Future of Ultracold and Ultrafast Dynamics

Dynamic polarizabilities of cesium Rydberg states, explicitly $nS_{1/2}$, $nP_{1/2}$, $nP_{3/2}$, $nD_{3/2}$, and $nD_{5/2}$, where the principal quantum number $n$ is $40$ to $70$, are presented for linearly polarized light. The dynamic polarizability is calculated using the sum-over-states approach. We identify double magic wavelengths in the range of $1,000-2,000$~nm for simultaneous trapping of the ground state and a Rydberg state, which are, respectively, red-detuned and blue-detuned with respect to a low-lying excited auxiliary state. Based on calculations of the radiative lifetime, blackbody radiation induced transitions, and population transfer out of the Rydberg and auxiliary states (estimated within two-state as well as master equation models), we conclude that magic wavelength trapping is particularly promising experimentally for the $nD_{J,|M_J|}$ Rydberg series with angular momentum $J=3/2$ and projection quantum numbers $M_J=\pm 1/2$ (auxiliary state $8P_{1/2}$) and $M_J=\pm 3/2$ (auxiliary state $8P_{3/2}$), using trap depths as large as $10$~$\mu$K. Moreover, by tuning the angle between the quantization axis and the polarization vector of the light, we identify triple and quadruple magic wavelengths, for which the polarizabilities of the ground state, a Rydberg state, and, respectively, one and two low-lying excited states are equal. Our comprehensive theoretical study provides much needed guidance for on-going experimental efforts on cesium Rydberg-state based quantum simulations that operate on time scales up to several $\mu$s.

Experiments investigating the low-temperature electronic transport of a quantum hall system interacting with the vacuum field of a terahertz split ring resonator have uncovered a giant anisotropy of the longitudinal resistance. Remarkably, this effect is induced by the cavity, as transport is isotropic outside of the resonator. We interpret these experimental results as the orientational stabilization of fluctuating charge density wave order—the quantum hall stripes—by the vacuum field of the cavity. In particular, we show that the vacuum field energetically selects charge order to form perpendicular to the polarization of the fundamental mode of the cavity, providing an explanation of the experimental observations. Taken together, our results constitute a first step towards the vacuum cavity control of correlated electronic transport.

In this poster, we present a mechanism to induce superconductivity in atomically thin semiconductors where excitons mediate an effective attraction between electrons. Our model includes interaction effects beyond the paradigm of phonon-mediated superconductivity and connects to the well-established limits of Bose and Fermi polarons. By accounting for the strong-coupling physics of trions, we find that the effective electron-exciton interaction develops a strong frequency and momentum dependence accompanied by the system undergoing an emerging BCS-BEC crossover from weakly bound s-wave Cooper pairs to a superfluid of bipolarons. Even at strong-coupling the bipolarons remain relatively light, resulting in critical temperatures of up to 10% of the Fermi temperature. This renders heterostructures of two-dimensional materials a promising candidate to realize superconductivity at high critical temperatures set by electron doping and trion binding energies.

In various solid state systems, the excitation of an electron from a completely filled valence band into an empty conduction band leads to a small binding energy between the electron-hole pair, called exciton. In the past years, cold atom systems have emerged as versatile platforms for simulating two-dimensional structures, such as hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDs). Recent experiments have for example managed to map the full band structure of a cold atom system in a honeycomb lattice. With all necessary experimental methods available and successfully utilized, we now investigate the existence of a different effect inspired from the solid state systems: excitons under short range interaction. We predict the existence of a similar atom-hole pair (atomic exciton) in a system of single-component fermionic atoms with nearest neighbour interactions. Our 2D lattice similar to hBN has a hexagonal structure with two different lattice site types, leading to a native two-band structure with nonzero bandgap. We use variational methods to find the energy spectrum of zero-momentum excitons in the single particle band operator basis, which form around the K/K'-point. We show that excitonic effects can be found over a large range of system parameters, which also includes currently accessible regions of band gap, tunnelling rate and nearest neighbour interaction strength. Furthermore, we use Fermis Golden Rule to propose an already existing experimental procedure consisting of a perturbation operator followed by time-of-flight spectroscopy to probe the predicted states.

We report on our recent studies on ion-Rydberg atom interactions performed in the ultra-cold quantum regime using a high-resolution ion microscope. This apparatus provides temporal and spatial imaging of charged particles with a resolution of 200\,nm. Ion-Rydberg atom pair-states on the one hand allow for the observation of collisional dynamics on steep attractive potential energy curves. Avoided crossings with high-$l$ states can cause significant speed up in the dynamics which is dependent on the individual Landau-Zener probabilities. On the other hand, the avoided crossings also lead to potential wells that give rise to bound molecular states. These ultralong-range Rydberg molecules consisting of an ion and a Rydberg atom feature a large bond length that drastically influences the timescale on which internal dynamics are happening. This allows the direct observation of vibrational dynamics. Further, this binding mechanism is fundamentally not limited to diatomic molecules but can be extended to polyatomic molecules, for which we expect even more complex interactions.

