Physikalisches Institut
Im Neuenheimer Feld 226
69120 Heidelberg
Tel: 06221/ 54 19471
Fax: 06221/ 54 19545
"Strong photonphoton interactions, magnetic polarons, and strings with cold atoms and van der Waals materials"
Prof. Dr. Georg Bruun
Stronglycorrelated Rydberg quantum gases
Ultracold atomic gases are ideally suited for exploring the quantum physics of manybody systems and for investigating quantum matter and exotic quantum phenomena. We make use of the extraordinary properties of highlyexcited Rydberg atoms in dense atomic gases to explore the realm of stronglycorrelated manybody physics.
Far from equilibrium dynamics of a Rydberg spin system
The dynamics of stronglyinteracting quantum systems constitutes one of the most challenging problems in modern science, touching basic concepts of statistical physics, thermodynamics and quantum physics. These systems are particularly intriguing since disorder gives rise to a variety of collective phenomena like anomalously slow dynamics, the breakdown of thermalization and the emergence of novel glassy phases of matter. As a model system we use a frozen gas of Rydberg atoms where the spin is encoded in two electronically excited states. This system is ideally suited for exploring the dynamics of isolated quantum systems due to the huge dipolar interaction between Rydberg atoms. Using coherent laser beams and arbitrary pulse sequences of microwave radiation, we experimentally implement novel tools for quantum engineering and diagnostics.
Transport of Rydberg excitations
We study the transport of Rydberg excitations in a cold atomic cloud. A single excitation to an impurity Rydberg state couples via resonant dipoledipole interactions to the background atoms, which are dressed with another Rydberg state via electromagnetically induced transparency. The transparent background is therefore rendered absorptive around the impurity. This allows direct optical detection of transport dynamics. The imaging process presents a tunable degree of decoherence, which enables the study of different transport regimes.
Nonlinear photon interactions in a RydbergEIT medium
Making photons interact lies at the heart of modern quantum optics and has applications for example in alloptical quantum information
processing. Coupling light to an interacting gas of Rydberg atoms under conditions of electromagnetically induced transparency (EIT) provides
a strong optical nonlinearity, which allows for photonic interactions on the single photon level. This rapidly evolving field of RydbergEIT
paves the way for applications such as photon transistors and fosters investigations of manybody physics with interacting photons as well as
exotic states of light.
Our group recently predicted a twobody, twophoton resonance that enhances the optical nonlinearity in the regime of
low optical depth per blockade radius and allows to alter the absorptive and dispersive properties of the Rydberg gas.
Engineering Rydbergspin hamiltonians
Rydberg atoms constitute an ideal platform to perform quantum simulations of Heisenbergspin systems. By mapping the spin degree of freedom to
two atomic states, involving at least one highlying Rydberg state, Ising, XX and XXZmodels can be realized. Depending on the specific form
of the underlying Hamiltonian, interesting phases of matter can be observed that are yet not fully understood.
However, Rydberg quantum simulators (as well as simulators on other platforms) are usually only suited to study a specific class of Hamiltonians
and it is often quite demanding or even impossible to change the implemented Hamiltonian in a controlled way.
One approach to overcome this fundamental limitation is to engineer terms in a given interaction Hamiltonian by means of global control pulses.
Traditionally, these techniques have been developed in nuclear magnetic resonance (NMR) in order to filter interactions and single particle dephasing. Nowadays, the use of pulse sequences have been extended to not only cancel terms, but to modify them in a desired way. Our goal is to study relaxation dynamics in a disordered spin system under general XYZ Hamiltonians using microwave pulse sequences.
Latest news and research results 
Download the latest poster from the Rydberg group [pdf] 
Floquet Hamiltonian engineering of an isolated manybody spin system: paper published in Science!  28.11.2021 
This month, our paper on "Floquet Hamiltonian engineering of an isolated manybody spin system" got published in Science! In this work, by periodically driving an isolated spin system, we changed a naturally given manybody Hamiltonian into a desired target form. Using a sequence of timeperiodic microwave pulses, we change the Hamiltonian of our interacting Rydbergspin system from a Heisenberg XXmodel into an effective XYZmodel with tunable symmetry. As a consequence, the magnetization relaxation dynamic of the is drastically modified. The ability to engineering a wide range of Hamiltonians opens vast opportunities for implementing quantum simulation of nonequilibrium dynamics in a single experimental setting.

Reference: Floquet Hamiltonian engineering of an isolated manybody spin system, Science, Vol 374, Issue 6571, pp. 11491152, or see our full list of publications For more information: 
Nonlinear optical response in RydbergEIT medium: paper published in Phys. Rev. A  06.12.2019 
This month, our paper on a "Blockadeinduced resonant enhancement of the optical nonlinearity in a Rydberg medium" got published in Phys. Rev. A! We predict the enhancement of the nonlinear optical response of the Rydberg gas as a consequence of a twophoton process that resonantly couples electronic states of a pair of atoms dressed by a strong control field. Moreover, we propose a realistic experimental scenario to observe the resonance by performing transmission measurements

Reference: Blockadeinduced resonant enhancement of the optical nonlinearity in a Rydberg medium, Phys. Rev. A 100, 063812, (2019), or see our full list of publications 
Dr. Clément Hainaut is now an Alexander von Humboldt Fellow  28.02.2019 
Congratulations to Clément Hainaut who was awarded an Alexander von Humboldt Fellowship! Clément joined the Rydberg team as a postdoc for investigating relaxation dynamics of Rydberg spin systems as well as nonlinear photon interactions in a Rydberg EIT medium!

