Periodic Reporting for period 1 - QuALIHDS (Quantum Atom Light Interfaces in High Density Samples)
Okres sprawozdawczy: 2024-01-01 do 2025-12-31
The development of quantum technologies is driving fundamental and applied research to unforeseen limits. Diverse platforms, from atomic systems to integrated photonic circuits to nitrogen vacancy centers in diamond or superconducting qubits are investigated and pushed to the extremes of quantum mechanics due to the impressive degree of control and manipulation achieved. Some of the most promising technologies, e.g. quantum simulation, are based on ideal ‘closed’ quantum systems where only a handful of states are present with allowed transitions between them and no interaction with the rest of the universe. However, this ideal description is not real, and current technological applications are hindered by decoherence in the form of losses or decay. In this framework, the study of ‘open quantum systems’ where decoherence or dissipation is present becomes a necessity to further increase the efficacy and robustness of developing quantum technologies. A natural candidate to study the physics of open quantum systems are atom-light interfaces. Within the last two decades, research activity in atom-light interfaces, exploiting the unique properties of atomic systems and its interaction with light at the level of quantum mechanics allowed the development and demonstration of a wide range of quantum phenomena: from quantum protocols for Quantum Information, e.g. quantum memories of light, to Quantum Metrology protocols exploiting quantum nondemolition measurements, to the engineering of quantum states of matter for Quantum Simulation. This impressive progress was possible due to a thorough understanding of atom-atom and atom-light interactions. Yet, there are regimes that challenge our understanding of quantum atom-light interfaces and pose limitations to both technological and fundamental developments.
One example of these situations arises when a stream of photons interacts with a high-density atomic sample. The driving field mediates a dipole-dipole (DD) interaction among the atoms both in the form of real scattered photons and exchange of virtual photons. This DD interaction leads to the formation of collective states of matter, that is, states that show behavior governed by a collective property of the ensemble. Collective or cooperative scattering effects in high density media can lead to an optical behavior substantially different from the optical response of a single atom under the same driving conditions (see Figure 2). To date, collective scattering effects in ultracold atomic systems were studied primarily using alkaline atoms. The reported theoretical and experimental effects range from modified decay rates to resonance shifts, to vacuum Rabi splitting , to spectral broadening of the line, and the collective modes of these spatially disordered systems show interesting properties and phase transitions between diffusive transport and localization. Besides its implications for the scattered light properties, DD interactions were also proposed to generate non-classical states of light, to explore new quantum information protocols, or in many-body optical lattice systems as a tool to engineer the system Hamiltonian using alkaline-earth-like atoms such as Sr. More recently, several experiments started to investigate the collective scattering effects in ordered arrays, mainly using nanofibers in order to benefit from the increased directionality of the emitted fluorescence, and a lot of theoretical effort was devoted to study subradiant/superradiant atomic states in ordered arrays.
In this context, the project proposes to investigate the effect of the collective effects in driven-dissipative systems made of large, high-density clouds of ultra-cold atoms,to further our understanding of the light-mediated DD interactions. The ambition of this project is to exploit this knowledge to better control and enhance quantum metrology protocols for precision measurements.
One example of these situations arises when a stream of photons interacts with a high-density atomic sample. The driving field mediates a dipole-dipole (DD) interaction among the atoms both in the form of real scattered photons and exchange of virtual photons. This DD interaction leads to the formation of collective states of matter, that is, states that show behavior governed by a collective property of the ensemble. Collective or cooperative scattering effects in high density media can lead to an optical behavior substantially different from the optical response of a single atom under the same driving conditions (see Figure 2). To date, collective scattering effects in ultracold atomic systems were studied primarily using alkaline atoms. The reported theoretical and experimental effects range from modified decay rates to resonance shifts, to vacuum Rabi splitting , to spectral broadening of the line, and the collective modes of these spatially disordered systems show interesting properties and phase transitions between diffusive transport and localization. Besides its implications for the scattered light properties, DD interactions were also proposed to generate non-classical states of light, to explore new quantum information protocols, or in many-body optical lattice systems as a tool to engineer the system Hamiltonian using alkaline-earth-like atoms such as Sr. More recently, several experiments started to investigate the collective scattering effects in ordered arrays, mainly using nanofibers in order to benefit from the increased directionality of the emitted fluorescence, and a lot of theoretical effort was devoted to study subradiant/superradiant atomic states in ordered arrays.
