LKB SEMINARS/SÉMINAIRES DU LKB

Martin ZWIERLEIN

(Massachusetts Institute of Technology)

Owls to Athens: Recent experiments on Rotating Bose Gases

Abstract – Rapidly rotating quantum gases, pioneered at LKB, realize the physics of charged particles in high magnetic fields. We developed a novel protocol, geometric squeezing, that enables to create Bose-Einstein condensates in a single Landau gauge wavefunction of the lowest Landau level. Based on the non-commutativity of guiding center X and Y coordinates, geometric squeezing in a saddle potential is a real space analogue to squeezing in phase space of an inverted 1D harmonic oscillator. The condensate’s transverse width shrinks to the Heisenberg-limited ground-state extent of cyclotron motion. Removing the saddle enables studying the evolution of a Landau gauge condensate in « flat land » under the sole influence of interactions. Surprisingly, we find that Landau gauge condensates are unstable towards crystallization into arrays of droplets. This instability of states in the lowest Landau level has its classical analogy in the Kelvin-Helmholtz instability of counterflowing liquids. We explore the crossover of this instability from the lowest Landau level to the Thomas-Fermi regime. I will discuss experiments on observing edge states in confined geometries and prospects to extend this work beyond mean-field quantum Hall states of bosons.

Gerhard REMPE

(Max-Planck Institute of Quantum Optics, Germany)

Massively entangled photonic graph states

Abstract – Individually addressable atoms strongly coupled to optical cavities constitute light-matter quantum modules with numerous applications in quantum information processing. The talk will present a new capability of the system as an efficient, high-fidelity entanglement source for large photonic graph states. Unprecedented entanglement rates for more than a dozen photons are now state-of-the-art. This opens up new perspectives for measurement-based quantum computation and loss-tolerant quantum communication.

Nicolas GISIN

(Group of Applied Physics, University of Geneva, Switzerland)

Quantum Non-Locality in Networks

Abstract – Quantum non-locality, i.e. the violation of some Bell inequality, has proven to be an extremely useful concept in analyzing entanglement, quantum randomness and cryptography, among others. In particular, it led to the fascinating field of device-independent quantum information processing.

Historically, the idea was that the particles emitted by various quantum sources carry additional variables, known as local hidden variables. The more modern view, strongly influenced by computer science, refers to these additional variables as shared randomness. This, however, leads to ambiguity when there is more than one source, as in quantum networks. Should the randomness produced by each source be considered as fully correlated, as in most common analyses, or should one analyze the situation assuming that each source produces independent randomness, closer to the historical spirit?

The latter is known, for the case of n independent sources, as n-locality. For example, in entanglement swapping there are two sources, hence “quantumness” should be analyzed using 2-locality (or, equivalently, bi-locality). The situation when the network has loops is especially interesting. Recent results for triangular networks will be presented. Another result is that some quantum networks can’t be described by real-number Hilbert spaces.

Francesca FERLAINO

(University of Innsbruck and Institute for Quantum Optics and Quantum Information (IQOQI), Austrian Academy of Sciences)

Many-body physics in dipolar quantum gases

Abstract – Brought to quantum degeneracy, ultracold gases enable the study of many-body quantum phenomena, in which the interaction between atoms can be so carefully mastered as to determine the very state of matter. Typically, this interaction has a short-range and isotropic nature, the latter meaning that the atoms globally attract or repel each other. However, there is another possibility that naturally emerges for some specific atomic species, such as erbium and dysprosium, featuring an extraordinarily large magnetic dipole moment. Magnetic properties give rise to dipolar many-body interactions, qualitatively very different from others in that it is long-range and anisotropic, thus adding connectivity and directionality at the quantum level.

In the present talk I will retrace the fundamental steps in the study of dipolar gases, with emphasis on the Innsbruck results, from their creation to the new phenomena unveiled such as the emergence of rotonic excitations, so named by Landau, to the observation of a new and paradoxical state of matter with multiple spontaneous symmetry breaking, known as supersolid.

Martino TRASSINELLI

(Institut des NanoSciences de Paris, CNRS, Paris, France)

Testing quantum electrodynamics at extreme fields in the heaviest few-electron atoms

Abstract – Quantum electrodynamics (QED), the quantum field theory that describes the interaction between light and matter is commonly regarded as the best tested quantum theory in modern physics. However, this claim is mostly based upon extremely precise studies performed in the domain of relatively low-field strengths and light atoms/ions. In the realm of very strong electromagnetic fields, as present e.g. in the heaviest highly-charged ions, QED calculations enter a qualitatively different non-perturbative regime (in Za with Z the nuclear charge and a the fine structure constant) with quantum vacuum fluctuations effects such as vacuum polarisation and self-energy strongly affecting the atomic binding energies. Experimental studies on this regime are very challenging due to the required extremely high ionization state of the investigated ions. Therefore, at present, precision of QED tests in the non-perturbative strong-field domain still remains considerably lower as compared to the perturbative regime. For these endeavours, precise understanding of the structure of highly-charged ions is of great importance to test bound-state QED in strongest Coulomb fields. We present here the most recent experimental developments for measuring of transition energy in heavy, few-electron atoms. In particular, we present a recent experiment on high-accuracy x-ray spectroscopy of two-, three-, and four-electron uranium ions (Z=92, i.e. Za=0.7). The obtained experimental accuracy allows to test, on the one hand, two-loop quantum electrodynamics effects for the first time in heavy helium-like atoms, and, on the other hand, to isolate the effects of electron-electron interaction in heavy bound systems. Such measurements rule out some of the most recent theoretical predictions.

Silke OSPELKAUS

(Institut of Quantum Optics, Leibniz Universität Hannover, Germany)

Quantum gases of ultracold polar molecules

Abstract – In recent years, tremendous progress in the preparation and control of ultracold molecular gases in the quantum regime has been achieved and has opened exciting new research opportunities. Molecules rotate and oscillate and therefore offer many more quantum degrees of freedom than their atomic counterparts. Polar molecules interact via strong and long-range anisotropic interactions. These unique molecular properties lead to largely unexplored new possibilities and surprising results.  These range from peculiar scattering properties via the control of ultracold collisions and chemical reactions to strongly correlated dipolar quantum many-body systems.

In this talk I will take you on a journey through the world of cold and ultracold molecular systems and discuss future prospects.

Patrice BERTET

(Université Paris Saclay, CEA Saclay, SPEC)

Single-electron-spin-resonance detection by microwave photon counting

Abstract – Electron spin resonance (ESR) spectroscopy is the method of choice for characterizing paramagnetic impurities, with applications ranging from chemistry to quantum computing, but it gives access only to ensemble-averaged quantities due to its limited signal-to-noise ratio. The sensitivity needed to detect single electron spins has been reached so far using spin-dependent photoluminescence, transport measurements, or scanning-probe techniques. These methods are system-specific or sensitive only in a small detection volume, so that practical single spin detection remains an open challenge.

Here, we demonstrate single electron spin resonance at millikelvin temperature by spin fluorescence detection [1], using a microwave photon counter at cryogenic temperatures based on a superconducting transmon qubit [2]. We detect individual paramagnetic erbium ions in a scheelite crystal coupled to a small-mode-volume, high-quality factor superconducting resonator to enhance their radiative decay rate [3], with a signal-to-noise ratio of 1.9 in one second integration time. The fluorescence signal shows anti-bunching, proving that it comes from individual emitters [4]. Coherence times up to 3ms are measured, limited by the ion radiative lifetime. The method applies to arbitrary paramagnetic species with long enough non-radiative relaxation time, and offers large detection volumes; as such, it may find applications in magnetic resonance and quantum computing [4].

