High-performance, additively-manufactured atomic spectroscopy apparatus for portable
quantum technologies

Optics Express 30, 25753 (2022).



We demonstrate a miniaturised and highly robust system for performing Doppler-free spectroscopy on thermal atomic vapour for three frequencies as required for cold atom-basedquantum technologies. The application of additive manufacturing techniques, together with efficient use of optical components, produce a compact, stable optical system, with a volume of 0.089 L and a weight of 120 g. The device occupies less than a tenth of the volume of, and is considerably lower cost than, conventional spectroscopic systems, but also offers excellent stability against environmental disturbances. We characterise the response of the system to changes in environmental temperature between 7 and 35 ◦C and exposure to vibrations between 0 – 2000 Hz, finding that the system can reliably perform spectroscopic measurements despite substantial vibrational noise and temperature changes. Our results show that 3D-printed optical systems are an excellent solution for portable quantum technologies.

Experimental semi-device-independent certification of indefinite causal order

arXiv:2202.05346 [quant-ph]

Huan Cao, Jessica Bavaresco, Ning-Ning Wang, Lee A. Rozema, Chao Zhang, Yun-Feng Huang, Bi-Heng Liu, Chuan-Feng Li, Guang-Can Guo, Philip Walther

We report the first experimental certification of indefinite causal order that relies only on the characterization of the operations of a single party. We do so in the semi-device-independent scenario with the fewest possible assumptions of characterization of the parties’ local operations in which indefinite causal order can be demonstrated with the quantum switch. To achieve this result, we introduce the concept of semi-device-independent causal inequalities and show that the correlations generated in a tabletop optical implementation of the quantum switch, in which all parties are able to collect local outcome statistics, achieve a violation of this inequality of 224 standard deviations. This result consists of the experimental demonstration of indefinite causal order with the fewest device-characterization assumptions to date

Enhanced Photonic Maxwell’s Demon with Correlated Baths

Quantum (in print)


Guilherme L. Zanin, Michael Antesberger, Maxime J. Jacquet, Paulo H. Souto Ribeiro, Lee A. Rozema, Philip Walther

Maxwell’s Demon is at the heart of the interrelation between quantum information processing and thermodynamics. In this thought experiment, a demon extracts work from two thermal baths at equilibrium by gaining information about them at the single-particle level and applying classical feed-forward operations. Here we implement a photonic version of Maxwell’s Demon with active feed-forward in a fiber-based system using ultrafast optical switches. We experimentally show that, if correlations exist between the two thermal baths, the Demon can extract over an order of magnitude more work than without correlations. Our work demonstrates the great potential of photonic experiments — which provide a unique degree of control on the system — to access new regimes in quantum thermodynamics

Towards probing for hypercomplex quantum mechanics in a waveguide interferometer

New Journal of Physics, Vol 23 (2021)


S. Gstir, E. Chan, T. Eichelkraut, A. Szameit, R. Keil and G. Weihs

We experimentally investigate the suitability of a multi-path waveguide interferometer with mechanical shutters for performing a test for hypercomplex quantum mechanics. Probing the interferometer with coherent light, we systematically analyse the influence of experimental imperfections that could lead to a false-positive test result. In particular, we analyse the effects of detector nonlinearity, input-power and phase fluctuations on different timescales, closed-state transmissivity of shutters and crosstalk between different interferometer paths. In our experiment, a seemingly small shutter transmissivity in the order of about 2 × 10−4 is the main source of systematic error, which suggests that this is a key imperfection to monitor and mitigate in future experiments.

Quantum random number generation using a hexagonal boron nitride single photon emitter

Journal of Optcs, Vol 23, Num 1 (2020)


S . J U White1, F. Klauck, T. T. Tran, N. Schmitt, M. Kianinia, A. Steinfurth, M. Heinrich, M. Toth, A. Szameit, I. Aharonovich

Quantum random number generation (QRNG) harnesses the intrinsic randomness of quantum mechanical phenomena. On-chip photonic circuitry provides a robust and versatile platform that can address and explore fundamental questions in quantum as well as classical physics. Likewise, integrated waveguide-based architectures hold the potential for intrinsically scalable, efficient and compact implementations of photonic QRNG. Here, we harness the quantum emission from the two-dimensional material hexagonal boron nitride an emerging atomically thin medium that can generate single photons on demand while operating at room temperature. By means of a customized splitter arrangement, we achieve true random number generation through the measurement of single photons exiting one of four designated output ports, and subsequently verify the randomness of the sequences in accordance with the National Institute of Standards and Technology benchmark suite.

Our results clearly demonstrate the viability and efficiency of this approach to on-chip deterministic random number generators.

