In this talk we will introduce the use of $V_B^-$ point defects in hBN for quantum strain sensing. The measurement of optically-detected magnetic resonance (ODMR) on $V_B^-$ ensembles enables to achieve sub-micrometer spatial resolution that, combined with micro-Raman spectroscopy, can provide a spatially-resolved map of in-plane and out-of-plane strain distribution [1]. In the second part of the talk we will address the important issue of enhancing photon emission from point defects in general and $V_B^-$ defects in particular. To achieve such an enhancement we will implement metallic (gold-based) nanotrenches in which the coupled effect of gap-plasmons and enhanced extraction efficiency results in a ~40-times emission enhancement with respect to a bare gold surface, while being compatible with ODMR measurements [2]. Overall, this enhancement should promote the role of $V_B^-$ in hBN as a practical quantum sensor.

[1] X. Lyu et al., Nano Lett. 22 (2022) 6553
[2] H. Cai et al., Nano Lett. 23 (2023) accepted for publication (10.1021/acs.nanolett.3c00849)

The field of two-dimensional (2D) materials-based nanophotonics has been growing at a rapid pace, triggered by the ability to design nanophotonic systems with in situ control, unprecedented number of degrees of freedom, and to build material heterostructures from the bottom up with atomic precision [1]. A wide palette of polaritonic classes have been identified, comprising ultraconfined optical fields, even approaching characteristic length-scales of a single atom. These advances have been a real boost for the emerging field of quantum nanophotonics, enabling quantum technologies harnessing single-photon generation, manipulation, and detection using 2D materials. Here, I will show several hybrid systems consisting in lifetime-limited single emitters [2, 3] (linewidth ~ 40 MHz) and 2D materials at sub-wavelength separation without degradation of the emission properties [4]. We have demonstrated that their nanoscale dimensions enable ultra-broadband tuning (tuning range > 400 GHz) and fast modulation (frequency ~ 100 MHz) of the emission energy [5], which renders it an integrated, ultra-compact tuneable SPS. I will also present recent results on unusual Stark tuning of ultra-narrow quantum emitter located at the edge of a graphene transistor and electrostatic engineering of excitons in 2D semiconductors.

[1] A. Reserbat-Plantey, I. Epstein, I. Torre, A. T. Costa, P. A. D. Gonçalves, N. A. Mortensen, M. Polini, J. C. W. Song, N. M. R. Peres, F. H. L. Koppens, Quantum Nanophotonics in Two-Dimensional Materials. ACS Photonics. 8, 85–101 (2021)
[2] K. G. Schädler, C. Ciancico, S. Pazzagli, P. Lombardi, A. Bachtold, C. Toninelli, A. Reserbat-Plantey, F. H. L. Koppens, Electrical Control of Lifetime-Limited Quantum Emitters Using 2D Materials. Nano Letters. 19, 3789–3795 (2019).
[3] C. Toninelli, I. Gerhardt, A. S. Clark, A. Reserbat-Plantey, et al. Single organic molecules for photonic quantum technologies. Nature Materials. 20, 1615. (2021).
[4] C. Ciancico, K. G. Schädler, S. Pazzagli, M. Colautti, P. Lombardi, J. Osmond, C. Dore, A. Mihi, A. P. Ovvyan, W. H. P. Pernice, E. Berretti, A. Lavacchi, C. Toninelli, F. H. L. Koppens, A. Reserbat-Plantey, Narrow Line Width Quantum Emitters in an Electron-Beam-Shaped Polymer. ACS Photonics. 6, 3120–3125 (2019).
[5] D. Cano, A. Ferrier, K. Soundarapandian, A. Reserbat-Plantey, M. Scarafagio, A. Tallaire, A. Seyeux, P. Marcus, H. de Riedmatten, P. Goldner, F. H. L. Koppens, K.-J. Tielrooij, Fast electrical modulation of strong near-field interactions between erbium emitters and graphene. Nature Communications. 11, 4094 (2020).

Quantum photonics is at the heart of new quantum technologies, enabling major advances in communications and quantum computing. It is now finding a new application in quantum metrology and sensors, exploiting the properties of entangled photonic states to accurately probe optical properties such as chromatic dispersion and refractive index. A new approach to quantum interferometry in white light is presented, improving the accuracy of characterisation of these properties. A comparison between the classical and quantum approaches, supported by experimental results, is also discussed.