Cold fermion systems in a cavity are known to exhibit a superradiant phase transition as the fermion-cavity coupling is increased. In this work we study the dynamical instability arising when the coupling is quenched across the superradiant phase transition. Within linearized equations of motion, we predict the exponential growth rate and show it can exceed the Fermi energy by orders of magnitude. This behavior being a consequence of infinite range interactions mediated by the cavity photons. Apart from computing the rate, we also provide detailed analysis of the processes setting the instability into motion and show they are dominated by density fluctuations of fermions, with a very non-monotonic dependence on coupling strength.

We consider a coupled atom-photon system described by the Tavis-Cummings dimer (two coupled cavities) in the presence of photon loss and atomic pumping, to investigate the quantum signature of dissipative chaos. The appropriate classical limit of the model allows us to obtain a phase diagram identifying different dynamical phases, especially the onset of chaos. Both classically and quantum mechanically, we demonstrate the emergence of a steady state in the chaotic regime and analyze its properties. The interplay between quantum fluctuation and chaos leads to enhanced mixing dynamics and dephasing, resulting in the formation of an incoherent photonic fluid. The steady state exhibits an intriguing phenomenon of subsystem thermalization even outside the chaotic regime; however, its effective temperature increases with the degree of chaos. Moreover, the statistical properties of the steady state show a close connection with the random matrix theory. Finally, we discuss the experimental relevance of our findings, which can be tested in cavity and circuit quantum electrodynamics setups.

We look at the spectral function of a two component fermi gas (BCS) interacting with a Rydberg impurity. Using a functional determinant approach, it is possible to exactly (numerically) obtain the Ramsey signal and the absorption spectrum. Consequently, studying the deformation of the absorption spectrum caused by the interaction between the components of the Fermi gas.

Abstract. The ultracold matter is a promising system for emerging technologies [1,2,3]. One of the most valuable traps in 2D is the circular waveguide, formed by overlaying a blue-detuned laser in the middle of harmonic confinement, allowing efficient control of the waveguide's radius [4]. Atomtronics, exhibited by Bose-Einstein Condensates (BECs) within these ring waveguides, manifests several fascinating physical phenomena, among which the phenomenon of fractional revivals (FR) is recently reported in this system [5]. FR is a well-studied effect in diverse quantum systems during their evolution [6-9]. The localized initial condensate spreads over time, and the counter-propagating parts interfere to produce various mini replicas of the initial condensate at different time instances [5,9]. This work studies the dynamics of a ring-trapped binary BEC in the mean-field regime. Including interspecies interaction enriches the FR-physics due to the emergence of two time scales that influence each other [10]. We identify the effects of mass imbalance and the tunability through ring radius on these two-time scales. Additionally, we provide a systematic study of the time evolution and report the suitable trap parameters and interspecies interaction strengths necessary for phase separation of the two-component BEC. Some additional applications of such setup will also be explicated. References: 1.Amico, L., Birkl, G., Boshier, M., & Kwek, L. C. (2017). Focus on atomtronics-enabled quantum technologies. New Journal of Physics, 19(2), 020201. 2.Gross, C., & Bloch, I. (2017). Quantum simulations with ultracold atoms in optical lattices. Science, 357(6355), 995-1001. 3.Amico, L., Boshier, M., Birkl, G., Minguzzi, A., Miniatura, C., Kwek, L. C., & Yakimenko, A. (2021). Roadmap on Atomtronics: State of the art and perspective. AVS Quantum Science, 3(3). 4. Ryu, C., Andersen, M. F., Clade, P., Natarajan, V., Helmerson, K., & Phillips, W. D. (2007). Observation of persistent flow of a Bose-Einstein condensate in a toroidal trap. Physical Review Letters, 99(26), 260401. 5.Bera, J., Ghosh, S., Salasnich, L., & Roy, U. (2020). Matter-wave fractional revivals in a ring waveguide. Phys. Rev. A, 102(6), 063323. 6.Vrakking, M. J., Villeneuve, D. M., & Stolow, A. (1996). Observation of fractional revivals of a molecular wave packet. Phys. Rev. A, 54(1), R37. 7.R. W. Robinett, Quantum wave packet revivals, Physics reports 392(1-2), 1 (2004). 8.J. Banerji and S. Ghosh, The role of ro-vibrational coupling in the revival dynamics of diatomic molecular wave packets, Journal of Physics B: Atomic, Molecular and Optical Physics 39(5), 1113 (2006). 9. Ghosh, S., Bera, J., Panigrahi, P. K., & Roy, U. (2019). Sub-fourier quantum metrology through bright solitary trains in Bose–Einstein condensate. Int. J. Quantum Inf., 17(02), 1950019. 10. Raghav, S., Ghosh, S., Halder, B., & Roy, U. (2023). Matter Wave Isotope Separation in a Ring Trap. arXiv preprint arXiv:2309.09846.

