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Preparation of Non-Classical Sources

Polarization squeezing and entanglement

Non-classical polarization states of light are particularly interesting due to their compatibility with the spin variables of atomic systems and their simple detection without additional local oscillator. By exploiting the Kerr effect we generate efficient polarization squeezing using ultrashort light pulses in a single pass through a birefringent fiber [1]. Recently we have achieved a new world record in fiber squeezing: we have measured a noise reduction of -6.8±0.3 dB, which when corrected for linear losses is -10.4 ± 0.8 dB [2]. Applying first-principles simulations showed excellent agreement with these measurements (see Fig. 1) as well as with earlier ones [3]. By interfering two independent polarization squeezed light beams on a symmetric beam splitter we have generated a highly efficient and robust polarization entanglement source [4].

Involved in this project are:
Ruifang Dong,   Mikael Lassen,   Denis Sych,   Christoph Marquardt,   Gerd Leuchs

J. Heersink, V. Josse, G. Leuchs, U. L. Andersen,
R.-F. Dong, J. Heersink, J. F. Corney, P. D. Drummond, U. L. Andersen, and G. Leuchs,
J. F. Corney, P. Drummond, J. Heersink, V. Josse, G. Leuchs, and U.L. Andersen,
R.-F. Dong, J. Heersink, J. Yoshikawa, O. Glöckl, U. L. Andersen, and G. Leuchs,
Simulation of polarization squeezing

Fig. 1: In recent experiments we reached the world record of -6.8dB in fiber squeezing [2]. First-principles simulations fully explain these results [3].

Polarization squeezing and GAWBS in Photonic Crystal Fibers

An alternative tool for the generation of squeezed states are Photonic Crystal Fibers (PCFs) since the fiber’s microstructure allows for tailoring a multitude of parameters. For example, due to a higher non-linear effect the same amount of squeezing can be produced in a shorter fiber or with less power. Squeezed states thus accumulate less excess noise caused by phonon scattering [1], such as Guided Acoustic Wave Brillouin Scattering (GAWBS). Moreover, we have demon-strated that the microstructure of a PCF mechanically decouples the light-guiding fiber core from its environment. Less photon-phonon interactions thus lead to an additional GAWBS-noise reduction in commercially available PCFs
(see Fig. 3) [2, 3]. Further improvements can be achieved by tailoring the microstructure.

Involved in this project are:
Michael Förtsch,   Mikael Lassen,   Josip Milanovic,   Christoffer Wittmann,   Christoph Marquardt,   Gerd Leuchs

J. Milanovic, J. Heersink, Ch. Marquardt, A. Huck, U.L. Andersen, G. Leuchs,
D. Elser, U. L. Andersen, A. Korn, O. Glöckl, S. Lorenz, Ch. Marquardt, and G. Leuchs,
D. Elser, Ch. Wittmann, U. L. Andersen, O. Glöckl, S. Lorenz, Ch. Marquardt, and G. Leuchs,
Simulation of GAWBS

Fig. 3: Simulation of the acoustic energy density in a photonic crystal fiber (blue=low energy, red=high energy). The air holes prevent the harmful acoustic vibrations from entering the light-guiding core [2].

Resonant and non-resonant interactions in filled hollow core photonic crystal fibers

As an alternative to Kerr squeezing in fibers the high non-linearities of atoms at the resonance are promising. Self-induced transparency (SIT) can hereby prevent the light from being absorbed. We filled the core of hollow core PCFs with Rubidium (see Fig. 4) and plan to evaporate it to obtain a microscopic vapor cell. In addition to the generation of squeezing via SIT, filled hollow core fibers can act as a versatile tool for light-atom interactions.

Involved in this project is:
Wenjia Zhong,   Christoph Marquardt,   Gerd Leuchs

Fig. 4: Rubidium-filled hollow-core fiber

Whispering Gallery Modes

We study nonlinear optical processes in whispering gallery mode (WGM) resonators to explore their quantum-optical aspects. The WGM resonators tend to have very high Q-factors as well as small optical mode volume, which greatly enhances the nonlinear frequency conversion processes. Such processes as e.g. second harmonic generation, parametric down-conversion, and others, then become efficient for very low optical powers and ultimately for individual photons. In this regime, the non-classical nature of the generated or converted light fields is most strongly revealed.

Involved in this project are:
Dmitry Strekalov,   Josef Fürst,   Michael Förtsch,   Christoffer Wittmann,   Christoph Marquardt,   Gerd Leuchs

Fig. 5: WGM resonator

Cylindrically polarized modes of light

Fig. 6: The two orthogonally polarized Hermite-Gaussian basis modes of a squeezed cylindrically polarized mode are not only entangled in the polarization and spatial DOF but also between these two DOFs.

Cylindrically polarized modes have the intriguing property that they combine a complex polarization with a complex spatial pattern. In [1] we have shown that these two degrees of freedom (DOFs) are already inseparable in classical description of the modes. When a cylindrically polarized mode is quadrature squeezed this structural inseparability leads to entanglement not only in the polarization and spatial DOF but also between these two DOFs [2]. Currently we are working on the direct detection of this hybrid-entanglement [3]. Furthermore, we are investigating how these modes can be used for cluster state generation. Due to their unique properties they have the potential to generate cluster states which are compact and addressable. These are two features essential for successful quantum computing. In another experiment we are generating non-classical states of light in higher-order modes with the help of a spatial light modulator. This device allows one not only to generate cylindrically polarized modes, but also any desired higher-order mode.

Involved in this project are:
Stefan Berg-Johansen
,   Marion Semmler,   Peter Banzer,   Andrea Aiello,  
Ioannes Rigas,   Christoph Marquardt,   Gerd Leuchs

[1] A. Holleczek, A. Aiello, C. Gabriel, Ch. Marquardt and G. Leuchs,
Optics Express 19, 9714 (2011). [preprint]

[2] C. Gabriel, A. Aiello, W. Zhong, T. G. Euser, N. Y. Joly, P. Banzer, M. Förtsch, D. Elser, U. L. Andersen, Ch. Marquardt, P. St.J. Russell and G. Leuchs,
     Phys. Rev. Lett. 106 (6), 060502 (2011). [preprint]

[3] C. Gabriel, A. Aiello, S. Berg-Johansen, Ch. Marquardt, G. Leuchs,
     Eur. Phys. J D 66 (7) (2012). [preprint]