Session N6: Quantum Optics

Optical Pattern Formation in Spatially Bunched Atoms: A Self-Consistent Model and Experiment

The nonlinear optics and optomechanical physics communities use different theoretical models to describe how optical fields interact with a sample of atoms. There does not yet exist a model that is valid for finite atomic temperatures but that also produces the zero temperature results that are generally assumed in optomechanical systems. We present a self-consistent model that is valid for all atomic temperatures and accounts for the back-action of the atoms on the optical fields. Our model provides new insights into the competing effects of the bunching-induced nonlinearity and the saturable nonlinearity. We show that it is crucial to keep the fifth and seventh-order nonlinearities that arise when there exists atomic bunching, even at very low optical field intensities. We go on to apply this model to the results of our experimental system where we observe spontaneous, multimode, transverse optical pattern formation at ultra-low light levels. We show that our model accurately predicts our experimentally observed threshold for optical pattern formation, which is the lowest threshold ever reported for pattern formation.   

Radially and azimuthally polarized nonparaxial Bessel beams made simple

We present a method for the realization of radially and azimuthally polarized nonparaxial Bessel beams in a rigorous but simple manner. This result is achieved by using the concept of Hertz vector potential to generate exact vector solutions of Maxwell's equations from scalar Bessel beams. The scalar part of the Hertz potential is built by analogy with the paraxial case as a linear combination of Bessel beams carrying a unit of orbital angular momentum. In this way we are able to obtain spatial and polarization patterns analogous to the ones exhibited by the standard cylindrically polarized paraxial beams. Applications of these beams are discussed.   

A new class of optical structures: Supersymmetric mode converters

Originally developed in the area of quantum field theory, the concept of supersymmetry (SUSY) can be exploited to systematically design a new class of mode converters. In our work, we show for the first time how supersymmetric optical structures can be utilized to control the flow of light for mode division multiplexing applications. Optical potentials and their superpartner configurations are experimentally realized in coupled waveguide arrays using the direct laser-writing technology. This key method allows a flexible and precise tuning of coupling and propagation constants in our optical network. Fluorescence microscopy is used for a direct observation of light dynamics in such systems. In our experiments we show that the fundamental mode of a multimode optical structure can be removed, while establishing global phase matching conditions for the remaining set of modes. SUSY may serve as a promising platform for a new generation of versatile optical components with novel properties and functionalities or even synthesize artificial optical structures that exhibit properties not found in nature.   

Realization of Hadamard, Pauli-X and rotation gate for polarization-encoded qubits on chip

The implementation of quantum operations in photonic devices plays a central role towards quantum computing, as light is a logical choice when low decoherence and stability is of interest. Particular interest is thereby given to polarization-encoded photonic qubits on chip that are superior in terms of stability and size. However, to date waveguide-based modulating polarization states was beyond technological capabilities, preventing from the implementation of universal quantum computing algorithms on chip. In our work we close this gap and present integrated Hadamard, Pauli-X and rotation gates of high fidelity for photonic polarization qubits by employing a reorientation of the birefringent waveguide's optical axis. To this end, we employ laser-written waveguides and use several of their unique features. Due to the impact of an artificial stress field created by an additional defect close to the waveguide, its optical axis is rotated. Further on, by adjusting the length of the defect, the retardation between ordinary and extraordinary field components is precisely tunable including half-wave plate and quarter-wave plate operations. A theoretical treatment as well as characterization of the implemented gates by classical and quantum light are presented.   

Room Temperature Memory for Few Photon Polarization Qubits

We have developed a room temperature quantum memory device based on Electromagnetically Induced Transparency capable of reliably storing and retrieving polarization qubits on the few photon level. Our system is realized in a vapor of $^{87}$Rb atoms utilizing a $\Lambda$-type energy level scheme. We create a dual-rail storage scheme mediated by an intense control field to allow storage and retrieval of any arbitrary polarization state. Upon retrieval, we employ a filtering system to sufficiently remove the strong pump field, and subject retrieved light states to polarization tomography. To date, our system has produced signal-to-noise ratios near unity with a memory fidelity of $>$80$\%$ using coherent state qubits containing four photons on average. Our results thus demonstrate the feasibility of room temperature systems for the storage of single-photon-level photonic qubits. Such room temperature systems will be attractive for future long distance quantum communication schemes.   

High-Order Photonic W-states for Random Number Generation

Multipartite entanglement plays a key role in a number of counter-intuitive phenomena in quantum me- chances. A particular type of multipartite entangled states are the so called W-states which are in generalized form a coherent superposition of N single qubit states exhibiting equal probability amplitudes. The entanglement carried by these quantum entities has the remarkable property of being intrinsically robust to decoherence in one of the qubits. In our work, we experimentally realize high order W-states by forcing single photons to exist in a uniform coherent superposition of N (up to 16) spatial optical modes within a multi-port integrated system. Interestingly, in the generated W-states, a single photon will emerge from any of the N output ports with exactly the same probability. Based on that fact we have additionally developed a scheme for the generation of genuine random bits on chip, without the need of any post-processig. The authenticity of the random numbers is validated by applying the fifteen statistical tests suggested by National Institute of Standard Technology.   

Phase Synchronization between Superradiant Lasers

Bad-cavity (or superradiant) lasers using highly forbidden atomic transitions are expected to achieve coherence lengths on the order of the earth-sun distance, with the potential to improve optical atomic clocks, long-baseline interferometry, and other precision measurements. Cold-atom Raman lasers operating deep in the superradiant regime have been demonstrated, where the effective atomic linewidth is much narrower than the cavity linewidth. We explore the interactions of two independently driven superradiant Raman lasers emitting into a single mode of an optical cavity. In particular, we present experimental studies of the time dynamics of phase synchronization of the two collective atomic dipoles after an external phase perturbation has been applied. Also, the two lasers are shown to undergo a transition from lasing at distinct frequencies to lasing at a common frequency as the relative detuning between their natural emission frequencies is decreased. This work with a model Raman system will inform strategies for mitigating phase noise in future superradiant lasers that will use highly forbidden optical transitions.   

Experimental realization of Coherent Perfect Rotation in TGG

Coherent Perfect Rotation is the reversible generalization of the anti-laser process that can occur in optical systems with Faraday rotation. We describe the first experiment to verify CPR using a TGG resonator, and give an assessment of the experimentally achievable contrast ratio of the CPR resonance and remark on its utility in optical devices and related future experiments.