Carlos Sánchez Muñoz

Coherent Generation of Nonclassical Light on Chip via Detuned Photon Blockade.

The on-chip generation of nonclassical states of light is a key requirement for future optical quantum hardware. In solid-state cavity quantum electrodynamics, such nonclassical light can be generated from self-assembled quantum dots strongly coupled to photonic crystal cavities. Their anharmonic strong light-matter interaction results in large optical nonlinearities at the single photon level, where the admission of a single photon into the cavity may enhance (photon tunneling) or diminish (photon blockade) the probability for a second photon to enter the cavity. Here, we demonstrate that detuning the cavity and quantum-dot resonances enables the generation of high-purity nonclassical light from strongly coupled systems. For specific detunings we show that not only the purity but also the efficiency of single-photon generation increases significantly, making high-quality single-photon generation by photon blockade possible with current state-of-the-art samples.

Macroscopic Two-Dimensional Polariton Condensates

We report a record-size, two-dimensional polariton condensate of a fraction of a millimeter radius free from the presence of an exciton reservoir. This macroscopically occupied state is formed by the ballistically expanding polariton flow that relaxes and condenses over a large area outside of the excitation spot. The density of this trap-free condensate is <1u2009u2009polariton/μm^{2}, reducing the phase noise induced by the interaction energy. Moreover, the backflow effect, recently predicted for the nonparabolic polariton dispersion, is observed here for the first time in the fast-expanding wave packet.Dario Ballarini,1 Davide Caputo,1, 2 Carlos Sánchez Muñoz,3 Milena De Giorgi,1 Lorenzo Dominici,1 Marzena H. Szymańska,4 Kenneth West,5 Loren N. Pfeiffer,5 Giuseppe Gigli,1, 2 Fabrice P. Laussy,3, 6 and Daniele Sanvitto1 CNR NANOTEC—Institute of Nanotechnology, Via Monteroni, 73100 Lecce, Italy University of Salento, Via Arnesano, 73100 Lecce, Italy Departamento de Física Teórica de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom PRISM, Princeton Institute for the Science and Technology of Materials, Princeton Unviversity, Princeton, NJ 08540 Russian Quantum Center, Novaya 100, 143025 Skolkovo, Moscow Region, Russia

Topological order and thermal equilibrium in polariton condensates

The Berezinskii-Kosterlitz-Thouless phase transition from a disordered to a quasi-ordered state, mediated by the proliferation of topological defects in two dimensions, governs seemingly remote physical systems ranging from liquid helium, ultracold atoms and superconducting thin films to ensembles of spins. Here we observe such a transition in a short-lived gas of exciton-polaritons, bosonic light-matter particles in semiconductor microcavities. The observed quasi-ordered phase, characteristic for an equilibrium two-dimensional bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. This is made possible thanks to long polariton lifetimes in high-quality samples and in a reservoir-free region. Our results show that the joint measurement of coherence both in space and time is required to characterize driven-dissipative phase transitions and enable the investigation of topological ordering in open systems.We report the observation of the Berezinskii-Kosterlitz-Thouless transition for a 2D gas of exciton-polaritons, and through the joint measurement of the first-order coherence both in space and time we bring compelling evidence of a thermodynamic equilibrium phase transition in an otherwise open driven/dissipative system. This is made possible thanks to long polariton lifetimes in high-quality samples with small disorder and in a reservoir-free region far away from the excitation spot, that allow topological ordering to prevail. The observed quasi-ordered phase, characteristic for an equilibrium 2D bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. Finally, measurements in the weak-coupling regime confirm that polariton condensates are fundamentally different from photon lasers and constitute genuine quantum degenerate macroscopic states.The Berezinskii-Kosterlitz-Thouless phase transition from a disordered to a quasi-ordered state, mediated by the proliferation of topological defects in two dimensions, governs seemingly remote physical systems ranging from liquid helium, ultracold atoms and superconducting thin films to ensembles of spins. Here we observe such a transition in a short-lived gas of exciton-polaritons, bosonic light-matter particles in semiconductor microcavities. The observed quasi-ordered phase, characteristic for an equilibrium two-dimensional bosonic gas, with a decay of coherence in both spatial and temporal domains with the same algebraic exponent, is reproduced with numerical solutions of stochastic dynamics, proving that the mechanism of pairing of the topological defects (vortices) is responsible for the transition to the algebraic order. This is made possible thanks to long polariton lifetimes in high-quality samples and in a reservoir-free region. Our results show that the joint measurement of coherence both in space and time is required to characterize driven-dissipative phase transitions and enable the investigation of topological ordering in open systems.