Quantum computation with near-future devices will remain limited by the noise in the physical components. The only way forward, to get to large-scale and useful quantum computers, is be through active error correction and fault-tolerant methods. I will discuss some aspects of this, touching on the current status of the subject, and areas worthy of further exploration.

Quantum optics offers a viable and fascinating path to scalable, fault-tolerant, measurement-based quantum computing. I will present our endeavors and results toward this goal: the scalable continuous-variable entanglement of the resonant cavity (qu)modes of an optical parametric oscillator, and their de-Gaussification by photon-number-resolving detection, into resources states such as Gottesman-Kitaev-Preskill qubit codes.

We give a direct product theorem for the entanglement-assisted interactive quantum communication complexity of an l-player predicate $V$. In particular we show that for a distribution $p$ that is product across the input sets of the l players, the success probability of any entanglement-assisted quantum communication protocol for computing n copies of $V$, whose communication is $o(n\log(eff∗(V,p)))$, goes down exponentially in $n$. Here $eff∗(V,p)$ is a distributional version of the quantum efficiency or partition bound introduced by Laplante, Lerays and Roland (2014), which is a lower bound on the distributional quantum communication complexity of computing a single copy of $V$ with respect to $p$. Applying our direct product theorem for small communication and techniques related to $eff∗$, we show that it is possible to do device-independent (DI) quantum cryptography without the assumption that devices do not leak any information. First, we analyze the parallel DI quantum key distribution protocol given by Jain, Miller and Shi (2020), and show that when the protocol is carried out with devices that are compatible with $n$ copies of the Magic Square game, it is possible to extract $Ω(n)$ bits of key from it, even in the presence of $O(n)$ bits of leakage. Second, we show that it is possible to do sequential versions of the Jain, Miller and Shi protocol, which give a better key rate for QKD with leakage, and let us do sequential DI randomness expansion with leakage. Third, we show that proofs of quantumness with two entangled provers are resistant to leakage, i.e., classical players who communicate $O(n)$ bits with each other cannot convince the verifier that they share entanglement.

This talk recalls the current operation mode of security protocols that use traditional cryptography. We next describe the changes brought by the advent of quantum computers and the so-called quantum cryptography. We define what is covered by the name « post-quantum cryptography » and some ideas about the security goals that can be fulfilled. We conclude with the description of a hybrid protocol that could be implemented in current security libraries used by the Internet.

Feynman diagram expansions are key to perturbative approaches to quantum systems. I will survey various developments that enrich classical diagrammatics by taking into account what happens in perturbative expansions for interacting systems (that is, more concretely, when averages are not taken over the ground state of the system) and/or when dealing with quantum probabilities (that is, algebras of noncommutative random variables). Based on joint works with Ch. Brouder and K. Ebrahimi-Fard.

Quantum energetics explores the flows of energy, entropy and information in situations of quantum physics where temperature is not necessarily defined. This is the case, e.g. in measurement-powered engines or in driven-dissipative systems. In this talk, I will focus on the canonical case of two coupled systems, otherwise isolated. This captures most situations of quantum optics, where a qubit is coupled to a light field injected in one or several electromagnetic modes. Global energy conservation allows to characterize the nature of energy exchanges, yielding work-like and heat-like quantities. I apply this framework to study the energy cost of work extraction and the role of quantum coherence in quantum batteries, and present recent experimental results obtained with superconducting and semiconducting devices. I finally show how energetic quantities can help probing the quantum nature of a light field.

Obtaining the expectation value of an observable on a quantum computer is a crucial step in the variational quantum algorithms. For complicated observables such as molecular electronic Hamiltonians, a common strategy is to present the observable as a linear combination of measurable fragments. The main problem of this approach is a large number of measurements required for accurate estimation of the observable's expectation value. We consider several partitioning schemes based on grouping of commuting multi-qubit Pauli products with the goal of minimizing the number of measurements. Three main directions are explored: 1) grouping commuting operators using the greedy approach, 2) involving non-local unitary transformations for measuring, and 3) taking advantage of compatibility of some Pauli products with several measurable groups. The last direction gives rise to a general framework that not only provides improvements over previous methods but also connects measurement grouping approaches with recent advances in techniques of shadow tomography. Following this direction, we develop two new measurement schemes that achieve a severalfold reduction in the number of measurements for a set of model molecules compared to previous state-of-the-art methods.

