Ulrik L. Andersen

Naturally phase matched second harmonic generation in a whispering gallery mode resonator

We observed conversion efficiencies of 9% at 30µW pump power in LiNbO<inf>3</inf>, as well as self-limiting effects at high powers. The continuous-wave pump at a wavelength of 1064nm and the second-harmonic feature Q > 10⁷.

Colloquium: The Einstein-Podolsky-Rosen paradox: From concepts to applications

This Colloquium examines the field of the Einstein, Podolsky, and Rosen (EPR) gedanken experiment, from the original paper of Einstein, Podolsky, and Rosen, through to modern theoretical proposals of how to realize both the continuous-variable and discrete versions of the EPR paradox. The relationship with entanglement and Bells theorem are analyzed, and the progress to date towards experimental confirmation of the EPR paradox is summarized, with a detailed treatment of the continuous-variable paradox in laser-based experiments. Practical techniques covered include continuous-wave parametric amplifier and optical fiber quantum soliton experiments. Current proposals for extending EPR experiments to massive-particle systems are discussed, including spin squeezing, atomic position entanglement, and quadrature entanglement in ultracold atoms. Finally, applications of this technology to quantum key distribution, quantum teleportation, and entanglement swapping are examined.

Controlling the coupling of a single nitrogen vacancy center to a Silver nanowire

We report on the controlled coupling of a single nitrogen-vacancy (NV) center to a surface plasmon mode propagating along a chemically grown silver nanowire (NW). We locate and optically characterize a single NV center in a uniform dielectric environment before we controllably position this emitter in the close proximity of the NW. We are thus able to control the coupling of this particular emitter to the NW and directly compare the photon emission properties before and after the coupling. The excitation of single plasmonic modes is witnessed and a total rate enhancement by a factor of up to 4.6 is demonstrated.

High-rate measurement-device-independent quantum cryptography

Stefano Pirandola, Carlo Ottaviani, Gaetana Spedalieri, Christian Weedbrook, Samuel L. Braunstein, Seth Lloyd, Tobias Gehring, Christian S. Jacobsen, and Ulrik L. Andersen Department of Computer Science, University of York, York YO10 5GH, United Kingdom Department of Physics, University of Toronto, Toronto M5S 3G4, Canada and QKD Corp., 112 College St., Toronto M5G 1L6, Canada MIT – Department of Mechanical Engineering and Research Laboratory of Electronics, Cambridge MA 02139, USA Department of Physics, Technical University of Denmark, Fysikvej, 2800 Kongens Lyngby, Denmark

A generator for unique quantum random numbers based on vacuum states

Researchers demonstrate random-number generation by exploiting the intrinsic randomness of vacuum states. The approach may lead to reliable and high-speed quantum random-number generators for applications ranging from gambling to cryptography.

Quantum light from a whispering-gallery-mode disk resonator.

Optical parametric down-conversion has proven to be a valuable source of nonclassical light. The process is inherently able to produce twin-beam correlations along with individual intensity squeezing of either parametric beam, when pumped far above threshold. Here, we present for the first time the direct observation of intensity squeezing of -1.2  dB of each of the individual parametric beams in parametric down-conversion by use of a high quality whispering-gallery-mode disk resonator. In addition, we observed twin-beam quantum correlations of -2.7  dB with this cavity. Such resonators feature strong optical confinement and offer tunable coupling to an external optical field. This work exemplifies the potential of crystalline whispering-gallery-mode resonators for the generation of quantum light. The simplicity of this device makes the application of quantum light in various fields highly feasible.

Surpassing the Standard Quantum Limit for Optical Imaging Using Nonclassical Multimode Light

Using continuous wave superposition of spatial modes, we demonstrate experimentally displacement measurement of a light beam below the standard quantum limit. Multimode squeezed light is obtained by mixing a vacuum squeezed beam and a coherent beam that are spatially orthogonal. Although the resultant beam is not squeezed, it is shown to have strong internal spatial correlations. We show that the position of such a light beam can be measured using a split detector with an increased precision compared to a classical beam. This method can be used to improve the sensitivity of small displacement measurements.

Low-threshold optical parametric oscillations in a whispering gallery mode resonator

Whispering gallery mode (WGM) resonators feature strong optical confinement, small mode volume, and offer tunable coupling to an external optical field. Fabricating WGM resonators from lithium niobate one can take advantage of these properties to achieve very strong optical nonlinear response, e. g. parametric downconversion (PDC). This process offers a highly wavelength tunable light source and is used as a state of the art source for nonclassical light. Driving PDC in cavities, also referred to as an optical parametric oscillator (OPO), provides efficient wavelength conversion. Thus, it is intruiging to investigate PDC in a WGM resonator, although phase matching conditions become involved in spherical geometry. In the process of higher harmonic generation, quasi phase-matching has already been demonstrated in a lithium niobate WGM cavity [1, 2], showing the potential of these resonators. Here we present a highly efficient OPO with a WGM resonator using natural temperature phase matching, where the individual optical fields have narrow optical linewidth [3].

Experimental entanglement distillation of mesoscopic quantum states

Two independent experiments demonstrate that quantum entanglement that has been lost in decoherence processes can be recovered. For the first time such ‘entanglement distillation’ has been achieved for states of light that are entangled in continuous variables, which should help to increase the distance over which quantum information can be distributed.

Demonstration of quadrature squeezed surface-plasmons in a gold waveguide

The first experiment demonstrating the quantum optical properties of SPPs was the preservation of entanglement under plasmon assisted transmission through sub-wavelength holes in a conductor [1,2].