[1] E. Albertinale et al., Nature 600, 434 (2021) 

[2] R. Lescanne et al., Phys. Rev. X 10, 021038 (2020) 

[3] A. Bienfait et al., Nature 531, 74 (2016) 

[4] Z. Wang et al., arXiv:2301.02653 (2023) 

Philipp TREUTLEIN

(Department of Physics, University of Basel, Switzerland)

An Einstein-Podolsky-Rosen experiment with two Bose-Einstein condensates

Abstract – In 1935, Einstein, Podolsky and Rosen (EPR) conceived a Gedankenexperiment which became a cornerstone of quantum technology and still challenges our understanding of reality and locality today. While the experiment has been realized with small quantum systems, a demonstration of the EPR paradox with spatially separated, massive many-particle systems had so far remained elusive. In our experiment, we observe the EPR paradox with two spatially separated Bose-Einstein condensates containing about 700 Rubidium atoms each. Entanglement between the two condensates results in strong correlations between measurement outcomes of their collective spins, satisfying Reid’s criterion for the EPR paradox. Our results show that the conflict between quantum mechanics and local realism does not disappear as the system size is increased to over a thousand massive particles. Furthermore, EPR entanglement in conjunction with individual manipulation of the two condensates on the quantum level, as demonstrated here, constitutes an important resource for quantum metrology and information processing with many-particle systems.

P. Colciaghi, Y. Li, P. Treutlein, and T. Zibold, Einstein-Podolsky-Rosen experiment with two Bose-Einstein condensates, arXiv:2211.05101.

Jacqueline BLOCH

(C2N Palaiseau*)

Excitonic polaritons in semiconductor lattices: emulating condensed matter physics

Abstract – Photonic resonators, coupled within a lattice, have appeared in the recent years as a powerful synthetic platform to imprint on light some of the fascinating physical properties that can emerge in condensed matter, or even to go beyond what exists in nature. For instance, light can become superfluid, present spin orbit coupling, spin Hall effect or propagate along topologically protected edge states. New physical properties may emerge when drive and dissipation come into play. Such realizations are not only interesting from a fundamental point of view, but also inspire innovative photonic devices.

After a general introduction to exciton-polariton (polariton) physics and polariton lattices [1], I will present some recent experiments we have performed at C2N. Using lattices of semiconductor microcavities, we explore single and many body physics of photons in 1D or 2D lattices and the emergence of novel physics related to the openness of the system [2]. Topological physics can be investigated when non-linearities come into play [3]. Interestingly, our photonic platform also enables exploring the physics of out of equilibrium Bose Einstein Condensates. We have recently shown that the coherence of these BECs features universal scaling related to the Kardar–Parisi–Zhang universality class [4].

[1] C. Ciuti and I. Carusotto, Quantum fluids of light, Rev. Mod. Phys. 85, 299 (2013) 

[2] A. Amo and J. Bloch, Exciton-polaritons in lattices: A non-linear photonic simulator, Comptes Rendus de l’Académie des Sciences 8, 805 (2016) (Elsevier) 

[3] N. Pernet et al., Topological gap solitons in a 1D non-Hermitian lattice, Nature Physics 18, 678 (2022) 

[4] Q. Fontaine et al., Observation of KPZ universal scaling in a one-dimensional polariton condensate, Nature 608, 687 (2022) 

* Center for Nanoscience and Nanotechnology (C2N), Université Paris Saclay / CNRS
Palaiseau, France

Immanuel BLOCH

(Max Planck Garching)

Quantum Simulations using Ultracold Atoms in Optical Lattices

Abstract – 40 years ago, Richard Feynman outlined his vision of a quantum simulator for carrying out complex calculations of physical problems. Today, his dream has become a reality and a highly active field of research across different platforms ranging from ultracold atoms and ions, to superconducting qubits and photons. In my lecture, I will outline how ultracold atoms in optical lattices started this vibrant and interdisciplinary research field 20 years ago and now allow probing quantum phases in- and out-of-equilibrium with fundamentally new tools and single particle resolution. Novel (hidden) order parameters, entanglement properties, full counting statistics or topological features can now be measured routinely and provide deep new insight into the world of correlated quantum matter. I will introduce the measurement and control techniques and a few recent applications regarding quantum simulations of condensed matter systems, namely the Fermi Hubbard model, which plays an essential role in the context of High-Tc superconductivity, experiments on new dynamical phases of matter, as well as topological systems.

Andreas WALLRAFF

(Department of Physics, ETH Zurich, Switzerland)

Realizing Quantum Error Correction with Superconducting Circuits*

Abstract – Superconducting electronic circuits are ideally suited for studying quantum physics and its applications. Since complex circuits containing hundreds or thousands of elements can be designed, fabricated, and operated with relative ease, they are one of the prime contenders for realizing quantum computers. Currently, both academic and industrial labs vigorously pursue the realization of universal fault-tolerant quantum computers. However, building systems which can address commercially relevant computational problems continues to require significant conceptual and technological progress.

For fault-tolerant operation quantum computers must correct errors occurring due to unavoidable decoherence and limited control accuracy. Here, we demonstrate quantum error correction using the surface code, which is known for its exceptionally high tolerance to errors. Using 17 physical qubits in a superconducting circuit we encode quantum information in a distance-three logical qubit building up on our recent distance-two error detection experiments [1]. In an error correction cycle taking only 1.1 µs, we demonstrate the preservation of four cardinal states of the logical qubit. Repeatedly executing the cycle, we measure and decode both bit- and phase-flip error syndromes using a minimum-weight perfect-matching algorithm in an error-model-free approach and apply corrections in postprocessing. We find a low logical error probability of 3 % per cycle [2].

The measured characteristics of our device agree well with a numerical model. Our demonstration of repeated, fast, and high-performance quantum error correction cycles, together with recent advances in ion traps, support our understanding that fault-tolerant quantum computation will be practically realizable.

[1] C. Kraglund Andersen et al., Nature Physics 16, 875–880 (2020) 

[2] S. Krinner, N. Lacroix et al., Nature 605, 669–674 (2022) 

* Work done in collaboration with Sebastian Krinner, Nathan Lacroix, Ants Remm, Agustin Di Paolo, Elie Genois, Catherine Leroux, Christoph Hellings, Stefania Lazar, Francois Swiadek, Johannes Herrmann, Graham J. Norris, Christian Kraglund Andersen, Markus Müller, Alexandre Blais, Christopher Eichler, and Andreas Wallraff

Päivi TÖRMÄ

(Aalto University – Helsinki, Finland)

New perspectives on quantum geometry, superconductivity and Bose-Einstein condensation

Abstract – Superconductivity, superfluidity and Bose-Einstein condensation (BEC) are many-body phenomena where quantum statistics are crucial and the effect of interactions may be intriguing. Superconductors are already widely applied, but theoretical understanding of superconductivity and condensation in several real world systems is still a challenge, and superconductivity at room temperature remains a grand goal. We have discovered that superconductivity (superfluidity) has a connection to quantum geometry [1]. Namely, the superfluid weight in a multiband system has a previously unnoticed component which we call the geometric contribution. It is proportional to the quantum metric of the band. Quantum metric is connected to the Berry curvature, and this allows to relate superconductivity with the topological properties of the band. Using this theory, we have shown that superconductivity is possible also in a flat band where individual electrons would not move. Recently, we and other groups have shown [2,3] that these results are essential in explaining the intriguing observation of superconductivity in bilayer graphene and may eventually help realize superconductors at elevated temperatures. We have also explored the effect of quantum geometry on Bose-Einstein condensation [4].

