# Cavity Quantum Electrodynamics

# Cavity Quantum E-Dyn.

## Helmut Ritsch's Theory Group | Universität Innsbruck

# CQED Theory

## Helmut Ritsch

# Research

This page details our research efforts and interests. Below you will find a selection of topics we are currently working on. For a list of publications please see the links.

#### General research statement

Our research covers the fields of theoretical quantum optics and ultra cold gas physics with strong connections to quantum information theory, foundations of quantum physics and quantum theory of condensed matter systems. We focus on full quantum descriptions of matter and light waves, which are strongly coupled by momentum exchange. Our aim is an effective theoretical description of real physical systems in a close connection with experiments, where genuine quantum phenomena as quantum phase transitions, entanglement and macroscopic super positions can be studied in a well-controlled and understood environment.

#### Publications

For an (almost) complete list of publications that have emerged from our research, please visit

or contact us directly.

## Lightforces in high-Q cavities

**Cavity Cooling**

Freezing particles in very small regions in space is the prerequisite for any investigation (like spectroscopy) or manipulation (like quantum computation). As photons carry energy and momentum, absorption, emission, and coherent scattering of light creates forces on the involved particles. For light enclosed between two highly reflective mirrors, i.e. an optical resonator, these forces are strongly enhanced. They can be tailored to control the quantum motion of the particles. This allows for trapping atoms and molecules as well as cooling them to temperatures very close to the absolute zero.

## Crystallization of atoms bound by light

**Self-Organization**

Photons carry momentum and their scattering thus not only modifies light propagation but induces forces on particles. Confining mobile scatterers and light in an optical resonator thus generates complex coupled nonlinear dynamics. An illumination at sufficient power induces a phase transition from random order to a crystalline structure. This minimizes the optical potential energy of the particles in concurrence with a maximization of light scattering into the resonator. Applying several illumination colors and cavity modes particles adapt to various illumination conditions dynamically to ensure maximal simultaneous scattering of all frequencies. This constitutes a self-optimizing light collection system with a built-in memory.

Such adaptive self-ordering dynamics in optical resonators can be implemented in a wide range of systems from cold atoms and molecules to mobile nanoparticles in solution. In the quantum regime it enables the exploration of uncharted regions of multi-particle phases allowing for a simulation of the Dicke superradiant phase transition, Hopfield networks, associative memories or generalized Hamiltonian mean field models.

Recently we have shown that closely related self-ordering phenomena appear in nanooptical geometries as tapered optical nanofibres, which mediate long-range dipole-dipole interaction and collective scattering.

## Forces from thermal light

**Attractive optical forces from blackbody radiation**

Typical quantum optics employs laser light to manipulate the motion of atoms. But examples such as Kepler's observation of radiation pressure effects in comet tails call our attention to forces generated by natural sources of light. In astrophysics it is well known that the scattering force is of great importance for the dynamics of gaseous clouds or the stability of stars.

In atomic physics thermal light fields usually play a role as an undesired source of blackbody Stark shifts limiting the precision of atomic clocks. But localised sources of thermal radiation generate an environment, where atoms see different Stark shifts at different locations, thus generating a potential landscape.

This leads to a weak, typically attractive force, pulling the atom towards hot objects. As this effect is measurable in atom interferometers, it has to be considered as a possible source of error in upcoming high-precision experiments. Returning to astrophysics, we are also interested in exploring possible implications for the interaction between hot dust particles and molecular clouds.

## The smallest possible laser

**Superradiant Lasing**

A single inverted atom in an optical micro resonator forms the smallest possible laser system as a coherent source of light. The laser light generated by the atom through stimulated emission exhibits strong forces which in turn provide for self-trapping and cooling of the atom. When operated on an ultra narrow clock transition one enters the super radiant operating regime which allows precise and accurate operation on the atomic clock transition wavelength.

## Protected subspace Ramsey spectroscopy

**Quantum Metrology: Making time measurements more precise**

Quantum physics tells us that the energies an electron can have within an atom are directly associated with frequencies and can therefore be used to count time. This is usually done in an atomic clock, a setup that has been well established over the last 50 years.

As a matter of fact, atomic clocks have become that good that even the smallest interactions between atoms now constitute a perturbation that fundamentally limits the accuracy of such a device. In our work, we have suggested an alteration of the standard measurement procedure, the so-called Ramsey spectroscopy method, that leverages the unwanted interactions amongst the atoms and turns them into an advantage, yielding much better sensitivities and ultimately a more precise clock.

In a nutshell, the suggested alteration makes use of subradiant, i.e. slowly decaying states, that emerge as a consequence of the mutual dipole-dipole interaction amongst the atoms. Therby population can be retained much longer during the phase of free evolution during the Ramsey measurement procedure.

This, of course, has many applications ranging from fundamental physics over high-speed communications to improved GPS navigation and beyond.

## QuantumOptics.jl: A Julia framework for simulating open quantum systems

**Simulating the dynamics of open quantum systems**

QuantumOptics.jl is an open source computational framework geared towards the efficient numerical investigation of open quantum systems written in the Julia programming language. Built exclusively in Julia and based on standard quantum optics notation, the toolbox offers speed comparable to low-level statically typed languages such as FORTRAN or C, without compromising on the accessibility and code readability found in dynamic languages.

More details, installation instructions and an extensive documentation with many examples can be found on the QuantumOptics.jl homepage.