Exploring complex graphs using three-dimensional quantum walks of correlated photons

Science Advances, Vol 7, Issue 9, 2021


M. Ehrhard, R. Keil, L.J. Maczewsky, Ch. Dittel, M. Heinrich and A. Szameit

Graph representations are a powerful concept for solving complex problems across natural science, as patterns of connectivity can give rise to a multitude of emergent phenomena. Graph-based approaches have proven particularly fruitful in quantum communication and quantum search algorithms in highly branched quantum networks. Here, we introduce a previously unidentified paradigm for the direct experimental realization of excitation dynamics associated with three-dimensional networks by exploiting the hybrid action of spatial and polarization degrees of freedom of photon pairs in complex waveguide circuits with tailored birefringence. This testbed for the experimental exploration of multiparticle quantum walks on complex, highly connected graphs paves the way toward exploiting the applicative potential of fermionic dynamics in integrated quantum photonics.

Observation of PT-symmetric quantum interference

Nature Photonics 13, 883-887 (2019)


F. Klauck, L. Teuber, M. Ornigotti, M. Heinrich, S. Scheel & A. Szameit

A common wisdom in quantum mechanics is that the Hamiltonian has to be Hermitian in order to ensure a real eigenvalue spectrum. Yet, parity–time (PT)-symmetric Hamiltonians are sufficient for real eigenvalues and therefore constitute a complex extension of quantum mechanics beyond the constraints of Hermiticity. However, as only single-particle or classical wave physics has been exploited so far, an experimental demonstration of the true quantum nature of PT symmetry has been elusive. In our work, we demonstrate two-particle quantum interference in a PT-symmetric system. We employ integrated photonic waveguides to reveal that the quantum dynamics of indistinguishable photons shows strongly counterintuitive features. To substantiate our experimental data, we analytically solve the quantum master equation using Lie algebra methods. The ideas and results presented here pave the way for non-local PT-symmetric quantum mechanics as a novel building block for future quantum devices.

Embedded nanograting-based waveplates for polarization control in integrated photonic circuits

Optical Materials Express, Vol. 9, Issue 6, pp. 2560-2572 (2019)


K. Lammers, M. Ehrhardt, T, Malendevych, X. Xu, Ch. Vetter, A.Alberucci, A. Szameit, and S. Nolte

Femtosecond laser direct writing (FLDW) enables precise three-dimensional structuring of transparent host materials such as fused silica. With this technique, reliable integrated optical circuits can be written, which are also a possible candidate for future quantum technologies. We demonstrate the manufacturing of integrated waveplates with arbitrary orientations and various phase delays by combining embedded birefringent nanograting structures and FLDW waveguides in fused silica glass. These waveplates can be used both for classical applications and for quantum gates.

Unraveling Two-Photon Entanglement via the Squeezing Spectrum of Light Traveling through Nanofiber-Coupled Atoms

Phys. Rev. Lett. 127, 123602 (2021)


Jakob Hinney, Adarsh S. Prasad, Sahand Mahmoodian, Klemens Hammerer, Arno Rauschenbeutel, Philipp Schneeweiss, Jürgen Volz, and Max Schemmer

We observe that a weak guided light field transmitted through an ensemble of atoms coupled to an optical nanofiber exhibits quadrature squeezing. From the measured squeezing spectrum we gain direct access to the phase and amplitude of the energy-time entangled part of the two-photon wave function which arises from the strongly correlated transport of photons through the ensemble. For small atomic ensembles we observe a spectrum close to the line shape of the atomic transition, while sidebands are observed for sufficiently large ensembles, in agreement with our theoretical predictions. Furthermore, we vary the detuning of the probe light with respect to the atomic resonance and infer the phase of the entangled two-photon wave function. From the amplitude and the phase of the spectrum, we reconstruct the real and imaginary part of the time-domain wave function. Our characterization of the entangled two-photon component constitutes a diagnostic tool for quantum optics devices.

Bragg condition for scattering into a guided optical mode

Phys. Rev. A 104, 043517 (2021)


B. Olmos, C. Liedl, I. Lesanovsky, P. Schneeweiss

We theoretically investigate light scattering from an array of atoms into the guided modes of a waveguide. We observe that the scattering of a plane-wave laser field into the waveguide modes is dramatically enhanced for angles that deviate from the geometric Bragg angle. This modified Bragg condition arises from the dispersive interactions between the guided light and the atoms. We analytically identify various parameter regimes in which the scattering rate features a qualitatively different dependence on the atom number, such as linear, quadratic, oscillatory, or constant behavior. In combination with rigorous numerical calculations, we demonstrate that these scalings are independent of a possible asymmetry of the atom-light coupling. Finally, we show that our findings are robust against voids in the atomic array, facilitating their experimental observation and potential applications. Our work sheds light on collective light scattering and the interplay between geometry and interaction effects, with implications reaching beyond the optical domain.