Bose-Einstein condensation has been realized for various particles or quasi-particles, such as atoms, molecules, photons, magnons and semiconductor exciton polaritons. We have experimentally realized a new type of condensate: a BEC of hybrids of surface plasmons and light in a nanoparticle array [5]. The condensate forms at room temperature and shows ultrafast dynamics, and the system provides easy tunability of the lattice and unit cell geometry and symmetries. Recently, we have observed formation of polarization textures and domain walls, and obtained the BEC phase for the first time using a phase retrieval algorithm [6]. Our measurements of spatial and temporal coherence show a change from exponential decay to power-law when crossing to the BEC phase [7]. We have also observed topological bound states of continuum (BICs) and loss-driven topological transitions in nanoparticle arrays [8].

[1] S. Peotta, P. Törmä, Nature Commun. 6, 8944 (2015); A. Julku, S. Peotta, T.I. Vanhala, D.-H. Kim, P. Törmä, Phys. Rev. Lett. 117, 045303 (2016); P. Törmä, L. Liang, S. Peotta, Phys. Rev. B 98, 220511(R) (2018): K.-E. Huhtinen, J. Herzog-Arbeitman, A. Chew, B.A. Bernevig, P. Törmä, Phys. Rev. B Editor’s Suggestion arXiv:2203.11133 (2022) 

[2] A. Julku, T.J. Peltonen, L. Liang, T.T. Heikkilä, P. Törmä, Phys. Rev. B 101, 060505(R) (2020); X. Hu, T. Hyart, D.I. Pikulin, E. Rossi, Phys. Rev. Lett. 123, 237002 (2019); F. Xie, Z. Song, B. Lian, B.A. Bernevig, Phys. Rev. Lett. 124, 167002 (2020); for a news article see L. Classen, Physics 13, 23 (2020) https://physics.aps.org/articles/v13/23 

[3] P. Törmä, S. Peotta, B.A. Bernevig, Nat. Rev. Phys., https://doi.org/10.1038/s42254-022-00466-y (2022) 

[4] A. Julku, G.M. Bruun, P. Törmä, Phys. Rev. Lett., 127, 170404 (2021) 

[5] T.K. Hakala, A.J. Moilanen, A.I. Väkeväinen, R. Guo, J.-P. Martikainen, K.S. Daskalakis, H.T. Rekola, A. Julku, P. Törmä, Nature Phys. 14, 739 (2018); A.I. Väkeväinen, A.J. Moilanen, M. Necada, T.K. Hakala, P. Törmä, Nature Commun. 11, 3139 (2020) 

[6] J.M. Taskinen, P. Kliuiev, A.J. Moilanen, P. Törmä, Nano Letters 21, 5202 (2021) 

[7] A.J. Moilanen, K.S. Daskalakis, J.M. Taskinen, P. Törmä, Phys. Rev. Lett. 127, 255301 (2021) 

[8] R. Heilmann, G. Salerno, J. Cuerda, T.K. Hakala, P. Törmä, ACS Photonics 9, 224 (2022); G. Salerno, R. Heilmann, K. Arjas, K. Aronen, J.-P. Martikainen, P. Törmä, arXiv:2206.08897 (2022) 

Gerardo ADESSO

(University of Nottingham)

Every quantum helps

Abstract – What makes quantum technologies tick? Advantages in communication, computation, metrology and other applications can be traced back to distinct manifestations of quantum theory, such as coherence and entanglement. We characterise these elusive quantum signatures and their operational power under the unifying lens of resource theories. We then show that every (convex) quantum resource yields an advantage in a channel discrimination task, enabling a strictly greater success probability than what is achievable by any state without the given resource. We further discuss recent progress in untapping these benefits for practical quantum-enhanced imaging and sensing tasks

Rainer BLATT

(University of Innsbruck)*

The Quantum Way of Doing Computations, Simulations and Measurements 

Abstract – In this talk, the basic functional principles of quantum information processing are reviewed and the state-of-the-art of the Innsbruck trapped-ion quantum computer is reported. With strings of trapped ions, we implement a quantum information processor and perform quantum operations. We present an overview on the available quantum toolbox and discuss the scalability of the approach. The quantum way of doing computations is illustrated with analog and digital quantum simulations. Employing universal quantum computations, we investigate the dynamics of the Lattice Schwinger model [1], a gauge theory of 1D quantum electrodynamics and using a hybridclassical ansatz, we determine steady-state properties of the Hamiltonian [2]. Using tailored quantum operations, we obtain optimized measurements for spectroscopy [3].

[1] E. A. Martinez et al., Nature 534, 516 (2016). 

[2] C. Kokail et al., Nature 569, 355–360 (2019). 

[3] C. Marciniak et al., Nature 603, 604 (2022). 

*Inst. for Experimental Physics, University of Innsbruck and IQOQI Innsbruck, Austrian Academy of Sciences

Barbara TERHAL

(TU Delft)*

Quantum Error Correction: Dream or Nightmare

Abstract – I will discuss the ideas behind quantum error correction. I will give an overview of some recent research on quantum error correction with superconducting qubits. In particular I will discuss the problem of qubit leakage as well as the alternative of bosonic error correction.

*QuTech & EEMCS Dept. Delft University of Technology

Klaus Blaum

(Max-Planck Heidelberg)

Precision Tests of Fundamental Interactions and Their Symmetries using Exotic Ions in Penning Traps An overview is given on recent mass and g-factor measurements with extreme precision on single or few cooled ions stored in Penning traps. On the one hand, mass measurements provide crucial information for atomic, nuclear and neutrino physics as well as for testing fundamental interactions and their symmetries. On the other hand, g-factor measurements of the bound electron in highly charged hydrogen-like ions allow for the determination of fundamental constants and for constraining Quantum Electrodynamics. For example, the most stringent test of CPT symmetry in the baryonic sector could be performed by mass comparison of the antiproton with H- and the knowledge of the electron atomic mass could be improved by a factor of 13.

Robin Kaiser (University of Cambridge)

Zoran Hadzibabic (University of Cambridge)

05/25/22

Klaus Mølmer (University of Aarhus, Denmark)

David Clément (Laboratoire Charles Fabry – Institut d’Optique)

13H45

ENS

Piet O. Schmidt

(Universität Hannover)

Highly charged ions (HCI) have many favorable properties for tests of fundamental physics and as potential next-generation optical atomic frequency standards [1]. For example, narrow optical fine-structure transitions have smaller polarizabilities and electric quadrupole moments, but much stronger relativistic, QED and nuclear size contributions to their binding energy compared to their (near) neutral counterparts. Therefore, HCI have been found to be among the most sensitive atomic species to probe for a possible variation of the fine-structure constant or dark matter coupling.
HCI can readily be produced and stored in an electron beam ion trap (EBIT). There, the most accurate laser spectroscopy on any HCI was performed on the 17 Hz wide fine-structure transition in Ar¹³⁺ with 400 MHz resolution, lagging almost twelve orders of magnitude behind state-of-the-art optical clocks. This was primarily limited by Doppler broadening of the megakelvin hot ion plasma in the EBIT [2]. The lack of a suitable optical transition for laser cooling and detection can be overcome through sympathetic cooling with a co-trapped Be⁺ ion [3]. Techniques developed for quantum information processing with trapped ions can be used to perform quantum logic spectroscopy [4]: A series of laser pulses transfers the internal state information of the Ar¹³⁺ ion after spectroscopy onto the Be⁺ ion for efficient readout.
We present the first coherent laser spectroscopy of an HCI. Ar¹³⁺ are extracted from a compact EBIT [5], charge-to-mass selected and injected into a cryogenic Paul trap containing a crystal of laser-cooled Be⁺ ions [6]. By removing excess Be⁺ ions, a crystal composed of a Be⁺/Ar¹³⁺ ion pair is obtained. Results on sympathetic ground state cooling and quantum logic spectroscopy of the Ar¹³⁺ P_1/2-P_3/2 fine-structure transition at 441 nm will be presented, improving the precision of the observed line center by more than eight orders of magnitude. Furthermore, excited state lifetimes and the first high-accuracy measurement of excited state g-factor demonstrate the versatility of the technique to access all relevant atomic parameters [7]. Finally, we have started to perform frequency measurements of this transition, including first estimates of systematic uncertainties.

