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 Measurement-feedback control of chiral photon emission from an atom chain into a nanofiber

Journal of the Optical Society of America B 38, 1470 (2021)

G. Buonaiuto, I. Lesanovsky and B. Olmos

We theoretically investigate measurement-based feedback control of a laser-driven one-dimensional atomic chain interfaced with a nanofiber. The interfacing leads to all-to-all interactions among the atomic emitters and induces chirality (i.e., the directional emission of photons into a preferred guided mode of the nanofiber). In the setting we consider, the measurement of guided light—conducted either by photon counting or through homodyne detection of the photocurrent quadratures—is fed back into the system through modulation of the driving laser field. We investigate how this feedback scheme allows control of the statistics of the photon counting and the quadratures of the light, as well as the many-body state of the atom chain. In particular, we identify regimes where both the photon counting rate and its fluctuations are dramatically enhanced. Moreover, we find that the action of homodyne detection feedback allows the alteration of the stationary state of the chain from a pure, dimer state, to a fully mixed one. Our results provide insights on how to control and engineer dynamics in light–matter networks realizable with state-of-the-art experimental setups.

Paper on Interaction signatures and non-Gaussian photon states from a strongly driven atomic ensemble coupled to a nanophotonic waveguide

Physical Review A 102, 043711 (2020)

We study theoretically a laser-driven one-dimensional chain of atoms interfaced with the guided optical modes of a nanophotonic waveguide. The period of the chain and the orientation of the laser field can be chosen such that emission occurs predominantly into a single guided mode. We find that the fluorescence excitation line shape changes as the number of atoms is increased, eventually undergoing a splitting that provides evidence for the waveguide-mediated all-to-all interactions. Remarkably, in the regime of strong driving the light emitted into the waveguide is nonclassical with a significant negativity of the associated Wigner function. We show that both the emission properties and the non-Gaussian character of the light are robust against voids in the atom chain, enabling the experimental study of these effects with present-day technology. Our results offer a route towards novel types of fiber-coupled quantum light sources and an interesting perspective for probing the physics of interacting atomic ensembles through light.

Paper on Witnessing nonclassicality through large deviations in quantum optics

Physical Review Research 2, 023078 (2020)

D. Cilluffo, G. Buonaiuto,  S. Lorenzo, G. M. Palma, F. Ciccarello, F. Carollo and I. Lesanovsky

Nonclassical correlations in quantum optics as resources for quantum computation are important in the quest for highly specialized quantum devices. Here, we put forward a methodology to witness nonclassicality of the output field from a generic quantum optical setup via the statistics of time-integrated photocurrents. Specifically, exploiting the thermodynamics of quantum trajectories, we express a known nonclassicality witness for bosonic fields fully in terms of the source master equation, thus bypassing the explicit calculation of the output light state.

Paper on Collectively enhanced chiral photon emission from an atomic array near a nanofiber

Physical Review Letters 124, 093601 (2020)

R. Jones, G. Buonaiuto, B. Lang, I. Lesanovsky and B. Olmos

Emitter ensembles interact collectively with the radiation field. In the case of a one-dimensional array of atoms near a nanofiber, this collective light-matter interaction does not only lead to an increased photon coupling to the guided modes within the fiber, but also to a drastic enhancement of the chirality in the photon emission. We show that near-perfect chirality can be achieved already for moderately sized ensembles, containing 10 to 15 atoms, by phase matching a superradiant collective guided emission mode via an external laser field. This is of importance for developing an efficient interface between atoms and waveguide structures with unidirectional coupling, with applications in quantum computing and communication such as the development of nonreciprocal photon devices or quantum information transfer channels.

Paper on Dynamical creation and detection of entangled many-body states in a chiral atom chain

New Journal of Physics 21, 113021 (2019)

G. Buonaiuto, R. Jones, B. Olmos and I. Lesanovsky

Open quantum systems with chiral interactions can be realized by coupling atoms to guided radiation modes in photonic waveguides or optical fibers. In their steady state these systems can feature intricate many-body phases such as entangled dark states, but their detection and characterization remains a challenge. Here we show how such collective phenomena can be uncovered through monitoring the record of photons emitted into the guided modes. This permits the identification of dark entangled states but furthermore offers novel capabilities for probing complex dynamical behavior, such as the coexistence of a dark entangled and a mixed phase. Our results are of direct relevance for current optical experiments, as they provide a framework for probing, characterizing and classifying classical and quantum dynamical features of chiral light–matter systems.

Paper on Controlled multi-photon subtraction with cascaded Rydberg superatoms as single-photon absorbers

Nat. Commun. 12, 4328 (2021)

Nina Stiesdal, Hannes Busche, Kevin Kleinbeck, Jan Kumlin, Mikkel G. Hansen, Hans Peter Büchler and Sebastian Hofferberth

The preparation of light pulses with well-defined quantum properties requires precise control at the individual photon level. Here, we demonstrate exact and controlled multi-photon subtraction from incoming light pulses. We employ a cascaded system of tightly confined cold atom ensembles with strong, collectively enhanced coupling of photons to Rydberg states. The excitation blockade resulting from interactions between Rydberg atoms limits photon absorption to one per ensemble and rapid dephasing of the collective excitation suppresses stimulated re-emission of the photon. We experimentally demonstrate subtraction with up to three absorbers. Furthermore, we present a thorough theoretical analysis of our scheme where we identify weak Raman decay of the long-lived Rydberg state as the main source of infidelity in the subtracted photon number and investigate the performance of the multi-photon subtractor for increasing absorber numbers in the presence of Raman decay.

Paper on Nonexponential decay of a collective excitation in an atomic ensemble coupled to a one-dimensional waveguide

Phys. Rev. A 102, 063703 (2021)

Jan Kumlin, Kevin Kleinbeck, Nina Stiesdal, Hannes Busche, Sebastian Hofferberth, and Hans Peter Büchler

We study the dynamics of a single excitation coherently shared among an ensemble of atoms and coupled to a one-dimensional wave guide. The coupling between the matter and the light field gives rise to collective phenomena such as superradiant states with an enhanced initial decay rate, but also to the coherent exchange of the excitation between the atoms. We find that the competition between the two phenomena provides a characteristic dynamics for the decay of the excitations, and remarkably exhibits an algebraic behavior, instead of the expected standard exponential one, for a large number of atoms. The analysis is first performed for a chiral waveguide, where the problem can be solved analytically. Remarkably, we demonstrate that a bidirectional waveguide exhibits the same behavior for large number of atoms and, therefore, it is possible to experimentally access characteristic properties of a chiral waveguide also within a bidirectional waveguide.