Periodic Reporting for period 4 - DYNAMIQS (Relaxation dynamics in closed quantum systems)
Période du rapport: 2020-11-01 au 2022-04-30
Statistical mechanics, a century-old theory, is probably one of the most powerful constructions of physics. It predicts that the equilibrium properties of any system composed of a large number of particles depend only on a handful of macroscopic parameters, no matter how the particles exactly interact with each other. From the viewpoint of classical physics, equilibration occurs because the microstate of the system uniformly explores the phase space over time — this is the long-known concept of ergodicity. But in quantum mechanics things are different: because quantum dynamics constrains the microstate to evolve along a periodic orbit in the Hilbert space, which is the quantum equivalent of the classical phase space. In the absence of classical ergodicity, what mechanism can then lead to the equilibration of an isolated quantum system? And how long will it take? Answering these questions is not only of fundamental interest. It will also help understand what limits the speed at which quantum information can be transported, or how fast one can change the state of a quantum system, with direct impact on future quantum technologies.
The concept of this project is to take advantage of the great versatility offered by ultra-cold atom systems to investigate the relaxation dynamics in regimes well beyond the boundaries of our current knowledge. We focus our attention on two-dimensional systems and systems with both short- and long-range interactions. Specifically, we will set the system out of equilibrium by a sudden change of the interaction parameter, a ‘quantum quench’, and characterise both the relaxation dynamics and the final state through the measurement of two-point correlation functions. The realisation of the project hinges on the construction of a new-generation quantum gas microscope experiment for Strontium gases enabling us to induce long-range interactions between the atoms. Beside the construction of this apparatus, our main scientific objectives are: (i) to confront the locality of the dynamics to a two-dimensional geometry, where most of the existing results have been obtained in a one-dimension geometry; (ii) to explore situations where quasiparticles are absent or short-lived, and see whether correlations still propagate in ballistic manner; (iii) to study the relaxation dynamics in the presence of long-range interactions, where counter-intuitive behaviours have been predicted, but not yet observed.
The concept of this project is to take advantage of the great versatility offered by ultra-cold atom systems to investigate the relaxation dynamics in regimes well beyond the boundaries of our current knowledge. We focus our attention on two-dimensional systems and systems with both short- and long-range interactions. Specifically, we will set the system out of equilibrium by a sudden change of the interaction parameter, a ‘quantum quench’, and characterise both the relaxation dynamics and the final state through the measurement of two-point correlation functions. The realisation of the project hinges on the construction of a new-generation quantum gas microscope experiment for Strontium gases enabling us to induce long-range interactions between the atoms. Beside the construction of this apparatus, our main scientific objectives are: (i) to confront the locality of the dynamics to a two-dimensional geometry, where most of the existing results have been obtained in a one-dimension geometry; (ii) to explore situations where quasiparticles are absent or short-lived, and see whether correlations still propagate in ballistic manner; (iii) to study the relaxation dynamics in the presence of long-range interactions, where counter-intuitive behaviours have been predicted, but not yet observed.
At the end of the project, we have finished building the experimental apparatus that will enable the study of the relaxation dynamics in ultracold quantum gases. It can be divided in several parts:
I. A laser system for cooling the atoms from the oven temperature (between 500 and 600°C) to about 1µK.
II. A laser system for transporting the cold gas from the magneto-optical trap chamber to the science chamber and confining and precisely positioning the atoms underneath the microscope.
III. An ultra-high vacuum system to isolate the gas from the environment, comprising: an oven producing an atomic beam; a collimation area to reduce the beam's divergence; a Zeeman slower to decrease the beam's longitudinal velocity; a chamber for the magneto-optical trap; and finally a 'science' chamber where the microscope objective is located.
IV. A fluorescence microscopy system capable of resolving individual atoms separated by half a µm.
V. A control software for programming and synchronizing with microsecond accuracy the numerous devices (lasers, RF signal generators, cameras) involved in an experimental cycle.
Because of many technical difficulties, we spent the entire duration of the project on the construction of the apparatus and we couldn't address the other scientific objectives so far. Still, we have achieved several significant results:
a. We have demonstrated a new and efficient "shelving" spectroscopy scheme for the 689-nm strontium intercombination line in collaboration with Martin Robert-de-Saint-Vincent and Bruno Laburthe (Université Sorbonne Paris Nord). This result was published in a peer-reviewed journal (doi:10.1088/1361-6455/ab707f).
b. We have developed an advanced deconvolution algorithm for the florescence microscopy images performing well in a regime of low optical resolution and signal-to-noise ratio. We have positively benchmarked its performance against the conventional Wiener deconvolution method, as well as a UNet convolutional neural network. This work was performed in collaboration with Matthieu Boffety, Caroline Kulcsar (Institut d'Optique Graduate School) and Pauline Trouvé (ONERA). We are currently wrapping up our findings in view of writing an article.
c. We have devised a laser cooling scheme to simultaneously cool and make the atoms fluoresce in a shallow optical lattice and using a standard wavelength of 1064 nm for the lattice lasers. We have also investigated the cooling mechanisms at work in our scheme and identified several regimes of interest for our community. We will publish our findings in the coming months.
Finally, another important achievement of this project is to have trained 4 PhD students and one postdoc to high-standard scientific research, among whom two have now obtained a position in the industry, two are still working on their thesis and the last has started new academic studies in a different field.
