arXiv subjects

Michael Köhl

Publications and source records attributed to Michael Köhl.

At least 19 recordsLinked to original sources

arXiv2026

Two-phase driving of a linear radio-frequency ion trap

A linear radio-frequency Paul trap is traditionally driven with one diagonal pair of electrodes grounded and the other connected to a high-voltage radio-frequency source. This method simplifies impedance matching of the voltage source to the trap. However, for several architectures it leads to increasing the axial micromotion amplitude, for example, when the capacitance between radio-frequency and end-cap electrodes is not negligible. Here, we present a technique to generate two high-voltage radio-frequency signals \SI{180}{\degree} out of phase to drive a linear Paul trap with opposite voltages between neighbouring electrodes. Using this, we have successfully trapped and cooled a chain of Ytterbium ions in a linear radio-frequency Paul trap.

quant-ph↗

arXiv2024

Floquet-driven crossover from density-assisted tunneling to enhanced pair tunneling

We investigate the experimental control of pair tunneling in a double-well potential using Floquet engineering. We demonstrate a crossover from a regime with density-assisted tunneling to dominant pair tunneling by tuning the effective interactions. Furthermore, we show that the pair tunneling rate can be enhanced not only compared to the Floquet-reduced single-particle tunneling but even beyond the static superexchange rate, while keeping the effective interaction in a relevant range. This opens possibilities to realize models with explicit pair tunneling in ultracold atomic systems.

cond-mat.quant-gas↗

arXiv2024

Cooling a strongly-interacting quantum gas by interaction modulation

We present a cooling method for a strongly-interacting trapped quantum gas. By applying a magnetic field modulation with frequencies close to the binding energy of a molecular bound state we selectively remove dimers with high kinetic energy from the sample. We demonstrate cooling of the sample over a wide range of interaction strengths and measure a high cooling efficiency of $γ=4$ that exceeds all previous cooling near Feshbach resonances.

cond-mat.quant-gas↗

arXiv2024

Exciting the Higgs mode in a strongly-interacting Fermi gas by interaction modulation

We study the Higgs mode of a strongly-interacting Fermi gas in the crossover regime between a fermionic and bosonic superfluid. By periodically modulating the interaction strength of the gas, we parametrically excite the Higgs mode and study its resonance frequency and line width as a function of both interaction strength and temperature. We find that the resonance frequency at low temperature agrees with a local-density approximation of the pairing gap. Both frequency and line width do not exhibit a pronounced variation with temperature, which is theoretically unexpected, however, in qualitative agreement with a different recent study.

cond-mat.quant-gas↗

arXiv2023

Coupled high-finesse optical Fabry-Perot microcavities

Optical fiber Fabry-Perot cavities have been a development facilitating the efficient integration of high-finesse cavities into fiber-optic assemblies. In this work, we demonstrate coupling of two high-finesse fiber cavities by direct photon tunneling between them. We detect the coupled mode spectra and demonstrated the variability of coupling strength and dissipation rates for different transverse modes. Moreover, we observe very narrow spectral features resulting from a dynamical generalizaton of electromagnetically-induced transparency showing that even dissipative systems without metastable states can feature long-lived coherences.

physics.optics↗

arXiv2023

Competing magnetic orders in a bilayer Hubbard model with ultracold atoms

Fermionic atoms in optical lattices have served as a compelling model system to study and emulate the physics of strongly-correlated matter. Driven by the advances of high-resolution microscopy, the recent focus of research has been on two-dimensional systems in which several quantum phases, such as anti-ferromagnetic Mott insulators for repulsive interactions and charge-density waves for attractive interactions have been observed. However, the aspired emulations of real materials, such as bilayer graphene, have to take into account that their lattice structure composes of coupled layers and therefore is not strictly two-dimensional. In this work, we realize a bilayer Fermi-Hubbard model using ultracold atoms in an optical lattice and demonstrate that the interlayer coupling controls a crossover between a planar anti-ferromagnetically ordered Mott insulator and a band insulator of spin-singlets along the bonds between the layers. Our work will enable the exploration of further fascinating properties of coupled-layer Hubbard models, such as theoretically predicted superconducting pairing mechanisms.

