Unit Seminar Series

Past Seminars

Tuesday 19th March 2024: 1 - 2:30pm on Zoom

Title: Atom-interferometry based Quantum gravimeter for field applications

Speaker: Dr. Ravi Kumar, Atomionics Pte, Ltd, Singapore

Abstract:
Atom-interferometry is one the most sensitive tools available today for the measurement of inertial forces. The talk will cover principles of atom-interferometry in the context of gravity measurement and discuss some of the field trials conducted by a quantum gravimeter developed on this technology by Atomionics Pte. Ltd, Singapore.

Monday 18th March 2024: 5 - 6:30pm on Zoom

Title: Ultracold atoms carrying orbital angular momentum in lattices of rings

Speaker: Dr. Verónica Ahufinger, Universitat Autònoma de Barcelona, Spain

Abstract:
Tunnelling is one of the paradigms of quantum mechanics and its control in the context of ultracold neutral atoms has been, recently, a topic of intense research. In this seminar, we will consider a system formed by non-interacting ultracold atoms loaded into the manifold of l = 1 Orbital Angular Momentum (OAM) states of a diamond lattice, in which complex tunnelling amplitudes appear naturally [1]. Through a series of successive basis rotations [2,3], we show that the OAM degree of freedom induces phases in some tunnelling amplitudes of the tightbinding model that are equivalent to a net π flux through the plaquettes and give rise to a topologically non-trivial band structure and protected edge states. In addition, we demonstrate that the system also exhibits Aharonov-Bohm caging, which consists of the confinement of specifically prepared wave packets due to quantum interference. We will also show the experimental observation of Aharonov-Bohm caging using OAM modes in a system of coupled optical waveguides forming a diamond structure [4].

[1] J. Polo, J. Mompart and V. Ahufinger, Geometrically induced complex tunnelings for ultracold atoms carrying orbital angular momentum. Phys. Rev. A 93, 033613 (2016).
[2] G. Pelegrí, A. M. Marques, R. G. Dias, A. J. Daley, V. Ahufinger and J. Mompart, Topological edge states with ultracold atoms carrying orbital angular momentum in a diamond chain. Phys. Rev. A 99, 023612 (2019).
[3] G. Pelegrí, A. M. Marques, R. G. Dias, A. J. Daley, J. Mompart and V. Ahufinger, Topological edge states and Aharonov-Bohm caging with ultracold atoms carrying orbital angular momentum. Phys. Rev. A 99, 023613 (2019).
[4] C. Jörg, G. Queraltó, M. Kremer, G. Pelegrí, J. Schulz, A. Szameit, G. von Freymann, J. Mompart and V. Ahufinger, Artificial gauge field switching using orbital angular momentum modes in optical waveguides. Light: Science & Applications 9, 150 (2020).

Monday 11th March 2024: 5-6pm on Zoom

Title: Dipolar quantum gases: From rotons to supersolids

Speaker: Dr. Manfred Mark, Institut für Experimentalphysik, Universität Innsbruck, Austria

Abstract:
The field of ultracold atomic gases has seen a tremendous evolution over the past 25 years since the first successful creation of a Bose-Einstein condensate in 1995. Such systems offer an unprecedented control over essentially all degrees of freedom, making them an ideal platform for what is now called quantum simulation. The advent of quantum gases made from lanthanide atoms 12 years ago opened up the new field of strongly dipolar systems, as they feature a strong magnetic dipole moment, giving rise to long-range and anisotropic interactions between the atoms. This type of interaction has proven to be a rich source of new and fascinating many-body phenomena.
In this talk I will recap some of the most recent discoveries, starting with the observation of the rotonized excitation spectrum. This effect, first proposed from Landau for liquid helium, was predicted to occur also in strongly dipolar quantum gases, and finally got observed recently in our lab. This triggered the interest of many groups, as it was believed that the roton mode is a precursor for crystallization, opening up the possibility to have spontaneous density modulations while being superfluid - a so-called supersolid state. Already a year later, three groups showed the existence of this long-sought state, including two realizations in our group. Our comparatively long lifetimes allowed us to investigate this state in terms of its dynamical response to phase excitations, where we observed rapid dephasing/rephasing of the system. We were also able to study the lifecycle of the state, from its initial forming directly out of a thermal cloud until its decay to a Bose-Einstein condensate due to particle loss. Finally we studied its dependence on the trap geometry and managed to observe supersolidity where the translational symmetry is broken along two directions in a pancake-shaped trap.

Related papers:
Two-dimensional supersolidity in a dipolar quantum gas,M. A. Norcia, C. Politi, L. Klaus, E. Poli, M. Sohmen, M. J. Mark, R. Bisset, L. Santos, F. Ferlaino,
Nature, 596, 357-361, 2021, http://dx.doi.org/10.1038/s41586-021-03725-7

Birth, life, and death of a dipolar supersolid,
M. Sohmen, C. Politi, L. Klaus, L. Chomaz, M. J. Mark, M. A. Norcia, F. Ferlaino,
Phys. Rev. Lett., 126, 233401, 2021, http://dx.doi.org/10.1103/PhysRevLett.126.233401