• G. Kozlov et al., Rev. Mod. Phys. 90, 045005 (2018).
• Draganić et al., Phys. Rev. Lett. 91, (2003).
• Schmöger et al., Science 347, 1233–1236 (2015).
• O. Schmidt et al., Science 309, 749–752 (2005).
• Micke et al., Rev. Sci. Instrum. 89, 063109 (2018).
• Leopold et al., Rev. Sci. Instrum. 90, 073201 (2019).
• Micke et al., Nature 578 60-65 (2020).

Tilman Pfau

(University of Stuttgart)

Dipolar interactions are fundamentally different from the usual van der Waals forces in real gases. Besides the anisotropy the dipolar interaction is nonlocal and as such allows for self organized structure formation. In 2005 the first dipolar effects in a quantum gas were observed in an ultracold Chromium gas. By the use of a Feshbach resonance a purely dipolar quantum gas was observed three years after [1]. Recently it became possible to study degenerate gases of lanthanide atoms among which one finds the most magnetic atoms. Similar to the Rosensweig instability in classical magnetic ferrofluids self-organized structure formation was expected. In our experiments with quantum gases of Dysprosium atoms we could observe the formation of a droplet crystal [2]. In contrast to theoretical mean field based predictions the super-fluid droplets did not collapse. We find that this unexpected stability is due to beyond mean-field quantum corrections of the Lee-Huang-Yang type [3,4]. We observe and study self-bound droplets [5] which can interfere with each other. We also observe self-organized stripes in a confined geometry [6] and collective scissors mode oscillations of dipolar droplets [7Recently in the striped phase also phase coherence was observed in Dysprosium and Erbium experiments, which is evidence for a supersolid state of matter [8]. Upon crossing the transition to the dipolar supersolid a Goldstone mode appears, which we have observed recently [9]. The existence of this mode proofs the superfluid stiffness or the so-called phase rigidity of this new state of matter. Despite the lack of symmetry protection, also a Higgs mode was predicted to be observable in a finite system [10].

References

[1] T. Lahaye, et al., Rep. Prog. Phys. 72, 126401 (2009)

[2] H. Kadau, et al., Nature 530, 194 (2016)

[3] T.D. Lee, K. Huang, and C. N. Yang, Phys. Rev. 106, 1135 (1957),

D.S. Petrov, Phys. Rev. Lett. 115, 155302 (2015).

[4] I. Ferrier-Barbut, et al., Phys. Rev. Lett. 116, 215301 (2016)

[5] M. Schmitt, et al., Nature 539, 259 (2016)

[6] M. Wenzel, et al., Phys. Rev. A 96 053630 (2017)

[7] I. Ferrier-Barbut, et al., Phys. Rev. Lett. 120, 160402 (2018)

[8] F. Böttcher, et al. Phys. Rev. X. 9, 011051 (2019), see also L. Tanzi, et al. Phys. Rev. Lett. 122, 130405 (2019), L. Chomaz et al., Phys. Rev. X 9, 021012 (2019)

[9] M. Guo, et al. Nature. 574, 386–389 (2019)

[10] J. Hertkorn et al., Phys. Rev. Lett. 123, 193002 (2019

Valery Nesvizhevsky

(Institut Laue-Langevin)

Gravitational Quantum States (GQS) and Whispering-Gallery Quantum (WGS) states of light neutral particles are a useful tool for high-precision measurements. Such quantum states of neutrons have been observed [1] and used ourdays by several research groups to constrain fundamental short-range interactions. Analogous states of ultracold atoms and antiatoms have been predicted [2]; they appear due to quantum reflection of ultracold (anti)atoms from surface [3]. Studies with hydrogen is the goal of a new GRASIAN collaboration.

Those with antihydrogen can be pursuit in the future by GBAR collaboration at CERN in more precise experiments, which test the equivalence principle with antimatter [4]. In all these cases, a long observation time thus a much better energy resolution and precision can be achieved in a novel Magneto-Gravitational Trap (MGT), where the particles will be trapped vertically by gravity and a mirror, and horizontally by a magnetic field [5]. The ultralow energies of (anti)atoms and long lifetimes of GQS provide unique conditions for precision gravitational, optical and hyperfine spectroscopy of (anti)atoms.

In this talk we will discuss some experimental and theoretical results as well as prospects of these activities

1. V.V. Nesvizhevsky et al, Quantum states of neutrons in the Earth’s gravitational field, Nature 415 (2002) 297; Neutron whispering gallery, Nature Phys. 6 (2010) 114.

2. A.Yu. Voronin et al, Gravitational quantum states of antihydrogen, Phys. Rev. A 83 (2011) 032903.

3. P.-P. Crepin et al, Quantum reflection of antihydrogen from a liquid helium bulk, Hyperf. Int. 240 (2019) 58.

4. P.-P. Crepin et al, Quantum interference test of the equivalence principle of antihydrogen, Phys. Rev. A 99 (2019) 042119.

5. V.V. Nesvizhevsky et al, A magneto-gravitational trap for precision studies of gravitational quantum states ), arXiv:200106332 (2020).

Juliette Simonet

(University of Hamburg)

Ultrashort laser pulses grant access to an instantaneous and controlled creation of electrons and ions in a quantum gas. A single femtosecond laser pulse can ionize up to several thousand atoms, thus triggering the formation of strongly coupled ultracold plasmas.

We report on the observation of ultrafast electron cooling in an expanding micro-plasma from 5000 K to about 1 K within less than one picosecond. Our experimental setup allows measuring the electronic kinetic energy distribution with meV resolution. Furthermore, we have performed numerical simulations of the plasma dynamics that are in excellent agreement with the measurements and provide insights into the thermalization mechanisms on sub-nanosecond timescales.

Markus Aspelmeyer

(University of Vienna)

The increasing level of control over motional quantum states of massive, solid-state mechanical devices opens the door to an hitherto unexplored parameter regime of macroscopic quantum physics. I will report on our recent progress towards controlling levitated solids in the quantum regime. I will discuss the prospects of using these systems for fundamental

tests of physics, including the interface between quantum and gravitational physics.

Michael Köhl

(University of Bonn)

Understanding strongly correlated quantum many-body systems remains a challenge both for experiment and theory. In particular, the interplay of interactions, kinetic energy, and dimensionality is complex and governs the occurrence and properties of low-temperature quantum phases. A paradigmatic example of a strongly correlated many-body problem is that of interacting spin-1/2 fermions on a periodic lattice. Depending on the Hamiltonian parameters, different quantum phases are realized. For example, for weak interactions and low filling of the lattice, the fermions delocalize into Bloch waves and constitute a metallic state with finite charge compressibility. In contrast, for strong repulsive interactions at half filling the fermions form a Mott insulator that occurs when the kinetic energy can no longer overcome the energy gap due to repulsive on-site interactions. In this talk, I will review the latest results from our lab of exploring this physics.

