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.

Paper on Additively manufactured ultra-high vacuum chamber for portable quantum technologies

Additive Manufacturing, 40, 101898 (2020).


N. Cooper, L. Coles, S. Everton, R. Campion, S. Madkhaly, C. Morley, W. Evans, R. Saint, P. Krüger, F. Oručević, C. Tuck, R. Wildman, T. M. Fromhold, L. Hackermueller,

Additive manufacturing is having a dramatic impact on research and industry across multiple sectors, but the production of additively manufactured systems for ultra-high vacuum applications has so far proved elusive and widely been considered impossible. We demonstrate the first additively manufactured vacuum chamber operating at a pressure below 10−10 mbar, measured via an ion pump current reading, and show that the corresponding upper limit on the total gas output of the additively manufactured material is 3.6 × 10−13 mbar l/(s mm2). The chamber is produced from AlSi10Mg by laser powder bed fusion. Detailed surface analysis reveals that an oxidised, Mg-rich surface layer forms on the additively manufactured material and plays a key role in enabling vacuum compatibility. Our results not only enable lightweight, compact versions of existing systems, but also facilitate rapid prototyping and unlock hitherto inaccessible options in experimental science by removing the constraints that traditional manufacturing considerations impose on component design. This is particularly relevant to the burgeoning field of portable quantum sensors — a point that we illustrate by using the chamber to create a magneto-optical trap for cold 85Rb atoms — and will impact significantly on all application areas of high and ultra-high vacuum.

Paper on Universal scheme for integrating cold atoms into optical waveguides

Phys. Rev. Res. 2, 033098, (2020)


E. Da Ros, N. Cooper, J. Nute and L. Hackermueller

Hybrid quantum devices, incorporating both atoms and photons, can exploit the benefits of both to enable scalable architectures for quantum computing and quantum communication, as well as chip-scale sensors and single-photon sources. Production of such devices depends on the development of an interface between their atomic and photonic components. This should be compact, robust, and compatible with existing technologies from both fields. Here we demonstrate such an interface. Cold cesium atoms are trapped inside a transverse, 30μm-diameter through hole in an optical fiber, created via laser micromachining. When the guided light is on resonance with the cesium D2 line, up to 87% of it is absorbed by the atoms. The corresponding optical depth per unit length is ∼700 cm−1, higher than any reported for a comparable system. This is important for miniaturization and scalability. The technique can be equally effective in optical waveguide chips and other existing photonic systems, providing a promising platform for fundamental research.

Paper on Prospects for strongly coupled atom-photon quantum nodes

Nature Sci. Reports, 9, 7798 (2019).


N. Cooper, C. Briddon, E. da Ros, V. Naniyil, M. Greenaway and L Hackermueller

We discuss the trapping of cold atoms within microscopic voids drilled perpendicularly through the axis of an optical waveguide. The dimensions of the voids considered are between 1 and 40 optical wavelengths. By simulating light transmission across the voids, we find that appropriate shaping of the voids can substantially reduce the associated loss of optical power. Our results demonstrate that the formation of an optical cavity around such a void could produce strong coupling between the atoms and the guided light. By bringing multiple atoms into a single void and exploiting collective enhancement, cooperativities ~400 or more should be achievable. The simulations are carried out using a finite difference time domain method. Methods for the production of such a void and the trapping of cold atoms within it are also discussed.

Paper on Nanofiber-based high-Q microresonator for cryogenic applications published

Johanna Hütner, Thomas Hoinkes, Martin Becker, Manfred Rothhardt, Arno Rauschenbeutel, and Sarah M. Skoff

We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free optical microresonator. The compatibility with low temperatures expands its possible applications to the wide field of solid-state quantum optics, where a cryogenic environment is often a requirement. At a temperature of 4.6 K we obtain a quality factor of (9.9 ± 0.7) × 106. In conjunction with the small mode volume provided by the nanofiber, this cavity can be either used in the coherent dynamics or the fast cavity regime, where it can provide a Purcell factor of up to 15. Our resonator is therefore suitable for significantly enhancing the coupling between light and a large variety of different quantum emitters and due to its proven performance over a wide temperature range, also lends itself for the implementation of quantum hybrid systems.

Paper on Interaction signatures and non-Gaussian photon states published

B. Olmos, G. Buonaiuto, P. Schneeweiss, and I. Lesanovsky

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.