Paper on Asymmetric Rydberg blockade of giant excitons in Cuprous Oxide

Nature Communications 12, 3556 (2021)

J. Heckötter, V. Walther, S. Scheel, M. Bayer, T. Pohl and M. Aßmann 

The ability to generate and control strong long-range interactions via highly excited electronic states has been the foundation for recent breakthroughs in a host of areas, from atomic and molecular physics to quantum optics and technology. Rydberg excitons provide a promising solid-state realization of such highly excited states, for which record-breaking orbital sizes of up to a micrometer have indeed been observed in cuprous oxide semiconductors. Here, we demonstrate the generation and control of strong exciton interactions in this material by optically producing two distinct quantum states of Rydberg excitons. This is made possible by two-color pump-probe experiments that allow for a detailed probing of the interactions. Our experiments reveal the emergence of strong spatial correlations and an inter-state Rydberg blockade that extends over remarkably large distances of several micrometers. The generated many-body states of semiconductor excitons exhibit universal properties that only depend on the shape of the interaction potential and yield clear evidence for its vastly extended-range and power-law character.

Paper on Nonlinear absorption in interacting Rydberg electromagnetically-induced-transparency spectra on two-photon resonance

Phys. Rev. A 103, 063710 (2021)

A. Tebben, C. Hainaut, A. Salzinger, S. Geier, T. Franz, T. Pohl, M. Gärttner, G. Zürn, and M. Weidemüller

We experimentally investigate the nonlinear transmission spectrum of coherent light fields propagating through a Rydberg EIT medium with strong atomic interactions. In contrast to previous investigations, which have largely focused on resonant control fields, we explore here the full two-dimensional spectral response of the Rydberg gas. Our measurements confirm previously observed spectral features for a vanishing control-field detuning, but also reveal significant differences on two-photon resonance. In particular, we find qualitative deficiencies of mean-field models and rate-equation simulations as well as a third-order nonlinear susceptibility that accounts for pairwise interaction effects at low probe-field intensities in describing the nonlinear probe-field response under EIT conditions. Our results suggest that a more complete understanding of Rydberg EIT and emerging photon interactions requires to go beyond existing simplified models as well as few-photon theories.

Paper on Sonochemical synthesis of large two-dimensional Bi2O2CO3 nanosheets for hydrogen evolution in photocatalytic water splitting

arXiv:2105.09885 (2021)

T. Gupta, N. Rosza,M. Sauer ,A. Goetz,M. Winzely,J. Rath,S. Naghdi,D. H. Apaydin,G. Friedbacher, A.Foelske, S. M. Skoff, B. C. Bayer and D. Eder

Laterally large (~3 micrometers), atomically-thin two-dimensional (2D) Bi2O2CO3 nanosheets (2D bismuth oxycarbonate, 2D bismutite) are fabricated via sonochemically-assisted template-free synthesis. Key to the synthesis of the freestanding, laterally large 2D Bi2O2CO3 nanosheets from bulk Bi powder is choice of suspension medium, controlled reaction temperatures and several hours processing time. Lateral sizes of 2D Bi2O2CO3 can be controlled between micrometer-sized nanosheets and tens of nm sized nanoflakes solely based on the choice of suspension medium. The here introduced 2D Bi2O2CO3 nanosheets/-flakes are then hybridized by a simple mix-and-match approach with TiO2 nanoparticles for testing in suspension-type photocatalytic hydrogen production via water splitting. This introduces the 2D Bi2O2CO3 with TiO2 as a promising noble-metal-free co-catalyst for photocatalytic hydrogen evolution. Our results enrich the fabrication toolbox of emerging 2D pnictogen oxycarbonates towards large 2D nanosheets and demonstrate the promising potential of 2D Bi2O2CO3 as an advantageous (co-)catalyst for hydrogen evolution in photocatalytic water splitting.

Paper on Thermal tuning of a fiber-integrated Fabry-Pérot cavity

arXiv:2105.12560 (2021)

C. Singer, A. Goetz, A. S. Prasad, M. Becker , M. Rothhardt and S. M. Skoff

Here, we present the thermal tuning capability of an alignment-free fiber-integrated Fabry-Pérot cavity.The two mirrors are made of fiber Bragg gratings that can be individually temperature stabilized and tuned. We show the temperature tuning of the resonance wavelength of the cavity without any degradation of the finesse and the tuning of the individual stop bands of the fiber Bragg gratings. This not only permits for the cavity’s finesse to be optimized post-fabrication but also makes this cavity applicable as a narrowband filter with a FWHM spectral width of 0.07 (0.02) pm and a suppression of more than -15 dB that can be wavelength tuned. Further, in the field of quantum optics, where strong light-matter interactions are desirable, quantum emitters can be coupled to such a cavity and the cavity effect can be reversibly omitted and re-established. This is particularly useful when working with solid-state quantum emitters where such a reference measurement is often not possible once an emitter has been permanently deposited inside a cavity.

Paper on Quantum Emitter Localization in Layer-Engineered Hexagonal Boron Nitride

J. C. Stewart, Y. Fan, J. S. H. Danial, A.Goetz, A. S. Prasad, O. J. Burton, J. A. Alexander-Webber, S. F. Lee, S. M. Skoff, V. Babenko and S. Hofmann

Hexagonal boron nitride (hBN) is a promising host material for room-temperature, tunable solid-state quantum emitters. A key technological challenge is deterministic and scalable spatial emitter localization, both laterally and vertically, while maintaining the full advantages of the 2D nature of the material. Here, we demonstrate emitter localization in hBN in all three dimensions via a monolayer (ML) engineering approach. We establish pretreatment processes for hBN MLs to either fully suppress or activate emission, thereby enabling such differently treated MLs to be used as select building blocks to achieve vertical (z) emitter localization at the atomic layer level. We show that emitter bleaching of ML hBN can be suppressed by sandwiching between two protecting hBN MLs, and that such thin stacks retain opportunities for external control of emission. We exploit this to achieve lateral (x–y) emitter localization via the addition of a patterned graphene mask that quenches fluorescence. Such complete emitter site localization is highly versatile, compatible with planar, scalable processing, allowing tailored approaches to addressable emitter array designs for advanced characterization, monolithic device integration, and photonic circuits.

