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Communications Physics

Ultrafast quantum control of atomic excited states via interferometric two-photon Rabi oscillations

Yudong Chen, Sainan Peng, Zongyuan Fu, Liyang Qiu, Guangyu Fan, Yi Liu, Saijun Wu, Xinhua Xie, Zhensheng Tao

Published Mar 16, 2024• 12 citations
Publisher

Abstract

Abstract Quantum-state manipulation through coherent interaction with a radiation field is a fundamental process with broad implications in quantum optics and quantum information processing. However, current quantum control methods are limited by their operation at Rabi frequencies below the gigahertz range, which restricts their applicability to systems with long coherence times. To overcome this limitation, alternative approaches utilizing ultrafast driving lasers have garnered great interest. In this work, we demonstrate two-photon Rabi oscillations in the excited states of argon operating at terahertz frequencies driven by ultrafast laser pulses. Leveraging quantum-path interferometry, we are able to measure and manipulate both the amplitudes and phases of the transition dipoles by exploiting the intensity and polarization state of the driving laser. This precise control enables femtosecond population transfer and coherent accumulation of geometric phase. Our findings provide valuable insights into the all-optical manipulation of extreme-ultraviolet radiations and demonstrate the possibility of ultrafast quantum control through interferometric multiphoton transitions.

DOI
10.1038/s42005-024-01585-x

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•Jan 1, 2008•0 citations
Abstract Quantum-state manipulation through coherent interaction with a radiation field is a fundamental process with broad implications in quantum optics and quantum information processing. However, current quantum control methods are limited by their operation at Rabi frequencies below the gigahertz range, which restricts their applicability to systems with long coherence times. To overcome this limitation, alternative approaches utilizing ultrafast driving lasers have garnered great interest. In this work, we demonstrate two-photon Rabi oscillations in the excited states of argon operating at terahertz frequencies driven by ultrafast laser pulses. Leveraging quantum-path interferometry, we are able to measure and manipulate both the amplitudes and phases of the transition dipoles by exploiting the intensity and polarization state of the driving laser. This precise control enables femtosecond population transfer and coherent accumulation of geometric phase. Our findings provide valuable insights into the all-optical manipulation of extreme-ultraviolet radiations and demonstrate the possibility of ultrafast quantum control through interferometric multiphoton transitions.
Coherent light-matter interaction drives a variety of interesting processes in a quantum system. Rabi oscillation is one such process offering a periodic population transfer between quantum states in an ultrashort time scale. Here we demonstrate a robust and simple scheme to achieve femtosecond twophoton Rabi oscillations by intense NIR laser pulses, using excited He atoms prepared by EUV-FEL. The extension to multiphoton regime will expand the applicability to highly excited or dipole-forbidden states.
In coherent light-matter interactions the coherent state produced by the light pulse remains coherent during the entire period of observation. One of the well-known examples of such interactions is the Rabi oscillation—a periodic transfer of population in a two-level system induced by an external electromagnetic field. While these oscillations have been studied in light-matter interactions since the discovery of laser itself, most of the studies are limited to long wavelengths, typically ranging from microwave to visible radiation. Here, we discuss how such oscillations can be generated at shorter wavelengths, for example in the extreme ultraviolet domain, using intense femtosecond pulses from seeded free-electron lasers. When a two-level system is undergoing Rabi oscillations, it can be described via an ‘atom + photon’ dressed-state, the properties of which can be further manipulated to study quantum entanglement between massive particles. Our results offer opportunities to employ these intense laser pulses for generating and controlling coherent light-matter interactions across ultrashort timescales.
Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field. They are ubiquitous in physics with applications in different areas such as photonics, nano-electronics, electron microscopy, and quantum information. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light-matter interaction within the rotating wave approximation\cite. Although intense nanometer-wavelength light-sources have been available for more than a decade, Rabi dynamics at such short wavelengths have not been observed directly. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser can drive Rabi oscillations between the ground state and an excited state in helium atoms. The measured photoemission signal revealed an Autler-Townes doublet as well as an avoided crossing, phenomena that are both trademarks of quantum optics. Using theoretical analyses that go beyond the strong-field approximation, we found that the ultrafast build-up of the doublet structure follows from a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond and few-femtosecond extreme-ultraviolet pulses, our results offer opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.
