Periodic Reporting for period 4 - RyD-QMB (Rydberg dressed quantum many-body systems)
Okres sprawozdawczy: 2020-12-01 do 2021-05-31
In the project "Rydberg dressed quantum many-body systems" (RyD-QMB) we studied synthetic quantum many-body systems with long-ranged interactions. Compared to contact interactions, their long-range counterpart introduces a new length scale to quantum many-body problems which can lead to rich new physics. This includes new forms of matter such as supersolids in two or three dimensions or exotic quantum liquids in one dimension. The interactions can be made state dependent, such that magnetic quantum systems can be explored. This allows us to experimentally explore various models for quantum magnets in a highly controlled laboratory environment and provides a basis for neutral atom based quantum simulation and computation.
We induce long-ranged interactions between neutral atoms by laser coupling to so called Rydberg states. These Rydberg states are weakly bound atomic quantum states, where the weak binding between the nucleus and the outer electron leads to a large polarizability and in turn to extremely strong dipole interactions between these atoms. The advantage of the laser controlled interactions is their intrinsic switchability: When the light is switched off, the interactions are absent. By tailoring the laser parameters, one can generate various different forms of interactions, not only differing in their strength, but also in their shape. The objectives of RyD-QMB are to demonstrate that: 1) Megahertz scale optical coupling can be achieved and that the Rydberg interactions can be controlled by the parameters of the laser, 2) the interactions lead to coherent many-body physics in the continuum with the aim to realize new states of quantum matter and 3) to use these interactions for the study of quantum magnets.
Our atomic species of choice is potassium, for which we were the first to trap individual atoms in optical tweezers and to demonstrate coherent Rydberg coupling of the atoms in the array. The laser light for the coupling is derived from novel UV laser system providing more than 1W output at 286nm with very low line width and stable amplitude. This provides enough intensity to reach the Megahertz scale in the optical coupling as required for Rydberg dressing. We and others discovered a collective decoherence channel for Rydberg dressed atoms. An atom in the Rydberg state can decay to a nearby state of opposite parity, opening a very strong dipolar interaction channel. We used our perfectly defined array to study this important effect in detail, which allowed us to demonstrate that it indeed arises from dipolar interactions. In quantum magnets we observed novel types of interactions with non-monotoneous spatial dependence. This required double dressing of two atomic ground states, which define the quantum magnets up and down state.
With these achievements we successfully reached most goals of RyD-QMB and now have one instance of a Rydberg tweezer platform at hand, which is one of the most promising candidates to quantum computing. Future will show if the technology can be advanced far enough to realize a quantum computer with practical use for industrial applications.
We induce long-ranged interactions between neutral atoms by laser coupling to so called Rydberg states. These Rydberg states are weakly bound atomic quantum states, where the weak binding between the nucleus and the outer electron leads to a large polarizability and in turn to extremely strong dipole interactions between these atoms. The advantage of the laser controlled interactions is their intrinsic switchability: When the light is switched off, the interactions are absent. By tailoring the laser parameters, one can generate various different forms of interactions, not only differing in their strength, but also in their shape. The objectives of RyD-QMB are to demonstrate that: 1) Megahertz scale optical coupling can be achieved and that the Rydberg interactions can be controlled by the parameters of the laser, 2) the interactions lead to coherent many-body physics in the continuum with the aim to realize new states of quantum matter and 3) to use these interactions for the study of quantum magnets.
Our atomic species of choice is potassium, for which we were the first to trap individual atoms in optical tweezers and to demonstrate coherent Rydberg coupling of the atoms in the array. The laser light for the coupling is derived from novel UV laser system providing more than 1W output at 286nm with very low line width and stable amplitude. This provides enough intensity to reach the Megahertz scale in the optical coupling as required for Rydberg dressing. We and others discovered a collective decoherence channel for Rydberg dressed atoms. An atom in the Rydberg state can decay to a nearby state of opposite parity, opening a very strong dipolar interaction channel. We used our perfectly defined array to study this important effect in detail, which allowed us to demonstrate that it indeed arises from dipolar interactions. In quantum magnets we observed novel types of interactions with non-monotoneous spatial dependence. This required double dressing of two atomic ground states, which define the quantum magnets up and down state.
With these achievements we successfully reached most goals of RyD-QMB and now have one instance of a Rydberg tweezer platform at hand, which is one of the most promising candidates to quantum computing. Future will show if the technology can be advanced far enough to realize a quantum computer with practical use for industrial applications.
The experimental study of Rydberg induced long-range interactions among ultracold atoms first requires the construction of a dedicated and optimized experimental platform. This platform is a next generation ultracold quantum gas machine based on potassium atoms, where the choice of potassium offers specific advantages when studying quantum magnets and a greater flexibility due to the availability of both a fermionic and a bosonic isotope. During the first half of the project the focus was fully on the construction of this experimental platform.
