My current research interests cover a variety of areas in the field of quantum optics ranging from quantum descriptions of light matter interactions of atomic and molecular systems in the framework of open quantum systems, interacting manybody systems to quantum optomechanics. Especially collective phenomena are at the focus of my research in recent time. Working in experimental quantum optics for many years my research interest has shifted to theoretical studies in recent years.
Molecular polaritonics in dense mesoscopic disordered ensembles
Christian Sommer, Michael Reitz, Francesca Mineo, Claudiu Genes
We study the dependence of the vacuum Rabi splitting (VRS) on frequency disorder, vibrations, nearfield effects and density in molecular polaritonics. In the mesoscopic limit, static frequency disorder introduces a loss mechanism from polaritonic states into a dark state reservoir, which we
quantitatively describe, providing an analytical scaling of the VRS on the level of disorder. The combination of disorder, vibronic coupling, dipoledipole interactions and vibrational relaxation induces an incoherent FRET (Forster resonance energy transfer) migration of excitations from
donortype to acceptortype molecules within the collective molecular state. This is equivalent to a dissipative disorder and has the effect of saturating and even reducing the VRS in the mesoscopic, highdensity limit.
Multimode colddamping optomechanics with delayed feedback
Christian Sommer, Alekhya Ghosh, Claudiu Genes
Physical Review Research
2
033299
(2020)

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We investigate the role of time delay in colddamping optomechanics with multiple mechanical resonances.
For instantaneous electronic response, it was recently shown by C. Sommer and C. Genes [Phys. Rev. Lett. 123,
203605 (2019)] that a single feedback loop is sufficient to simultaneously remove thermal noise from many
mechanical modes. While the intrinsic delayed response of the electronics can induce singlemode and mutual
heating between adjacent modes, we propose to counteract such detrimental effects by introducing an additional
time delay to the feedback loop. For lossy cavities and broadband feedback, we derive analytical results for the
final occupancies of the mechanical modes within the formalism of quantum Langevin equations. For modes
that are frequency degenerate collective effects dominate, mimicking behavior similar to Dicke super and
subradiance. These analytical results, corroborated with numerical simulations of both transient and steady state
dynamics, allow us to find suitable conditions and strategies for efficient singlemode or multimode feedback
optomechanics.
Moleculephoton interactions in phononic environments
Michael Reitz, Christian Sommer, Burak Gürlek, Vahid Sandoghdar, DiegoMartin Cano, Claudiu Genes
Physical Review Research
2
033270
(2020)

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Molecules constitute compact hybrid quantum optical systems that can interface photons, electronic degrees of freedom, localized mechanical vibrations, and phonons. In particular, the strong vibronic interaction between electrons and nuclear motion in a molecule resembles the optomechanical radiation pressure Hamiltonian. While molecular vibrations are often in the ground state even at elevated temperatures, one still needs to get a handle on decoherence channels associated with phonons before an efficient quantum optical network based on optovibrational interactions in solidstate molecular systems could be realized. As a step towards a better understanding of decoherence in phononic environments, we take here an open quantum system approach to the nonequilibrium dynamics of guest molecules embedded in a crystal, identifying regimes of Markovian versus nonMarkovian vibrational relaxation. A stochastic treatment, based on quantum Langevin equations, predicts collective vibronvibron dynamics that resembles processes of sub and superradiance for radiative transitions. This in turn leads to the possibility of decoupling intramolecular vibrations from the phononic bath, allowing for enhanced coherence times of collective vibrations. For molecular polaritonics in strongly confined geometries, we also show that the imprint of optovibrational couplings onto the emerging output field results in effective polariton crosstalk rates for finite bath occupancies.
Prospects of reinforcement learning for the simultaneous damping of many mechanical modes
Christian Sommer, Muhammad Asjad, Claudiu Genes
Scientific Reports
10(2623)
(2020)

