arXiv subjects

Andreas Hemmerich

Publications and source records attributed to Andreas Hemmerich.

At least 19 recordsLinked to original sources

arXiv2026

Fluctuation-based evidence for number--phase dynamics in a frustrated orbital superfluid

Frustrated quantum matter can host intertwined orders rooted in symmetry-related low-energy landscapes, yet static order parameters alone do not reveal how fluctuations are organized among competing configurations. Here we measure mode-resolved shot-to-shot population fluctuations in a $p$-orbital triangular-lattice superfluid with a tunable bias among three valleys. We observe a bias-tuned evolution from enhanced, anticorrelated fluctuations of two minority valleys toward strong confinement of relative-population fluctuations in a selected two-valley stripe phase. The dominant fluctuation structure is captured by an effective canonical model that includes interactions among the condensed modes, supporting a quasi-equilibrium description of the coherent three-valley condensate. Together, the data and model reveal a quantum--thermal regime shaped by pair-tunneling-induced number--phase dynamics, in which relative-phase scrambling softens effective barriers in the minority-valley regime, while phase rigidity gives rise to macroscopic harmonic confinement in the stripe phase. Our results establish mode-resolved fluctuation measurements as a probe of hidden number--phase back-action in frustrated quantum fluids.

cond-mat.quant-gas↗

arXiv2025

Controlled bit-flip of period-doubling and discrete time crystalline states in open systems

In this work, we explore the robustness of a bit-flip operation against thermal and quantum noise for bits represented by the symmetry-broken pairs of the period-doubled (PD) states in a classical parametric oscillator and discrete time crystal (DTC) states in a fully-connected open spin-cavity system, respectively. The bit-flip operation corresponds to switching between the two PD and DTC states induced by a defect in a periodic drive, introduced in a controlled manner by linearly ramping the phase of the modulation of the drive. In the absence of stochastic noise, strong dissipation results in a more robust bit-flip operation in which slight changes to the defect parameters do not significantly lower the success rate of bit-flips. The operation remains robust even in the presence of stochastic noise when the defect duration is sufficiently large. The fluctuations also enhance the success rate of the bit-flip below the critical defect duration needed to induce a switch. By considering parameter regimes in which the DTC states in the spin-cavity system do not directly map to the PD states, we reveal that this robustness is due to the system being quenched by the defect towards a new phase that has enough excitation to suppress the effects of the stochastic noise. This allows for precise control of the bit-flip operations by tuning into the preferred intermediate state that the system will enter during a bit-flip operation. We demonstrate this in a modified protocol based on precise quenches of the driving frequency.

quant-ph↗

arXiv2025

Fiber-coupled external cavity DFB-laser

Narrow-linewidth, compact and robust laser systems are of high interest, especially with prospects for portable quantum metrological applications. We realized an external cavity DFB-laser incorporating an intra-cavity EOM in a fully fiber-coupled manner. Additional active feedback on the EOM, by referencing a high finesse cavity, enabled us to reach low kHz-linewidths. We analyzed the laser spectrum by means of delayed self-heterodyne measurements to reconstruct the power spectral density. This is achieved by utilizing a data-driven power spectrum equalization approach. The presented laser setup is successfully used to create an ultracold gas by laser cooling on a narrow atomic transition of ${}^{40}$Ca at 1978 nm.

physics.optics↗

arXiv2025

Observation of Brownian Motion of a Bose-Einstein Condensate

We report on the experimental observation of classical Brownian motion in momentum space by a Bose-Einstein condensate (BEC) of Rubidium atoms prepared in a hexagonal optical lattice. Upon suddenly increasing the effective atomic mass, the BEC as a whole behaves as a classical rigid body with its center-of-mass receiving random momentum kicks by a Langevin force arising from atom loss and interactions with the surrounding thermal cloud. Physically, this amounts to selective heating of the BEC center-of-mass degree of freedom by a sudden quench, while with regard to the relative coordinates, the BEC is stablized by repulsive atomic interactions, and its internal dynamics is suppressed by forced evaporative cooling induced by atom loss. A phenomenological theory is developed that well explains the experimental data quantitatively.

