In whispering gallery mode (WGM) resonators light is guided by continuous total internal reflection along a curved surface. Fabricating such resonators from an optically nonlinear material one takes advantage of their exceptionally high quality factors and small mode volumes to achieve extremely efficient optical frequency conversion. Our analysis of the phase matching conditions for optical parametric down conversion (PDC) in a spherical WGM resonator shows their direct relation to the sum rules for photons' angular momenta and predicts a very low parametric oscillations threshold. We realized such an optical parametric oscillator (OPO) based on naturally phase-matched PDC in Lithium Niobate. We demonstrated a single-mode, strongly non-degenerate OPO with a threshold of 6.7 µW and linewidth under 10 MHz. This work demonstrates the remarkable capabilities of WGM-based OPOs and opens the perspectives for their applications in quantum and nonlinear optics, particularly for the generation of squeezed light.

We report on the efficient generation, propagation, and re-emission of squeezed long-range surfaceplasmon polaritons (SPPs) in a gold waveguide. Squeezed light is used to excite the non-classical SPPs and the re-emitted quantum state is fully quantum characterized by complete tomographic reconstruction of the density matrix. We find that the plasmon-assisted transmission of non-classical light in metallic waveguides can be described by a Hamiltonian analogue to a beam splitter. This result is explained theoretically.PACS numbers: 42.50.Lc, 42.50.Nn, 73.20.Mf Enormous interest has recently been devoted to the emergent field of quantum plasmonics due to its unique capabilities in the way electromagnetic radiation can be localized and manipulated at the nanoscale. In particular, integrated quantum technologies based on surface plasmons hold great promises for quantum information processing, since it allows for scalability, miniaturization, and coherent coupling to single emitters [1,2,3,4,5]. To enable these quantum information processing technologies with high fidelity, it is of paramount importance, that the nonclassicality of the plasmonic modes is preserved in propagation. The first experiment verifying the preservation of entanglement in plasmonic nanostructures was carried out by Alterwischer et al. [6]. They demonstrated the survival of polarization entanglement after plasmonic propagation through subwavelength holes in a metal film. The preservation of energy time entanglement in a perforated metal film as well as in a thin conducting waveguide was later demonstrated by Fasel et al. [7]. These experiments have witnessed the preservation of probabilistically prepared entanglement (thus neglecting the vacuum contribution) described in the two dimensional Hilbert space.In the present Letter we investigate the compatibility of the quantum plasmonic technology with the continuous variable quantum domain (described in the infinite dimensional Hilbert space) by demonstrating the plasmonic excitation, propagation, and detection of deterministically prepared quadrature squeezed vacuum states. We show that a squeezed vacuum state excite an electron resonance on the surfaces of a metallic gold waveguide to form a surface plasmon polariton (SPP). Despite loss and decoherence in the plasmonic mode we demonstrate that quadrature squeezing is retained in the retrieved light state. Importantly, we fully characterize the input state and output state by performing a complete quantum tomographic reconstruction of the states density matrix. This is in strong contrast to previous experiments on plasmon assisted quantum state transmission, where only a certain property of the quantum state was investigated.SPPs are combined electron oscillations and electromagnetic waves propagating along the interface between a conductor and a dielectric medium [8]. By reducing the thickness of the metal film the SPP from the upper interface and the lower interface can couple. This coupling results in the formation of either long-range (LR) ...

A quantum key distribution (QKD) system must fulfill the requirement of universal composability to ensure that any cryptographic application (using the QKD system) is also secure. Furthermore, the theoretical proof responsible for security analysis and key generation should cater to the number N of the distributed quantum states being finite in practice. Continuous-variable (CV) QKD based on coherent states, despite being a suitable candidate for integration in the telecom infrastructure, has so far been unable to demonstrate composability as existing proofs require a rather large N for successful key generation. Here we report a Gaussian-modulated coherent state CVQKD system that is able to overcome these challenges and can generate composable keys secure against collective attacks with N ≈ 2 × 108 coherent states. With this advance, possible due to improvements to the security proof and a fast, yet low-noise and highly stable system operation, CVQKD implementations take a significant step towards their discrete-variable counterparts in practicality, performance, and security.

We report on a novel and efficient source of polarization squeezing using a single pass through an optical fiber. Simply passing this Kerr squeezed beam through a carefully aligned λ/2 waveplate and splitting it on a polarization beam splitter, we find polarization squeezing of up to 5.1 ± 0.3 dB. The experimental setup allows for the direct measurement of the squeezing angle.