Coherent Control Scheme Research Articles

Synthesis of robust memory modes for linear quantum systems with unknown inputs

In this paper, the synthesis of robust memory modes for linear quantum passive systems in the presence of unknown inputs has been studied, aimed at facilitating secure storage and communication of quantum information. In particular, we can switch on decoherence-free (DF) modes in the storage stage by placing the poles on the imaginary axis via a coherent feedback control scheme, and these memory modes can further be simultaneously made robust against perturbations to the system parameters by minimizing the condition number associated with imaginary poles. The DF modes can also be switched off by tuning the controller parameters to place the poles in the left half of the complex plane in the writing/reading stage. We develop explicit algebraic conditions guiding the design of such a coherent quantum controller, which involves employing an augmented system model to counter the influence of unknown inputs. Examples are provided to illustrate the procedure of synthesizing robust memory modes for linear optical quantum systems.

Terahertz sum-frequency excitation of coherent optical phonons in the two-dimensional semiconductor WSe2

Driving fundamental excitations via strong light fields is one of the most important issues in solid state physics, which opens up new avenues to control material properties. Two-dimensional materials are fruitful platforms for future semiconductor applications, including opto-electronic and phononic devices, yet the phonon dynamics and nonlinear phonon–phonon coupling remain under-explored. Here, we demonstrate coherent phonon excitation in thin films of the layered two-dimensional semiconductor WSe2 induced by intense and broadband ultrafast terahertz (THz) pulses. We performed THz Kerr effect spectroscopy and observed coherent phonon oscillations assigned to the E2g optical phonon mode. The phonon amplitude displays a quadratic THz field strength dependence, indicating a sum-frequency THz excitation process. Furthermore, pump–probe polarization and crystal orientation relationships, supported by symmetry analysis of the nonlinear susceptibility and Raman tensors, provide helpful insight into nonlinear phonon–phonon interactions and potential coherent control schemes for the manipulation of phonon polarization and material properties in WSe2.

Tuning quantum pathway interference in two-color laser photoemission using DC bias

Coherent control of quantum systems depends on the manipulation of quantum interference through external fields. In this work, we investigate the effects of DC bias field on coherent control of quantum pathways in two-color laser photoemission using exact analytical solutions of the one-dimensional time dependent Schrödinger equation. Increasing DC bias lowers and narrows the surface potential barrier, shifting the dominant emission to lower order multiphoton photoemission, photo-assisted tunneling and then direct tunneling. Those lower order photon absorption processes result in fewer possible pathways, and therefore modulation of photoemission current can be suppressed as DC field increases. It is shown that a maximum modulation depth of 99.4% can be achieved for a gold emitter at local DC bias F 0 = 0.5 V nm−1, fundamental (800 nm) laser field F 1 = 2.6 V nm−1 and second harmonic laser field F 2 = 0.25 V nm−1 . For a given set of input parameters, the total photoemission consists of different k-photon processes, each of which has their own different multiple possible pathways and interference effects. However, the quantum pathways and their interference for the dominant k-photon process and for the total photoemission probability show the same trends. This study demonstrates strong flexibility in tuning two-color lasers induced photoemission using a DC bias and provides insights into coherent control schemes of general quantum systems.

Femtosecond Polarization Shaping of Free-Electron Laser Pulses.

We demonstrate the generation of extreme-ultraviolet (XUV) free-electron laser (FEL) pulses with time-dependent polarization. To achieve polarization modulation on a femtosecond timescale, we combine two mutually delayed counterrotating circularly polarized subpulses from two cross-polarized undulators. The polarization profile of the pulses is probed by angle-resolved photoemission and above-threshold ionization of helium; the results agree with solutions of the time-dependent Schrödinger equation. The stability limit of the scheme is mainly set by electron-beam energy fluctuations, however, at a level that will not compromise experiments in the XUV. Our results demonstrate the potential to improve the resolution and element selectivity of methods based on polarization shaping and may lead to the development of new coherent control schemes for probing and manipulating core electrons in matter.

Non-Abelian gauge invariance from dynamical decoupling

Lattice gauge theories are fundamental to such distinct fields as particle physics, condensed matter or quantum information theory. The recent progress in the control of artificial quantum systems already allows for studying Abelian lattice gauge theories in tabletop experiments. However, the realization of non-Abelian models remains challenging. Here, we employ a coherent quantum control scheme to enforce non-Abelian gauge invariance, and discuss this idea in detail for a one-dimensional SU(2) lattice gauge system. We comment on how to extend our scheme to other non-Abelian gauge symmetries and higher spatial dimensions. Because of its wide applicability, the presented coherent control scheme provides a promising route for the quantum simulation of non-Abelian lattice gauge theories.