In this context, the project proposes to investigate the effect of the collective effects in driven-dissipative systems made of large, high-density clouds of ultra-cold atoms,to further our understanding of the light-mediated DD interactions. The ambition of this project is to exploit this knowledge to better control and enhance quantum metrology protocols for precision measurements.
The project started with an experimental study of light diffusion in optically dense atomic ensembles. Light diffusion in optically dense media is an old problem in the physics of complex media. One of the main challenges in the study of light transport in cold atoms is related to the finite size of the sample. In this study we developed a technique based on optical manipulation with an auxiliary beam to address the atoms at the center of the cloud to optically excite them. Then, we were able to follow the propagation dynamics of this initially launched photons through multiple scattering events. By changing the optical density of the sample and measuring key parameters of the system, namely the diffusion coefficient and transport velocity, we identified a diffusive regime, finding a good agreement with diffusion models. In order to be able to spatially resolve the dynamics of the photons within the medium, we implemented a depletion (or shelving) imaging method based on absorption imaging that enables direct time- and space-resolved observation of excited state population, allowing the measurement of light diffusion in atomic ensembles. This technique exploits the V-type internal energy structure of alkaline-earth-like atoms
The successful implementation of the depletion imaging technique opened the door to revisit some well-known collective effects from a different perspective. In particular, we investigated the subradiant dynamics in cold atomic clouds with spatial resolution.
We were able to directly detect the subradiant dynamics of the excited state population in an ultra-cold atomic cloud of 174Yb atoms, and compare it with measurements of the scattered light under the same conditions. We characterized the decay dynamics of the subradiant modes using both the excited state population and scattered light intensity, finding good quantitative agreement with numerical predictions from simulations of two-level atomic ensembles. A numerical model of atom-light interactions was implemented to support the findings of these experiences. The model included the light-mediated dipole-dipole interactions, and takes a mean-field approach to go beyond the linear approximation. This model allowed the computation of the excited state population of the sample.
The goals of QUAHLIDS were related to the study of how collective effects can affect metrology protocols. One of the main metrology schemes is based on Rabi oscillations. With the insight gained on the aforementioned investigations, we turned to the investigation of frequency shifts induced in the Rabi oscillations of an optically dense media. The experience started by setting up a detection scheme based on Single Photon Counting modules and a time-tagger card to detect the fluorescence of the atomic cloud at different observation angles. Preliminary numerical simulations showed that important modifications to the expected Rabi frequency would be present for clouds with optical densities > 10 and strong driving intensities I/Isat > 1. We performed several experiments to observe modified Rabi frequencies during the driving of an optically dense atomic cloud. The results of this study indicate that a complex dynamics takes place during the driving. Collective effects predicted by numerical simulations are entangled with spatial effects stemming from the attenuation of the probe beam intensity through the cloud and the slow diffusion of the light within the atomic ensemble. This led to a big angular dependency of the Rabi frequencies measured at different observation angles. In addition, the driving dynamics was not qualitatively well predicted by the numerical models, turning the analysis and assessment of the results extremely hard.
Then we turned to the study of collective effects in strongly driven cold atomic clouds with population and light measurements. Using the numerical model already mentioned as a guide, we performed an experimental investigation of the collective early-time decay rates of a strongly driven and optically dense cold atomic cloud. We prepared the atomic ensemble by driving the system to its steady state with varying Rabi frequencies \Omega that go from the weak \Omega/\Gamma<<1 to the strong driving regime, where \Gamma is the single-atom decay rate. We investigate the early-time dynamics in the transition between the strong and weak driving regimes using the angular-dependent observables such as the light emitted by the cloud, and the excited state population. We found that when driving the cloud on-resonance, as a function of the probe beam intensity, the behavior of the collected light at certain angles transitions from the single-photon subradiant regime to a superradiant regime while the behavior of the excited state population does not show superradiance. The experiment shows good agreement with numerical predictions in the regime of parameters under study.
In summary, this project has generated several investigations related to collective effects in optically dense media, gaining insight in the complex problem of atom-light interactions, extending the regime of parameters under study to previously unexplored regimes, and introducing appealing techniques and methodology to further the investigation of collective effects in cold atomic ensembles.
The successful implementation of the depletion imaging technique opened the door to revisit some well-known collective effects from a different perspective. In particular, we investigated the subradiant dynamics in cold atomic clouds with spatial resolution.