Scaling today’s photonic quantum technologies to realize practical computation and simulation faces critical challenges. It requires complex systems with thousands to millions of components, which are difficult to implement using traditional bulk optics. Integrated photonics is likely the only solution. In this talk, we will describe our research efforts on developing integrated photonic devices for scalable quantum information processing. We will first introduce thin-film lithium niobate (TFLN) as a promising material platform for quantum applications. It offers many attractive properties that are critically missing in existing leading platforms, such as large electro-optic and piezoelectric coefficients, strong second-order nonlinearity, and engineerable ferroelectric domains. We will describe how these properties can enable key functionalities such as efficient quantum light generation, modulation, spectral control, and coherent transduction. We will then introduce superconducting nanowires as an effective solution for on-chip single-photon detection. Combining both can realize a fully integrated quantum photonic processor that allows single-photon generation, control, and detection on a single chip.

NbN is a superconducting material that has interesting properties for certain emblematic components in the field of quantum technologies. After a review of recent results on components (Qubits, SNSPDs) based on NbN, we will discuss the interest of developing these thin NbN films by epitaxy.

We report recent progress on nonlinear integrated photonic devices implemented on the ultra-silicon-rich nitride (USRN) platform. USRN’s high Kerr nonlinearity ($100\times$ larger than in stoichiometric silicon nitride), absence of two-photon absorption at telecommunications wavelengths and compatibility with complementary metal-oxide semiconductor processing makes it highly advantageous for applications in Bragg solitons and topological photonics. We report recent advances in the observation of Bragg soliton phenomena including Bragg soliton-effect compression and fission, optical parametric Bragg amplification, gap solitons and high spectro-temporal compression. We further demonstrate a topological nonlinear parametric amplifier on a chip. The strong localization of light to the boundary state in a Su-Schreiffer-Heeger waveguide is observed to be broken when subjected to a nonlinear perturbation. The ability for the Kerr effect to induce chiral symmetry breaking in the topological waveguide provides a new paradigm in which optical nonlinearities may be used to control topological states.

Nanophotonics has demonstrated over the years to be the solution to provide extremely large light matter interactions which are at the center of numerous applications in data communications and processing as well as quantum optics. Photonic crystals are certainly amongst the most emblematic nanophotonic structures. They enable the conception of the best photonic cavities with the highest measured Q factor over modal volume ratios which scale with intracavity electromagnetic field intensity. Our research activity is focused on the exploration of these interactions in III-V semiconductor/Silicon hybrid photonic crystal (PhC) structures and their exploitation for the achievement of smaller, smarter, faster energy-efficient optoelectronic components which will revolutionize our world, governed by information and communication technology.
Indeed, these hybrid structures combine the best of Silicon and III-V semiconductor photonics, merging the fantastic capacity of Silicon and especially Silicon on insulator (SOI) to provide an excellent way to achieve highly compact low loss passive optical circuitries, with the optical gain and nonlinearity of III-V semiconductor compounds. This is the key technology for achieving the convergence of microelectronics and photonics.
Amongst all the devices to be developed, sources are of utmost importance as they provide the necessary electro-optical conversion functionality. In this talk, I will describe our latest work on nanolaser diodes [1] as well as the first demonstration of a nanocavity based optical parametric oscillator [2]. I will also show that the latter can also be used as a heralded single photon source [3].

[1] G. Crosnier et al, Nature Phot, 11, 297–300 (2017).
[2] G. Marty et al, Nature Phot. 15, 53–58 (2021).
[3] A. Chopin et al, Communications Physics 6, 77 (2023).

(pdf version with figures)

Spatiotemporal mode-locking is a promising lasing regime for developing coherent sources for multimode nonlinear photonics. We show that a degenerate-cavity Vertical External-Cavity Surface-Emitting Lasers (VECSELs) can be operated in this regime. The emitted pulses exhibit a spatial profile which depends on the resonator parameters. Approaching the self-imaging condition, we observe mainly two kinds of non homotetic patterns: hexagons and rolls. These pulsating patterns are temporally localized, i.e., they can be individually addressed by shining optical pump pulses. Our result reveals that large-aspect-ratio VECSELs offer unique opportunities for studying fully developed spatiotemporal dynamics and for applications to multidimensional control of light. As an example, we provide a proof of principle of a VECSEL capable of generating spatio-temporally reconfigurable light.

