selectivity of double-exciton states which is not possible using classical stochastic light with the same power spectrum. [ABSTRACT FROM AUTHOR]

The coincidence rate in two detectors that register signal and idler photons in parametric down-conversion is calculated using a quantum description of the radiation field that extends the semiclassical theory to the resonance regime. The signal is given by a sum over paths in the joint field plus matter space, each involving a pair of sites (molecules) and six radiation field modes. It may not be factorized into amplitudes representing one-site, three-mode paths. All effects of multiple vacuum modes are accounted for by theses paths without adding a Langevin noise source. The molecular information is given by a time-ordered superoperator Green's function rather than the susceptibility &khgr;(2) used in the semiclassical theory. [ABSTRACT FROM AUTHOR]

We present a diagrammatic technique for calculating quantum-detected nonlinear optical signals using loop diagrams that act in the joint matter plus field space. This formalism, which is based on time-ordered superoperators keeps track of the entangled matter plus field state, making it most suitable for spectroscopy applications. Photon counting is recast as a time and frequency convolution of a bare signal, which is given by a con-elation function of matter, and a gating spectrogram. [ABSTRACT FROM AUTHOR]

We show that photocell power can be substantially enhanced by quantum coherence in the model proposed by [M. O. Scully Phys. Rev. Lett. 104, 207701 (2010)]. Here coherence is induced by an external microwave drive. We show that although such coherence requires an extra energy input, the amount of extra input power can be much smaller than produced enhancement of the photovoltaic power. We demonstrate that for certain parameters power enhancement is governed by quantum coherence and is not just a result of population transfer due to the driving field. This corroborates and extends the earlier work of Scully. [ABSTRACT FROM AUTHOR]

We show that coherence induced by Fano interference can enhance power generated by a photovoltaic device as compared to the same system with no coherence. No additional external energy source is necessary to create such induced coherence. In the present model, coherence increases the flow of electrons through the load, which reduces radiative recombination and enhances cell power. [ABSTRACT FROM AUTHOR]

In this work we report a quantum interference mediated control of the resonance profiles in a generic three-level system and investigate its effect on key quantum interference (QI) phenomena. Namely in a three-level configuration with doublets in the ground or excited states, we show control over enhancement and suppression of the emission (absorption) profiles. This is achieved by manipulation of the strength of QI and the energy spacing of the doublets. We analyze the application of such QI-induced control of the resonance profile in the framework of two limiting cases of lasing without inversion and photodetection. [ABSTRACT FROM AUTHOR]

We discuss the proper definition of the term "quantum spectroscopy." Since the response of matter to coherent states of the field is not necessarily classical, positivity of the Glauber-Sudarshan P representation as used by Kira et al. [Nat. Phys. 7, 799 (2011)] does not imply that the response is classical. We show that classical and quantum spectroscopies may be unambiguously distinguished by formulating the signals in terms of multipoint field correlation functions. [ABSTRACT FROM AUTHOR]