We were able to directly detect the subradiant dynamics of the excited state population in an ultra-cold atomic cloud of 174Yb atoms, and compare it with measurements of the scattered light under the same conditions. We characterized the decay dynamics of the subradiant modes using both the excited state population and scattered light intensity, finding good quantitative agreement with numerical predictions from simulations of two-level atomic ensembles. A numerical model of atom-light interactions was implemented to support the findings of these experiences. The model included the light-mediated dipole-dipole interactions, and takes a mean-field approach to go beyond the linear approximation. This model allowed the computation of the excited state population of the sample.
The goals of QUAHLIDS were related to the study of how collective effects can affect metrology protocols. One of the main metrology schemes is based on Rabi oscillations. With the insight gained on the aforementioned investigations, we turned to the investigation of frequency shifts induced in the Rabi oscillations of an optically dense media. The experience started by setting up a detection scheme based on Single Photon Counting modules and a time-tagger card to detect the fluorescence of the atomic cloud at different observation angles. Preliminary numerical simulations showed that important modifications to the expected Rabi frequency would be present for clouds with optical densities > 10 and strong driving intensities I/Isat > 1. We performed several experiments to observe modified Rabi frequencies during the driving of an optically dense atomic cloud. The results of this study indicate that a complex dynamics takes place during the driving. Collective effects predicted by numerical simulations are entangled with spatial effects stemming from the attenuation of the probe beam intensity through the cloud and the slow diffusion of the light within the atomic ensemble. This led to a big angular dependency of the Rabi frequencies measured at different observation angles. In addition, the driving dynamics was not qualitatively well predicted by the numerical models, turning the analysis and assessment of the results extremely hard.
Then we turned to the study of collective effects in strongly driven cold atomic clouds with population and light measurements. Using the numerical model already mentioned as a guide, we performed an experimental investigation of the collective early-time decay rates of a strongly driven and optically dense cold atomic cloud. We prepared the atomic ensemble by driving the system to its steady state with varying Rabi frequencies \Omega that go from the weak \Omega/\Gamma<<1 to the strong driving regime, where \Gamma is the single-atom decay rate. We investigate the early-time dynamics in the transition between the strong and weak driving regimes using the angular-dependent observables such as the light emitted by the cloud, and the excited state population. We found that when driving the cloud on-resonance, as a function of the probe beam intensity, the behavior of the collected light at certain angles transitions from the single-photon subradiant regime to a superradiant regime while the behavior of the excited state population does not show superradiance. The experiment shows good agreement with numerical predictions in the regime of parameters under study.
In summary, this project has generated several investigations related to collective effects in optically dense media, gaining insight in the complex problem of atom-light interactions, extending the regime of parameters under study to previously unexplored regimes, and introducing appealing techniques and methodology to further the investigation of collective effects in cold atomic ensembles.
QuALIHDS attempted to further our understanding of atom-light interactions in a dissipative quantum many body set up of many two level systems, constituted by cold atoms, through experimental observations. The project has generated several investigations related to collective effects in optically dense media. The main results that go beyond the state of the art are:
- Study of the early-time decay dynamics in a strongly, optically driven atomic sample, exploring signatures beyond the linear perturbation theory both theoretically and experimentally, extending the regime of parameters under study to previously unexplored regimes and gaining insight in the complex problem of atom-light interactions.
- Experimental demonstration of differences in dynamical observations of collective effects using different observables, namely excited state population of the sample and the scattered light.
- Spatially resolved characterization of subradiant decay dynamics in a cold atomic cloud.
- Numerical prediction and experimental signatures of modified Rabi frequencies in optically dense samples under strong driving.
These results represent significant progress in understanding dissipative quantum many body problems, which are among the most challenging open issues in quantum technologies.
- Study of the early-time decay dynamics in a strongly, optically driven atomic sample, exploring signatures beyond the linear perturbation theory both theoretically and experimentally, extending the regime of parameters under study to previously unexplored regimes and gaining insight in the complex problem of atom-light interactions.
- Experimental demonstration of differences in dynamical observations of collective effects using different observables, namely excited state population of the sample and the scattered light.
- Spatially resolved characterization of subradiant decay dynamics in a cold atomic cloud.
- Numerical prediction and experimental signatures of modified Rabi frequencies in optically dense samples under strong driving.
These results represent significant progress in understanding dissipative quantum many body problems, which are among the most challenging open issues in quantum technologies.