In this talk I will introduce the use of solid-state ensembles consisting of rare-earth ion-doped crystals for the storage and manipulation of photonic quantum information. I will introduce the atomic frequency comb protocol and highlight its strengths. Despite their record coherence time (up to a few hours), rare-earth ions are subject to interaction with the matrix host ions possessing a nuclear spin, and I will present a model simulating dephasing due to this magnetic dipole-dipole interaction. Comparisons between experimental and numerically simulated echo decays will be shown for different rare-earth species, showing similar behaviour and comforting our model. Finally, fabrication techniques for integrating such quantum memories will be shown, relying on a ridge waveguide architecture.

We will discuss the on-going developments on III-nitride integrated photonics and their prospectives. Recent results on efficient nonlinear conversion in III-nitride waveguides and on the dynamics in microlasers will be presented.

Systems at the onset of a disorder-induced metal-insulator transition exhibit eigenstate intensity fluctuations in real space characterised by so-called multifractal dimensions. In this talk, I will explain how the dynamics of the Coherent Forward Scattering (CFS) peak, a smoking gun of Anderson localisation in momentum space, can be used to reveal the multifractal dimensions $D_1$ and $D_2$. At criticality, the only relevant time scale of the system is the Heisenberg time $\tau_H$ associated to the system size and the CFS peak exhibit two different dynamical behaviours. For $t\gg\tau_H$ (infinite time limit) the rise of the CFS peak is driven by the system boundaries and the height-to-background relative contrast of the CFS peak goes to 1 with a finite-size correction related to the multifractal dimension $D_2$. For $t\ll\tau_H$ (infinite size limit), the rise of the CFS peak is governed by the nonergodicity of the eigenstates and the height-to-background relative contrast of the CFS peak reaches the spectral compressibility given by the Bogolmonyi-Giraud conjecture, $\kappa = 1 - D_1/d$, with a temporal power-law related to $D_2$. Careful numerical simulations, done on different critical models, namely the Power-Law Random Banded Matrix Hamiltonian model, the 3D Random Kicked Rotor and the Ruijsenaars-Schneider model, confirm our analytical predictions.

A wave packet launched in a disordered system, in the regime of strong (Anderson) localization, first moves away, but then comes back and then stops in the original position. This phenomenon has been called the quantum boomerang effect. Such effect persists in models with pseudorandom potentials and it is also present in the kicked rotor. We have shown, theoretically and experimentally, that in the kicked rotor it is possible to control the final state of the wave packet, by breaking the time-reversal symmetry of the system.

Superfluidity is the ability of a fluid to move without dissipation through a possibly complex environment. It is a property shared by many different systems ruled out by non-linear wave equations such as the non linear Schrodinger equation. On the other hand, wave thermalisation may occur due to non linear interaction between waves. During this talk, I will discuss the competition between these different phenonema in various physical systems such as, Bose-Einstein condensates, light propagating in non linear media or quantum chaotic systems (modified one dimensional kicked rotor).

The Bose-Hubbard (BH) model quantifies the quantum matter phases accessible to strongly correlated bosons confined in lattice potentials. In its elementary form the BH Hamiltonian is restricted to on-site interactions and quantifies the transition from superfluid to Mott insulating phases. Extending the BH model to additional degrees of freedom naturally provides a route to broaden the range of accessible quantum matter states. In this presentation we introduce a new platform to experimentally emulate the Bose-Hubbard model extended by nearest neighbour interactions. In particular, we emphasise dipolar excitons of GaAs bilayers, confined in electrostatic lattice potentials. Then we realise first a Mott insulating phase at unitary lattice filling, and a second quantum insulator at half lattice filling that exhibits the signatures of a checkerboard solid [1].

