Session W1: Photonic Systems and Entanglement Generation

Efficient quantum computing using coherent photon conversion

Single photons make very good quantum information carriers, but current schemes for photonic quantum information processing (QIP) are inefficient. We describe a new scheme, \emph{coherent photon conversion (CPC)}, using classically pumped nonlinearities to generate and process complex multiquanta states\footnote{Published in Nature \textbf{478}, 360 (2011)}. One example based on four-wave mixing provides a full suite of QIP tools for scalable quantum computing from a single, versatile process, including: deterministic multiqubit entanglement gates based on a novel photon-photon interaction, high-quality heralded multiphoton states without higher-order imperfections, and robust, high-efficiency detection. Using photonic crystal fibres, we present observations of quantum correlations from a four-colour nonlinear process suitable for CPC and study the feasibility of reaching the deterministic regime with current technology. The scheme could also be implemented in optomechanical, electromechanical and superconducting systems.   

Dephasing of multiparticle Rydberg excitations for fast entanglement generation

We propose an approach to fast entanglement generation based on Rydberg dephasing of collective excitations (spin waves) in large, optically thick atomic ensembles. Rather than trying to prevent multiple excitations via the Rydberg blockade mechanism, our idea is to allow multiple Rydberg level excitations to self-interact and dephase. The strong interaction required to dephase multiple excitations is induced by mixing adjacent, opposite-parity Rydberg levels with a microwave field. These levels experience resonant $1/r^3$ dipole-dipole interactions ($ns + n'p \rightarrow n'p + ns$) that extend over the whole ensemble in contrast to the weaker, short range $1/r^6$ Van der Waals coupling due to non-resonant processes ($ns + ns \rightarrow np + (n-1)p$). The interaction-induced phase shifts suppress the contribution of multiply excited states in phase matched optical retrieval. The dephasing mechanism therefore permits isolation and manipulation of individual spin wave excitations. High quality single photons can be created with $1/e$ maximum efficiency in few microseconds. The dephasing mechanism is shown to have favorable, approximately exponential, scaling. Required long coherence times are achieved via four-photon excitation and read-out of long wavelength spin waves.   

Unconventional ultra-efficient photon blockade and single-photon emitters from weakly nonlinear systems based on coupled cavities

Single photons are usually generated by non-resonant excitation of single (artificial) atoms or by resonant excitation of Kerr systems with a giant nonlinear interaction much larger than the losses of the system (standard photon blockade). Here, we present a general class of destructive quantum interference effects [1,2], which provide a robust protocol to achieve strong photon antibunching and single-photon emission in a double cavity system, where one resonantly driven cavity is coupled to an auxiliary nonlinear cavity. An original scheme [2] shows the single-photon emission can be also produced by the auxiliary cavity with orthogonal polarization with respect to the pump beam, hence providing a direct way to get spatial and polarization selection. \\[4pt] [1] M. Bamba, A. Imamo\u{g}lu, I. Carusotto, C. Ciuti, {\it Origin of strong photon antibunching in weakly nonlinear photonic molecules}, Phys. Rev. A {\bf 83}, 021802 (2011) and references therein. \\[0pt] [2] M. Bamba, C. Ciuti, {\it Counter-polarized single-photon generation from the auxiliary cavity of a weakly nonlinear photonic molecule}, Appl. Phys. Lett. {\bf 99}, 171111 (2011).   

Anomalous Stokes scattering by an atomic ensemble: detection of entanglement and vector metrology of field gradient

We investigate the collective Stokes scattering by a typical atomic cloud, i.e. with size much larger than the light wavelength and with density much lower than that required for Dicke superradiance. We show that the diffraction pattern of the Stokes photon can be used to detect entanglement in the atomic ensemble. When the atomic cloud is placed in a static magnetic field or electric field, the change of diffraction pattern by the evolution in the field can also provide sensitive vector metrology of the spatial gradient of the field.   

Unique Properties and Prospects: Quantum Theory of the Orbital Angular Momentum of Ince-Gauss Beams

The Ince-Gauss modes represent a new addition to the standard solutions to the paraxial wave equation. Parametrized by the ellipticity of the beam, they span the solution space between the Hermite-Gauss and the Laguerre-Gauss modes. These beams may be decomposed in either basis, and single photons in the Ince-Gauss modes exist naturally as superpositions of either Laguerre-Gauss or Hermite-Gauss modes. We present the fully quantum theory of the orbital angular momentum of these beams. Interesting features that arise are: stable beams with fractional orbital angular momentum, non-monotonic behavior of the OAM with respect to ellipticity, and the possibility of orthogonal modes possessing the same OAM. We believe that these modes may open up a fully new parameter space for quantum informatics and communication, and thus are worthy of thorough study.   

