71 research outputs found

### Master equation approach for interacting slow- and stationary-light polaritons

A master equation approach for the description of dark-state polaritons in
coherently driven atomic media is presented. This technique provides a
description of light-matter interactions under conditions of
electromagnetically induced transparency (EIT) that is well suited for the
treatment of polariton losses. The master equation approach allows us to
describe general polariton-polariton interactions that may be conservative,
dissipative or a mixture of both. In particular, it enables us to study
dissipation-induced correlations as a means for the creation of strongly
correlated polariton systems. Our technique reveals a loss mechanism for
stationary-light polaritons that has not been discussed so far. We find that
polariton losses in level configurations with non-degenerate ground states can
be a multiple of those in level schemes with degenerate ground states

### Three-body bound states in dipole-dipole interacting Rydberg atoms

We show that the dipole-dipole interaction between three identical Rydberg
atoms can give rise to bound trimer states. The microscopic origin of these
states is fundamentally different from Efimov physics. Two stable trimer
configurations exist where the atoms form the vertices of an equilateral
triangle in a plane perpendicular to a static electric field. The triangle edge
length typically exceeds $R\approx 2\,\mu\text{m}$, and each configuration is
two-fold degenerate due to Kramers' degeneracy. The depth of the potential
wells and the triangle edge length can be controlled by external parameters. We
establish the Borromean nature of the trimer states, analyze the quantum
dynamics in the potential wells and describe methods for their production and
detection.Comment: 5 pages, 3 figures and supplementary material; to appear in PR

### A polynomial Ansatz for Norm-conserving Pseudopotentials

We show that efficient norm-conserving pseudopotentials for electronic
structure calculations can be obtained from a polynomial Ansatz for the
potential. Our pseudopotential is a polynomial of degree ten in the radial
variable and fulfills the same smoothness conditions imposed by the
Troullier-Martins method [Phys. Rev. B 43, 1993 (1991)] where pseudopotentials
are represented by a polynomial of degree twenty-two. We compare our method to
the Troullier-Martins approach in electronic structure calculations for diamond
and iron in the bcc structure and find that the two methods perform equally
well in calculations of the total energy. However, first and second derivatives
of the total energy with respect to atomic coordinates converge significantly
faster with the plane wave cutoff if the standard Troullier-Martins potentials
are replaced by the pseudopotentials introduced here.Comment: 7 pages, 3 figure

### Probing microscopic models for system-bath interactions via parametric driving

We show that strong parametric driving of a quantum harmonic oscillator
coupled to a thermal bath allows one to distinguish between different
microscopic models for the oscillator-bath coupling. We consider a bath with an
Ohmic spectral density and a model where the system-bath interaction can be
tuned continuously between position and momentum coupling via the coupling
angle $\alpha$. We derive a master equation for the reduced density operator of
the oscillator in Born-Markov approximation and investigate its quasi-steady
state as a function of the driving parameters, the temperature of the bath and
the coupling angle $\alpha$. We find that the time-averaged variance of
position and momentum exhibits a strong dependence on these parameters. In
particular, we identify parameter regimes that maximise the $\alpha$-dependence
and provide an intuitive explanation of our results.Comment: 13 pages, 8 figure

### Quantum mechanical calculation of Rydberg-Rydberg autoionization rates

We present quantum mechanical calculations of Auger decay rates for two
Rubidium Rydberg atoms with weakly overlapping electron clouds. We neglect
exchange effects and consider tensor products of independent atom states
forming an approximate basis of the two-electron state space. We consider large
sets of two-atom states with randomly chosen quantum numbers and find that the
charge overlap between the two Rydberg electrons allows one to characterise the
magnitude of the Auger decay rates. If the electron clouds overlap by more than
one percent, the Auger decay rates increase approximately exponentially with
the charge overlap. This finding is independent of the energy of the initial
state.Comment: 8 pages, 5 figure

### Tensor network reduced order models for wall-bounded flows

We introduce a widely applicable tensor network-based framework for developing reduced order models describing wall-bounded fluid flows. As a paradigmatic example, we consider the incompressible Navier-Stokes equations and the lid-driven cavity in two spatial dimensions. We benchmark our solution against published reference data for low Reynolds numbers and find excellent agreement. In addition, we investigate the short-time dynamics of the flow at high Reynolds numbers for the liddriven and doubly-driven cavities. We represent the velocity components by matrix product states and find that the bond dimension grows logarithmically with simulation time. The tensor network algorithm requires at most a few percent of the number of variables parameterizing the solution obtained by direct numerical simulation, and approximately improves the runtime by an order of magnitude compared to direct numerical simulation on similar hardware. Our approach is readily transferable to other flows, and paves the way towards quantum computational fluid dynamics in complex geometries

### Mott polaritons in cavity-coupled quantum materials

We show that strong electron-electron interactions in cavity-coupled quantum
materials can enable collectively enhanced light-matter interactions with
ultrastrong effective coupling strengths. As a paradigmatic example we consider
a Fermi-Hubbard model coupled to a single-mode cavity and find that resonant
electron-cavity interactions result in the formation of a quasi-continuum of
polariton branches. The vacuum Rabi splitting of the two outermost branches is
collectively enhanced and scales with $g_{\text{eff}}\propto\sqrt{2L}$, where
$L$ is the number of electronic sites, and the maximal achievable value for
$g_{\text{eff}}$ is determined by the volume of the unit cell of the crystal.
We find that $g_{\text{eff}}$ for existing quantum materials can by far exceed
the width of the first excited Hubbard band. This effect can be experimentally
observed via measurements of the optical conductivity and does not require
ultra-strong coupling on the single-electron level. Quantum correlations in the
electronic ground state as well as the microscopic nature of the light-matter
interaction enhance the collective light-matter interaction compared to an
ensemble of independent two-level atoms interacting with a cavity mode.Comment: 11 pages, 4 figures. arXiv admin note: text overlap with
arXiv:1806.0675

### Pulse-splitting in light propagation through $N$-type atomic media due to an interplay of Kerr-nonlinearity and group velocity dispersion

We investigate the spatio-temporal evolution of a Gaussian probe pulse
propagating through a four-level $N$-type atomic medium. At two-photon
resonance of probe-and control fields, weaker probe pulses may propagate
through the medium with low absorption and pulse shape distortion. In contrast,
we find that increasing the probe pulse intensity leads to a splitting of the
initially Gaussian pulse into a sequence of subpulses in the time domain. The
number of subpulses arising throughout the propagation can be controlled via a
suitable choice of the probe and control field parameters. Employing a simple
theoretical model for the nonlinear pulse propagation, we conclude that the
splitting occurs due to an interplay of Kerr nonlinearity and group velocity
dispersion.Comment: 9 pages, 7 figure

### Steady-state negative Wigner functions of nonlinear nanomechanical oscillators

We propose a scheme to prepare nanomechanical oscillators in nonclassical
steady states, characterized by a pronounced negative Wigner function. In our
optomechanical approach, the mechanical oscillator couples to multiple laser
driven resonances of an optical cavity. By lowering the resonance frequency of
the oscillator via an inhomogeneous electrostatic field, we significantly
enhance its intrinsic geometric nonlinearity per phonon. This causes the
motional sidebands to split into separate spectral lines for each phonon number
and transitions between individual phonon Fock states can be selectively
addressed. We show that this enables the preparation of the nanomechanical
oscillator in a single phonon Fock state. Our scheme can for example be
implemented with a carbon nanotube dispersively coupled to the evanescent field
of a state of the art whispering gallery mode microcavity

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