Olivier Arcizet

(Institut Néel)

We will present our recent developments in the realisation of ultrasensitive vectorial force field sensors based on suspended silicon carbide nanowires, at room and dilution temperatures. We will introduce a universal measurement method where the 2D force field gradients are determined from the modifications of the nanowire eigenfrequencies and eigenvector orientations they induce [1]. In particular we will analyse the impact of non-conservative (rotational) force fields on the nanowire dynamics: leading to eigenmode orthogonality breaking, noise reduction and violation of the fluctuation dissipation relation [2]. We will then present how those exceptional force sensors can be implemented in a cavity nano-optomechanical experiment where we giant coupling strength achieved opens the road towards optomechanical experiments in the single photon regime.

[1] L. Mercier de Lépinay et al, Nature Nanotech. 12, 156 (2017) 

[2] L. Mercier de Lépinay et al, Nature Comm. 9, 1401(2018) 

[3] F. Fogliano et al, arxiv 1904.01140 (2019) 

Arno Rauschenbeutel

(Humboldt-Universität – Berlin)

Typical schemes for generating correlated states of light require a highly nonlinear medium that is strongly coupled to an optical mode. However, unavoidable dissipative processes, which cause photon loss and blur nonlinear quantum effects, often impede such methods. In this seminar, I will report on a recent experimental demonstration of the opposite approach [1]. Using a strongly dissipative, weakly coupled medium, we generate and study strongly correlated states of light. Specifically, we study the transmission of resonant light through an ensemble of non-interacting atoms that weakly couple to a guided optical mode. Dissipation removes uncorrelated photons while preferentially transmitting highly correlated photons created through collectively enhanced nonlinear interactions. As a result, the transmitted light constitutes a strongly correlated many-body state of light, revealed in the second-order correlation function. The latter exhibits strong antibunching or bunching, depending on the optical depth of the atomic ensemble. The demonstrated mechanism opens a new avenue for generating nonclassical states of light and for exploring correlations of photons in non-equilibrium systems using a mix of nonlinear and dissipative processes. Furthermore, our scheme may turn out transformational in quantum information science. For example, it offers a fundamentally new approach to realizing single photon sources, which may outperform sources based on single quantum emitters with comparable coupling strength.

Carsten Klempt

(Institut für Quantenoptik, Leibniz Universität Hannove)

Spin-changing collisions can be employed for the generation of entanglement in spinor Bose-Einstein condensates, in close analogy to optical parametric down-conversion. I will present the creation of two types of entangled states, Twin-Fock states and two-mode squeezed states. Both states can be applied for interferometry beyond the Standard Quantum Limit. We have demonstrated that such entangled states can be separated in the spatial domain to transfer the entanglement from internal to external degrees of freedom [1]. I will discuss methods to employ spin-entangled Bose-Einstein condensates for inertially sensitive atom interferometers.

[1] K. Lange, J. Peise, B. Lucke, I. Kruse, G. Vitagliano, I. Apellaniz, M. Kleinmann, G. Toth, C. Klempt, Entanglement between two spatially separated atomic modes, Science 360, 416 (2018). 

(LENS, Florence)

Ultracold gases of neutral atoms provide a powerful technological platform for engineering synthetic many-body quantum systems. In a “quantum simulation” perspective, it is possible to control the atomic state to provide direct experimental realizations of prototypical theoretical models and to achieve “extreme” states of matter with no counterpart in conventional materials.

I will give an introduction to the new approaches that are opened by the coherent, optical manipulation of internal states in ultracold Fermi gases of two-electron atoms. I will review quantum simulation approaches for the realization of gauge fields and topological systems based on the concept of “synthetic dimensions” and I will discuss ongoing experiments with multicomponent Hubbard systems with SU(N) and SU(N)-broken interaction symmetries.

(Durham-Newcastle)

(University Sapienza – Rome, IT)

(Institut Néel)

References

[1] Elouard, Herrera-Marti, Huard, Auffèves, “Extracting work from quantum measurement in Maxwell’s demon engines”, PRL 118, 260603 (2017)
[2] Grangier and Auffèves, “What is quantum in quantum randomness”, Phil. Trans. Roy. Soc. A 376 :2017/0322 (2018)
[3] Monsel, Fellous-Asiani, Huard and Auffèves, “A coherent quantum engine based on bath and battery engineering”, arXiv: 1907.00812 (2019)
[4] Elouard, Herrera-Marti, Clusel, Auffèves, “The role of quantum measurement in stochastic thermodynamics”, npj QI 3: 9 (2017) 

(Stanford)

(Braunschweig University of Technology)

Pacôme Delva
(Observatoire de Paris)

Leticia Tarruell
(ICFO Barcelona)

Albert Schliesser
(Niels Bohr Institute)

In this talk I will review the recent advances in the theoretical and experimental study of quantum fluids of light. This novel form of quantum matter consists of assemblies of photons that display collective many-body effects as a result of the optical nonlinearity of the underlying material.
I will start with a brief survey of early results on Bose-Einstein condensation and superfluid hydrodynamical effects with an emphasis on those non-equilibrium features and non-equilibrium phase transitions that originate from the intrinsically driven-dissipative nature of optical systems.
I will then present the on-going challenge of using quantum fluids of light to generate strongly correlated states of matter such as Mott insulators or fractional quantum Hall fluids of light in a completely new non-equilibrium framework. The main challenges and the potential of optical systems in this direction will be highlighted, with a special emphasis on connection to the growing field of topological photonics and synthetic magnetism.

[1] Ar-39 dating with small samples provides new key constraints on ocean ventilation, Nature Comm. 9, 5046 (2018).[2] Observation of universal dynamics in a spinor Bose gas far from equilibrium, Nature 563, 217 (2018) 

The propagation of coherent light through a thick layer of scattering material is an extremely complex physical process. However, it remains linear, and under certain conditions, if the incoming beam is spatially modulated to encode some data, the output as measured on a sensor can be modeled as a random projection of the input, i.e. its multiplication by an iid random matrix. One can leverage this principle for compressive imaging, and more generally for any data processing pipeline involving large-scale random projections. This talk will discuss recent technological developments of optical co-processors within the startup LightOn, and present a series of proof of concept experiments in machine learning, such as transfer learning, change point detection, or recommender systems.

In the so called bottom-up approach to controlled atomic quantum matter small quantum systems are synthesized involving single, few or many atom systems. I will discuss two examples:

I will show that controlled interaction of atoms with a so called fast optical resonators leads not only to a strongly preferred emission of light into optical wave guides (Purcell effect) but also to speed higher than conventional atomic decay rates. This device will be useful for future interconnects in hybrid quantum networks: We have taken first steps towards coupling broad band (e.g. quantum dot) photons to narrow band fiber coupled atomic memories. Quantum networks will have to rely on so called quantum repeaters for large scale distribution of quantum states. Quantum repeaters remain an enormous challenge for experimenters.

With quantum walks – i. e. driven discrete transport on a lattice conditioned on the spin state – we operate a tool realizing controlled coherent transport of atoms over tens of lattice sites – up to the so called quantum speed limit available. I will present the experimental realization of “ideal negative measurements” showing strong violation of the Leggett-Gard inequality. The experiment distinguishes quantitatively the macro realist’s world from the quantum world. A few more examples including e.g. the creation of artificial magnetic fields will be given. The ultimate aim of these experiments is the creation of quantum cellular automata.

