arXiv subjects

Johannes Schachenmayer

Publications and source records attributed to Johannes Schachenmayer.

At least 37 records · Page 2Linked to original sources

arXiv2020

Adiabatic preparation of entangled, magnetically ordered states with cold bosons in optical lattices

We analyze a scheme for preparation of magnetically ordered states of two-component bosonic atoms in optical lattices. We compute the dynamics during adiabatic and optimized time-dependent ramps to produce ground states of effective spin Hamiltonians, and determine the robustness to decoherence for realistic experimental system sizes and timescales. Ramping parameters near a phase transition point in both effective spin-1/2 and spin-1 models produces entangled spin-symmetric states that have potential future applications in quantum enhanced measurement. The preparation of these states and their robustness to decoherence is quantified by computing the Quantum Fisher Information of final states. We identify that the generation of useful entanglement should in general be more robust to heating than it would be implied by the state fidelity, with corresponding implications for practical applications.

cond-mat.quant-gas↗

arXiv2020

Dynamics of rotated spin states and magnetic ordering with two-component bosonic atoms in optical lattices

The microscopic control available over cold atoms in optical lattices has opened new opportunities to study the properties of quantum spin models. While a lot of attention is focussed on experimentally realizing ground or thermal states via adiabatic loading, it would often be more straightforward to prepare specific simple product states and to probe the properties of interacting spins by observing their dynamics. We explore this possibility for spin-1/2 and spin-1 models that can be realized with bosons in optical lattices, and which exhibit \textit{XY}-ferromagnetic (or counterflow spin superfluid) phases. We consider the dynamics of initial spin-rotated states corresponding to a mean-field version of the phases of interest. Using matrix product state methods in one dimension, we compute both non-equilibrium dynamics and ground/thermal states for these systems. We compare and contrast their behaviour in terms of correlation functions and induced spin currents, which should be directly observable with current experimental techniques. We find that although spin correlations decay substantially at large distances and on long timescales, for induction of spin currents, the rotated states behave similarly to the ground states on experimentally observable timescales.

cond-mat.quant-gas↗

arXiv2020

Ensemble-induced strong light-matter coupling of a single quantum emitter

We discuss a technique to strongly couple a single target quantum emitter to a cavity mode, which is enabled by virtual excitations of a nearby mesoscopic ensemble of emitters. A collective coupling of the latter to both the cavity and the target emitter induces strong photon non-linearities in addition to polariton formation, in contrast to common schemes for ensemble strong coupling. We demonstrate that strong coupling at the level of a single emitter can be engineered via coherent and dissipative dipolar interactions with the ensemble, and provide realistic parameters for a possible implementation with SiV$^{-}$ defects in diamond. Our scheme can find applications, amongst others, in quantum information processing or in the field of cavity-assisted quantum chemistry.

quant-ph↗

arXiv2019

Adiabatic elimination for ensembles of emitters in cavities with dissipative couplings

We consider an ensemble of cavity-coupled two-level emitters interacting via full (coherent and dissipative) dipole-dipole interactions. Using an adiabatic elimination procedure we derive effective equations of motion for a subsystem consisting of the cavity and a single emitter. Those equations can be used for schemes to enhance the cavity-coupling of single emitters as shown in [S. Schütz, J. Schachenmayer, D. Hagenmüller, G. K. Brennen, T. Volz, V. Sandoghdar, T. W. Ebbesen, C. Genes, and G. Pupillo, e-print arXiv:1904.08888]. We analyze limitations of effective subsystem parameters, and study how joint dissipative decay processes in the subsystem affect cavity-coupling properties of the single emitter and cavity transmission spectra.

quant-ph↗

arXiv2019

Exploring Superconductivity under Strong Coupling with the Vacuum Electromagnetic Field

Light-matter interactions have generated considerable interest as a means to manipulate material properties. Light-induced superconductivity has been demonstrated using pulsed lasers. An attractive alternative possibility is to exploit strong light-matter interactions arising by coupling phonons to the vacuum electromagnetic field of a cavity mode as has been suggested and theoretically studied. Here we explore this possibility for two very different superconductors, namely YBCO (YBa$_2$Cu$_3$O$_{6+x}$) and Rb$_3$C$_{60}$, coupled to surface plasmon polaritons, using a novel cooperative effect based on the presence of a strongly coupled vibrational environment allowing efficient dressing of the otherwise weakly coupled phonon bands of these compounds. By placing the superconductor-surface plasmon system in a SQUID magnetometer, we find that the superconducting transition temperatures ($T_{c}$) for both compounds are modified in the absence of any external laser field. For YBCO, $T_{c}$ decreases from 92 K to 86 K while for Rb$_3$C$_{60}$, it increases from 30 K to 45 K at normal pressures. In the latter case, a simple theoretical framework is provided to understand these results based on an enhancement of the electron-phonon coupling. This proof-of-principle study opens a new tool box to not only modify superconducting materials but also to understand the mechanistic details of different superconductors.