cond-mat.quant-gas↗

arXiv2025

Torus bifurcation of a dissipative time crystal

Using a quantum gas setup consisting of a Bose-Einstein condensate strongly coupled to a high-finesse optical cavity by a transverse pump laser, we experimentally observe an instability of a dissipative continuous time crystal (CTC) towards a time crystalline state exhibiting two prominent oscillation frequencies. Applying a mean-field approximation model and a Floquet analysis, we theoretically confirm that this transition is a manifestation in a many-body system of a torus bifurcation between a limit cycle (LC) and a limit torus (LT). We theoretically illustrate the LC and LT attractors using the minimal model and experimentally reconstruct them using Takens' embedding theorem applied to the non-destructively measured intracavity photon dynamics.

cond-mat.quant-gas↗

arXiv2025

Observation of a zero-energy excitation mode in the open Dicke model

Approaching phase boundaries in many-body systems can give rise to intriguing signatures in their excitation spectra. Here, we explore the excitation spectrum of a Bose-Einstein condensate strongly coupled to an optical cavity and pumped by an optical standing wave, which simulates the famous Dicke-Hepp-Lieb phase transition of the open Dicke model with dissipation arising due to photon leakage from the cavity. For weak dissipation, the excitation spectrum displays two strongly polaritonic modes. Close to the phase boundary, we observe an intriguing regime where the lower-energetic of these modes, instead of showing the expected roton-type mode softening, is found to approach and persist at zero energy, well before the critical pump strength for the Dicke-Hepp-Lieb transition boundary is reached. Hence, a peculiar situation arises, where an excitation is possible at zero energy cost, but nevertheless no instability of the system is created.

cond-mat.quant-gas↗

arXiv2024

Observation of a phase transition from a continuous to a discrete time crystal

Discrete (DTCs) and continuous time crystals (CTCs) are novel dynamical many-body states, that are characterized by robust self-sustained oscillations, emerging via spontaneous breaking of discrete or continuous time translation symmetry. DTCs are periodically driven systems that oscillate with a subharmonic of the external drive, while CTCs are continuously driven and oscillate with a frequency intrinsic to the system. Here, we explore a phase transition from a continuous time crystal to a discrete time crystal. A CTC with a characteristic oscillation frequency $ω_\mathrm{CTC}$ is prepared in a continuously pumped atom-cavity system. Modulating the pump intensity of the CTC with a frequency $ω_{\mathrm{dr}}$ close to $2\,ω_\mathrm{CTC}$ leads to robust locking of $ω_\mathrm{CTC}$ to $ω_{\mathrm{dr}}/2$, and hence a DTC arises. This phase transition in a quantum many-body system is related to subharmonic injection locking of non-linear mechanical and electronic oscillators or lasers.

quant-ph↗

arXiv2024

Realizing limit cycles in dissipative bosonic systems

We propose a general mechanism for generating limit cycle (LC) oscillations by coupling a linear bosonic mode to a dissipative nonlinear bosonic mode. By analyzing the stability matrix, we show that LCs arise due to a supercritical Hopf bifurcation. We find that the existence of LCs is independent of the sign of the effective nonlinear interaction. The LC phase can be classified as a continuous time crystal (CTC), if it emerges in a many-body system. The bosonic model can be realised in three-level systems interacting with a quantised light mode as realised in atom-cavity systems. Using such a platform, we experimentally observe LCs for the first time in an atom-cavity system with attractive optical pump lattice, thereby confirming our theoretical predictions.

cond-mat.quant-gas↗

arXiv2023

Enhancing exotic quantum fluctuations in a strongly entangled cavity BEC system

We show that the strong coupling of a quantum light field and correlated quantum matter induces exotic quantum fluctuations in the matter sector. We determine their spectral characteristics and reveal the impact of the atomic s-wave scattering. In particular, we derive the dissipative Landau and Beliaev processes from the microscopic Hamiltonian using imaginary time path integrals. By this, their strongly sub-Ohmic nature is revealed analytically. A competition between damping and antidamping channels is uncovered. Their intricate influence on physical observables is quantified analytically and the Stokes shift of the critical point is determined. This illustrates the tunability of the quantum matter fluctuations by exploiting strong light-matter coupling.

