Paper on Dynamical phases and quantum correlations in an emitter-waveguide system with feedback


G. Buonaiuto, F. Carollo, B. Olmos and I. Lesanovsky

We investigate the creation and control of emergent collective behavior and quantum correlations using feedback in an emitter-waveguide system using a minimal model. Employing homodyne detection of photons emitted from a laser-driven emitter ensemble into the modes of a waveguide allows to generate intricate dynamical phases. In particular, we show the emergence of a time-crystal phase, the transition to which is controlled by the feedback strength. Feedback enables furthermore the control of many-body quantum correlations, which become manifest in spin squeezing in the emitter ensemble. Developing a theory for the dynamics of fluctuation operators we discuss how the feedback strength controls the squeezing and investigate its temporal dynamics and dependence on system size. The largely analytical results allow to quantify spin squeezing and fluctuations in the limit of large number of emitters, revealing critical scaling of the squeezing close to the transition to the time-crystal. Our study corroborates the potential of integrated emitter-waveguide systems — which feature highly controllable photon emission channels — for the exploration of collective quantum phenomena and the generation of resources, such as squeezed states, for quantum enhanced metrology.

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.

Paper on Observation of collective decay dynamics of a single Rydberg superatom

Phys. Rev. Research 2, 043339 (2020)

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

We experimentally investigate the collective decay of a single Rydberg superatom, formed by an ensemble of thousands of individual atoms supporting only a single excitation due to the Rydberg blockade. Instead of observing a constant decay rate determined by the collective coupling strength to the driving field, we show that the enhanced emission of the single stored photon into the forward direction of the coupled optical mode depends on the dynamics of the superatom before the decay. We find that the observed decay rates are reproduced by an expanded model of the superatom which includes coherent coupling between the collective bright state and subradiant states.

Paper on Fiber-compatible photonic feed-forward with 99% fidelity

Optics Express, 29, 3425-3437 (2021)

Zanin, G. L., Jacquet, M. J., Spagnolo, M., Schiansky, P., Calafell, I. A., Rozema, L. A. & Walther, P.

Both photonic quantum computation and the establishment of a quantum internet require fiber-based measurement and feed-forward in order to be compatible with existing infrastructure. Here we present a fiber-compatible scheme for measurement and feed-forward, whose performance is benchmarked by carrying out remote preparation of single-photon polarization states at telecom-wavelengths. The result of a projective measurement on one photon deterministically controls the path a second photon takes with ultrafast optical switches. By placing well-calibrated bulk passive polarization optics in the paths, we achieve a measurement and feed-forward fidelity of (99.0 ± 1)%, after correcting for other experimental errors. Our methods are useful for photonic quantum experiments including computing, communication, and teleportation.