Coherent Control of the Nonlinear Emission of Single Plasmonic Nanoantennas by Dual‐Beam Pumping (Advanced Optical Materials 20/2022)

An asymmetric plasmonic nanoantenna mediates the nonlinear mixing between ultrashort infrared laser pulses at frequency ω and 2ω. The third harmonic at ω + ω + ω and the sum frequency at ω + 2ω are coherent and degenerate at the visible frequency 3ω. Consequently, a relative time delay between the ω and 2ω pumps brings a strong modulation of the power at 3ω due to interference. This phase-dependent modulation, as presented in article number 2200757 by Michele Celebrano and co-workers, realizes an ultrafast coherent control scheme of nonlinear emission.

Influence of an atomic resonance on the coherent control of the photoionization process

In coherent control schemes, pathways connecting an initial and a final state can be independently controlled by manipulating the complex amplitudes of their transition matrix elements. For paths characterized by the absorption of multiple photons, these quantities depend on the magnitude and phase between the intermediate steps, and are expected to be strongly affected by the presence of resonances. We investigate the coherent control of the photoemission process in neon using a phase-controlled two-color extreme ultraviolet pulse with frequency in proximity of an excited energy state. Using helium as a reference, we show that the presence of such a resonance in neon modifies the amplitude and phase of the asymmetric emission of photoelectrons. Theoretical simulations based on perturbation theory are in fair agreement with the experimental observations.Received 25 July 2021Revised 1 August 2022Accepted 5 August 2022DOI:https://doi.org/10.1103/PhysRevResearch.4.033231Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMultiphoton or tunneling ionization & excitationPhotoemissionSingle- and few-photon ionization & excitationUltrafast phenomenaUltrashort pulsesAtomic, Molecular & Optical

Observation of optical coherence in a disordered metal-molecule interface by coherent optical two-dimensional photoelectron spectroscopy

Coherent multidimensional optical spectroscopy methods overcome some of the limitations found in their one-dimensional counterparts and allow us, for example, to resolve overlapped spectral features, to separate homogeneous and inhomogeneous broadening, and to track the energy transfer kinetics and coherent dynamics in complex quantum systems. In their most common configurations, signal detection is achieved by optical means, making these techniques widespread in studies of bulk systems but less common in the surface and interface sciences. In this paper, we demonstrate an inherently surface-sensitive two-dimensional coherent spectroscopy scheme, based on photoelectron detection, by studying the interface formed between tris(8-hydroxyquinolinato)aluminium (${\mathrm{Alq}}_{3}$) and a ferromagnetic Co surface. Despite the inhomogeneous linewidth broadening (\ensuremath{\approx}800 meV) in the ensemble of disordered molecules, we resolve two narrow resonances (\ensuremath{\approx}10 meV linewidth) with an energy spacing of \ensuremath{\approx}80 meV. By combining experimental data and simulations, using the Lindblad master equation, we identify these resonances as lowest unoccupied molecular orbital (LUMO) to $\mathrm{LUMO}+1$ transitions in chemically decoupled second-layer ${\mathrm{Alq}}_{3}$ molecules and deduce related optical coherence lifetimes of at least 120 and 240 fs. These observations establish that ${\mathrm{Alq}}_{3}$ molecules in a disordered adsorbate layer exhibit well-defined and rather homogeneous internal electronic transitions, although the absolute energetic positions of the involved states with respect to the substrate reference are significantly affected by the disorder. The results indicate that inhomogeneous line broadening in a disordered ${\mathrm{Alq}}_{3}$ layer and pure dephasing have only a minor impact on optical transitions in individual molecules. This opens interesting opportunities for coherent control schemes and emphasizes the importance of optical coherences for all electron dynamics, even in disordered hybrid metal-molecule interfaces.

Enabling impedance-based physical human–multi–robot collaboration: Experiments with four torque-controlled manipulators

Robotics research into multi-robot systems so far has concentrated on implementing intelligent swarm behavior and contact-less human interaction. Studies of haptic or physical human-robot interaction, by contrast, have primarily focused on the assistance offered by a single robot. Consequently, our understanding of the physical interaction and the implicit communication through contact forces between a human and a team of multiple collaborative robots is limited. We here introduce the term Physical Human Multi-Robot Collaboration (PHMRC) to describe this more complex situation, which we consider highly relevant in future service robotics. The scenario discussed in this article covers multiple manipulators in close proximity and coupled through physical contacts. We represent this set of robots as fingers of an up-scaled agile robot hand. This perspective enables us to employ model-based grasping theory to deal with multi-contact situations. Our torque-control approach integrates dexterous multi-manipulator grasping skills, optimization of contact forces, compensation of object dynamics, and advanced impedance regulation into a coherent compliant control scheme. For this to achieve, we contribute fundamental theoretical improvements. Finally, experiments with up to four collaborative KUKA LWR IV+ manipulators performed both in simulation and real world validate the model-based control approach. As a side effect, we notice that our multi-manipulator control framework applies identically to multi-legged systems, and we execute it also on the quadruped ANYmal subject to non-coplanar contacts and human interaction.