quant-ph↗

arXiv2023

Evidence for Quantum Stripe Ordering in a Triangular Optical Lattice

Understanding strongly correlated quantum materials, such as high $T_\textrm{c}$ superconductors, iron-based superconductors, and twisted bilayer graphene systems, remains to be one of the outstanding challenges in condensed matter physics. Quantum simulation with ultra-cold atoms in particular optical lattices, which provide orbital degrees of freedom, is a powerful tool to contribute new insights to this endeavor. Here, we report the experimental realization of an unconventional Bose-Einstein condensate of $^{87}$Rb atoms populating degenerate $p$-orbitals in a triangular optical lattice, exhibiting remarkably long coherence times. Using time-of-flight spectroscopy, we observe that this state spontaneously breaks the rotational symmetry and its momentum spectrum agrees with the theoretically predicted coexistence of exotic stripe and loop current orders. Like certain strongly correlated electronic systems with intertwined orders, as high-$T_\textrm{c}$ cuprate superconductors, twisted bilayer graphene, and the recently discovered chiral density-wave state in kagome superconductors $\textrm{AV}_3 \textrm{Sb}_5$ (A=K, Rb, Cs), the newly demonstrated quantum state, in spite of its markedly different energy scale and the bosonic quantum statistics, exhibits multiple symmetry breakings at ultralow temperatures. These findings hold the potential to enhance our comprehension of the fundamental physics governing these intricate quantum materials.

cond-mat.quant-gas↗

arXiv2023

Two-colour laser cooling for 40K-87Rb quantum gas mixtures

We present an efficient cooling scheme for fermionic $^{40}{\text{K}}$ atoms, using laser light red and blue detuned with respect to the $\mathcal{D}2$ and $\mathcal{D}1$ principle flourescence lines, respectively. The cooling scheme is found to significantly increase the saturation level for loading of a $^{40}{\text{K}}$ magneto-optical trap (MOT), resulting in increased atom numbers or decreased cycle times. While the attainable $^{40}{\text{K}}$ atom number is approximately doubled if exclusively $^{40}{\text{K}}$ atoms are cooled, the scheme is particularly powerful for dual-species MOTs, for example, if $^{40}{\text{K}}$ and $^{87}{\text{Rb}}$ atoms are cooled simultaneously in the same MOT configuration. The typical atom losses due to light-assisted hetero-nuclear collisions between $^{40}{\text{K}}$ and $^{87}{\text{Rb}}$ seem to be reduced giving rise to a threefold improvement of the $^{40}{\text{K}}$ atom number as compared to that in a conventional dual-species MOT, operating merely with $\mathcal{D}2$ light. Our scheme can be a useful extension to most dual-species experiments, aiming to reach simultaneous degeneracy of both species.

cond-mat.quant-gas↗

arXiv2023

Condensate formation in a dark state of a driven atom-cavity system

We demonstrate condensate formation in a dark state in an ultracold quantum gas coupled to a high-finesse cavity and pumped by a shaken optical lattice. We show experimentally and theoretically that the atoms in the dark state display a strong suppression of the coupling to the cavity. On the theory side, this is supported by solving the dynamics of a minimal three-level model and of the full atom-cavity system. The symmetry of the condensate wave function is anti-symmetric with respect to the potential minima of the pump lattice, and displays a staggered sign along the cavity direction. This symmetry decouples the dark state from the cavity, and is preserved when the pump intensity is switched off.

cond-mat.quant-gas↗

arXiv2023

Ultracold Feshbach molecules in an orbital optical lattice

Quantum gas systems provide a unique experimental platform to study a fundamental paradigm of quantum many-body physics: the crossover between Bose-Einstein condensed (BEC) molecular pairs and Bardeen Cooper Schrieffer (BCS) superfluidity. Some studies have considered quantum gas samples confined in optical lattices, however, focusing on the case, when only the lowest Bloch band is populated, such that orbital degrees of freedom are excluded. In this work, for the first time, ultracold Feshbach molecules of fermionic $^{40}K$ atoms are selectively prepared in the second Bloch band of an optical square lattice, covering a wide range of interaction strengths including the regime of unitarity. Binding energies and band relaxation dynamics are measured by means of a method resembling mass spectrometry. The longest lifetimes arise for strongly interacting Feshbach molecules at the onset of unitarity with values around 300 ms for the lowest band and 100 ms for the second band. In the case of strong confinement in a deep lattice potential, we observe bound dimers also for negative values of the s-wave scattering length, extending previous findings for molecules in the lowest band. Our work prepares the stage for orbital BEC-BCS crossover physics.

