Vibrational molecular quantum computing: basis set independence and theoretical realization of the Deutsch-Jozsa algorithm

The phase of quantum gates is one key issue for the implementation of quantum algorithms. In this paper we first investigate the phase evolution of global molecular quantum gates, which are realized by optimally shaped femtosecond laser pulses. The specific laser fields are calculated using the multitarget optimal control algorithm, our modification of the optimal control theory relevant for application in quantum computing. As qubit system we use vibrational modes of polyatomic molecules, here the two IR-active modes of acetylene. Exemplarily, we present our results for a Pi gate, which shows a strong dependence on the phase, leading to a significant decrease in quantum yield. To correct for this unwanted behavior we include pressure on the quantum phase in our multitarget approach. In addition the accuracy of these phase corrected global quantum gates is enhanced. Furthermore we could show that in our molecular approach phase corrected quantum gates and basis set independence are directly linked. Basis set independence is also another property highly required for the performance of quantum algorithms. By realizing the Deutsch-Jozsa algorithm in our two qubit molecular model system, we demonstrate the good performance of our phase corrected and basis set independent quantum gates.     

The influence of laser field noise on controlled quantum dynamics

The influence of laser noise on the dynamics of simple quantum systems is analyzed. An anharmonic ladder is chosen for illustration and several pulses are obtained that optimize the yield of a quantum transition by constraining the laser parameters. The following models of laser noise are introduced: Amplitude white noise, phase white noise, frequency white noise and shot-to-shot static noise in the different pulse parameters. It is shown that the optimal pulses are robust to white amplitude noise, since the system acts as a dynamical filter. White phase noise affects the optimal pulses in a similar way by reducing the pulse area. This effect can be easily compensated for by pulse amplitude rescaling, up to a high level of noise. White frequency noise reduces the pulse area and induces spectral broadening, more strongly affecting the high frequency components. It can be partially compensated for by amplitude rescaling. The effects of static noise in the parameters cannot be easily corrected. It is shown that optimal pulses that drive n-photon transitions become more sensitive to noise in the amplitude and less sensitive to noise in the frequency as n increases. The effects of noise in the relative phase rapidly become constant for a large number of interfering pathways.     

Monitoring the effect of a control pulse on a conical intersection by time-resolved photoelectron spectroscopy

We have previously shown how femtosecond angle- and energy-resolved photoelectron spectroscopy can be used to monitor quantum wavepacket bifurcation at an avoided crossing or conical intersection and also how a symmetry-allowed conical intersection can be effectively morphed into an avoided crossing by photo-induced symmetry breaking. The latter result suggests that varying the parameters of a laser to modify a conical intersection might control the rate of passage of wavepackets through such regions, providing a gating process for different chemical products. In this paper, we show with full quantum mechanical calculations that such optical control of conical intersections can actually be monitored in real time with femtosecond angle- and energy-resolved photoelectron spectroscopy. In turn, this suggests that one can optimally control the gating process at a conical intersection by monitoring the photoelectron velocity map images, which should provide far more efficient and rapid optimal control than measuring the ratio of products. To demonstrate the sensitivity of time-resolved photoelectron spectra for detecting the consequences of such optical control, as well as for monitoring how the wavepacket bifurcation is affected by the control, we report results for quantum wavepackets going through the region of the symmetry-allowed conical intersection between the first two (2)A' states of NO(2) that is transformed to an avoided crossing. Geometry- and energy-dependent photoionization matrix elements are explicitly incorporated in these studies. Time-resolved photoelectron angular distributions and photoelectron images are seen to systematically reflect the effects of the control pulse.     

Effect of diatomic molecular properties on binary laser pulse optimizations of quantum gate operations

The importance of the ro-vibrational state energies on the ability to produce high fidelity binary shaped laser pulses for quantum logic gates is investigated. The single frequency 2-qubit ACNOT(1) and double frequency 2-qubit NOT(2) quantum gates are used as test cases to examine this behaviour. A range of diatomics is sampled. The laser pulses are optimized using a genetic algorithm for binary (two amplitude and two phase parameter) variation on a discretized frequency spectrum. The resulting trends in the fidelities were attributed to the intrinsic molecular properties and not the choice of method: a discretized frequency spectrum with genetic algorithm optimization. This is verified by using other common laser pulse optimization methods (including iterative optimal control theory), which result in the same qualitative trends in fidelity. The results differ from other studies that used vibrational state energies only. Moreover, appropriate choice of diatomic (relative ro-vibrational state arrangement) is critical for producing high fidelity optimized quantum logic gates. It is also suggested that global phase alignment imposes a significant restriction on obtaining high fidelity regions within the parameter search space. Overall, this indicates a complexity in the ability to provide appropriate binary laser pulse control of diatomics for molecular quantum computing.     