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We apply adaptive feedback for the partial refrigeration of a mechanical resonator, i.e. with the aim to
simultaneously cool the classical thermal motion of more than one vibrational degree of freedom. The
feedback is obtained from a neural network parametrized policy trained via a reinforcement learning
strategy to choose the correct sequence of actions from a fnite set in order to simultaneously reduce
the energy of many modes of vibration. The actions are realized either as optical modulations of the
spring constants in the socalled quadratic optomechanical coupling regime or as radiation pressure
induced momentum kicks in the linear coupling regime. As a proof of principle we numerically illustrate
efcient simultaneous cooling of four independent modes with an overall strong reduction of the total
system temperature.
Partial optomechanical refrigeration via multimode colddamping feedback
We provide a fully analytical treatment for the partial refrigeration of the thermal motion of a quantum mechanical resonator under the action of feedback. As opposed to standard cavity optomechanics where the aim is to isolate and cool a single mechanical mode, the aim here is to extract the thermal energy from many vibrational modes within a large frequency bandwidth. We consider a standard colddamping technique, where homodyne readout of the cavity output field is fed into a feedback loop that provides a cooling action directly applied on the mechanical resonator. Analytical and numerical results predict that low final occupancies are achievable independent of the number of modes addressed by the feedback, as long as the cooling rate is smaller than the intermode frequency separation. For resonators exhibiting a few nearly degenerate pairs of modes, cooling is less efficient and a weak dependence on the number of modes is obtained. These scalings hint toward the design of frequencyresolved mechanical resonators, where efficient refrigeration is possible via simultaneous colddamping feedback.
Laser refrigeration of gas filled hollowcore fibres
Christian Sommer, Nicolas Y. Joly, Helmut Ritsch, Claudiu Genes
We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered
refrigeration device is based on exciplexmediated frequency upconversion inside hollowcore fibers pressurized with a dopant  buffer
gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the
ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during
atomic collisions inducing light assisted exciplex formation followed by blueshifted spontaneous emission on the atomic line of the bare
dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large
fraction of collision energy to be extracted in each cycle. The hollowcore fiber plays a crucial role as it allows for strong lightmatter
interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while
limiting reabsorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations
and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding
solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the
exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification
of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific
applications.
Langevin Approach to Quantum Optics with Molecules
We investigate the interaction between light and molecular systems modeled as quantum emitters coupled to a multitude of vibrational modes via a Holsteintype interaction. We follow a quantum Langevin equations approach that allows for analytical derivations of absorption and fluorescence profiles of molecules driven by classical fields or coupled to quantized optical modes. We retrieve analytical expressions for the modification of the radiative emission branching ratio in the Purcell regime and for the asymmetric cavity transmission associated with dissipative cross talk between upper and lower polaritons in the strong coupling regime. We also characterize the Förster resonance energy transfer process between donoracceptor molecules mediated by the vacuum or by a cavity mode.
Enhanced collective Purcell effect of coupled quantum emitter systems
David Plankensteiner, Christian Sommer, Michael Reitz, Helmut Ritsch, Claudiu Genes
Cavityembedded quantum emitters show strong modifications of free space radiation properties such as an enhanced decay known as the Purcell effect. The central parameter is the cooperativity C  the ratio of the square of the coherent cavity coupling strength over the product of cavity and emitter decay rates. For a single emitter, C is independent of the transition dipole moment and dictated by geometric cavity properties such as finesse and mode waist. In a recent work Phys. Rev. Lett. 119, 093601 (2017) we have shown that collective excitations in ensembles of dipoledipole coupled quantum emitters show a disentanglement between the coherent coupling to the cavity mode and spontaneous free space decay. This leads to a strong enhancement of the cavity cooperativity around certain collective subradiant antiresonances. Here, we present a quantum Langevin equations approach aimed at providing results beyond the classical coupled dipoles model. We show that the subradiantly enhanced cooperativity imprints its effects onto the cavity output field quantum correlations while also strongly increasing the cavityemitter system's collective Kerr nonlinear effect.
Attosecond Control of Restoration of Electronic Structure Symmetry
Chun Mei Liu, Jörn Manz, Kenji Ohmori, Christian Sommer, Nobuyuki Takei, Jean Christophe Tremblay, Yichi Zhang
Laser pulses can break the electronic structure symmetry of atoms and molecules by preparing a superposition of states with different irreducible representations. Here, we discover the reverse process, symmetry restoration, by means of two circularly polarized laser pulses. The laser pulse for symmetry restoration is designed as a copy of the pulse for symmetry breaking. Symmetry restoration is achieved if the time delay is chosen such that the superposed states have the same phases at the temporal center. This condition must be satisfied with a precision of a few attoseconds. Numerical simulations are presented for the C6H6 molecule and Rb87 atom. The experimental feasibility of symmetry restoration is demonstrated by means of highcontrast timedependent Ramsey interferometry of the Rb87 atom.
Ramsey interferometry of Rydberg ensembles inside microwave cavities
Christian Sommer, Claudiu Genes
JOURNAL OF PHYSICS BATOMIC MOLECULAR AND OPTICAL PHYSICS
51(11)
115502
(2018)