Wednesday 21st February 2024: 4-5pm on Zoom

Title: Guiding Light to Non-Classicality

Speaker: Dr Philipp Schneeweiss, Scientific Staff, Humboldt University, Germany

Abstract:
The interaction between light and matter is an important topic in physics research and relevant to areas ranging from photo synthesis to quantum communication. One of the most fundamental settings is the scattering of a coherent laser beam on a single quantum emitter. A peculiarity of this interaction is that the scattered light never contains two photons at the same time, i.e., it is showing photon-antibunching. In my talk, I will show that photon-antibunching arises from quantum interference between two types of two-photon scattering processes, typically referred to as coherent and incoherent scattering. By spectrally rejecting the coherently scattered part of the fluorescence light from a single two-level atom, we show experimentally that the remaining part of the light consists of photon pairs that have been simultaneously scattered by the emitter [1]. Furthermore, by transmitting a guided laser light field through an ensemble of cold atoms coupled to an optical nanofiber, we demonstrate a collective enhancement of this two-photon scattering process [2]. By changing the dispersion and the optical depth of the atomic ensemble, we can tailor the second-order correlation function of the transmitted light and adjust its photon statistics from photon bunching to perfect antibunching [3]. Our results contribute to understanding the fundamentals of light-matter interaction and might contribute to developing a novel type of source of non-classical light.

[1] L. Masters et al., Nat. Phot. 17, 972 (2023)
[2] A. S. Prasad et al., Nat. Phot. 14, 722 (2020), News & Views: P. Solano, Nat. Phot. 14, 716 (2020)
[3] M. Cordier et al., Phys. Rev. Lett. 131, 183601 (2023)

Wednesday 21st February 2024: 2-3pm, C209, Center Building

Title: Ultrafast Rydberg experiments with ultracold atoms

Speaker: Dr Sylvain de Léséleuc, Research Associate Professor, Institute for Molecular Science, National Institutes of Natural Sciences, Okazaki, Japan

Abstract:Rydberg atoms, with their giant electronic orbitals, exhibit dipole-dipole interaction reaching the GHz range at a distance of a micron (C3 ~ GHz.μm3), making them a prominent contender for realizing ultrafast quantum operations. However, such strong interactions have never been harnessed so far because of the stringent requirements on the fluctuation of the atom positions and the necessary excitation strength. Here, we introduce novel techniques to enter and explore this ultrafast Rydberg regime [1,2].
I will first introduce the Rydberg timescale to position the various fundamental limits and opportunities set by atomic physics properties, as well as the technical challenges in reaching them with today’s experimental tools. We will then look at how we excite Rydberg atoms as fast as physically possible (~10 picoseconds) by pulsed lasers, non-linear optics and spectral optimization. With the atoms now in the Rydberg states, we will revisit how fast they can interact with each other through long-range dipole-dipole interaction and demonstrate coherent dynamics in the nanosecond timescale. Finally, we will consider how the internal electronic Rydberg dynamics driven by interaction couples coherently to the external motional degrees of freedom (position, momentum). I will show signatures of this effective “spin-motion” coupling on experiments with atoms trapped in optical tweezers and optical lattices, and opportunities offered by quantum control of the motional states on both platform [3].

Sponsored by SPIE and Optica Student Chapters

Wednesday 14th February 2024: 4-5pm on Zoom

Title: Quantum computing with optical tweezer trapped arrays of neutral atoms

Speaker: Dr Ratnesh Kumar Gupta, 5th Quantum Institute, University of Stuttgart, Germany

Abstract:
The global pursuit of developing a fully functional quantum computer is intensifying, with various underlying physical principles under exploration for achieving scalability and practical problem-solving capabilities. While the most effective approach remains undetermined, recent advancements in quantum logic gates utilizing Rydberg atoms position them competitively alongside ion traps and superconductors within the quantum computing landscape. Rydberg atoms, characterized by highly excited states, exhibit strong and controllable interactions with external electromagnetic field as well as neighboring Rydberg atoms, forming the basis for rapid quantum logic gates. This project aims to advance the field by realizing a quantum computer demonstrator with individually controlled Rydberg atoms, aiming to compete with established platforms. Leveraging excitation to Rydberg states enables rapid switching of interactions, facilitating fast and high-fidelity gate operations within an optical tweezer architecture. The project explores novel aspects, including the potential to dynamically reshuffle the qubit array during quantum computations.

Monday 5th February 2024: 5-6pm on Zoom

Title: Loading of cavities in vacuum using femtosecond laser melt-out technique

Speaker: Dr Nafia Rahaman, Senior Postdoc, Quantum Nanophysics Group, University of Vienna, Austria

Abstract:
The synthesis of nanoparticles, characterized by precise size and shape control, has posed a significant challenge in the realms of both nanotechnology and the growing field of levitated optomechanics. In this study, we unveil a novel approach encompassing the fabrication, launch, and detection of nanoparticles in both gaseous and vacuum environments. Our methodology involves the utilization of a highly focused ultra-fast laser beam with substantial intensity yet minimal energy to liquefy, form, and release individual particles during each optical pulse. Surface tension plays a crucial role in facilitating the formation of spherical particles from molten droplets, allowing for the manipulation of their radii within the range of r = 80 - 150 nm by adjusting pulse intensity. The particle generation system is compact and simple and enables operation at repetition rates exceeding 10 kHz.

An exceptional aspect of our method lies in its ability to produce pristine silicon spheres directly in a vacuum environment, otherwise unattainable in air due to rapid oxidation. Silicon, with its high infrared polarizability, tunable electrical conductivity, and low work function, holds substantial significance in levitated optomechanics, cavity cooling, and emerging quantum interference experiments. Furthermore, coupling our particle source with an infrared cavity demonstrates the temporary trapping of gold nanoparticles.