飞秒泵浦-探测实验中量子拍频现象的数据处理

The pump-probe experiments for 1,3-dichlorobenzene in gas phase have been performed by femtosecond multiphoton ionization mass spectra at a pump pulse of 270 nm and a probe pulse of 810 nm by using our homebuilt femtosecond laser system. The models for the exponential process and quantum beat are presented to extract the photodissociation dynamics information from the data. A fitting and analyzing software which is programmed by graphical language LabVIEW has been developed for the experimental data. The Convolution, Levenberg Marquardt,and Deconvolution mathematics VIs of LabVIEW are used in this program to make the program much simpler than using other programming languages. The experimental data have been well fitted with this program. The results provide the lifetime of excited-state S1 and the frequency of quantum beat.     

Laser pulse trains for controlling excited state dynamics of adenine in water

We investigate theoretically the control of the ultrafast excited state dynamics of adenine in water by laser pulse trains, with the aim to extend the excited state lifetime and to suppress nonradiative relaxation processes. For this purpose, we introduce the combination of our field-induced surface hopping method (FISH) with the quantum mechanical-molecular mechanical (QM/MM) technique for simulating the laser-driven dynamics in the condensed phase under explicit inclusion of the solvent environment. Moreover, we employ parametric pulse shaping in the frequency domain in order to design simplified laser pulse trains allowing to establish a direct link between the pulse parameters and the controlled dynamics. We construct pulse trains which achieve a high excitation efficiency and at the same time keep a high excited state population for a significantly extended time period compared to the uncontrolled dynamics. The control mechanism involves a sequential cycling of the population between the lowest and higher excited states, thereby utilizing the properties of the corresponding potential energy surfaces to avoid conical intersections and thus to suppress the nonradiative decay to the ground state. Our findings provide a means to increase the fluorescence yield of molecules with an intrinsically very short excited state lifetime, which can lead to novel applications of shaped laser fields in the context of biosensing.     

Femtosecond lasers in gas phase chemistry

This critical review is intended to provide an overview of the state-of-the-art in femtosecond laser technology and recent applications in ultrafast gas phase chemical dynamics. Although "femtochemistry" is not a new subject, there have been some tremendous advances in experimental techniques during the last few years. Time-resolved photoelectron spectroscopy and ultrafast electron diffraction have enabled us to observe molecular dynamics through a wider window. Attosecond laser sources, which have so far only been exploited in atomic physics, have the potential to probe chemical dynamics on an even faster timescale and observe the motions of electrons. Huge progress in pulse shaping and pulse characterisation methodology is paving the way for exciting new advances in the field of coherent control.     

Control of molecular fragmentation using shaped femtosecond pulses

The possibility that chemical reactions may be controlled by tailored femtosecond laser pulses has inspired recent studies that take advantage of their short pulse duration, comparable to intramolecular dynamics, and high peak intensity to fragment and ionize molecules. In this article, we present an experimental quest to control the chemical reactions that take place when isolated molecules interact with shaped near-infrared laser pulses with peak intensities ranging from 1013 to 1016 W/cm2. Through the exhaustive evaluation of hundreds of thousands of experiments, we methodically evaluated the molecular response of 16 compounds, including isomers, to the tailored light fields, as monitored by time-of-flight mass spectrometry. Analysis of the experimental data, taking into account its statistical significance, leads us to uncover important trends regarding the interaction of isolated molecules with an intense laser field. Despite the energetics involved in fragmentation and ionization, the integrated second-harmonic generation of a given laser pulse (ISHG), which was recorded as an independent diagnostic parameter, was found to be linearly proportional to the total ion yield (IMS) generated by that pulse in all of our pulse shaping measurements. Order of magnitude laser control over the relative yields of different fragment ions was observed for most of the molecules studied; the fragmentation yields were found to vary monotonically with IMS and/or ISHG. When the extensive changes in fragmentation yields as a function of IMS were compared for different phase functions, we found essentially identical results. This observation implies that fragmentation depends on a parameter that is responsible for IMS and independent from the particular time-frequency structure of the shaped laser pulse. With additional experiments, we found that individual ion yields depend only on the average pulse duration, implying that coherence does not play a role in the observed changes in yield as a function of pulse shaping. These findings were consistently observed for all molecules studied (p-, m-, o-nitrotoluene, 2,4-dinitrotoluene, benzene, toluene, naphthalene, azulene, acetone, acetyl chloride, acetophenone, p-chrolobenzonitrile, N,N-dimethylformamide, dimethyl phosphate, 2-chloroethyl ethyl sulfide, and tricarbonyl-[eta5-1-methyl-2,4-cyclopentadien-1-yl]-manganese). The exception to our conclusion is that the yield of small singly-charged fragments resulting from a multiple ionization process in a subset of molecules, were found to be highly sensitive to the phase structure of the intense pulses. This coherent process plays a minimal role in photofragmentation; therefore, we consider it an exception rather than a rule. Changes in the fragmentation process are dependent on molecular structure, as evidenced in a number of isomers, therefore femtosecond laser fragmentation could provide a practical dimension to analytical chemistry techniques.     

