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An ultracold gas plus one ion: advancing Quantum Simulations of in- and out-of-equilibrium many-body physics

Periodic Reporting for period 4 - PlusOne (An ultracold gas plus one ion: advancing Quantum Simulations of in- and out-of-equilibrium many-body physics)

Reporting period: 2019-11-01 to 2020-04-30

Ultracold atoms and trapped ions are among the most powerful tools to study quantum physics. On the one hand, ultracold atoms provide an exceptional resource for studying many-body physics, since a relatively large number of particles, typically from a few tens of thousands to several million, can be brought to quantum degeneracy. On the other hand, trapped ions provide a great resource to explore the physics of small quantum systems. They provide one of the most successful hardware for a quantum computer, and clocks made of trapped ions are among the most precise.
Only recently, though, ultracold atoms and trapped ions have been brought together in a single experimental setup. The main goal is to exploit atom-ion interactions, which are much more long-ranged with respect to the interactions between ultracold atoms (scaling with R^-4 instead of R^-6, where R is the internuclear separation), to advance different fields, including quantum simulation, quantum computation, and quantum chemistry.
In the PlusOne project a new generation atom-ion machine was conceived, simulated and realized. The development of this novel apparatus was made it possible after tackling a number of technical issues. Among the concrete results of the project, a new ion trap was designed and realized, a new design for lasers was conceived and demonstrated, a new control apparatus made of both hardware and software was realized. These results will have a strong impact not only in the field of atom-ion physics, but in the wider field of experimental atomic physics and optical spectroscopy.
This new apparatus will open the way to quantum simulations of a many-body system in the presence of one or more localized impurities, by exploiting fundamental atom-ion interactions in the ultracold regime. An hybrid quantum system of atoms and ions interacting in a - so far unexplored - full quantum regime will realize a brand new quantum system with the possibility of tuning most of the parameters of the system, so that this can be used to study existing problems and new problems of physics from a completely novel standpoint.
The goal of the project is to realize for the first time a quantum mixture of atoms and ions in which the atom-ion collisions are at a sufficiently low energy that the atom-ion mixture evolves coherently. To this end, an utterly new experimental apparatus was conceived and realized. Most of the activities that were pursued in the project were devoted to the design and realization of this apparatus. The apparatus is composed of a large number of sources of light (lasers), a vacuum system in which an ultra-high vacuum (UHV) is created, electrodes and coils realizing electric and magnetic fields used to trap and manipulate ions and atoms, respectively, and a large number of electronic equipment pieces. Importantly, the apparatus’ design is based on a number of original concepts in the realization of an ion trapping potential, in the laser cooling of neutral atoms, and in the realization of many technical parts composing the overall experimental setup, like the lasers for cooling, trapping and detecting the atoms and the ions, the electronics for controlling the experiment, the creation of intense RF fields for driving an ion Paul trap.
The main results of this strong experimental effort were reported on a number of publications (3 published paper, 2 papers in preparation, 1 manuscript in the arxive) and on a patent application on a new design for lasers. Moreover, an ERC-PoC grant was awarded to the PlusOne project’s PI for a pre-commercialization study of these new lasers. Additionally, new types of equipment, like a new control apparatus for experiments on composite quantum systems, were developed and realized, and some of the results, like the software for controlling the experiment, are in a public repository and available to anyone free of charge.
The realization of an innovative experimental apparatus was made possible by the development of novel strategies for the realization of several fundamental and technical processes that are at the base of the experiment.
These advancements are or will be soon reported in scientific publications, which will be relevant not only for the atom-ion community, but for a large portion of the wide atomic physics community. These achievements are mainly five.
1. A new ion trap for confining charged particles by using a combination of an electrical static field and an optical field. This new trap does not make use of radiofrequencies, and it therefore lifts the main limitation that has so far made it impossible to produce ultracold atom-ion mixtures, i.e. to bring atoms and ions together to temperatures at which the mixture undergoes a quantum coherent evolution. The techical details of the trap, and of its simulated behaviors are available in a scientific publication: Perego et al. Appl. Sci. 2020, 10(7), 2222
2. A new electronic and software control system, based on programmable FPGA chips. While commercial electronic boards typically execute pre-determined temporal sequences of electric signals, therefore not allowing any action on the electrical outputs while the sequence is executed, the new control software and electronic boards designed by the research team make this possible. As a result, the machine-time of the experimental setup can be optimized so that two experiments can be performed at the same time. This new control system was reported in a publication in Review of Scientific Instruments (Perego et al. Rev. Sci. Instrum. 89, 113116 (2018) ).
3. A new RF drive providing the electric signal to operate a Paul trap. Paul traps are the most commonly used traps for confining charged particles. In order to function, the electrodes of the trap must be fed with an intense RF electric signal. Typically, this is realized either by using large amplifiers, or by implementing bulky resonant circuits. Instead, we have developed a new, compact and inexpensive RF drive that is confined in a single electronic board (smaller than a hand), based on a low-noise amplifier on on low-losses ferrites used to realize highly performing inductors. This new RF drive was reported in a publication in Review of Scientific Instruments (Detti et al. Rev. Sci. Instrum. 90, 023201 (2019) ).
4. A new laser source, made by a diode laser in an extended cavity configuration. The new laser source solves existing problems of instability that affect similar sources, and allows us to realize laser sources for atomic and molecular spectroscopy in a cheap and reliable way. A patent is currently pending on this novel laser design, and a scientific publication is under preparation.
5. We theoretically investigated a new strategy for realizing one-photon Li sideband cooling, with a scheme that provides a simplification with respect to the existing schemes for Li sideband cooling, which are based on the action of two lasers. The theoretical study was performed by the PI and by one member of the research team, who was awarded the best poster prize at the course 206 of the Enrico Fermi Physics School in Varenna, Italy. The prize consisted in the right of publishing a short article in the School book. The article, which has been recently submitted, is available online (arXiv:1912.08104).
A picture of the ion trap illuminated by the lasers for cooling the ions