arXiv subjects

Mikkel F. Andersen

Publications and source records attributed to Mikkel F. Andersen.

12 recordsLinked to original sources

arXiv2026

Spin-entanglement of an atomic pair through coupling to their thermal motion

The spin-dynamics of two alkali atoms in an optical tweezer is driven by spin-changing collisions that couple the spin-state of the atoms to their relative motion. This paper experimentally studies the resulting spin-states when the relative motion is in a thermal state with k B T much larger than the energies of the spin-states that take part in the dynamics. We find that an initially unentangled spin-state can evolve into an entangled state. This is contrary to the common case when coupling a quantum system to hot degrees of freedom leads to loss of entanglement and not its generation. Moreover, we show that the generated entanglement is technologically useful as it, in principle, can enhance the sensitivity of measurements beyond the standard quantum limit. This may provide a promising avenue for robust entanglement generation for future technologies.

quant-ph↗

arXiv2026

Simple Magneto-Optical and Magnetic Traps for Dysprosium

Dysprosium (Dy) is the most magnetic element on the periodic table, making it excellent for studying dipolar atom-atom interactions. We report on a simple Dy MOT that captures atoms directly from the thermal beam using a single diode-laser system to generate the light. Additionally, the atoms are magnetically confined by the quadrupole magnetic field that also facilitates the MOT. The MOT loading time is $τ_\text{b} = 26~\text{ms}$. Atoms can decay to a dark state that is magnetically trapped. The time constant for loading into this magnetic trap is $τ_\text{d} = 410~\text{ms}$. The total magnetically trapped population is $1.14\times10^{5}$ atoms, with $85\%$ residing in the dark states. The magnetically trapped atoms have a temperature of $28~μ\text{K}$, significantly below the Doppler limit. This population fulfills the requirements for a range of future experiments.

physics.atom-ph↗

arXiv2026

Counterdiabatic driving for random-gap Landau-Zener transitions

The Landau--Zener (LZ) model describes a two-level quantum system that undergoes an avoided crossing. In the adiabatic limit, the transition probability vanishes. An auxiliary control field $H_\text{CD}$ can be reverse-engineered so that the full Hamiltonian $H_0 + H_\text{CD}$ reproduces adiabaticity for all parameter values. Our aim is to construct a single control field $H_1$ that drives an ensemble of LZ-type Hamiltonians with a distribution of energy gaps. $H_1$ works best statistically, minimizing the average transition probability. We restrict our attention to a special class of $H_1$ controls, motivated by $H_\text{CD}$. We found a systematic trade-off between instantaneous adiabaticity and the final transition probability. Certain limiting cases with a linear sweep can be treated analytically; one of them being the LZ system with Dirac $δ(t)$ function. Comprehensive and systematic numerical simulations support and extend the analytic results.

quant-ph↗

arXiv2022

Classical model for survival resonances close to Talbot time

We present a classical approximation for the peaks of survival resonances occurring when diffracting matter waves from absorption potentials. Generally our simplified model describes the absorption-diffraction process around the Talbot time very well. Classical treatments of this process are presently lacking. For purely imaginary potentials, the classical model duplicates quantum mechanical calculations. The classical model allows for simple evolution of phase-space probability densities, which in the limit of the effective Planck's constant going to zero allows for a compact analytical expression of the survival probability as a function of remaining parameters. Our work extends the range of processes that can be described through classical analogues.

quant-ph↗

arXiv2019

Thermally-robust spin correlations between two 85Rb atoms in an optical microtrap

The complex collisional properties of atoms fundamentally limit investigations into a range of processes in many-atom ensembles. In contrast, the bottom-up assembly of few- and many-body systems from individual atoms offers a controlled approach to isolating and studying such collisional processes. Here, we use optical tweezers to individually assemble pairs of trapped $^{85}$Rb atoms, and study the spin dynamics of the two-body system in a thermal state. The spin-2 atoms show strong pair correlation between magnetic sublevels on timescales exceeding one second, with measured relative number fluctuations $11.9\pm0.3$ dB below quantum shot noise, limited only by detection efficiency. Spin populations display relaxation dynamics consistent with simulations and theoretical predictions for $^{85}$Rb spin interactions, and contrary to the coherent spin waves witnessed in finite-temperature many-body experiments and zero-temperature two-body experiments. Our experimental approach offers a versatile platform for studying two-body quantum dynamics and may provide a route to thermally-robust entanglement generation.

quant-ph↗

arXiv2019

Atom interferometry using $δ$-kicked and finite duration pulse-sequences

We investigate an atom interferometer in which large momentum differences between the arms are obtained by using quantum resonances in the atom optics $δ$-kicked rotor. The interferometer can potentially measure the Talbot time (from which $h/m$ can be deduced), the local gravitational field, or can serve as a narrow velocity filter. We present an analytical analysis in the short pulse limit, and a numerical investigation for finite pulse durations. The sensitivity of the interferometer is improved by a moderate violation of the short pulse limit. Remarkably simple relations predict the optimal pulse duration, and the sensitivity of the interferometer.