Use of the Gerchberg–Saxton algorithm in optimal coherent anti-Stokes Raman spectroscopy

We are utilizing recent advances in ultrafast laser technology and recent discoveries in optimal shaping of laser pulses to significantly enhance the stand-off detection of explosives via control of molecular processes at the quantum level. Optimal dynamic detection of explosives is a method whereby the selectivity and sensitivity of any of a number of nonlinear spectroscopic methods are enhanced using optimal shaping of ultrafast laser pulses. We have recently investigated the Gerchberg–Saxton algorithm as a method to very quickly estimate the optimal spectral phase for a given analyte from its spontaneous Raman spectrum and the ultrafast laser pulse spectrum. Results for obtaining selective coherent anti-Stokes Raman spectra (CARS) for an analyte in a mixture, while suppressing the CARS signals from the other mixture components, are compared for the Gerchberg–Saxton method versus previously obtained results from closed-loop machine-learning optimization using evolutionary strategies.     

Quantum Control of Gas‐Phase and Liquid‐Phase Femtochemistry

Active control of chemical reactions on a microscopic (molecular) level, that is, the selective breaking or making of chemical bonds, is an old dream. However, conventional control agents used in chemical synthesis are macroscopic variables such as temperature, pressure or concentration, which gives no direct access to the quantum‐mechanical reaction pathway. In quantum control, by contrast, molecular dynamics are guided with specifically designed light fields. Thus it is possible to efficiently and selectively reach user‐defined reaction channels. In the last years, experimental techniques were developed by which many breakthroughs in this field were achieved. Femtosecond laser pulses are manipulated in so‐called pulse shapers to generate electric field profiles which are specifically adapted to a given quantum system and control objective. The search for optimal fields is guided by an automated learning loop, which employs direct feedback from experimental output. Thereby quantum control over gas‐phase as well as liquid‐phase femtochemical processes has become possible. In this review, we first discuss the theoretical and experimental background for many of the recent experiments treated in the literature. Examples from our own research are then used to illustrate several fundamental and practical aspects in gas‐phase as well as liquid‐phase quantum control. Some additional technological applications and developments are also described, such as the automated optimization of the output from commercial femtosecond laser systems, or the control over the polarization state of light on an ultrashort timescale. The increasing number of successful implementations of adaptive learning techniques points at the great versatility of computer‐guided optimization methods. The general approach to active control of light–matter interaction has also applications in many other areas of modern physics and related disciplines.     

Isotope selective photoionization of NaK by optimal control: theory and experiment

We present a joint theoretical and experimental study of the maximization of the isotopomer ratio (23)Na(39)K(23)Na(41)K using tailored phase-only as well as amplitude and phase modulated femtosecond laser fields obtained in the framework of optimal control theory and closed loop learning (CLL) technique. A good agreement between theoretically and experimentally optimized pulse shapes is achieved which allows to assign the optimized processes directly to the pulse shapes obtained by the experimental isotopomer selective CLL approach. By analyzing the dynamics induced by the optimized pulses we show that the mechanism involving the dephasing of the wave packets between the isotopomers (23)Na (39)K and (23)Na (41)K on the first excited state is responsible for high isotope selective ionization. Amplitude and phase modulated pulses, moreover, allow to establish the connection between the spectral components of the pulse and corresponding occupied vibronic states. It will be also shown that the leading features of the theoretically shaped pulses are independent from the initial conditions. Since the underlying processes can be assigned to the individual features of the shaped pulses, we show that optimal control can be used as a tool for analysis.     

用超短脉冲激光研究1,3-二氯苯激发态动力学

利用啁啾脉冲放大技术建立了一套掺钛蓝宝石飞秒激光放大系统,该系统输出中心波长808nm,单脉冲能量8mJ,脉冲宽度60fs,脉冲重复频率20Hz.利用飞秒激光泵浦-探测及分子束技术,结合飞行时间质谱,对1,3-二氯苯分子的激发态动力学过程进行了研究,实验中观察到该分子能级间的量子拍频现象,并获得了第一单重激发态寿命及其拍频频率,阐述了飞秒激光场下间位二氯苯分子的电离解离机理.     

Systematic control of nonlinear optical processes using optimally shaped femtosecond pulses.