The Rydberg team
Carlos Brandl Sebastian Geier David Grimshandl Renato Ferracini Alves Titus Franz André Salzinger Annika Tebben Dr. Gerhard Zürn Prof. Matthias Weidemüller ... and you? 
Publications by the Rydberg project
Semiclassical simulations predict glassy dynamics for disordered Heisenberg models, to appear in Phys. Rev. B Letter (arXiv:2107.13314) (2022)
Glassy quantum dynamics of disordered Ising spins, Phys. Rev. B 105, L020201 (2022)
2021Observation of glassy dynamics in a disordered quantum spin system, Phys. Rev. X 11, 011011 (2021)
Nonlinear absorption in interacting Rydberg electromagneticallyinducedtransparency spectra on twophoton resonance, Phys. Rev. A 103, 063710 (2021)
Floquet Hamiltonian Engineering of an Isolated ManyBody Spin System, Science 374, 1149 (2021)
2020Photon correlation transients in a weakly blockaded Rydberg ensemble, J. Phys. B: At. Mol. Opt. Phys. 53, 084004 (2020)
Depletion imaging of Rydberg atoms in cold atomic gases, J. Phys. B: At. Mol. Opt. Phys. 53, 084005 (2020)
2019 Blockadeinduced resonant enhancement of the optical nonlinearity in a Rydberg medium, Phys. Rev. A 100, 063812 (2019)
Diffusive to nonergodic dipolar transport in a dissipative atomic medium, Phys. Rev. Lett. 123, 213606 (2019)
2018 Relaxation of an Isolated DipolarInteracting Rydberg Quantum Spin System, Phys. Rev. Lett. 120, 063601 (2018)
2016 Direct observation of ultrafast manybody electron dynamics in an ultracold Rydberg gas, Nature Comm. 7, 13449 (2016)
Density matrix reconstruction of threelevel atoms via Rydberg electromagnetically induced transparency, Journal of Physics B: Atomic, Molecular and Optical Physics 49, 164002 (2016) [pdf]
Interaction Enhanced Imaging of Rydberg P states, The European Physical Journal Special Topics 225, 28632889 (2016) [pdf]
2015 Correlated Exciton Transport in RydbergDressedAtom Spin Chains, Phys. Rev. Lett. 115, 093002 (2015) [pdf]
2014 Full Counting Statistics of Laser Excited Rydberg Aggregates in a OneDimensional Geometry, Phys. Rev. Lett. 112, 013002 (2014)
An experimental approach for investigating manybody phenomena in Rydberginteracting quantum systems, Frontiers of Physics 9, 571586 (2014)
2013 Observing the Dynamics of DipoleMediated Energy Transport by InteractionEnhanced Imaging, Science 342, 954956 (2013)
Spontaneous avalanche ionization of a strongly blockaded Rydberg a gas, Phys. Rev. Lett. 110, 045004 (2013) [pdf]
Atomic Interactions at a Distance, Physics 6, 71 (2013) [pdf]
Quantum physics: Spooky action gets collective, Nature 498, 438 (2013)
SubPoissonian Statistics of RydbergInteracting DarkState Polaritons, Phys. Rev. Lett. 110, 203601 (2013) [pdf]
2012 Interaction enhanced imaging of individual atoms embedded in dense a atomic gases, Phys. Rev. Lett 108, 013002 (2012) [pdf]
2011 Quantum interference in interacting threelevel Rydberg gases: coherent a population trapping and electromagnetically induced transparency, J. Phys. B: At. Mol. Opt. Phys. 44, 184018 (2011) [pdf]
Highprecision semiconductor wavelength sensor based on a doublelayer a photo diode, Review of Scientific Instruments 82, 093111 (2011)
2010 Coherent population trapping with controlled interparticle interactions, Phys. Rev. Lett. 104, 173602 (2010)
Evidence of Antiblockade in an Ultracold Rydberg Gas, Phys. Rev. Lett. 104, 013001 (2010)
2009 Rydberg atoms  There can be only one, Nature Physics 5 (2009)
Autoionization of an ultracold Rydberg gas through resonant dipole a coupling, Eur. Phys. J. D 53, 329 (2009)
Frozen Rydberg Gases, in: Cold Atoms and Molecules, Wiley VCH (2009)
2008 Rabi oscillations between ground and Rydberg states and van der Waals a blockade in a mesoscopic frozen Rydberg gas, New J. Phys. 10, 045026 (2008)
2007 Modeling fewbody phenomena in an ultracold Rydberg gas, Nucl. Phys. A 790, 728c (2007)
Mechanical effect of van der Waals interactions observed in real a time in an ultracold Rydberg gas, Phys. Rev. Lett. 98, 023004 (2007)
2006 Coherent excitation of Rydberg atoms in an ultracold gas, Opt. Comm. 264, 293 (2006)
Prospects of ultracold Rydberg gases for quantum information processing, Fortschr. Phys. 54, 776 (2006)
Motion in an ultralongrange potential in coldRydbergatom collisions, Phys. Rev. A 73, 034703 (2006)
2005 Longrange interactions between alkali Rydberg atom pairs correlated a to the nsns, npnp and ndnd asymptotes, J. Phys. B 38, S295 (2005)
Spectroscopy of an ultracold Rydberg gas and signatures of RydbergRydberg a interactions, J. Phys. B 38, S321 (2005)
UltralongRange Interactions and Blockade of Excitation in a Cold a Rydberg Gas, in: Atomic Physics XIX, pp. 157163 (2005)
Interactions in an Ultracold Gas of Rydberg Atoms, in: Laser Spectroscopy XVII, pp. 264274 (2005)
2004 Spectral Broadening and Suppression of Excitation Induced by UltralongRange a Interactions in a Cold Gas of Rydberg Atoms, Phys. Rev. Lett. 93, 163001 (2004)