Random scattering of light, which takes place in paper, paint and biological tissue is an obstacle to imaging and focusing of light and thus hampers many applications. At the same time scattering is a phenomenon of basic physical interest as it allows the study of fascinating interference effects such as open transport channels, which enable lossless transport of waves through strongly scattering materials. These speckle correlation effects are associated with a relatively broad bandwidth, raising the question whether they are associated with light that has undergone a less-than average number of scattering events.
A thorough understanding of these open channels and the correlations between scattered and ballistic waves may help imaging methods to penetrate deeper into volume scattering media.

Precise determination of transition frequencies of simple atomic systems are required for a number of fundamental applications such as tests of quantum electrodynamics (QED), the determination of fundamental constants and nuclear charge radii. The sharpest transition in atomic hydrogen occurs between the metastable 2S state and the 1S ground state with a natural line width of only 1.3 Hz. Its transition frequency has been measured with almost 15 digits accuracy using an optical frequency comb and a cesium atomic clock as a reference [1]. A measurement of the Lamb shift in muonic hydrogen is in significant contradiction to the hydrogen data if QED calculations are assumed to be correct [2]. In order to shed light on this discrepancy the transition frequency of one of the broader lines in atomic hydrogen has to be measured with very good accuracy. For this purpose we have employed our previous 1S-2S apparatus as a cold source of laser excited 2S atoms in order to perform spectroscopy on the 2S-4P transitions. With a natural line width of 12.7 MHz, large Doppler effects, quantum interference etc. a good line shape analysis is mandatory to identify the true transition frequency. Our result on this transition yields a value for the proton radius that is compatible with the value obtained from muonic hydrogen with an uncertainty that is comparable to the previous hydrogen world data [3]. Meanwhile Helene Fleurbaey and her team at the Laboratoire Kastler Brossel, Paris have re-measured the 1S-3S transition frequency with a significantly improved accuracy and find the previous “regular hydrogen charge radius” [4]. At our lab we have also been working on this transition with a different method. We hope to be ready to report a result soon. This will provide a unique opportunity to compare two highly accurate measurements obtained at different labs.

References

C. G. Parthey et al., Phys. Rev. Lett. 107, 203001 (2011)
A. Antognini et al., Science 339, 417, (2013)
A. Beyer et al., Science 358, 79 (2017)
H. Fleurbaey et al. PRL 120, 183001 (2018)

We study suspended, 300-nm-long single-walled carbon nanotubes (SWCNT) contacted using MoRe leads. Good contact transparency of the superconductor-nanotube interface allows for the observation of proximity-induced superconductivity in our SWCNT devices. The magnitude of the switching supercurrent ranges up to 50 nA and can be tuned periodically by gate-induced charge. The gate charge modulates the retrapping current even more strongly, and its magnitude becomes vanishingly small far away from the charge degeneracy point.

Under rf irradiation, our SWCNT devices display clear Shapiro steps, the shape of which depend on the rf frequency and power. Under certain conditions the observed steps become hysteretic, indicating small dissipation on the Josephson phase dynamics at the rf frequency.

In these SWCNT devices we find mechanical resonances around 1.5 GHz, while the Q factors amount up to 15000 near the charge degeneracy point. Mechanical modes can be observed also in the superconducting regime either by mixing by the current-phase relation or by inducing Shapiro steps resonantly with the mechanical mode, which both reflect the interplay between the Josephson dynamics and the mechanical degrees of freedom. In addition, we find bifurcation of Shapiro steps around the mechanical resonance frequency, both at fundamental and subharmonic resonance drives. The understanding of these phenomena within the framework of current biased RCSJ model will be discussed.

La métrologie moderne est basée sur la physique quantique et la relativité. La seconde est définie comme un nombre entier de périodes d’une transition atomique et le mètre est déduit de la valeur fixée pour la vitesse de la lumière dans le vide. Très prochainement, la constante de Planck et la charge élémentaire vont être fixées avec des valeurs exactes qui remplaceront les anciennes définitions du kilogramme et de l’ampère. Le nouveau SI va réaliser le programme d’universalité des unités engagé lors de la création du système métrique. Il pose aussi des questions nouvelles à l’interface entre physique quantique, relativité et métrologie. On discutera ces questions et on présentera une esquisse de cadre théorique où des observables sont définies pour le temps, l’espace et la masse de manière à être compatibles avec les exigences relativistes et quantiques.

We present theoretical results on hydrogen atoms and hydrogen-like ions.
Concerning atoms, we explore in detail the van der Waals interactions between a ground-state and an excited-state atom, focusing especially (but not exclusively) on excited $s$ states. The appearance of oscillating long-range ($1/R^2$) tails in the large-separation limit is highlighted. The role of quasi-degenerate, and lower-lying excited states, is described in detail.
Concerning ions, we discuss the computation of a set of two-loop QED corrections to the bound electron g-factor. These corrections involve the so-called ‘magnetic loop’ sub-diagram, which, at the lowest non-vanishing order, involves the light-by-light scattering process. Preliminary results indicate that these corrections can be observable at high-Z, provided that the presumably larger, two-self-energy-loops corrections are computed and benchmarked first. With the development of experimental facilities aiming at very high-precision measurements of $g$-factors in heavy highly charged ions, such exotic QED corrections as presented here, can become relevant for the determination of fundamental constants.

Measurements of the fine-structure constant are powerful tests of the consistency of theory and experiment in physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded a measurement of the fine-structure constant α = 1/137.035999046(27) at 0.20 parts per billion accuracy using multiphoton interactions such as Bragg diffraction and Bloch oscillations. Comparison with Penning trap measurements of the electron gyromagnetic anomaly via the Standard Model of particle physics has implications for dark-sector candidates and electron substructure. We will close with an outlook on future applications of matterwave interferometry.

Atomic physics has been revolutionised by the introduction of laser techniques to cool atoms far below the Doppler limit. Now, it has become possible to laser cool molecules, to collect them in a magneto-optical trap, to cool them below the Doppler limit [1,2] and to trap them with modest magnetic fields [3]. These ultracold molecules open up a wide vista of future applications. To give a few examples, they can be optically or magnetically trapped to form arrays for quantum simulation, they can make a molecular fountain for testing fundamental physics at unprecedented levels of sensitivity, and they open a new energy range for the study of ultracold collisions and ultracold chemistry. I will review the current status of this field.

We report the observation of the opto-mechanical strain applied to ultra-cold rubidium atoms when shined on by an intense, far detuned homogenous laser beam. This force acts perpendicular to the laser beams direction and depends on the atomic cloud density profile, effectively generating interactions between the atoms. We refer to this force as electrostriction, since it resembles shape changes of materials under the application of a static electric field. We experimentally demonstrate the basic features of electrostriction, distinguishing it from the well-established scattering and dipole forces, and proving it is a new type of force [1]. We also report on experimental study the anomalous real-space and phase-space dynamics of ultra-cold atoms in a one dimension where the spatial distribution exhibits fractional self-similarity and Lévy statistics [2]. We show that the position-velocity correlation function builds up on a timescale related to the initial conditions of the ensemble and then decays asymptotically as a power-law, following a simple scaling theory involving the power-law asymptotic dynamics of position and velocity [3]. Finally, for trapped atoms we demonstrate a breakdown of equipartition theorem [4] predicted for anomalous dynamics in [5]. 1. N. Matzliah, H. Edri, A. Sinay, R. Ozeri, and N. Davidson, Phys. Rev. Lett. 119, 163201 (2017). 2. Y. Sagi, M. Brook, I. Almog, and N. Davidson, Phys. Rev. Lett. 108, 093002 (2012). 3. G. Afek, J. Coslovsky, A. Courvoisier, O. Livneh, and N. Davidson, Phys. Rev. Lett. 119, 060602 (2017). 4. G. Afek, A. Courvoisier, A. Cheplev, and N. Davidson, submitted. 5. Dechant, D. Kessler, and E. Barkai, Phys. Rev. Lett. 115, 173006 (2015).