Coherent dynamical control of quantum processes

Manipulation of a quantum system requires the knowledge of how it evolves. To impose that the dynamics of a system becomes a particular target operation (for any preparation of the system), it may be more useful to have an equation of motion for the dynamics itself, rather than the state. Here we develop a Markovian master equation for the process matrix of an open system interacting with a reservoir, which resembles the Lindblad Markovian master equation. We employ this equation to introduce a scheme for optimal local coherent process control at target times and extend the Krotov technique to obtain optimal control. We illustrate utility of this framework through several quantum coherent-control scenarios, such as optimal decoherence suppression, gate simulation, and passive and indirect control of the reservoir, in all of which we aim to simulate a given terminal process at a given final time.

Generation of maximal three-state field-free molecular orientation with terahertz pulses

We present a combined analytical and numerical investigation to show how an optimal control field can be designed to generate maximum field-free orientation of molecules for three populated rotational states. Based on a model involving pure rotational ladder-climbing excitation between rotational states, a set of optimal amplitude and phase conditions are analytically derived for the applied control fields. The maximum degree of orientation can be achieved when the field satisfies amplitude and phase conditions at the two transition frequencies. Multiple optimal solutions exist and to examine these conditions, we devise a quantum coherent control scheme using two terahertz pulses and successfully apply it to the linear polar molecule HCN at ultracold temperature. The sensitivity of both populations and phases of rotational states to control field parameters, i.e., the detuning, bandwidth, and time delay, is analyzed for understanding the optimal orientation mechanism. This work thus examines the frequency domain landscape belonging to optimal pulses.

Coherent control at gold needle tips approaching the strong-field regime

Abstract We demonstrate coherent control in photoemission from a gold needle tip using an ω − 2ω field composed of strong few-cycle laser pulses with a nearfield intensity of ∼4 TW/cm2. We obtain the nearfield intensity from electron energy spectra, showing the tell-tale plateau of field-driven electron rescattering at the metal surface induced by the fundamental field. Changing the relative phase between the fundamental field centered at 1560 nm and its second harmonic modulates the total emitted photocurrent with visibilities of up to 80% despite the strong and broadband excitation of the plasmonic material. Our work combines a two-color coherent control scheme and strong-field physics enabled by a nanoplasmonic emitter.

Tuning the electronic band structure of metal surfaces for enhancing high-order harmonic generation

High-harmonic generation (HHG) from the condensed matter phase holds promise to promote future cutting-edge research in the emerging field of attosecond nanoscopy. The key for the progress of the field relies on the capability of the existing schemes to enhance the harmonic yield and to push the photon energy cutoff to the extreme-ultraviolet (XUV, 10-100eV) regime and beyond toward the spectral "water window" region (282-533eV). Here, we demonstrate a coherent control scheme of HHG, which we show to give rise to quantum modulations in the XUV region. These modulations are shown to be caused by quantum-path interferences and are found to exhibit a strong sensitivity to the delocalized character of bulk states of the material. The control scheme is based on exploring surface states in transition-metal surfaces and, specifically, tuning the electronic structure of the metal surface itself together with the use of optimal chirped pulses. Moreover, we show that the use of such pulses having moderate intensities permits us to push the harmonic cutoff further to the spectral water window region and that the extension is found to be robust against the change in the intrinsic properties of the material. The scenario is numerically implemented using a minimal model by solving the time-dependent Schrödinger equation for the metal surface Cu(111) initially prepared in the surface state. Our findings elucidate the importance of metal surfaces for generating coherent isolated attosecond XUV and soft-x-ray pulses and for designing compact solid-state HHG devices.

Quantum control of entangled photon-pair generation in electron-atom collisions driven by laser-synthesized free-electron wave packets

We propose an extension of coherent control using laser-synthesized free-electron matter waves. In contrast to coherent control schemes exploiting optical coherences to steer the dynamics of matter waves, we analyze the opposite and investigate the control of quantum light emission driven by laser-sculpted coherent free-electron matter waves. We apply this concept to the control of entangled photon-pair emission in electron-atom collisions, in which the incident electron wave packet, colliding with a target atom $B$, is engineered by interferometric resonantly-enhanced multiphoton ionization of a parent atom $A$. Each ionization pathway leads to electron wave packets that coherently interfere during temporal evolution in the continuum. Their mutual coherence can be controlled by adjusting the relative phases or time delays of the frequency components of the ionizing field contributing to the interfering pathways. We report coherent control of entangled photon-pair generation in radiative photo-cascade emission upon decay of the target atom after inelastic excitation triggered by the collision with the synthesized electron wave packet.