[1] C. Lagoin et al., Nature 609, 485 (2022)

Ultracold gas dressed with laser fields are ideal platforms to generate artificial gauge fields, which in turn allow simulating problems in topological-condensed matter and high-energy physics.
In this talk, I will present our recent experiment on SU(2) and SU(3) non-Abelian gauge fields. With the former, we generalize the concept of Zitterbewegung effect of the Dirac equation to non-Abelian gauge fields, and found new way to manipulate matter-wave by geometrical means. In the latter, we performed Cartan-Weyl transformations, and introduce dynamical color-orbit coupling.

Quantum fluids of exciton-polaritons constitute a class of driven-dissipative light-like quantum fluids, that have the particularity of living within a solid-state crystalline lattice. At finite temperature the latter is subject to thermal vibrations that couple to the condensate. In a joint theoretical and experimental analysis, we show that this coupling generates a steady-state flux of Bogoliubov elementary excitations, that provides a quantitative measurement of the quantum correlations that characterize them, and we show that this correlation significantly renormalizes the thermal energy transfer between the condensate and the crystalline lattice. We also highlight the relevance of these phenomena to other families of quantum fluids such as weakly interacting ultracold atoms.

Photonic quantum fluids have been recently identified as an outstanding system for quantum simulation, ranging from condensed matter to astrophysics [1]. Through the emergence of many-body physics and nonlinear optics, fluids of light reveal quantum hydrodynamic features of light when it propagates in a nonlinear media. Fluids of light result from the formal analogy between the Gross-Pitaevskii equation for quantum fluids and the wave equation for the paraxial propagation of an optical field in a nonlinear transparent medium. In this analogy, the spatial evolution of the optical field in the propagation direction substitutes to the temporal evolution of the wave function of a quantum gas. Simply put, each transverse plane in the nonlinear medium is equivalent to a snapshot of the temporal dynamics of at wo-dimensional quantum fluid. To fulfil this analogy, photons need to acquire an effective mass and be in a fully controlled effective (repulsive) interaction, two features that are allowed in properly engineered photonic systems. They thus behave collectively as a quantum fluid, and share remarkable common features with other platforms helium4 [2,3], atomic Bose-Einstein condensates [4] such as superfluidity and quantum turbulence. One of the most remarkable evidence of light behaving as an interacting fluid is its ability to carry itself as a superfluid. At the Institut de Physique de Nice, we developed a state-of-the-art experimental set-up based on a versatile photonic platform [5] and recently reported a direct experimental detection of the transition to superfluidity in the flow of a fluid of light past an obstacle in a bulk nonlinear crystal. In this cavity less all-optical system, we extract a direct optical analog of the drag force exerted by the fluid of light and measure the associated displacement of the obstacle. We observe that both quantities drop to zero in the superfluid regime [6]. In this context, we recently realized an experimental phase diagram gathering the different hydrodynamic regimes observed such as vortex pairs generation, snaking instabilities as well as a solitonic regime in a simple configuration [7].

[1]I.Carusotto and C.Cuiti, “Quantum fluids of light”, Rev.Mod.Phys. 85,299(2013).
[2]J.F.Allen and A.D.Misener, Flow of Liquid Helium II,Nature,vol.141,no.3558,pp.75–75, 1938.
[3]P.Kapitza, Viscosity of Liquid Helium below the -Point,Nature,vol.141,no.3558,pp.74–74, 1938.
[4]I.Bloch, J.Dalibard and S.Nascimbne, Quantum simulations with ultracold quantum gases, NaturePhysics, vol.8,no.4,2012.
[5]O.Boughdad et al, “Anisotropic nonlinear refractive index measurement of a photorefractive crystal via spatial self-phase modulation”, Opt.Express 27,21,30360(2019).
[6] C.Michel et al,“Superfluid motion and drag-force cancellation in a fluid of light”, Nat.Commun. 9, 2108(2018).
[7]A.Eloy et al, Experimental observation of turbulent coherent structures in a superfluid of light, arXiv2012.13280, 2020