cond-mat.supr-con↗

arXiv2019

Doublon dynamics of Bose-Fermi mixtures in optical lattices

We study the out-of-equilibrium dynamics of a dilute, lattice-confined Bose-Fermi mixture initialized in a highly excited state consisting of boson-fermion pairs (doublons) occupying single lattice sites. This system represents a paradigmatic case for studying relaxation dynamics in strongly correlated systems, and provides a versatile platform for studying thermalization and localization phenomena. We provide analytical expressions for the short-time decay of isolated doublons and small doublon clusters due to the competition between tunneling and interparticle interactions. We also discuss a mechanism for long-time decay that crucially depends on the quantum statistics of the particles constituting the doublon, namely, the conversion of pairs of neighboring doublons into an unpaired fermion and a site with a fermion and two bosons. Building on these insights, we develop a cluster expansion method to describe the dynamics in extended systems and compare it to numerically exact matrix product state simulations in one dimension. Finally, we discuss how our predictions can be observed in experiments with ultracold heteronuclear molecules.

cond-mat.quant-gas↗

arXiv2019

A generalized phase space approach for solving quantum spin dynamics

Numerical techniques to efficiently model out-of-equilibrium dynamics in interacting quantum many-body systems are key for advancing our capability to harness and understand complex quantum matter. Here we propose a new numerical approach which we refer to as GDTWA. It is based on a discrete semi-classical phase-space sampling and allows to investigate quantum dynamics in lattice spin systems with arbitrary $S\geq 1/2$. We show that the GDTWA can accurately simulate dynamics of large ensembles in arbitrary dimensions. We apply it for $S>1/2$ spin-models with dipolar long-range interactions, a scenario arising in recent experiments with magnetic atoms. We show that the method can capture beyond mean-field effects, not only at short times, but it also correctly reproduces long time quantum-thermalization dynamics. We benchmark the method with exact diagonalization in small systems, with perturbation theory for short times, and with analytical predictions made for closed system which feature quantum-thermalization at long times. By computing the Renyi entropy, currently an experimentally accessible quantifier of entanglement, we reveal that large $S$ systems can feature larger entanglement than corresponding $S=1/2$ systems. Our analyses demonstrate that the GDTWA can be a powerful tool for modeling complex spin dynamics in regimes where other state-of-the art numerical methods fail.

cond-mat.quant-gas↗

arXiv2019

Enhancement of the electron-phonon scattering induced by intrinsic surface plasmon-phonon polaritons

We investigate light-matter coupling in metallic crystals where plasmons coexist with phonons exhibiting large oscillator strength. We demonstrate theoretically that this coexistence can lead to strong light-matter interactions without external resonators. When the frequencies of plasmons and phonons are comparable, hybridization of these collective matter modes occurs in the crystal. We show that the coupling of these modes to photonic degrees of freedom gives rise to intrinsic surface plasmon-phonon polaritons, which offer the unique possibility to control the phonon properties by tuning the electron density and the crystal thickness. In particular, dressed phonons with reduced frequency and large wave vectors arise in the case of quasi-2D crystals, which leads to large enhancements of the electron-phonon scattering in the vibrational ultrastrong coupling regime. This suggests that photons can play a key role in determining the quantum properties of certain materials. A non-perturbative self-consistent Hamiltonian method is presented that is valid for arbitrarily large coupling strengths.

cond-mat.mes-hall↗

arXiv2018

Cavity-assisted mesoscopic transport of fermions: Coherent and dissipative dynamics

We study the interplay between charge transport and light-matter interactions in a confined geometry, by considering an open, mesoscopic chain of two-orbital systems resonantly coupled to a single bosonic mode close to its vacuum state. We introduce and benchmark different methods based on self-consistent solutions of non-equilibrium Green's functions and numerical simulations of the quantum master equation, and derive both analytical and numerical results. It is shown that in the dissipative regime where the cavity photon decay rate is the largest parameter, the light-matter coupling is responsible for a steady-state current enhancement scaling with the cooperativity parameter. We further identify different regimes of interest depending on the ratio between the cavity decay rate and the electronic bandwidth. Considering the situation where the lower band has a vanishing bandwidth, we show that for a high-finesse cavity, the properties of the resonant Bloch state in the upper band are transfered to the lower one, giving rise to a delocalized state along the chain. Conversely, in the dissipative regime with low cavity quality factors, we find that the current enhancement is due to a collective decay of populations from the upper to the lower band.

