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Journal of Physics B Atomic Molecular and Optical Physics

Excitation-free quantum-state manipulation via Floquet engineering

Haoxuan Chen, Hailang Wei, Xiaosong Zhu, Pengfei Lan, Peixiang Lu, Manfred Lein

Published Aug 5, 2026• 0 citations
Publisher

Abstract

Abstract We present a non-adiabatic approach to transfer a quantum state between different superpositions of the eigenstates within a degenerate subspace, namely between eigenstates of various spatial orientations, while avoiding excitation. This is achieved by inducing different accumulated dynamical phases for the left- and right-rotating components of the state, in analogy with optical rotation. The control of the dynamical phase is based on the asymmetric Floquet energy shift induced by a monochromatic circularly polarized external laser field. In contrast to adiabatic control methods demanding an extremely slowly varying carrier wave of the laser field for excitation-free control, our approach requires only moderately slow variation in the envelope. This methodology promises fast quantum-state manipulation and control of electron dynamics in biological and chemical reactions.

DOI
10.1088/1361-6455/ae95a5

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Abstract We present a non-adiabatic approach to transfer a quantum state between different superpositions of the eigenstates within a degenerate subspace, namely between eigenstates of various spatial orientations, while avoiding excitation. This is achieved by inducing different accumulated dynamical phases for the left- and right-rotating components of the state, in analogy with optical rotation. The control of the dynamical phase is based on the asymmetric Floquet energy shift induced by a monochromatic circularly polarized external laser field. In contrast to adiabatic control methods demanding an extremely slowly varying carrier wave of the laser field for excitation-free control, our approach requires only moderately slow variation in the envelope. This methodology promises fast quantum-state manipulation and control of electron dynamics in biological and chemical reactions.
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The manipulation of population transfer and the creation of an arbitrary superposition between two atomic states in the framework of a closed inverse Y-type four-level system driven by a series of pulse pairs and an external control field is reported. It is shown that when the laser fields are tuned to resonance with respective transitions, the non-adiabatic coupling between two degenerate adiabatic states can be reduced tremendously and complete population transfer from the initial state to the target state can be achieved in a piecewise manner. Moreover, an arbitrary superposition between the initial state and target state can be created by suitably tuning the Rabi frequency of the control laser field, inter-pair time delay, pulse number or intra-pair time delay between the pump and Stokes pulses. This scheme combines stimulated Raman adiabatic passage and temporal coherent control, and may be useful for quantum information processing and control of chemical reactions.
The main goal of this PhD work is an experimental study of coherent excitation of atomic and molecular wavepackets, i.e. superpositions of many quantum eigenstates, by shaped femtosecond pulses. Approaches allowing nearly complete population transfer between quantum eigenstates were well studied in the past within the two level approximation. In this work we focus on adiabatic and non-adiabatic methods of population transfer beyond the two-level approximation. Excitation of multi-level target states is possible due to broad spectrum of an ultrashort pulse which contains frequencies needed for multiple transitions to different states in the final superposition. At the same time, the spectrum of an ultrashort pulse can be modified, or ``shaped'', in order to affect the excitation process and control the amplitudes in the final superposition. Both non-adiabatic and quasi-adiabatic methods were first implemented and studied in electronic wavepackets in alkali atoms. The non-adiabatic approach revealed features linked to the strong-field perturbations of the energy level structure of the quantum system. An adiabatic method was implemented for the first time on a femtosecond time scale, and was thoroughly characterized. The control over complex amplitudes in the target superposition was demonstrated as well as completeness of the population transfer. In the second part of this work, we focused on coherent control of rotational wavepackets in diatomic molecules. Rotational excitation by a periodic train of femtosecond pulses was investigated in the context of ``delta-kicked'' rotor - a paradigm system for studying quantum chaos, and the effect of quantum resonance was demonstrated for the first time in a system of true quantum rotors. Control of uni-directional molecular rotation was proposed and demonstrated with a novel ``chiral pulse train'' - a sequence of femtosecond pulses with polarization rotating from pulse to pulse by a predefined angle. All the developed techniques offer new tools in coherent control of atomic and molecular wavepackets on an ultrashort time scale.