cond-mat.quant-gas↗

arXiv2022

Achievements and Perspectives of Optical Fiber Fabry-Perot Cavities

Fabry-Perot interferometers have stimulated numerous scientific and technical applications ranging from high resolution spectroscopy over metrology, optical filters to interfaces of light and matter at the quantum limit and more. End facet machining of optical fibers has enabled the miniaturization of optical Fabry-Perot cavities. Integration with fiber wave guide technology allows for small yet open devices with favorable scaling properties including mechanical stability and compact mode geometry. These Fiber Fabry-Perot Cavities (FFPCs) are stimulating extended applications in many fields including cavity quantum electrodynamics, optomechanics, sensing, nonlinear optics and more. Here we summarize the state of the art of devices based on Fiber Fabry-Perot Cavities, provide an overview of applications and conclude with expected further research activities.

physics.optics↗

arXiv2021

Secure distribution of a certified random quantum key using an entangled memory qubit

Random generation and confidential distribution of cryptographic keys are fundamental building blocks of secure communication. Using quantum states in which the transmitted quantum bit is entangled with a stationary memory quantum bit allows the secure generation and distribution of keys to be based on fundamental properties of quantum mechanics. At the same time, the reach of secure communication networks can be enhanced, in particular, since this architecture would be compatible with quantum repeaters which are an integral part for scaling quantum networks. Here, we realize a true single-photon quantum key distribution protocol (BBM92 protocol) at a second-order temporal correlation of $ {g^{(2)}(0)=0.00(5)}$ involving an entangled memory qubit which enables us to produce a certified random secret key on both endpoints of the quantum communication channel. We certify the randomness of the key using the min-entropy of the atom-photon state arising from the violation of the CHSH version of the Bell inequality of $ 2.33(6)$.

quant-ph↗

arXiv2021

Radio-frequency driving of an attractive Fermi gas in a one-dimensional optical lattice

We investigate the response to radio-frequency driving of an ultracold gas of attractively interacting fermions in a one-dimensional optical lattice. We study the system dynamics by monitoring the driving-induced population transfer to a third state, and the evolution of the momentum density and pair distributions. Depending on the frequency of the radio-frequency field, two different dynamical regimes emerge when considering the evolution of the third level population. One regime exhibits (off)resonant many-body oscillations reminiscent of Rabi oscillations in a discrete two-level system, while the other displays a strong linear rise. Within this second regime, we connect, via linear response theory, the extracted transfer rate to the system single-particle spectral function, and infer the nature of the excitations from Bethe ansatz calculations. In addition, we show that this radio-frequency technique can be employed to gain insights into this many-body system coupling mechanism away from equilibrium. This is done by monitoring the momentum density redistributions and the evolution of the pair correlations during the drive. Capturing such non-equilibrium physics goes beyond a linear response treatment, and is achieved here by conducting time-dependent matrix product state simulations.

cond-mat.quant-gas↗

arXiv2020

Decay and revival of a transient trapped Fermi condensate

We study experimentally and theoretically the response of a two-component Fermi condensate in the strongly-interacting regime to a quench of the interaction strength. The quench is realized using a radio-frequency $π$-pulse to a third internal level with a different interaction strength. We find that the quench excites the monopole mode of the trap in the hydrodynamic regime and that an initial change of the condensate properties takes place on a time scale comparable or even larger than the quasi-particle relaxation time.

cond-mat.quant-gas↗

arXiv2020

Exponentially improved detection and correction of errors in experimental systems using neural networks

We introduce the use of two machine learning algorithms to create an empirical model of an experimental apparatus, which is able to reduce the number of measurements necessary for generic optimisation tasks exponentially as compared to unbiased systematic optimisation. Principal Component Analysis (PCA) can be used to reduce the degrees of freedom in cases for which a rudimentary model describing the data exists. We further demonstrate the use of an Artificial Neural Network (ANN) for tasks where a model is not known. This makes the presented method applicable to a broad range of different optimisation tasks covering multiple fields of experimental physics. We demonstrate both algorithms at the example of detecting and compensating stray electric fields in an ion trap and achieve a successful compensation with an exponentially reduced amount of data.

quant-ph↗

arXiv2020

Deterministic spin-photon entanglement from a trapped ion in a fiber Fabry-Perot cavity

The development of efficient network nodes is a key element for the realisation of quantum networks which promise great capabilities as distributed quantum computing or provable secure communication. We report the realisation of a quantum network node using a trapped ion inside a fiber-based Fabry-Perot cavity. We show the generation of deterministic entanglement at a high fidelity of $ 91.2(2) $\,\% between a trapped Yb--ion and a photon emitted into the resonator mode. We achieve a success probability for generation and detection of entanglement for a single shot of $ 2.5 \cdot 10^{-3}$ resulting in 62\,Hz entanglement rate.