Abstract Rabi oscillations are periodic modulations of populations in two-level systems interacting with a time-varying field1. They are ubiquitous in physics with applications in different areas such as photonics2, nano-electronics3, electron microscopy4 and quantum information5. While the theory developed by Rabi was intended for fermions in gyrating magnetic fields, Autler and Townes realized that it could also be used to describe coherent light–matter interactions within the rotating-wave approximation6. Although intense nanometre-wavelength light sources have been available for more than a decade7–9, Rabi dynamics at such short wavelengths has not been directly observed. Here we show that femtosecond extreme-ultraviolet pulses from a seeded free-electron laser10 can drive Rabi dynamics between the ground state and an excited state in helium atoms. The measured photoelectron signal reveals an Autler–Townes doublet and an avoided crossing, phenomena that are both fundamental to coherent atom–field interactions11. Using an analytical model derived from perturbation theory on top of the Rabi model, we find that the ultrafast build-up of the doublet structure carries the signature of a quantum interference effect between resonant and non-resonant photoionization pathways. Given the recent availability of intense attosecond12 and few-femtosecond13 extreme-ultraviolet pulses, our results unfold opportunities to carry out ultrafast manipulation of coherent processes at short wavelengths using free-electron lasers.
The four-wave mixing produced with two ultrashort phase-locked !-3! laser pulses propagating coherently in a two-level system in the infrared spectral region is shown to depend on the pulses relative phase. The Maxwell-Bloch equations are solved beyond the rotating-wave approximation to account for field frequencies which are largely detuned from the atomic resonance. The relative phase dominating the efficiency of the coupling to the 5! anti-Stokes Raman component is determined by sign of the total ac Stark shift induced in the system, in such a way that the phase influence disappears precisely where the ac Stark effect due to both pulses is compensated. This fundamental quantum interference effect can be the basis for nonlinear ultrafast optical spectroscopy techniques. Phenomena arising from the coherent control of nonlinear interactions are of importance in fields as diverse as optoelectronics and materials research, in high harmonic generation, in photoionization or molecular dissociation, and in biological applications such as spectroscopy and imaging, among others [1, 2, 3, 4, 5, 6, 7]. Optical quantum coherent control is based on the fact that the phases of interfering transition amplitudes in light-matter interactions can be controlled through the optical phase of coherent light sources that drive the interaction, in such a way that the transition rates to final states and the dynamics at various stages of the process can be modified [1]. In a recent paper [8], a theoretical investigation on the quantum coherent control of the optical transient four-wave mixing of two intense phase-locked femtosecond laser pulses of central angular frequencies ω and 3ω propagating in a two-level atom (TLA) was reported. It was shown how the nonlinear (χ (3) ) coupling to the antiStokes Raman field at frequency 5ω depends critically on the initial relative phase φ of the propagating pulses. In Ref. [8], the study was centered to intense pulses in the visible and ultraviolet spectral regions, with frequencies at resonance or lower than the atomic transition. The phenomena observed in [8], however, can be scaled to various laser and material parameters. In the infrared spectral range, for instance, experiments on ultrafast molecular dynamics are frequently performed by help of twocolor pump probe nonlinear spectroscopy techniques. In this type of studies, due to the high intensities inherent to ultrashort (subpicosecond) pulses, nonlinear effects such as stimulated Raman processes may become important and are often utilized as a complementary tool to gain information [9]. In this Letter, we address the conditions for quantum coherent control of transient four-wave mixing interactions with two phase-locked ω-3ω femtosecond laser pulses in the mid-IR spectral region. Our purpose is to reveal the basic physics for a strict two-level medium and to this end the Maxwell-Bloch TLA will be considered. We will study pulses with a duration of 300 fs (with a w 5w