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arXiv:2104.04700 · The Journal of Chemical Physics

Pulse area dependence of multiple quantum coherence signals in dilute thermal gases

Benedikt Ames, Andreas Buchleitner, Edoardo G. Carnio, Vyacheslav Shatokhin

Published Jul 28, 2021• 6 citations
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Abstract

In the general framework of open quantum systems, we assess the impact of the pulse area on single and double quantum coherence (1QC and 2QC) signals extracted from fluorescence emitted by dilute thermal gases. We show that 1QC and 2QC signals are periodic functions of the pulse area, with distinctive features that reflect the particles' interactions via photon exchange, the polarizations of the laser pulses, and the observation direction.

DOI
10.1063/5.0053673

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Publisher’s Note: “Pulse area dependence of multiple quantum coherence signals in dilute thermal gases” [J. Chem. Phys. 155, 044306 (2021)]

Benedikt Ames, Andreas Buchleitner, Edoardo G. Carnio +1 more

The Journal of Chemical Physics•Aug 17, 2021•0 citations
In the general framework of open quantum systems, we assess the impact of the pulse area on single and double quantum coherence (1QC and 2QC) signals extracted from fluorescence emitted by dilute thermal gases. We show that 1QC and 2QC signals are periodic functions of the pulse area, with distinctive features that reflect the particles' interactions via photon exchange, the polarizations of the laser pulses, and the observation direction.
Abstract Manifestations of dipole–dipole interactions in dilute thermal gases are difficult to sense because of strong inhomogeneous broadening. Recent experiments reported signatures of such interactions in fluorescence detection-based measurements of multiple quantum coherence (MQC) signals, with many characteristic features hitherto unexplained. We develop an original open quantum systems theory of MQC in dilute thermal gases, which allows us to resolve this conundrum. Our theory accounts for the vector character of the atomic dipoles as well as for driving laser pulses of arbitrary strength, includes the far-field coupling between the dipoles, which prevails in dilute ensembles, and effectively incorporates atomic motion via a disorder average. We show that collective decay processes—which were ignored in previous treatments employing the electrostatic form of dipolar interactions—play a key role in the emergence of MQC signals.
We investigated the first and second-order correlations of the light scattered near-resonantly by a quantum dot under excitation by a frequency comb, i.e., a periodically pulsed laser source. In contrast to its monochromatic counterpart, the pulsed resonance fluorescence spectrum features a superposition of sidebands distributed around a central peak with maximal sideband intensity near the Rabi frequency. Distinguishing between the coherently and incoherently scattered light reveals pulse-area dependent Rabi oscillations evolving with different phase for each component. Our observations, which can be reproduced theoretically, may impact schemes for remote entanglement based on pulsed two-photon interference.
We consider finite-sized atomic systems with varying number of particles which have dipolar interactions among them and are also under the collective driving and dissipative effect of a thermal photon environment. Focusing on the simple case of two atoms, we investigate the impact of different parameters of the model on the coherence contained in the system. We observe that, even though the system is initialized in a completely incoherent state, it evolves to a state with a finite amount of coherence and preserves that coherence in the long-time limit in the presence of thermal photons. We propose a scheme to utilize the created coherence in order to change the thermal state of a single two-level atom by having it repeatedly interact with a coherent atomic beam. Finally, we discuss the scaling of coherence as a function of the number of particles in our system up to N=7N=7N=7.
The double quantum coherence (DQC) signal in nonlinear spectroscopy gives information about the many-body correlation effects not easily available by other methods. The signal is short-lived, consequently, a significant part of it is generated during the pulse overlap. Since the signal is at two times the laser frequency, one may intuitively expect that the pulse overlap-related artifacts are filtered out by the Fourier transform. Here, we show that this is not the case. We perform explicit calculations of phase-modulated two-pulse experiments of a two-level system where the DQC is impossible. Still, we obtain a significant signal at the modulation frequency, which corresponds to the DQC, while the Fourier transform over the pulse delay shows a double frequency. We repeat the calculations with a three-level system where the true DQC signal occurs. We conclude that with realistic dephasing times, the pulse-overlap artifact can be significantly stronger than the DQC signal. Our results call for great care when analyzing such experiments. As a rule of thumb, we recommend that only delays larger than 1.5 times the pulse length should be used.