I. A laser system for cooling the atoms from the oven temperature (between 500 and 600°C) to about 1µK.
II. A laser system for transporting the cold gas from the magneto-optical trap chamber to the science chamber and confining and precisely positioning the atoms underneath the microscope.
III. An ultra-high vacuum system to isolate the gas from the environment, comprising: an oven producing an atomic beam; a collimation area to reduce the beam's divergence; a Zeeman slower to decrease the beam's longitudinal velocity; a chamber for the magneto-optical trap; and finally a 'science' chamber where the microscope objective is located.
IV. A fluorescence microscopy system capable of resolving individual atoms separated by half a µm.
V. A control software for programming and synchronizing with microsecond accuracy the numerous devices (lasers, RF signal generators, cameras) involved in an experimental cycle.
Because of many technical difficulties, we spent the entire duration of the project on the construction of the apparatus and we couldn't address the other scientific objectives so far. Still, we have achieved several significant results:
a. We have demonstrated a new and efficient "shelving" spectroscopy scheme for the 689-nm strontium intercombination line in collaboration with Martin Robert-de-Saint-Vincent and Bruno Laburthe (Université Sorbonne Paris Nord). This result was published in a peer-reviewed journal (doi:10.1088/1361-6455/ab707f).
b. We have developed an advanced deconvolution algorithm for the florescence microscopy images performing well in a regime of low optical resolution and signal-to-noise ratio. We have positively benchmarked its performance against the conventional Wiener deconvolution method, as well as a UNet convolutional neural network. This work was performed in collaboration with Matthieu Boffety, Caroline Kulcsar (Institut d'Optique Graduate School) and Pauline Trouvé (ONERA). We are currently wrapping up our findings in view of writing an article.
c. We have devised a laser cooling scheme to simultaneously cool and make the atoms fluoresce in a shallow optical lattice and using a standard wavelength of 1064 nm for the lattice lasers. We have also investigated the cooling mechanisms at work in our scheme and identified several regimes of interest for our community. We will publish our findings in the coming months.
Finally, another important achievement of this project is to have trained 4 PhD students and one postdoc to high-standard scientific research, among whom two have now obtained a position in the industry, two are still working on their thesis and the last has started new academic studies in a different field.
The results briefly described in the previous section all go beyond the state of the arts:
* Our shelving spectroscopy is more robust and easier to implement than the methods employed so far.
* In contrast to comparable experiments, our microscopy imaging system follows a low-cost design, with a commercial aspheric lens inside the vacuum chamber playing the role of a microscope objective. To compensate for the limited optical resolution, we have developed a fast and robust local deconvolution algorithm which compares positively to more common deconvolution techniques. Thanks to the use of machine-learning tools, we also provide a better characterisation of the performance of generic deconvolution algorithms in the context of quantum gas microscope experiments.
* We have demonstrated the possibility to cool and make the atoms fluoresce in a shallow optical lattice and using a standard wavelength for the lattice lasers. In the past, this task was either performed in a very deep optical lattice, or using specific lattice wavelengths where the polarisabilities of the ground and excited states. Our work therefore reduces severe experimental constraints on the construction of quantum gas microscope experiment and should thus have a significant impact on our community.
Now that we are equipped with a fully functional apparatus, we will start addressing the big questions that have motivate this project. We expect our findings to clarify the status of the scenario according to which the relaxation dynamics of isolated quantum systems is essentially local and the propagation of correlations propagate is driven by that of quasiparticles. Does this scenario describe a universal behaviour? Does it apply to all systems (geometry, range of interaction)? Does it hold for all type of observables (local or non-local, few or many particles)? Our experimental observations will help find an answer to these fundamental questions and thereby improve our understanding of the out-of-equilibrium dynamics of isolated quantum systems well beyond linear response theory.
* Our shelving spectroscopy is more robust and easier to implement than the methods employed so far.
* In contrast to comparable experiments, our microscopy imaging system follows a low-cost design, with a commercial aspheric lens inside the vacuum chamber playing the role of a microscope objective. To compensate for the limited optical resolution, we have developed a fast and robust local deconvolution algorithm which compares positively to more common deconvolution techniques. Thanks to the use of machine-learning tools, we also provide a better characterisation of the performance of generic deconvolution algorithms in the context of quantum gas microscope experiments.
* We have demonstrated the possibility to cool and make the atoms fluoresce in a shallow optical lattice and using a standard wavelength for the lattice lasers. In the past, this task was either performed in a very deep optical lattice, or using specific lattice wavelengths where the polarisabilities of the ground and excited states. Our work therefore reduces severe experimental constraints on the construction of quantum gas microscope experiment and should thus have a significant impact on our community.
Now that we are equipped with a fully functional apparatus, we will start addressing the big questions that have motivate this project. We expect our findings to clarify the status of the scenario according to which the relaxation dynamics of isolated quantum systems is essentially local and the propagation of correlations propagate is driven by that of quasiparticles. Does this scenario describe a universal behaviour? Does it apply to all systems (geometry, range of interaction)? Does it hold for all type of observables (local or non-local, few or many particles)? Our experimental observations will help find an answer to these fundamental questions and thereby improve our understanding of the out-of-equilibrium dynamics of isolated quantum systems well beyond linear response theory.