quant-ph↗

arXiv2017

Cavity-enhanced transport of charge

We theoretically investigate charge transport through electronic bands of a mesoscopic one-dimensional system, where inter-band transitions are coupled to a confined cavity mode, initially prepared close to its vacuum. This coupling leads to light-matter hybridization where the dressed fermionic bands interact via absorption and emission of dressed cavity-photons. Using a self-consistent non-equilibrium Green's function method, we compute electronic transmissions and cavity photon spectra and demonstrate how light-matter coupling can lead to an enhancement of charge conductivity in the steady-state. We find that depending on cavity loss rate, electronic bandwidth, and coupling strength, the dynamics involves either an individual or a collective response of Bloch states, and explain how this affects the current enhancement. We show that the charge conductivity enhancement can reach orders of magnitudes under experimentally relevant conditions.

quant-ph↗

arXiv2016

Laser noise imposed limitations of ensemble quantum metrology

Laser noise is a decisive limiting factor in high precision spectroscopy of narrow lines using atomic ensembles. In an idealized Doppler and differential light shift free magic wavelength lattice configuration, it remains as one distinct principal limitation beyond collective atomic decay. In this work we study the limitations originating from laser phase and amplitude noise in an idealized Ramsey pulse interrogation scheme with uncorrelated atoms. Phase noise leads to a saturation of the frequency sensitivity with increasing atom number while amplitude noise implies a scaling $1/\sqrtτ$ with $τ$ being the interrogation time. We employ a technique using decoherence free subspaces first introduced in New J. Phys. \textbf{14}, 043011 (2012) which can restore the scaling with the square root of the inverse particle number $1/\sqrt{N}$. Similar results and improvements are obtained numerically for a Rabi spectroscopy setup.

quant-ph↗

arXiv2016

Entanglement growth and correlation spreading with variable-range interactions in spin and fermionic tunnelling models

We investigate the dynamics following a global parameter quench for two 1D models with variable-range power-law interactions: a long-range transverse Ising model, which has recently been realised in chains of trapped ions, and a long-range lattice model for spinless fermions with long-range tunnelling. For the transverse Ising model, the spreading of correlations and growth of entanglement are computed using numerical matrix product state techniques, and are compared with exact solutions for the fermionic tunnelling model. We identify transitions between regimes with and without an apparent linear light cone for correlations, which correspond closely between the two models. For long-range interactions (in terms of separation distance r, decaying slower than 1/r), we find that despite the lack of a light-cone, correlations grow slowly as a power law at short times, and that -- depending on the structure of the initial state -- the growth of entanglement can also be sublinear. These results are understood through analytical calculations, and should be measurable in experiments with trapped ions.

cond-mat.quant-gas↗

arXiv2016

Collective atomic scattering and motional effects in a dense coherent medium

We investigate collective emission from coherently driven ultracold $ ^{88} $ Sr atoms. We perform two sets of experiments, using a strong and weak transition that are insensitive and sensitive, respectively, to atomic motion at one microKelvin. We observe highly directional forward emission with a peak intensity that is enhanced, for the strong transition, by > $ 10 ^3 $ compared to that in the transverse direction. This is accompanied by substantial broadening of spectral lines. For the weak transition, the forward enhancement is substantially reduced due to motion. Meanwhile, a density-dependent frequency shift of the weak transition (~10% of the natural linewidth) is observed. In contrast, this shift is suppressed to <1% of the natural linewidth for the strong transition. Along the transverse direction, we observe strong polarization dependences of the fluorescence intensity and line broadening for both transitions. The measurements are reproduced with a theoretical model treating the atoms as coherent, interacting, radiating dipoles.

physics.atom-ph↗

arXiv2015

Doublon dynamics and polar molecule production in an optical lattice

Ultracold polar molecules provide an excellent platform to study quantum many-body spin dynamics, which has become accessible in the recently realized low entropy quantum gas of polar molecules in an optical lattice. To obtain a detailed understanding for the molecular formation process in the lattice, we prepare a density distribution where lattice sites are either empty or occupied by a doublon composed of a bosonic atom interacting with a fermionic atom. By letting this disordered, out-of-equilibrium system evolve from a well-defined initial condition, we observe clear effects on pairing that arise from inter-species interactions, a higher partial wave Feshbach resonance, and excited Bloch-band population. When only the lighter fermions are allowed to tunnel in the three-dimensional (3D) lattice, the system dynamics can be well described by theory. However, in a regime where both fermions and bosons can tunnel, we encounter correlated dynamics that is beyond the current capability of numerical simulations. Furthermore, we show that we can probe the microscopic distribution of the atomic gases in the lattice by measuring the inelastic loss of doublons. These techniques realize tools that are generically applicable to heteronuclear diatomic systems in optical lattices and can shed light on molecule production as well as dynamics of a Bose-Fermi mixture.