cond-mat.quant-gas↗

arXiv2022

Bosons condensed in two modes with flavour-changing interaction

A quantum model is considered for $N$ bosons populating two orthogonal single-particle modes with tunable energy separation in the presence of flavour-changing contact interaction. The quantum ground state is well approximated as a coherent superposition (for zero temperature) or a mixture (at low temperature) of two quasi-classical states. In a mean field description, the systems realizes one of these states via spontaneous symmetry breaking. Both mean field states, in a certain parameter range, possess finite angular momentum and exhibit broken time-reversal symmetry in contrast to the quantum ground state. The phase diagram is explored at the mean-field level and by direct diagonalisation. The nature of the quantum ground state at zero and finite temperature is analyzed by means of the Penrose Onsager criterion. One of three possible phases shows fragmentation on the single-particle level together with a finite pair order parameter. Thermal and quantum fluctuations are characterized with respect to regions of universal scaling behavior. The non-equilibrium dynamics shows a sharp transition between a self-trapping and a pair-tunneling regime. A recently realized experimental implementation is discussed with bosonic atoms condensed in the two inequivalent energy minima $X_{\pm}$ of the second band of a bipartite two-dimensional optical lattice.

cond-mat.quant-gas↗

arXiv2022

Observation of a continuous time crystal

Time crystals are classified as discrete or continuous depending on whether they spontaneously break discrete or continuous time translation symmetry. While discrete time crystals have been extensively studied in periodically driven systems since their recent discovery, the experimental realization of a continuous time crystal is still pending. Here, we report the observation of a limit cycle phase in a continuously pumped dissipative atom-cavity system, which is characterized by emergent oscillations in the intracavity photon number. We observe that the phase of this oscillation is random for different realizations, and hence this dynamical many-body state breaks continuous time translation symmetry spontaneously. The observed robustness of the limit cycles against temporal perturbations confirms the realization of a continuous time crystal.

cond-mat.quant-gas↗

arXiv2022

Experimental realization of a high precision tunable hexagonal optical lattice

Hexagonal optical lattices offer a tunable platform to study exotic orbital physics in solid state materials. Here, we present a versatile high-precision scheme to implement a hexagonal optical lattice potential, which is engineered by overlapping two independent triangular optical sublattices generated by laser beams with slightly different wavelengths around 1064 nm. This enables us to precisely control the detailed structure of the hexagonal lattice by adjusting the relative position and the relative lattice depth of the two triangular optical sublattices. Taking advantage of the sensitive dependence of the second Bloch band on small lattice deformations, we propose a strategy to optimize the optical lattice geometry with an extremely high precision. This method can also be extended to other lattice configurations involving more than two sublattices. Our work provides the experimental requirements in the search for novel orbital physics of ultracold atoms, for example, in the flat $p$-band of the hexagonal optical lattice.

cond-mat.quant-gas↗

arXiv2022

Route towards classical frustration and band flattening via optical lattice distortion

We propose and experimentally explore a method for realizing frustrated lattice models using a Bose-Einstein condensate held in an optical square lattice. A small lattice distortion opens up an energy gap such the lowest band splits into two. Along the edge of the first Brillouin zone for both bands a nearly flat energy-momentum dispersion is realized. For the excited band a highly degenerate energy minimum arises. By loading ultracold atoms into the excited band, a classically frustrated $XY$ model is formed, describing rotors on a square lattice with competing nearest and next nearest tunnelling couplings. Our experimental optical lattice provides a regime, where a fully coherent Bose-Einstein condensate is observed, and a regime where frustration is expected. If we adiabatically tune from the condensate regime to the regime of frustration, the momentum spectra shows a complete loss of coherence. Upon slowly tuning back to the condensate regime, coherence is largely restored. Good agreement with model calculations is obtained.

cond-mat.quant-gas↗

arXiv2021

Parametrically driven-dissipative three-level Dicke model

We investigate the three-level Dicke model, which describes a fundamental class of light-matter systems. We determine the phase diagram in the presence of dissipation, which we assume to derive from photon loss. Utilizing both analytical and numerical methods we characterize the incommensurate time crystalline, light-induced, and light-enhanced superradiant states in the phase diagram for the parametrically driven system. As a primary application, we demonstrate that a shaken atom-cavity system is naturally approximated via a parametrically driven-dissipative three-level Dicke model.

cond-mat.quant-gas↗