Paper on Slow-Light-Enhanced Optical Imaging of Microfiber Radius Variations with Subangstrom Precision

Phys. Rev. Applied 14, 064052 (2020)

Michael Scheucher, Khaled Kassem, Arno Rauschenbeutel, Philipp Schneeweiss, and Jürgen Volz

Optical fibers play a key role in many different fields of science and technology. In particular, fibers with a diameter of several micrometers are intensively used in photonics. For these applications, it is often important to precisely know and control the fiber radius. Here we describe a technique to determine the radius profile of an optical microstructure with ultrahigh precision from a single optical image. Using a basic microscopy setup, we demonstrate our method by measuring the axial radius variation along a 30-μm-diameter silica fiber with precision less than 0.3 Å. The axial resolution is tens of micrometers, and the measurement range is more than 0.5 mm. Our method relies on imaging the fiber’s whispering-gallery modes in which the speed of light propagating along the fiber axis is strongly reduced. Imaging those whispering-gallery modes, we infer the local radius variations of the optical microstructure with ultrahigh precision. Because of the performance and simplicity of implementation, we are convinced that our scheme has high potential for precision metrology and optical sensing.

Paper on Coupling a Single Trapped Atom to a Whispering-Gallery-Mode Microresonator

Phys. Rev. Lett. 126 (2021)

Elisa Will, Luke Masters, Arno Rauschenbeutel, Michael Scheucher, and Jürgen Volz

We demonstrate trapping of a single 85Rb atom at a distance of about 200 nm from the surface of a whispering-gallery-mode bottle microresonator. The atom is trapped in an optical potential, which is created by retroreflecting a red-detuned focused laser beam from the resonator surface. We counteract the trap-induced light shift of the atomic transition frequency by superposing a second laser beam. This allows us to observe a vacuum Rabi splitting in the excitation spectrum of the coupled atom-resonator system. This first demonstration of stable and controlled interaction of a single atom with a whispering-gallery mode in the strong coupling regime opens up the route toward the implementation of quantum protocols and applications that harvest the chiral atom-light coupling present in this class of resonators.

Paper on Atomic spin-controlled non-reciprocal Raman amplification of fibre-guided light

Sebastian Pucher, Christian Liedl, Shuwei Jin, Arno Rauschenbeutel, Philipp Schneeweiss

In a non-reciprocal optical amplifier, gain depends on whether the light propagates forwards or backwards through the device. Typically, one requires either the magneto-optical effect, a temporal modulation, or an optical nonlinearity to break reciprocity. By contrast, here, we demonstrate non-reciprocal amplification of fibre-guided light using Raman gain provided by spin-polarized atoms that are coupled to the nanofibre waist of a tapered fibre section. The non-reciprocal response originates from the propagation direction-dependent local polarization of the nanofibre-guided mode in conjunction with polarization-dependent atom-light coupling. We show that this novel mechanism does not require an external magnetic field and that it allows us to fully control the direction of amplification via the atomic spin state. Our results may simplify the construction of complex optical networks. Moreover, suitable solid-state based quantum emitters provided, our scheme could be readily implemented in photonic integrated circuits.

Paper on Designing non-equilibrium states of quantum matter through stochastic resetting


G. Perfetto, F. Carollo, M. Magoni and I. Lesanovsky

We consider closed quantum many-body systems subject to stochastic resetting. This means that their unitary time evolution is interrupted by resets at randomly selected times. When a reset takes place the system is reinitialized to a state chosen from a set of reset states conditionally on the outcome of a measurement taken immediately before resetting. We construct analytically the resulting non-equilibrium stationary state, thereby establishing a novel connection between quantum quenches in closed systems and the emergent open system dynamics induced by stochastic resetting. We discuss as an application the paradigmatic transverse-field quantum Ising chain. We show that signatures of its ground-state quantum phase transition are visible in the steady state of the reset dynamics as a sharp crossover. Our findings show that a controlled stochastic resetting dynamics allows to design non-equilibrium stationary states of quantum many-body systems, where uncontrolled dissipation and heating can be prevented. These states can thus be created on demand and exploited, e.g., as a resource for quantum enhanced sensing on quantum simulator platforms.

Paper on Bragg condition for scattering into a guided optical mode


B. Olmos, C. Liedl, I. Lesanovsky and P. Schneeweiss

We theoretically investigate light scattering from an array of atoms into the guided modes of a waveguide. We show that the scattering of a plane wave laser field into the waveguide modes is dramatically enhanced for angles that deviate from the geometric Bragg angle. We derive a modified Bragg condition, and show that it arises from the dispersive interactions between the guided light and the atoms. Moreover, we identify various parameter regimes in which the scattering rate features a qualitatively different dependence on the atom number, such as linear, quadratic, oscillatory or constant behavior. We show that our findings are robust against voids in the atomic array, facilitating their experimental observation and potential applications. Our work sheds new light on collective light scattering and the interplay between geometry and interaction effects, with implications reaching beyond the optical domain.