cond-mat.quant-gas↗

arXiv2015

Synchronization of Interacting Quantum Dipoles

Macroscopic ensembles of radiating dipoles are ubiquitous in the physical and natural sciences. In the classical limit the dipoles can be described as damped-driven oscillators, which are able to spontaneously synchronize and collectively lock their phases. Here we investigate the correspond- ing phenomenon in the quantum regime with arrays of quantized two-level systems coupled via long-range and anisotropic dipolar interactions. Our calculations demonstrate that the dipoles may overcome the decoherence induced by quantum fluctuations and inhomogeneous couplings and evolve to a synchronized steady-state. This steady-state bears much similarity to that observed in classical systems, and yet also exhibits genuine quantum properties such as quantum correlations and quan- tum phase diffusion (reminiscent of lasing). Our predictions could be relevant for the development of better atomic clocks and a variety of noise tolerant quantum devices.

quant-ph↗

arXiv2015

Dynamics of correlations in two-dimensional quantum spin models with long-range interactions: A phase-space Monte-Carlo study

Interacting quantum spin models are remarkably useful for describing different types of physical, chemical, and biological systems. Significant understanding of their equilibrium properties has been achieved to date, especially for the case of spin models with short-range couplings. However, progress towards the development of a comparable understanding in long-range interacting models, in particular out-of-equilibrium, remains limited. In a recent work, we proposed a semiclassical numerical method to study spin models, the discrete truncated Wigner approximation (DTWA), and demonstrated its capability to correctly capture the dynamics of one- and two-point correlations in one dimensional (1D) systems. Here we go one step forward and use the DTWA method to study the dynamics of correlations in 2D systems with many spins and different types of long-range couplings, in regimes where other numerical methods are generally unreliable. We compute spatial and time-dependent correlations for spin-couplings that decay with distance as a power-law and determine the velocity at which correlations propagate through the system. Sharp changes in the behavior of those velocities are found as a function of the power-law decay exponent. Our predictions are relevant for a broad range of systems including solid state materials, atom-photon systems and ultracold gases of polar molecules, trapped ions, Rydberg, and magnetic atoms. We validate the DTWA predictions for small 2D systems and 1D systems, but ultimately, in the spirt of quantum simulation, experiments will be needed to confirm our predictions for large 2D systems.

quant-ph↗

arXiv2015

Cavity enhanced transport of excitons

We show that exciton-type transport in certain materials can be dramatically modified by their inclusion in an optical cavity: the modification of the electromagnetic vacuum mode structure introduced by the cavity leads to transport via delocalized polariton modes rather than through tunneling processes in the material itself. This can help overcome exponential suppression of transmission properties as a function of the system size in the case of disorder and other imperfections. We exemplify massive improvement of transmission for excitonic wave-packets through a cavity, as well as enhancement of steady-state exciton currents under incoherent pumping. These results may have implications for experiments of exciton transport in disordered organic materials. We propose that the basic phenomena can be observed in quantum simulators made of Rydberg atoms, cold molecules in optical lattices, as well as in experiments with trapped ions.

quant-ph↗

arXiv2015

Adiabatic cooling of bosons in lattices to magnetic ordering

We suggest and analyze a new scheme to adiabatically cool bosonic atoms to picokelvin temperatures which should allow the observation of magnetic ordering via superexchange in optical lattices. The starting point is a gapped phase called the spin Mott phase where each site is occupied by one spin-up and one spin-down atom. An adiabatic ramp leads to an xy-ferromagnetic phase. We show that the combination of time-dependent density matrix renormalization group methods with quantum trajectories can be used to fully address possible experimental limitations due to decoherence, and demonstrate that the magnetic correlations are robust for experimentally realizable ramp speeds. Using a microscopic master equation treatment of light scattering in the many-particle system, we test the robustness of adiabatic state preparation against decoherence. Due to different ground-state symmetries, we also find a metastable state with xy-ferromagnetic order if the ramp crosses to regimes where the ground state is a z-ferromagnet. The bosonic spin Mott phase as the initial gapped state for adiabatic cooling has many features in common with a fermionic band insulator, but the use of bosons should enable experiments with substantially lower initial entropies.

cond-mat.quant-gas↗