In the past two decades many schemes to actively control the dynamical evolution of a quantum system have been proposed and established. We focus attention on the use of assisted adiabatic processes to generate selected target states with high efficiency. Specific control protocols that fall in this category are sometimes called shortcuts to adiabaticity. We review two assisted adiabatic population transfer schemes, the counter-diabatic and fast-forward protocols, with the intent of calling attention to their conceptual basis and applications to real systems, specifically vibrational energy transfer in polyatomic molecules and manipulation of Bose-Einstein (BEC) condensates. The particular applications of the counter-diabatic and fast-forward protocols to improving the efficiency of population transfer between vibrational states of a polyatomic molecule that we have studied are (i) selective vibrational energy transfer in the nonrotating thiophosgene (SCCl2) molecule and (ii) enhancing the yield of HNC in the driven rotation free isomerization reaction HCN→CNH. A major component of our considerations concerns reduction of the degradation of population transfer efficiency within a subset of states that is generated by coupling of the subset of states to a manifold of background states. We have examined the influence of a manifold of background vibrational states on population transfer within a subset of embedded vibrational states, and on how that influence can be suppressed. We emphasize that any control protocol must be designed to minimize interference between the background states and excitations within the selected subset of states that are used to define initial and target states. We have also examined the application of the fast-forward protocol to the transport of population between sites of a BEC in an optical lattice. Through the applications we illustrate how the theoretical framework of an assisted adiabatic process depends on the character of the system studied, on the available control parameters, on the constraints imposed by the experimental setting, on the efficiency of population transfer (fidelity) to be achieved, and on the stability of the control process with respect to the variation of the external control fields.
The merging of coherent control (CC) and adiabatic passage (AP) and the type of problems that can be solved using the resulting coherently controlled adiabatic passage (CCAP) method are discussed. The discussion starts with the essence of CC as the guiding of a quantum system to arrive at a given final state via a number of different quantum pathways. The guiding is done by ``tailor-made'' external laser fields. Selectivity in a host of physical and chemical processes is shown to be achieved by controlling the interference between such quantum pathways. The AP process is then discussed, in which a system is navigated adiabatically along a single quantum pathway, resulting in a complete population transfer between two energy eigenstates. The merging of the two techniques (CCAP) is shown to achieve both selectivity and completeness. Application of CCAP to the solution of the nondegenerate quantum control problem is first discussed and shown that it is possible to completely transfer population from an initial wave packet of arbitrary shape, composed of a set of nondegenerate energy eigenstates, to a final arbitrary wave packet, also composed of nondegenerate states. The treatment is then extended to systems with degenerate states and shown how to induce isomerization between the broken-symmetry local minima of a Jahn-Teller Al3O{\mathrm{Al}}_{3}\mathrm{O}Al3​O molecule. These approaches can be further generalized to situations with many initial, intermediate, and final states and applied to quantum coding and decoding problems. CCAP is then applied to cyclic population transfer (CPT), induced by coupling three states of a chiral molecule in a cyclic fashion, $\ensuremath{\mid}1⟩\ensuremath{\leftrightarrow}\ensuremath{\mid}2⟩\ensuremath{\leftrightarrow}\ensuremath{\mid}3⟩\ensuremath{\leftrightarrow}\ensuremath{\mid}1⟩$. Interference between two adiabatic pathways in CPT allows for a complete population transfer, coupled with multichannel selectivity, by virtue of its phase sensitivity. CPT can be used to show the purification of mixtures of right-handed and left-handed chiral molecules. Finally, quantum-field coherent control is introduced, where CCAP is extended to the use of nonclassical light. This emerging field may be used to generate new types of entangled radiation-matter states.
We demonstrate a method of adiabatic population transfer from a single quantum state into a coherent superposition of states. The transfer is executed with femtosecond pulses, spectrally shaped in a simple and intuitive manner, which does not require iterative feedback-controlled loops. In contrast to nonadiabatic methods of excitation, our approach is not sensitive to the exact value of laser intensity. We show that the population transfer is complete, and analyze the possibility of controlling the relative phases and amplitudes of the excited eigenstates. We discuss the limitations of the proposed control methods due to the dynamic level shifts and suggest ways of reducing their influence.