Entanglement of Ince-Gauss Modes of Photons

Ince-Gauss modes are solutions of the paraxial wave equation in elliptical coordinates [1]. They are natural generalizations both of Laguerre-Gauss and of Hermite-Gauss modes, which have been used extensively in quantum optics and quantum information processing over the last decade [2]. Ince-Gauss modes are described by one additional real parameter -- ellipticity. For each value of ellipticity, a discrete infinite-dimensional Hilbert space exists. This conceptually new degree of freedom could open up exciting possibilities for higher-dimensional quantum optical experiments. We present the first entanglement of non-trivial Ince-Gauss Modes. In our setup, we take advantage of a spontaneous parametric down-conversion process in a non-linear crystal to create entangled photon pairs. Spatial light modulators (SLMs) are used as analyzers. [1] Miguel A. Bandres and Julio C. Guti\'{e}rrez-Vega ``Ince Gaussian beams", Optics Letters, Vol. 29, Issue 2, 144-146 (2004) [2] Adetunmise C. Dada, Jonathan Leach, Gerald S. Buller, Miles J. Padgett, and Erika Andersson, ``Experimental high-dimensional two-photon entanglement and violations of generalized Bell inequalities", Nature Physics 7, 677-680 (2011)   

Entangling two spatially separated qubits via interaction with nonclassical radiation

We propose a scheme for entangling two spatially separated noninteracting qubits using two-mode squeezed light in a cavity. Unlike other methods that typically require dipolar coupling for creating entanglement, our proposal relies solely on the interaction of the qubits with the squeezed cavity field. The squeezed field induces exchange of correlated photons which leads to transfer of entanglement from the field to discrete entanglement between qubits. Our scheme exhibits substantial steady-state entanglement which is robust against decoherence under the strong squeezing condition. In addition, we find that the entanglement generated between two asymmetric qubits is stronger than that generated by identical ones and crucially depends on the degree of squeezing.   

Generating coherent states of entangled spins

A coherent state of many spins contains quantum entanglement, which increases with the decrease of the collective spin value. We present a scheme to engineer this class of pure state based on incoherent spin pumping with several collective raising and lowering operators. In a pumping scenario aimed for maximum entanglement, the $N$-qubit steady state realizes the ideal resource for the $1\to N/2$ quantum telecloning. We show how the scheme can be implemented in the cold atomic system in an optical lattice. Error analysis shows that high-fidelity state engineering is possible for $N\sim O(100)$ spins in the presence of decoherence. The scheme can also prepare the large-scale Affleck-Kennedy-Lieb-Tasaki state.   

Non-destructively probing matter-photon correlations described by the Dicke-Hubbard Lattice model

The Dicke-Hubbard Lattice (DHL) Hamiltonian is a prototypical system to study photon matter entanglement across a symmetry breaking quantum phase transition in the matter subsystem. The model describes a cavity containing a periodic lattice, with a single mode photon field delocalized across the cavity. Like the Bose-Hubbard model, the Hamiltonian includes on-site repulsion between atoms and nearest neighbor hopping of an atom from one site to another. In addition, matter-light coupling in the DHL model can excite an atom to a higher band by absorbing a photon and the reverse process. We focus on the DHL model in a region of parameter space in which light is ``superradiant'' and matter is either a Mott-insulator or superfluid of both bands. Through mean field, exact diagonalization, and quantum Monte Carlo calculations we examine photon statistics across the matter quantum phase transition in order to elucidate how the photon statistics reflect the matter correlations. Doing so provides a novel technique to non-destructively probe the Mott-insulator to superfluid phase transition.   

The Impact of Geometry on the TM PBGs of Photonic Crystals and Quasicrystals

Here we demonstrate a novel quantitative procedure to pursue statistical studies on the geometric properties of photonic crystals and photonic quasicrystals (PQCs) which consist of separate dielectric particles. The geometric properties are quantified and correlated to the size of the photonic band gap (PBG) for wide permittivity range using three characteristic parameters: shape anisotropy, size distribution, and feature-feature distribution. Our concept brings statistical analysis to the photonic crystal research and offers the possibility to predict the PBG from a morphological analysis.   

A low-dimensional population-competition model for analyzing transverse optical patterns

Under favorable conditions, laser beams passing through a nonlinear medium (e.g. atomic vapors) can undergo directional instabilities, generating transverse optical patterns in the far field. In particular, a low intensity all-optical switching scheme using these transverse patterns in semiconductor quantum well microcavities was numerically demonstrated. Trying to understand the switching mechanism through the simulation results has turned out to be a complicated task. In this Contribution, we present a low-dimensional ``population-competition' model that (i) exhibits nearly all the pattern selection and switching behaviors and (ii) is simple enough to allow a comprehensive analysis of its solution structure in the relevant region of parameter space. We will explain the relation between this simple model and the realistic theory. Using elementary methods in Catastrophe Theory, we analyze the ``phase diagrams'' of our model's steady state solutions in parameter space, with the help of which we construct an organized picture of the behaviors of the realistic simulation results.   

Phonon effects on analog quantum simulation with ultracold ions in a linear Paul trap

Linear Paul traps have been used to simulate the transverse field Ising model with long-range spin couplings. Here, we study the effects of phonon creation on the spin state probability and spin entanglement. The effective spin models are created by applying a spin-dependent force with a laser that couples the spin state to the phonons of the ion crystal. Adiabatically removing the phonons creates an action described by a static spin Hamiltonian plus additional quantum time-dependent phases. In appropriate limits, the system is described predominantly by the static spin Hamiltonian. Here, we solve for the evolution of the coupled spin-phonon system exactly using exact diagonalization and examine the effect of phonon creation during the simulation on the probabilities of different spin states and on their entanglement. In particular, we examine phonon effects on the possibility for seeing the kink transition when the laser is detuned between the two phonon modes that lie below the COM mode.   

Entanglement Effects in Highly Dense Systems

We study the effect of entanglement in Jaynes-Cummings model using Neumann Entropy in the highly dense systems. We study a highly dense system such as neutron star as a good application of our results.