Coherent control in ferromagnets driven by microwave radiation and spin polarized current

Coherent control is a method used to manipulate the state of matter using oscillatory electromagnetic radiation which relies on the non-adiabatic interaction. It is commonly applied in quantum processing applications. This technique is interesting in the context of ferromagnetic materials because of the ability to combine it with spintronics for the purpose of fundamental spin transport research, low-power information processing, and potentially future quantum bit (Qubit) applications. In this work we address the theoretical grounds of coherent manipulation in practical ferromagnetic systems. We study electromagnetic radiation driven interaction that is enhanced in the presence of spin polarized currents and map the conditions that allow coherent manipulation for which Rabi oscillations take place. The role of the magnetic anisotropy field is shown to act as an additional oscillatory driving field. We discuss the Gilbert losses in the context of effective coherence decay rates and show that it is possible to control these rates by application of a static spin current. The case of coherent manipulation using oscillatory spin currents that is free of radiation is discussed as well. Our work paves the way towards spin current amplification as well as radiation-free coherent control schemes that may potentially lead to novel Qubits that are robust and scalable.

Tracking attosecond electronic coherences using phase-manipulated extreme ultraviolet pulses

The recent development of ultrafast extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter. Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution. However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within an XUV pulse sequence opens exciting possibilities for coherent control and multidimensional spectroscopy, but has not been accomplished. Here, we overcome these constraints in a highly time-stabilized and phase-modulated XUV-pump, XUV-probe experiment, which directly probes the evolution and dephasing of an inner subshell electronic coherence. This approach, avoiding any XUV optics for direct pulse manipulation, opens up extensive applications of advanced nonlinear optics and spectroscopy at XUV wavelengths.

Coherent control schemes for the photoionization of helium in the extreme ultraviolet spectral region

SynopsisWe present a theoretical study for coherent control based on resonant excitation to doubly excited states of helium. Bichromatic schemes are designed, using XUV light from the seeded free-electron laser FERMI, where the phase of the third harmonic relative to the fundamental can be controlled. In addition to demonstrate the interference effects, we provide detailed theoretical analyses to identify the FEL parameters, such as intensities and pulse durations, needed for experimentally observable coherent control of the photoionization yields.

Multicolor Quantum Control for Suppressing Ground State Coherences in Two-Dimensional Electronic Spectroscopy.

The measured multidimensional spectral response of different light harvesting complexes exhibits oscillatory features which suggest an underlying coherent energy transfer. However, making this inference rigorous is challenging due to the difficulty of isolating excited state coherences in highly congested spectra. In this work, we provide a coherent control scheme that suppresses ground state coherences, thus making rephasing spectra dominated by excited state coherences. We provide a benchmark for the scheme using a model dimeric system and numerically exact methods to analyze the spectral response. We argue that combining temporal and spectral control methods can facilitate a second generation of experiments that are tailored to extract desired information and thus significantly advance our understanding of complex open many-body structure and dynamics.

Analytic solutions for generalized -symmetric Rabi models**Project supported in part by the National Natural Science Foundation of China (Grant No. 11874251), the Scientific Research Starting Foundation of Shaanxi Normal University, China, and the Scientific Research Plan Funded by the Education Department of Shaanxi Province, China (Grant No. 17JK0786).

We theoretically investigate the exact solutions for generalized parity–time()-reversal-symmetric Rabi models driven by external fields with monochromatic periodic, linear, and parabolic forms, respectively. The corresponding exact solutions are presented in terms of the confluent Heun equations without any approximation. In principle, the analytic solutions derived here are valid in the whole parameter space. Such a kind of study may offer potential coherent control schemes of the -symmetric two-level systems.

Optimal control for multi-parameter quantum estimation with time-dependent Hamiltonians

We investigate simultaneous estimation of multi-parameter with time-dependent Hamiltonians. We analytically obtain the maximum quantum Fisher information matrix for two-parameter encoded in time-dependent three-level systems. The optimal coherent control scheme is proposed to increase the simultaneous multi-parameter estimation precisions. In an example of a spin-1 particle in a uniformly rotating magnetic field, the optimal coherent Hamiltonians for different parameters can be chosen to be completely same. However, in general, the optimal coherent Hamiltonians for different parameters are incompatible. In this situation, we suggest a variance method to obtain the optimal coherent Hamiltonian for simultaneously estimating multiple parameters, and obtain the optimal simultaneous estimation precision of two-parameter in a three-level Landau-Zener Hamiltonian.