Sensors using chemical chain reactions have been widely used in biological and chemical analysis. Contrary to these classical chemical reactions, at extremely low temperatures (~ 100 nK), ultracold chemical reactions may occur in Bose-Einstein condensates (BEC) where the coherent oscillatory dynamics may occur even for the simplest reactions like $A + A \leftrightarrow A_2$. In an atom-molecule BEC under ultracold chemical reactions $A + A \leftrightarrow A_2$, by calculating the quantum Fisher information (QFI) and the lower bound of the classical Fisher information (CFI), we show that one can estimate the magnitude of the homogeneous external perturbations by measuring the number of BEC molecules [1]. Counting the number of BEC molecules under ultracold chemical reactions is simpler compared to previous proposals that rely on counting the number of phonons in BEC to detect external perturbations. This is because spectroscopic techniques can be used to count the number of BEC molecules, while there are several challenges in counting the number of phonons in BEC [2, 3]. Also, in our system, the lower bound of the CFI can reach up to around 60% of the QFI, meaning that the sensor relying on counting the number of BEC molecules is close to the optimal sensor for detecting the homogeneous external perturbations. References [1] Seong-Ho Shinn, Uwe R. Fischer, and Daniel Braun, arXiv:2208.06380 [cond-mat.quant-gas] [2] Ralf Schützhold, Phys. Rev. D 98, 105019 (2018) [3] Dennis Rätzel and Ralf Schützhold, Phys. Rev. A 103, 063321 (2021)

The well-studied process of high-harmonic generation has recently been described using a quantum formulation for the driving light, instead of the very known and successful semiclassical description [1,2]. In the original papers, the source driving the phenomenon was always a coherent state of light. Recently, it was shown that if the initial state for the light is a sum of coherent states with the same intensity but with different phases, the harmonic spectrum is the same, meaning that it is not a good quantifier to know if the original pulse was a coherent one or not. We utilize a non-revealed-measurement protocol [4], which defines an entropic distance between the original state (for instance, the state of the system after the pulse has finished) with the state after the protocol, and this could reveal underlying differences between coherent and incoherent driving states. [1] Nature Physics 17, 1104 (2021); [2] Phys. Rev. B 109, 035203 (2024); [3] Phys. Rev. Research 6, L032033 (2024); [4] EPL 112 40005 (2015).

In my talk, I will present our latest work [1] on Bose-induced superconductivity in two-dimensional semiconductor heterostructures in which electrons and excitons can bind into trions, competing with the formation of Cooper pairs. Along the way, I will relate/motivate this to/from lessons learned in previous works on Fermi polarons [2], strongly-coupled Bose-Fermi mixtures [3] and precursors found in minimal few-body systems [4]. By accounting for the strong-coupling physics of trions, our theory includes interaction effects beyond the paradigm of phonon-mediated superconductivity and connects to the well-established limits of Bose and Fermi polarons. As a result, a BCS-BEC crossover from weakly bound s-wave Cooper pairs to a superfluid of bipolarons emerges. Even at strong-coupling, the bipolarons remain relatively light, resulting in critical temperatures of up to 10% of the Fermi temperature. This renders such systems a promising candidate to realize superconductivity at high critical temperatures set by electron doping and trion binding energies. [1] J. von Milczewski, X. Chen, A. Imamoglu, R. Schmidt, (2023)[arXiv:2310.10726]. [2] J. von Milczewski, F. Rose, R. Schmidt, Phys. Rev. A 105, 013317 (2022)[arXiv:2104.14017]. [3] M. Duda, X.-Y. Chen, A. Schindewolf, R. Bause, J. von Milczewski, R. Schmidt, I. Bloch, X.-Y. Luo, Nat. Phys. 19, 720–725 (2023)[arXiv:2111.04301]. [4] R. Li, J. von Milczewski, A. Imamoglu, R. Oldziejewski, R. Schmidt, Phys. Rev. B 107, 155135 (2023)[arXiv:2211.12495].

Despite being one of the most common and straightforward ways of generating entanglement between two particles, the creation of entanglement in collisions has never been comprehensively studied beyond 1D or toy models. Here, we seek to quantify the degree of entanglement generated in ultracold atomic collisions by computing the inter-particle purity, focusing first on the motional degree of freedom. As the entanglement generated in collisions depends rather sensitively on the initial conditions, we consider two elongated Gaussian wave packets as pre-collision states, to model the realistic experimental settings as possible. We will study how the degree of entanglement can be influenced by the initial momenta, the interaction between the particles, as well as the shapes of the Gaussian wave packets, and establish the relation between entanglement and resonance states.