The Siegert relation relates field and intensity temporal correlations [1,2]. In this presentation, I will discuss the validity of this relation in two different domains: in astronomy and for light scattered by quantum emitters.
We first discuss this relation in astronomy, for light coming from stars. We can show that this relation can be used to extract astrophysical information, such as the coherence time due to emission lines. Combined to spatial interferometry, this can be used to determine the angular diameter of the source. This is especially interesting to perform such measurements on emission lines to characterize the extended atmosphere of the star, which can give access to other fundamental parameters such as its distance [3].
Second, we verify the validity of this relation for the light scattered by cold atoms. To validate the Siegert relation, one needs to verify different assumptions. One of this assumption is that the emitted or scattered phase should be random and uncorrelated. While the process of phase randomization is obvious in stars with thermal radiation, it is a bit more complex for light scattered by quantum scatterers. The process that randomizes the phase actually depends on the scattering regime. On one hand, for low saturation parameter, scattering is mainly elastic and the phase randomization is due to temperature [4]. On the other hand, when the saturation parameter is large, light is mainly inelastically scattered [5]. The origin of the phase randomization is completely different since it comes from the finite lifetime of the two-level excited state. We will show how we probe this transition from the classical regime, where the loss of coherence is due the atomic thermal motion, to the quantum regime dominated by spontaneous emission [6].

[1] D. Ferreira et al., "Connecting field and intensity correlations: The Siegert relation and how to test it", AJP 88, 831 (2020)
[2] P. Lassègues et al., "Field and intensity correlations: the Siegert relation from stars to quantum emitters", EPJD 76, 246 (2022)
[3] E.S.G. de Almeida et al., "Combined spectroscopy and intensity interferometry to determine the distances of the blue supergiants P Cygni and Rigel", MNRAS 515, 1 (2022)
[4] A. Eloy et al., "Diffusing-wave spectroscopy of cold atoms in ballistic motion", Phys. Rev. A 97, 013810 (2018)
[5] L. Ortiz et al., "Mollow triplet in cold atoms" New Journal of Physics 21, 093019 (2019)
[6] P. Lassègues et al., "From classical to quantum loss of light coherence", arXiv:2210.01003 (2022)

The capability of satellite-based quantum communication is an important steppingstone towards a global quantum network. In this implementation, network nodes in space with the ability to overcome distance limitation observed on Earth can connect multiple nodes across different global ground points. To achieve such a global quantum network, we need an optical ground station and a compatible satellite that can communicate with the station in uplink or downlink configuration or utilizing both means. In this talk, we report on the optical ground station that we are establishing on National University of Singapore campus. Here we elaborate on the design of the optical ground station to perform quantum key distribution. More specifically, we discuss the building blocks and design techniques of an optical ground station that can receive quantum signals from a satellite and perform necessary analysis to generate secret keys in a quantum key distribution experiment. We emphasize the design considerations of different subsystems of the optical ground station namely the telescope system, quantum receiver, polarization correction system, and the pointing, acquisition, and tracking system. The working lab-configuration of the station is able to receive and analyze the state of photons around 800 nm. We envision our ground station to support a range of beacon wavelengths to ensure its compatibility with various similar satellite missions. We aim to commission the optical ground station by 2023

Quantum laser links between the ground and space seem to be the most appropriate solution for establishing long-distance links. In this presentation, we will describe the various laser links set up at the Observatoire de la Côte d'Azur between the ground and space, for satellite distance measurement and for laser communications in low orbit. We will then present the technological challenges involved in establishing these links and the first experiments to be carried out in free space at the Observatoire de la Côte d'Azur.

For over a decade there have been continual effort on implementing satellite to ground quantum communication in Singapore. To this end, SpooQy-1, a CubeSat based pathfinder mission, have successfully demonstrated a miniaturized entangled photon pair source deployed in the Low Earth Orbit (LEO). The scientific knowledge and technical expertise gained from this mission have been utilized to design and prototype future quantum key distribution (QKD) capable quantum instruments. In this talk, we present the result of lab-based emulation of satellite-QKD using our prototype entanglement source. In satellite-based QKD, the number of secret bits that can be generated in a single satellite pass over the ground station is severely restricted by the pass duration and losses in the free-space optical channel. But, our experimental results show that QKD can be feasibly performed form a Singapore based optical ground station. Using the model for our satellite-to-ground quantum link we also study how quantum communication gets affected when the satellite’s altitude is increased to higher than LEO orbits.

We present a fully operational, real field quantum key distribution link based on entanglement over 50km of deployed optical fiber. We implemented automated synchronization and real time post treatment to continuously generate secret keys.