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.

Paper on Novel single-mode narrow-band photon source of high brightness tuned to cesium D2 line

ALP Photonics, 4, 90804-6 (2019)

Moqanaki, A., Massa, F. & Walther, P.

Cavity-enhanced spontaneous parametric down-conversion is capable of efficient generation of single photons with suitable spectral properties for interfacing with the atoms. However, beside the remarkable progress of this technique, multimode longitudinal emission remains a major drawback. Here, we demonstrate a bright source of single photons that overcomes this limitation by a novel mode-selection technique based on the introduction of an additional birefringent element to the cavity. This enables us to tune the double resonance condition independent of the phase matching and thus to achieve single-mode operation without mode filters. Our source emits single-frequency-mode photons at 852 nm, which is compatible to the Cs D2 line, with a bandwidth of 10.9 MHz and a detected photon-pair rate of 2.5 kHz at 10 mW of pump power, while maintaining g(2)(0) = 0.13. The efficiency of our source is further underlined by detecting a four-photon rate of 0.28 Hz at 20 mW of pump power. These detected rates correspond to a photon-pair generation rate of 47.5 Hz, and a four-photon generation rate of 37 Hz. Such photon generation rates open up a variety of new applications reaching from hybrid light-matter interactions to optical quantum information tasks based on long temporal coherence.

Paper on Experimental Two-Way Communication with One Photon

Advanced Quantum Technologies, 2, 1900050 (2019)

Massa, F., Moqanaki, A., Bäumeler, Ä., Del Santo, F., Kettlewell, J. A., Dakic, B. & Walther, P.

Superposition of two or more states is one of the most fundamental concepts of quantum mechanics and provides a basis for several advantages offered by quantum information processing. This work reports the experimental demonstration of two-way communication between two distant parties that can exchange only a single particle once, an impossible task in classical physics. This is achieved through preparation of a single photon in a coherent superposition of the two parties’ locations. Furthermore, it is shown that this concept allows the parties to perform secure and anonymous quantum communication employing one particle per transmitted bit. These important features can lead to the realization of new quantum communication schemes, which are simultaneously anonymous, secure, and resource-efficient.

Paper on Quantum computing with graphene plasmons

npj Quantum Information, 5, 37 (2019)

Calafell, I. A., Cox, J. D., Radonjic, M., Saavedra, J. R. M., Garcia de Abajo, F. J., Rozema, L. A. & Walther, P.

Among the various approaches to quantum computing, all-optical architectures are especially promising due to the robustness and mobility of single photons. However, the creation of the two-photon quantum logic gates required for universal quantum computing remains a challenge. Here we propose a universal two-qubit quantum logic gate, where qubits are encoded in surface plasmons in graphene nanostructures, that exploits graphene’s strong third-order nonlinearity and long plasmon lifetimes to enable single-photon-level interactions. In particular, we utilize strong two-plasmon absorption in graphene nanoribbons, which can greatly exceed single-plasmon absorption to create a “square-root-of-swap” that is protected by the quantum Zeno effect against evolution into undesired failure modes. Our gate does not require any cryogenic or vacuum technology, has a footprint of a few hundred nanometers, and reaches fidelities and success rates well above the fault-tolerance threshold, suggesting that graphene plasmonics offers a route towards scalable quantum technologies.