quant-ph↗

arXiv2017

Interaction blockade for bosons in an asymmetric double well

The interaction blockade phenomenon isolates the motion of a single quantum particle within a multi-particle system, in particular for coherent oscillations in and out of a region affected by the blockade mechanism. For identical quantum particles with Bose statistics, the presence of the other particles is still felt by a bosonic stimulation factor $\sqrt{N}$ that speeds up the coherent oscillations, where $N$ is the number of bosons. Here we propose an experiment to observe this enhancement factor with a small number of bosonic atoms. The proposed protocol realises an asymmetric double well potential with multiple optical tweezer laser beams. The ability to adjust bias independently of the coherent coupling between the wells allows the potential to be loaded with different particle numbers while maintaining the resonance condition needed for coherent oscillations. Numerical simulations with up to three bosons in a realistic potential generated by three optical tweezers predict that the relevant avoided level crossing can be probed and the expected bosonic enhancement factor observed.

cond-mat.quant-gas↗

arXiv2017

Zeeman-insensitive cooling of a single atom to its two-dimensional motional ground state in tightly focused optical tweezers

We combine near--deterministic preparation of a single atom with Raman sideband cooling, to create a push button mechanism to prepare a single atom in the motional ground state of tightly focused optical tweezers. In the 2D radial plane, we achieve a large ground state fidelity for the entire procedure (loading and cooling) of $\sim$0.73, while the ground state occupancy is $\sim$0.88 for realizations with a single atom present. For 1D axial cooling, we attain a ground state fraction of $\sim$0.52. The combined 3D cooling provides a ground state population of $\sim$0.11. Our Raman sideband cooling variation is indifferent to magnetic field fluctuations, allowing wide--spread unshielded experimental implementations. Our work provides a pathway towards a range of coherent few body experiments.

physics.atom-ph↗

arXiv2017

Resonant transfer of large momenta from finite duration pulse sequences

We experimentally investigate the atom optics kicked particle at quantum resonance using finite duration kicks. Even though the underlying process is quantum interference it can be well described by an $ε$-pseudoclassical model. The $ε$-pseudoclassical model agrees well with our experiments for a wide range of parameters. We investigate the parameters yielding maximal momentum transfer to the atoms and find that this occurs in the regime where neither the short pulse approximation nor the Bragg condition is valid. Nonetheless, the momentum transferred to the atoms can be predicted using a simple scaling law, which provides a powerful tool for choosing optimal experimental parameters. We demonstrate this in a measurement of the Talbot time (from which $h/M$ can be deduced), in which we coherently split atomic wave-functions into superpositions of momentum states that differ by 200 photon recoils. Our work may provide a convenient way to implement large momentum difference beam splitters in atom interferometers.

quant-ph↗

arXiv2016

An $\mathbfε$-pseudoclassical model for quantum resonances in a cold dilute atomic gas periodically driven by finite-duration standing-wave laser pulses

Atom interferometers are a useful tool for precision measurements of fundamental physical phenomena, ranging from local gravitational field strength to the atomic fine structure constant. In such experiments, it is desirable to implement a high momentum transfer "beam-splitter," which may be achieved by inducing quantum resonance in a finite-temperature laser-driven atomic gas. We use Monte Carlo simulations to investigate these quantum resonances in the regime where the gas receives laser pulses of finite duration, and demonstrate that an $ε$-classical model for the dynamics of the gas atoms is capable of reproducing quantum resonant behavior for both zero-temperature and finite-temperature non-interacting gases. We show that this model agrees well with the fully quantum treatment of the system over a time-scale set by the choice of experimental parameters. We also show that this model is capable of correctly treating the time-reversal mechanism necessary for implementing an interferometer with this physical configuration.

quant-ph↗

arXiv2011

Counting atoms in a deep optical microtrap

We demonstrate a method to count small numbers of atoms held in a deep, microscopic optical dipole trap by collecting fluorescence from atoms exposed to a standing wave of light that is blue detuned from resonance. While scattering photons, the atoms are also cooled by a Sisyphus mechanism that results from the spatial variation in light intensity. The use of a small blue detuning limits the losses due to light assisted collisions, thereby making the method suitable for counting several atoms in a microscopic volume.

physics.optics↗

arXiv2008

Lattice Interferometer for Ultra-Cold Atoms

We demonstrate an atomic interferometer based on ultra-cold atoms released from an optical lattice. This technique yields a large improvement in signal to noise over a related interferometer previously demonstrated. The interferometer involves diffraction of the atoms using a pulsed optical lattice. For short pulses a simple analytical theory predicts the expected signal. We investigate the interferometer for both short pulses and longer pulses where the analytical theory break down. Longer pulses can improve the precision and signal size. For specific pulse lengths we observe a coherent signal at times that differs greatly from what is expected from the short pulse model. The interferometric signal also reveals information about the dynamics of the atoms in the lattice. We investigate the application of the interferometer for a measurement of $h/m_A$ that together with other well known constants constitutes a measurement of the fine structure constant.

physics.atom-ph↗