The contemporary spectroscopy of gas targets several objectives, such as the fine description of the molecular Hamiltonians, the trace detection of diverse species, and the fundamental physics. In simple molecules such as molecular hydrogen, the transitions between energy levels can be probed to test the Standard Model of the Physics (i.e., the Quantum ElectroDynamics), and beyond. For example, the proton-to-electron mass ratio and the proton-radius size can be challenged. On other side, the detection of trace of molecules is of crucial importance for understanding chemical processes and molecular line shape. Up-to-date spectroscopy techniques based on optical cavities associated with Optical Frequency Combs have been developed in the Near-Infrared range to challenge sensitivity and frequency precision. We will illustrate the capability of the Noise-Immune Cavity-Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS), from both theoretical and experimental approaches. Very recent results on a few molecular species (C2H2 and HD) will be discussed for questioning the ultimate resolution and sensitivity which can be targeted.

Nowadays, single-photon frequency upconversion detectors (UCDs) have recently drawn a great deal of attention because it can be utilized as a quantum interface that enable qubits to transfer from infrared to visible regime, while preserving the quantum state information and then use Si-APDs to count the visible sum-frequency replicas of the infrared photons with high detection efficiency, high signal-to-noise ratio, short dead time, low timing jitter. The UCDs are increasingly used in more fields of importance, such as quantum key distribution (QKD), quantum metrology, quantum computation and quantum tomography. However, when pulse duration of the signal photons are very short to femtosecond where the spectrum of signal photons is very broadband to fulfill the Fourier transformation, the conversion efficiency will be much decreased, because the spectral width from these source is much wider than the acceptance bandwidth of the PPLN waveguide (0.2 nm). Here, we demonstrated a high efficiency telecom wavelength broadband single-photon frequency upconversion in PPLN crystal. By optimizing the pump light spectral bandwidth, we got 19.54 % conversion efficiency with a signal spectral bandwidth of 7.9 nm.

There are now over 200 quantum simulators on at least 8 separate architectures with long coherence times and controlled dynamics. These experimental systems have generated tremendous excitement about driven interacting quantum systems resulting in physics ranging from time crystals to dynamical many-body localization. The quantum ratchet adds a new feature to periodic driving: a preferred direction in both time and space, i.e., parity and time-reversal symmetry-breaking. By studying weakly interacting ultracold bosons in a quantum ratchet on a ring in position, momentum, and Floquet representations, we demonstrate the limits of known measures of quantum chaos in a system with a clearly defined and rather famous semiclassical or mean-field limit, and moreover supporting experiments. We show that the usual Wigner-Dyson statistics used to identify chaos are smeared out as we couple non-resonant modes into the drive. In contrast, the entropy of entanglement, condensate depletion, and inverse participation ratio all serve as accurate alternate identifiers for the chaotic regime in which the current on the ring flip-flops with a positive Lyapunov exponent in the mean-field limit. The dimension of the strange attractor is found to depend on the local vs. global nature of the observables. Moreover, the growth of depletion indicates mean field theory breaks down at realistic experimental times scaling polynomially as in the chaotic regime. This study opens the door to beyond single-frequency many-body Floquet physics showing many surprises and subtleties in both the quantum many-body dynamics and the mean-field limit (or lack thereof). Our prediction of a concrete time at which depletion grows is experimentally observable via an interference experiment. The dynamics and emergent structure of higher order correlators remains an especially intriguing avenue of exploration as we find that, contrary to oft-stated popular opinion, chaos, at least in the quantum ratchet, does not lead to high entanglement.

The fastest possible collective response of a quantum many-body system is related to its excitations at the highest possible energy. In condensed matter systems, the time scale for such “ultrafast” processes is typically set by the Fermi energy. Taking advantage of fast and precise control of interactions between ultracold atoms, we observed nonequilibrium dynamics of impurities coupled to an atomic Fermi sea. Our interferometric measurements track the nonperturbative quantum evolution of a fermionic many-body system, revealing in real time the formation dynamics of quasi-particles and the quantum interference between attractive and repulsive states throughout the full depth of the Fermi sea. Ultrafast time-domain methods applied to strongly interacting quantum gases enable the study of the dynamics of quantum matter under extreme nonequilibrium conditions.

Je présenterai nos travaux expérimentaux concernant les mesures quantiques faites sur des systèmes post-sélectionnés. Notamment, nous nous servons d’atomes froids et de l’effet «EIT» pour créer des interactions photon-photon suffisamment puissantes qu’on observe la phase nonlinéaire imprimée par des impulsions au niveau du photon unique sur un fasiceau sonde. En s’appuyant sur le formalisme de «weak measurement» de Yakir Aharonov et al., nous montrons aussi que quand la mesure est effectuée sur un photon post-sélectionné dans un état final donné, l’effet sur la sonde peut être «amplifié», de sorte qu’une mesure du nombre de photons dans une région d’espace peut même dépasser le nombre de photons total.
Je parlerai des questions à la fois philosophiques et pratiques qui se posent autour de ce genre d’observation, et si le temps le permet, je dirai quelques mots à propos d’autres manips en cours, telle une expérience pour mesurer la durée de la traversée d’une barrière à effet tunnel par les atoms ultrafroids (~900 pK), et une application des idées venues de l’informatique quantique à des problèmes d’imagerie classique (super-résolution).

Quantum memories are integral to the realization of quantum information networks and quantum information processing. A promising platform is gradient echo memory (GEM) in cold atomic systems with demonstrated efficiencies of ~87%. We demonstrate the first application of a deep learning algorithm to a cold atomic system in order to increase the optical depth (OD) of our atomic trap and thus increase memory efficiency. We perform a 63 parameter optimisation and find solutions that are agnostic to considerations regarding monotonicity or continuity and vastly outperform human solutions increasing our optical depth by (81+-3)%. We also observe a physical change in the atomic cloud corresponding to the spatial distribution of the atomic ensemble and apply the optimisation to the GEM protocol.

I will present my recent work made in IQOQI. The main part of the talk is devoted to the quantum scanning microscope arXiv:1709.01530 (to be published in PRL)

We propose and analyze a scanning microscope to monitor `live’ the quantum dynamics of cold atoms in a Cavity QED setup. The microscope measures the atomic density with subwavelength resolution via dispersive couplings to a cavity and homodyne detection within the framework of continuous measurement theory. We analyze two modes of operation. First, for a fixed focal point the microscope records the wave packet dynamics of atoms with time resolution set by the cavity lifetime. Second, a spatial scan of the microscope acts to map out the spatial density of stationary quantum states. Remarkably, in the latter case, for a good cavity limit, the microscope becomes an effective quantum non-demolition (QND) device, such that the spatial distribution of motional eigenstates can be measured back-action free in single scans, as an emergent QND measurement.