This article reviews experimental efforts to control multiphoton transitions using shaped femtosecond laser pulses, and it lays out the systematic study being followed by us for elucidating the effect of phase on nonlinear optical laser-molecule interactions. Starting with a brief review of nonlinear optics and how nonlinear optical processes depend on the electric field inducing them, a number of conclusions can be drawn directly from analytical solutions of the equations. From a Taylor expansion of the phase in the frequency domain, we learn that nonlinear optical processes are affected only by the second- and higher-order terms. This simple result has significant implications on how pulse-shaping experiments are to be designed. If the phase is allowed to vary arbitrarily as a continuous function, then an infinite redundancy that arises from the addition of a linear phase function across the spectrum with arbitrary offset and slope could prevent us from carrying out a closed-loop optimization experiment. The early results illustrate how the outcome of a nonlinear optical transition depends on the cooperative action of all frequencies in the bandwidth of a laser pulse. Maximum constructive or destructive interference can be achieved by programming the phase using only two phase values, 0 and pi. This assertion has been confirmed experimentally, where binary phase shaping (BPS) was shown to outperform other alternative functions, sometimes by at least on order of magnitude, in controlling multiphoton processes. Here we discuss the solution of a number of nonlinear problems that range from narrowing the second harmonic spectrum of a laser pulse to optimizing the competition between two- and three-photon transitions. This Review explores some present and future applications of pulse shaping and coherent control.     

Femtosecond laser pulse control of multidimensional vibrational dynamics: Computational studies on the pyrazine molecule

The multiconfiguration time-dependent Hartree (MCTDH) method is combined with the optimal control theory (OCT) to study femtosecond laser pulse control of multidimensional vibrational dynamics. Simulations are presented for the widely discussed three-electronic-level vibronic coupling model of pyrazine either in a three or four vibrational coordinate version. Thus, for the first time OCT is applied to a four-coordinate system. Different control tasks are investigated and also some general aspects of the OCT-MCTDH method combination are analyzed.     

Quantum reaction boundary to mediate reactions in laser fields

Dynamics of passage over a saddle is investigated for a quantum system under the effect of time-dependent external field (laser pulse). We utilize the recently developed theories of nonlinear dynamics in the saddle region, and extend them to incorporate both time-dependence of the external field and quantum mechanical effects of the system. Anharmonic couplings and laser fields with any functional form of time dependence are explicitly taken into account. As the theory is based on the Weyl expression of quantum mechanics, interpretation is facilitated by the classical phase space picture, while no "classical approximation" is involved. We introduce a quantum reactivity operator to extract the reactive part of the system. In a model system with an optimally controlled laser field for the reaction, it is found that the boundary of the reaction in the phase space, extracted by the reactivity operator, is modulated with time by the effect of the laser field, to "catch" the system excited in the reactant region, and then to "release" it into the product region. This method provides new insights in understanding the origin of optimal control of chemical reactions by laser fields.     

Manganese pentacarbonyl bromide as candidate for a molecular qubit system operated in the infrared regime

Our concept for a quantum computational system is based on qubits encoded in vibrational normal modes of polyatomic molecules. The quantum gates are implemented by shaped femtosecond laser pulses. We adopt this concept to the new species manganese pentacarbonyl bromide [MnBr(CO)5] and show that it is a promising candidate in the mid-infrared (IR) frequency range to connect theory and experiment. As direct reference for the ab initio calculations we evaluated experimentally the absorption bands of MnBr(CO)5 in the mid-IR as well as the related transition dipole moments. The two-dimensional potential-energy surface spanned by the two strongest IR active modes and the dipole vector surfaces are calculated with density-functional theory. The vibrational eigenstates representing the qubit system are determined. Laser pulses are optimized by multitarget optimal control theory to form a set of global quantum gates: NOT, CNOT, Pi, and Hadamard. For all of them simply structured pulses with low pulse energies around 1 microJ could be obtained. Exemplarily for the CNOT gate we investigated the possible transfer to experimental shaping, based on the mask function for pulse shaping in the frequency regime as well as decomposition into a train of subpulses.     

LIAD-fs scheme for studies of ultrafast laser interactions with gas phase biomolecules

Laser induced acoustic desorption (LIAD) has been used for the first time to study the parent ion production and fragmentation mechanisms of a biological molecule in an intense femtosecond (fs) laser field. The photoacoustic shock wave generated in the analyte substrate (thin Ta foil) has been simulated using the hydrodynamic HYADES code, and the full LIAD process has been experimentally characterised as a function of the desorption UV-laser pulse parameters. Observed neutral plumes of densities >10(9) cm(-3) which are free from solvent or matrix contamination demonstrate the suitability and potential of the source for studying ultrafast dynamics in the gas phase using fs laser pulses. Results obtained with phenylalanine show that through manipulation of fundamental femtosecond laser parameters (such as pulse length, intensity and wavelength), energy deposition within the molecule can be controlled to allow enhancement of parent ion production or generation of characteristic fragmentation patterns. In particular by reducing the pulse length to a timescale equivalent to the fastest vibrational periods in the molecule, we demonstrate how fragmentation of the molecule can be minimised whilst maintaining a high ionisation efficiency.