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We study ensembles of Rydberg atoms in a confined electromagnetic environment such as is provided by a microwave cavity. The competition between standard free space Ising type and cavitymediated interactions leads to the emergence of different regimes where the particle particle couplings range from the typical van der Waals r(6) behavior to r(3) and to rindependence. We apply a Ramsey spectroscopic technique to map the twobody interactions into a characteristic signal such as intensity and contrast decay curves. As opposed to previous treatments requiring highdensities for considerable contrast and phase decay (Takei et al 2016 Nat. Comms. 7 13449; Sommer et al 2016 Phys. Rev. A 94 053607), the cavity scenario can exhibit similar behavior at much lower densities.
Ultrafast Coherent Control of Condensed Matter with Attosecond Precision
Hiroyuki Katsuki, Nobuyuki Takei, Christian Sommer, Kenji Ohmori
Accounts of Chemical Research
51(5)
11741184
(2018)

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Coherent control is a technique to manipulate wave functions of matter with light. Coherent control of isolated atoms and molecules in the gas phase is wellunderstood and developed since the 1990s, whereas its application to condensed matter is more difficult because its coherence lifetime is shorter. We have recently applied this technique to condensed matter samples, one of which is solid parahydrogen (pH2). Intramolecular vibrational excitation of solid pH2 gives an excited vibrational wave function called a “vibron”, which is delocalized over many hydrogen molecules in a manner similar to a Frenkel exciton. It has a long coherence lifetime, so we have chosen solid pH2 as our first target in the condensed phase. We shine a timedelayed pair of femtosecond laser pulses on pH2 to generate vibrons. Their interference results in modulation of the amplitude of their superposition. Scanning the interpulse delay on the attosecond time scale gives a high interferometric contrast, which demonstrates the possibility of using solid pH2 as a carrier of information encoded in the vibrons.
In the second example, we have controlled the terahertz collective phonon motion, called a “coherent phonon”, of a single crystal of bismuth. We employ an intensitymodulated laser pulse, whose temporal envelope is modulated with terahertz frequency by overlap of two positively chirped laser pulses with their adjustable time delay. This modulated laser pulse is shined on the bismuth crystal to excite its two orthogonal phonon modes. Their relative amplitudes are controlled by tuning the delay between the two chirped pulses on the attosecond time scale. Twodimensional atomic motion in the crystal is thus controlled arbitrarily. The method is based on the simple, robust, and universal concept that in any physical system, twodimensional particle motion is decomposed into two orthogonal onedimensional motions, and thus, it is applicable to a variety of condensed matter systems.
In the third example, the doublepulse interferometry used for solid pH2 has been applied to manybody electronic wave functions of an ensemble of ultracold rubidium Rydberg atoms, hereafter called a “strongly correlated ultracold Rydberg gas”. This has allowed the observation and control of manybody electron dynamics of more than 40 Rydberg atoms interacting with each other. This new combination of ultrafast coherent control and ultracold atoms offers a versatile platform to precisely observe and manipulate nonequilibrium dynamics of quantum manybody systems on the ultrashort time scale.
These three examples are digested in this Account.
01/2017  present
Postdoctoral researcher (theoretical physics), Max Planck Institute for the Science of Light, Erlangen, Germany
04/2014 – 04/2016
Assistant professor (experimental physics), Department of PhotoMolecular Science, Institute for Molecular Science (IMS), National Institutes of Natural Sciences (Prof. K. Ohmori), Okazaki, Japan
01/2012 – 04/2014
Postdoctoral Researcher (experimental physics), Department of PhotoMolecular Science, Institute for Molecular Science (IMS), National Institutes of Natural Sciences (Prof. K. Ohmori), Okazaki, Japan
07/2011 – 12/2011
Postdoctoral Researcher, (experimental physics) Max Planck Institute of Quantum Optics, Quantum Dynamics Group (Prof. G. Rempe), Garching, Germany
09/2006 – 06/2011
PhD (Dr. rer. nat.) (Summa Cum Laude) (experimental physics), Technical University of Munich (TUM), Max Planck Institute of Quantum Optics (MPQ), Quantum Dynamics Group (Prof. G. Rempe), Garching, Germany
01/2000 – 04/2006
Diploma in Physics (theoretical physics), University of Hamburg, II. Institute for Theoretical Physics (Prof. K. Fredenhagen), Hamburg, Germany Specialization: Mathematical physics, Quantum Field Theory
Click here for a detailed CV (pdf) including list of publications
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