In the final part of the talk I will present an overview of our ongoing work involving cold Rydberg atoms in regular arrays forming an optical antenna arXiv:1802.05592

We describe the design of an artificial `free space’ 1D-atom for quantum optics, where we implement an effective two-level atom in a 3D optical environment with a chiral light-atom interface, i.e. absorption and spontaneous emission of light is essentially unidirectional. This is achieved by coupling the atom of interest in a laser-assisted process to a `few-atom’ array of emitters with subwavelength spacing, which acts as a phased-array optical antenna. We develop a general quantum optical model based on Wigner-Weisskopf theory, and quantify the directionality of spontaneous emission in terms of a Purcell $\beta$-factor for a given Gaussian (paraxial) mode of the radiation field, predicting values rapidly approaching unity for `few-atom’ antennas in bi- and multilayer configurations. Our setup has for neutral atoms a natural implementation with laser-assisted Rydberg interactions, and we present a study of directionality of emission from a string of trapped ions with superwavelength spacing.

Jour II

Jour I

I will describe our work with Betzalel Bazak on the N+1-body fermionic problem in three dimensions [1] and N-boson problem in two dimensions [2]. By developing a new method of solving few-body integral equations we are able to obtain precise results for ground state energies. In particular, we predict a universal pentamer state and five-body Efimov effect for the 4+1 fermionic problem and quantify the few-to-many-body crossover for two-dimensional bosons. I point to Refs. [3] and [4] as a recommended reading.

[1] B. Bazak and D.S. Petrov, « Five-body Efimov effect and universal pentamer in fermionic mixtures », Phys. Rev. Lett. 118, 083002 (2017) [2] B. Bazak and D.S. Petrov, « Energy of N two-dimensional bosons with zero-range interactions », arXiv:1711.02345 [3] Y. Castin, C. Mora, and L. Pricoupenko, « Four-Body Efimov Effect for Three Fermions and a Lighter Particle », Phys. Rev. Lett. 105, 223201 (2010) [4] H.-W. Hammer and D. T. Son, « Universal Properties of Two-Dimensional Boson Droplets », Phys. Rev. Lett. 93, 250408 (2004)

Les fibres microstructurées sont un excellent outil pour l’optique non linéaire en raison de la possibilité de contrôler et modeler leurs propriétés de dispersion et leur non linéarité. Dans ce séminaire je présenterai différentes fibres dédiées à la génération d’états non classiques de la lumière.

Dans un premier temps je montrerai la création de faisceaux jumeaux corrélés, créés par instabilité modulationelle. Le système est particulièrement simple et versatile et se compose d’une fibre à cœur creux remplie d’argon gazeux pompée par un laser saphir-titane. L’utilisation de gaz comme élément non linéaire permet de modifier à souhait la dispersion du système qui, une fois ajustée correctement, permet à des impulsions (300 fs) de développer des bandes latérales corrélées. De plus la source est spatialement monomode et présente un petit nombre de modes temporels (<5).

Dans un deuxième temps, je présenterai un nouveau concept de fibre microstructurée permettant, en principe, la création d’état triplet. La génération d’état triplet, dans laquelle un photon donne spontanément naissance à trois photons, peut être considérée comme le processus inverse du triplement de fréquence et nécessite par conséquent les mêmes conditions d’accord de phase. La dispersion chromatique impose cependant l’utilisation de différents modes spatiaux, ce qui est techniquement délicat. Nous avons récemment proposé un concept de fibre présentant deux modes de guidance différents : aux grandes longueurs d’onde la guidance s’opère par réflexion totale alors que la troisième harmonique est guidée par effet de bande interdite. Les premières expériences montrent la possibilité d’un accord de phase entre deux modes fondamentaux.

Jour II

Jour I

The most frequency-stable electromagnetic radiation is now produced optically, with stable reference cavities demonstrating fractional frequency instabilities below 10^-16 at 1 second and optical clocks reaching 10^-18 at 10^4 seconds. This talk will cover recent work at NIST using optical frequency combs to transfer this stability across the optical domain at the level of 10^-18 at 1 second, as well as into the RF, microwave, and mm-wave domains at the level of 10^-15 to 10^-17 at 1 second. In addition to the optical frequency combs themselves, elements of compact ultrastable optical cavities, high-speed photodetection and broadband electronic synthesis will also be discussed.

The decades-long history of theoretical understanding of the phenomenon of superfludity—from the Tisza-Landau phenomenology to the modern picture based on the emergence of topological constant of motion—has been most dramatic and instructive. Critically overviewing main historic steps highlights the crucial importance of the topological language for revealing the origin the phenomenon, as well as for establishing general relations of superfluid hydrodynamics, Berezinskii-Kosterlitz-Thouless transition, and superfluid-insulator quantum phase transitions in one dimension.

We have recently discovered a macroscopic object composed of a material particle dynamically coupled to a wave packet. The particle is a droplet bouncing on the surface of a vertically vibrated liquid bath; its pilot-wave is the result of the superposition of the surface waves it excites. Above an excitation threshold, this symbiotic object, designated as a “walker” becomes self-propelled.
Such a walker exhibits several features previously thought to be specific to the microscopic realm. The unexpected appearance of both uncertainty and quantization behaviors at the macroscopic scale lies in the essence of its “classical” duality. The dynamics of the droplet depends on previously visited spots along its trajectory through the surface waves emitted during each bounce. This path memory dynamics gives a walker an intrinsic spatio-temporal non-locality.
I will discuss the characteristics of these objects that encode a wave memory. In particular, I will introduce the concept of temporal mirrors and time crystals to interpret the characteristics of the driving wave packet.

I will present two antithetic experimental studies, exploiting strongly interacting ultracold Fermi gases of 6Li atoms confined in optical potentials. In a first experiment, we create the analogous of a Josephson junction by bisecting BEC-BCS crossover superfluids by a thin optical barrier. We observe coherent dynamics in both the population and in the relative phase between the two superfluid reservoirs. For critical parameters, we see how the Josephson dynamics is affected by the presence of topological defects entering the superfluid bulk [1]. In a second experiment, we create an artificial ferromagnetic state by segregating degenerate spin mixtures into two initially disconnected reservoirs [2]. We study the spin dynamics for different interaction strengths and temperatures. For sufficiently high values of the inter-spin repulsive interactions and sufficiently low temperatures, we observe a softening of the spin dipole mode and a time-window during which spin diffusion is zeroed. Our measurements provide exciting new insights into the physics of attractive and repulsive Fermi gases.

[1] G. Valtolina et al., Science 350, 1505 (2015)
[2] G. Valtolina et al., arXiv:1605.07850v1 (2015)

Ultracold molecules open up new routes for precision measurements, quantum information processing and many-body quantum physics. In particular, dipolar molecules with long-range interactions promise the creation of novel states of matter, such as topological superfluids and quantum crystals. Dipolar bialkali molecules can be efficiently assembled from ultracold atoms. Using this approach we have created the first near-degenerate gases of strongly dipolar NaK molecules. At temperatures of few hundred nanokelvin, we prepare ensembles, in which all molecules occupy the rovibrational and hyperfine ground state.
In my talk, I will discuss our recent progress on coherent quantum control of trapped, ultracold NaK molecules. Starting from the absolute ground state, we demonstrate microwave transfer into excited rotational and hyperfine states, and develop a thorough understanding of NaK’s rich hyperfine structure in the presence of static magnetic and electric fields. Building on this analysis, we show coherent two-photon microwave coupling between the two lowest nuclear spin states of NaK. For superpositions of these states, we observe coherence times of up to one second, enabling Ramsey spectroscopy with Hertz-level resolution.