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 Dual-beam spectroscopy for compact systems and enhanced laser stabilisation

arXiv:2106.11014 (2021).

N. Cooper, S. A. Madkhaly, D. Johnson, D. Baldolini and L. Hackermueller

Vapour-cell spectroscopy is widely used for the frequency stabilisation of diode lasers relative to specific atomic transitions – a technique essential in cold atom and ion trapping experiments. Two laser beams, tuned to different frequencies, can be overlapped on the same spatial path as an aid to compactness; this method also enhances the resulting spectroscopic signal via optical pumping effects, yielding an increase in the sensitivity of spectroscopically-generated laser stabilisation signals. Doppler-free locking features become visible over a frequency range several hundred MHz wider than for standard saturated absorption spectroscopy. Herein we present the measured Doppler-free spectroscopy signals from an atomic vapour cell as a function of both laser frequencies, showing experimental data that covers the full, 2D parameter space associated with dual-frequency spectroscopy. We consider how dual-frequency spectroscopy could be used for enhanced frequency-stabilisation of one laser, or alternatively to frequency-stabilise two lasers simultaneously, and analyse the likely performance of such stabilisation methods based on our experimental results. We discuss the underlying physical mechanism of the technique and show that a simple rate-equation model successfully predicts the key qualitative features of our results.

Paper on Enhanced magetoassociation of 6Li in the quantum degenerate regime

arXiv:2102.01805 (2021)

V. Naniyil, Y. Zhou, G. Simmonds, N. Cooper, W. Li and L. Hackermueller

We study magnetic Feshbach resonance of ultracold 6Li atoms in a dipole trap close to quantum degeneracy. The experiment is carried out by linearly ramping down the magnetic field from the BCS to the BEC side around the broad resonance at Br=834.1G. The Feshbach molecule formation efficiency depends strongly on the temperature of the atomic gas and the rate at which the magnetic field is ramped across the Feshbach resonance. The molecular association process is well described by the Landau-Zener transition while above the Fermi temperature, such that two-body physics dominates the dynamics. However, we observe an enhancement of the atom-molecule coupling as the Fermionic atoms reach degeneracy, demonstrating the importance of many-body coherence not captured by the conventional Landau-Zener model. We develop a theoretical model that explains the temperature dependence of the atom-molecule coupling. Furthermore, we characterize this dependence experimentally and extract the atom-molecule coupling coefficient as a function of temperature, finding qualitative agreement between our model and experimental results. Accurate measurement of this coupling coefficient is important for both theoretical and experimental studies of atom-molecule association dynamics.

Paper on Performance-optimized components for quantum technologies via additive manufacturing

Phys. Rev. X Quantum, arXiv:2102.11874 (2021).

S. Madkhaly, L. A. Coles, C. Morley, C. Colqhoun, M. Fromhold and L. Hackermueller,

Novel quantum technologies and devices place unprecedented demands on the performance of experimental components, while their widespread deployment beyond the laboratory necessitates increased robustness and fast, affordable production. We show how the use of additive manufacturing, together with mathematical optimization techniques and innovative designs, allows the production of compact, lightweight components with greatly enhanced performance. We use such components to produce a magneto-optical trap that captures ∼2×108 rubidium atoms, employing for this purpose a compact and highly stable device for spectroscopy and optical power distribution, optimized neodymium magnet arrays for magnetic field generation, and a lightweight, additively manufactured ultra-high vacuum chamber. We show how the use of additive manufacturing enables substantial weight reduction and stability enhancement, while also illustrating the transferability of our approach to experiments and devices across the quantum technology sector and beyond.

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.