Observation of Rabi dynamics with a short-wavelength free-electron laser

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.

Control of Ion-Photoelectron Entanglement and Coherence Via Rabi Oscillations

“…In this case, however, the photoelectron wave packets from the two pathways do not interfere with each other since the photoelectron and ionic core constitute one composite whole quantum system, and the two ionization pathways leave different ionic states (2s –1 and 2p –1 , denoted by |2⟩ and |1⟩, respectively), or, in other words, the photoelectron and ionic subsystems are entangled as c 2α |2⟩|α⟩ + c 1β |1⟩|β⟩, with c 2α and c 1β being the amplitudes of the corresponding states. Such a correlation or entanglement between the photoelectron and ionic states is attracting increasing attention in attosecond science. − Now, since ℏω is equal to the energy difference between the 2s –1 and 2p –1 states, the fundamental pulse couples the two ionic states, inducing Rabi oscillations. − If the 2p –1 ionic state (|1⟩) is excited to 2s –1 (|2⟩) (i.e., if the 2s electron is excited to 2p) (Figure c) or vice versa (Figure d), the two photoelectron states |α⟩ and |β⟩ can interfere with each other. Hence, a question arises, “does the Rabi coupling convert the entanglement to coherent superposition, leading to, e.g., coherent control of the photoelectron angular distribution through the ω–2ω relative phase?”…”

A "thoughtful" Local Friendliness no-go theorem: a prospective experiment with new assumptions to suit

“…Back on the quantum side, breakthroughs in optical frequency comb control now enable atomic clocks to operate at optical frequencies [71,72] (beyond 10 1 5Hz). Further on the quantum side, 2022 also saw the demonstration of a Rabi cycle (that is, a single-qubit gate) taking only 52fs, using extreme-ultraviolet pulses acting on Helium atoms [73].…”

Optical Analogs of Rabi Splitting in Integrated Waveguide‐Coupled Resonators

“…Such an exchange, in the strong light–matter coupling regime, results in anti‐crossing between the atom‐like emitter and the cavity‐mode dispersion relations, which is described by the so‐called Rabi splitting. [ 2–4 ]…”

“…Such an exchange, in the strong light-matter coupling regime, results in anti-crossing between the atom-like emitter and the cavity-mode dispersion relations, which is described by the so-called Rabi splitting. [2][3][4] As a physical phenomenon arising from light-matter interactions in the strong coupling regime, Rabi splitting enables quantum coherent oscillations between the joint systems and the quantum superpositions of different quantum states. In the early 1980s, Rabi splitting was observed for many atoms.…”