Filtering multiphoton emission from state-of-the-art cavity quantum electrodynamics

Engineering multiphoton states is an outstanding challenge with applications in multiple fields such as quantum metrology, quantum lithography, or even biological sensing. State-of-the-art methods to obtain them rely on post-selection, multi-level systems, or Rydberg atomic ensembles. Recently, it was shown that a strongly driven two-level system interacting with a detuned cavity mode can be engineered to continuously emit n-photon states. In the present work, we show that spectral filtering of its emission relaxes considerably the requirements on the system parameters even to the more accessible bad-cavity situation, opening up the possibility of implementing this protocol in a much wider landscape of different platforms. This improvement is based on a key observation: in the imperfect case where only a certain fraction of emission is composed of n-photon states, these have a well-defined energy separated from the rest of the signal, which allows one to reveal and purify multiphoton emission just by frequency filtering. We demonstrate these results by obtaining analytical expressions for the relevant figures of merit of multiphoton emission, such as the n-photon coupling rate between cavity and emitter, the fraction of light emitted as n-photon states, and n-photon emission rates. This allows us to make a systematic study of such figures of merit as a function of the system parameters and demonstrate the viability of the protocol in several relevant types of cavity quantum electrodynamics setups, where we take into account the impact of their respective experimental limitations.

Enhanced two-photon emission from a dressed biexciton

Radiative two-photon cascades from biexcitons in semiconductor quantum dots under resonant two-photon excitation are promising candidates for the generation of photon pairs. In this work, we propose a scheme to obtain two-photon emission that allows to operate under very intense driving fields. This approach relies on the Purcell enhancement of two-photon virtual transitions between states of the biexciton dressed by the laser. The richness provided by the biexcitonic level structure allows to reach a variety of regimes, from antibunched and bunched photon pairs with polarization orthogonal to the driving field, to polarization entangled two-photon emission. This evidences that the general paradigm of two-photon emission from a ladder of dressed states can find interesting, particular implementations in a variety of systems.

First observation of the quantized exciton-polariton field and effect of interactions on a single polariton

Álvaro Cuevas,1, 2, ∗ Blanca Silva,1, 3, ∗ Juan Camilo López Carreño,3, 4 Milena de Giorgi,1, † Carlos Sánchez Muñoz,3 Antonio Fieramosca,1 Daniel G. Suárez-Forero,1 Filippo Cardano,5 Lorenzo Marrucci,5 Vittorianna Tasco,1 Giorgio Biasiol,6 Elena del Valle,3 Lorenzo Dominici,1 Dario Ballarini,1 Giuseppe Gigli,1 Paolo Mataloni,2 Fabrice P. Laussy,4, 7, ‡ Fabio Sciarrino,2 and Daniele Sanvitto1 CNR NANOTEC—Institute of Nanotechnology, Via Monteroni, 73100 Lecce, Italy Dipartimento di Fisica, Sapienza University of Rome, Piazzale Aldo Moro, 2, 00185 Rome, Italy Departamento de F́ısica Teórica de la Materia Condensada, Universidad Autónoma de Madrid, 28049 Madrid, Spain Faculty of Science and Engineering, University of Wolverhampton, Wulfruna St, Wolverhampton WV1 1LY, UK Università di Napoli Federico II, Napoli, Italy Istituto Officina dei Materiali CNR, Laboratorio TASC, I-34149 Trieste, Italy Russian Quantum Center, Novaya 100, 143025 Skolkovo, Moscow Region, Russia (Dated: September 30, 2018)Polaritons reach the quantum limit, providing a new and promising platform of strongly coherent and interacting particles. Polaritons are quasi-particles that originate from the coupling of light with matter and that demonstrate quantum phenomena at the many-particle mesoscopic level, such as Bose-Einstein condensation and superfluidity. A highly sought and long-time missing feature of polaritons is a genuine quantum manifestation of their dynamics at the single-particle level. Although they are conceptually perceived as entangled states and theoretical proposals abound for an explicit manifestation of their single-particle properties, so far their behavior has remained fully accounted for by classical and mean-field theories. We report the first experimental demonstration of a genuinely quantum state of the microcavity polariton field, by swapping a photon for a polariton in a two-photon entangled state generated by parametric downconversion. When bringing this single-polariton quantum state in contact with a polariton condensate, we observe a disentangling with the external photon. This manifestation of a polariton quantum state involving a single quantum unlocks new possibilities for quantum information processing with interacting bosons.

Pulse, polarization and topology shaping of polariton fluids

Here we present different approaches to ultrafast pulse and polarization shaping, based on a “quantum fluid” platform of polaritons. Indeed we exploit the normal modes of two dimensional polariton fluids made of strong coupled quantum well excitons and microcavity photons, by rooting different polarization and topological states into their sub-picosecond Rabi oscillations. Coherent control of two resonant excitation pulses allows us to prepare the desired state of the polariton, taking benefit from its four-component features given by the combination of the two normal modes with the two degrees of polarization. An ultrafast imaging based on the digital off-axis holography technique is implemented to study the polariton complex wavefunction with time and space resolution. We show in order coherent control of the polariton state on the Bloch sphere, an ultrafast polarization sweeping of the Poincaré sphere, and the dynamical twist of full Poincaré states such as the skyrmion on the sphere itself. Finally, we realize a new kind of ultrafast swirling vortices by adding the angular momentum degree of freedom to the two-pulse scheme. These oscillating topology states are characterized by one or more inner phase singularities tubes which spirals around the axis of propagation. The mechanism is devised in the splitting of the vortex into the upper and lower polaritons, resulting in an oscillatory exchange of energy and angular momentum and in the emitted time and space structured photonic packets.