quant-ph↗

arXiv2019

Correlated photon-pair generation in a liquid-filled microcavity

We report on the realization of a liquid-filled optical microcavity and demonstrate photon-pair generation by spontaneous four-wave mixing. The bandwidth of the emitted photons is $\sim 300$ MHz and we demonstrate tuning of the emission wavelength between 770 and 800 nm. Moreover, by employing a liquid as the nonlinear optical medium completely filling the microcavity, we observe more than a factor $10^3$ increase of the pair correlation rate per unit pump power and a factor of 1.7 improvement in the coincidence/accidental ratio as compared to our previous measurements.

quant-ph↗

arXiv2019

Slow interaction quench in BCS superconductors: emergence of pre-formed pairs

We investigate the non-equilibrium behavior of BCS superconductors subjected to slow ramps of their internal interaction strength. We identify three dynamical regimes as a function of ramp duration. For short ramp times, these systems become non-superconducting; however, fermions with opposite momenta remain paired albeit with reduced amplitudes, and the associated pair amplitude distribution is non-thermal. In this first regime, the disappearance of superconductivity is due to the loss of phase coherence between pairs. By contrast, for intermediate ramp times, superconductivity survives but the magnitude of the order parameter is reduced and presents long-lived oscillations. Finally, for long ramp times, phase coherence is almost fully retained during the slow interaction quench, and the steady-state is characterized by a thermal-like pair amplitude distribution. Using this approach, one can therefore dynamically tune the coherence between pairs in order to control the magnitude of the superconducting order parameter and even engineer a non-equilibrium state made of pre-formed pairs.

cond-mat.supr-con↗

arXiv2019

Ultraviolet Fabry-Perot cavity with stable finesse under ultrahigh vacuum conditions

We have constructed an apparatus containing a linear ion trap and a high-finesse optical cavity in the ultraviolet spectral range. In our construction, we have avoided all organic materials inside the ultrahigh vacuum chamber. We show that, unlike previously reported, the optical cavity does not degrade in performance over a time scale of 9 months.

quant-ph↗

arXiv2018

Correlated photon-pair emission from a cw-pumped Fabry-Perot microcavity

We study a dispersion-compensated high-finesse optical Fabry-Perot microcavity under high-intensity cw pumping. The Kerr non-linearity in the optical coatings causes a spontaneous four-wave mixing process, which leads to the emission of time-correlated photon pairs. The photon frequencies are shifted by $\pm 1$ free spectral range relative to the pump frequency. This setup allows for constructing a photon-pair source with precisely adjustable frequency difference between the emitted photons, which may have applications in quantum communication.

physics.optics↗

arXiv2016

Monolayer graphene as dissipative membrane in an optical resonator

We experimentally demonstrate coupling of an atomically thin, free-standing graphene membrane to an optical cavity. By changing the position of the membrane along the standing-wave field of the cavity we tailor the dissipative coupling between the membrane and the cavity, and we show that the dissipative coupling can outweigh the dispersive coupling. Such a system, for which controlled dissipation prevails dispersion, will prove useful for novel laser-cooling schemes in optomechanics. In addition, we have determined the continuous-wave optical damage threshold of free-standing monolayer graphene of 1.8(4)~MW/cm$^2$ at 780nm.

physics.optics↗

arXiv2015

Equation of state of the two-dimensional Hubbard model

Understanding the phases of strongly correlated quantum matter is challenging because they arise from the subtle interplay between kinetic energy, interactions, and dimensionality. In this quest it has turned out that even conceptually simple models of strongly correlated fermions, which often only approximately represent the physics of the solid state, are very hard to solve. Since the conjecture by P. W. Anderson that the two-dimensional Hubbard model describes the main features of high-T$_c$ superconductivity in the cuprates, there has been a major, yet inconclusive, research effort on determining its fundamental thermodynamic properties. Here we present an experimental determination of the equation of state of the repulsive two-dimensional Hubbard model over a broad range of interactions, $0\leq U/t \lesssim 20$, and temperatures, down to $k_BT/t=0.63(2)$, using high-resolution imaging of ultracold atoms in optical lattices. The equation of state fully characterizes the thermodynamics of the Hubbard model, and our results constitute benchmarks for state-of-the-art theoretical approaches.

cond-mat.quant-gas↗