During the second half of the action we further optimized our laser system for the generation of Watt-scale ultraviolett light. Much work was required and know-how needed to be generated to overcome severe degrading of the optical components due to photochemistry. Furthermore, we narrowed the line-width of the laser system below the 10kHz scale, which turned out to be crucial for the coherence time of our dressing experiments. We implemented an liquid-crystal based holographic spatial light modulator in our platform to generate a two-dimensional array of optical tweezers. The development of a temporally interleaved scheme of laser cooling and trapping enabled high-fidelity (above 95%) detection of single atoms in individual optical tweezers, which has not been achieved so far with potassium atoms. Our experimental studies of Rydberg dressing in the array revealed that inhomogeneities in the array are severly limiting the quantum simulation possibilities. To overcome this obstacle we implemented Raman-sideband cooling to prepare the individual atoms near their absolute ground state of the center of mass degree of freedom. These results have been published in SciPost Physics 10, 052 (2021). Armed with these new possibilities we were able to study Rydberg dressing with a focus on the coherence properties of a dressed many-body system in optical tweezers which lead to the surprising insight, that motion of impurities generated by black-body radiation are of prime importance to understand the dynamics of the decoherence process (arXiv:2103.14383 (2021)). During the final year of the action we focused on the implementation of Rydberg double dressing for novel interactions among elementary spins in a two-dimensional quantum magnet. We performed experiments to explore the various different kinds of magnetic interaction (ZZ, XY and flop-flop) that can be engineered by the choice of the laser parameters.
In collaboration with another experimental team at the MPQ we performed experiments to better understand Rydberg-dressed quantum gases. These insights are crucial to understand and push the limits to coherence in Rydberg coupled ensembles, a central goal of this project. This experiment revealed the existence of so called Rydberg macrodimers, an exotic form of molecules with huge internuclear distance in the order of Micrometers (Science 364, 664–667 (2019) and Phys. Rev. Research 3, 013252 (2021)). In a collaboration with theory colleagues we developed ideas for a new detection scheme which may allow for the measurement of correlations at different times (Quantum Science and Technology 4, 024005 (2019)).
Our results have been summarized in the publications mentioned above. They also have been presented in an international conference on Rydberg physics and on several national workshops. The COVID pandemic prevented further dissemination at conferences, which we plan to do as soon as it is possible again.
During the second half of the action we further optimized our laser system for the generation of Watt-scale ultraviolett light. Much work was required and know-how needed to be generated to overcome severe degrading of the optical components due to photochemistry. Furthermore, we narrowed the line-width of the laser system below the 10kHz scale, which turned out to be crucial for the coherence time of our dressing experiments. We implemented an liquid-crystal based holographic spatial light modulator in our platform to generate a two-dimensional array of optical tweezers. The development of a temporally interleaved scheme of laser cooling and trapping enabled high-fidelity (above 95%) detection of single atoms in individual optical tweezers, which has not been achieved so far with potassium atoms. Our experimental studies of Rydberg dressing in the array revealed that inhomogeneities in the array are severly limiting the quantum simulation possibilities. To overcome this obstacle we implemented Raman-sideband cooling to prepare the individual atoms near their absolute ground state of the center of mass degree of freedom. These results have been published in SciPost Physics 10, 052 (2021). Armed with these new possibilities we were able to study Rydberg dressing with a focus on the coherence properties of a dressed many-body system in optical tweezers which lead to the surprising insight, that motion of impurities generated by black-body radiation are of prime importance to understand the dynamics of the decoherence process (arXiv:2103.14383 (2021)). During the final year of the action we focused on the implementation of Rydberg double dressing for novel interactions among elementary spins in a two-dimensional quantum magnet. We performed experiments to explore the various different kinds of magnetic interaction (ZZ, XY and flop-flop) that can be engineered by the choice of the laser parameters.
In collaboration with another experimental team at the MPQ we performed experiments to better understand Rydberg-dressed quantum gases. These insights are crucial to understand and push the limits to coherence in Rydberg coupled ensembles, a central goal of this project. This experiment revealed the existence of so called Rydberg macrodimers, an exotic form of molecules with huge internuclear distance in the order of Micrometers (Science 364, 664–667 (2019) and Phys. Rev. Research 3, 013252 (2021)). In a collaboration with theory colleagues we developed ideas for a new detection scheme which may allow for the measurement of correlations at different times (Quantum Science and Technology 4, 024005 (2019)).
Our results have been summarized in the publications mentioned above. They also have been presented in an international conference on Rydberg physics and on several national workshops. The COVID pandemic prevented further dissemination at conferences, which we plan to do as soon as it is possible again.
We pushed the state of the art in several aspects. First, we developed a novel quantum simulation platform based on potassium Rydberg atoms in optical tweezer arrays - the first using potassium worldwide. Second, we developed a laser system with unprecedented power at 286nm, which at the same time is narrow in frequency and stable in its power level. As we have shown by our results, this is an ideal tool for Rydberg experiments. Third, we were the first to induce non-monotonic spin-spin interactions of various types among trapped atoms in the array. To do so we implemented a double dressing protocol for two hyperfine atomic ground states. This widens the toolbox available to the experimentalist and opens new possibilities for the simulation of quantum magnets and for atomic quantum technologies in general.