Quantum Up-Conversion of Squeezed Vacuum States
Authors: Christina E. Vollmer, Christoph Baune, Aiko Samblowski, Tobias Eberle, Vitus Händchen, Jaromír Fiurášek, Roman Schnabel
Affiliation: Institut für Gravitationsphysik der Leibniz Universität Hannover, Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut), Hannover, Germany
Squeezed vacuum states of light belong to the special class of ‘nonclassical states’ that can not be fully described by either a classical or a semi-classical model. Overlapped with a (bright) coherent laser beam, they are able to reduce (to ‘squeeze’) the photon counting statistics, i.e. the light’s shot noise, thereby enhancing the sensitivity of optical measurement devices. There are applications in spectroscopy  and imaging  and in particular in interferometric length measurements in gravitational wave detectors [3-6], once the kick-off for research in squeezed states. With squeezed vacuum states it is also possible to teleport quantum states , to generate so-called Schrödinger kitten states , and to improve quantum cryptography .
Fig.1: Schematic of the experiment. A squeezed vacuum state at 1550 nm is overlapped with a bright pump field at 810 nm inside a periodically poled KTP crystal inside an optical resonator for sum-frequency generation. The output is a squeezed vacuum state at 532 nm.
In our recent work  we demonstrated for the first time the frequency up-conversion of squeezed vacuum states of light in an external setup, i.e. ‘on the fly’. Our scheme can be applied to quantum networks that first use a squeezing wavelength of 1550 nm for transmission through optical fibres and then use a shorter wavelength to meet the requirements of a quantum memory for storing the squeezed state. In our experiment we converted a 4dB squeezed state at 1550nm to a 1.5dB squeezed state at 532nm. The degradation was due to optical loss and in full agreement with our model.
With our experiment we also demonstrated a scheme that provides access to short squeezing wavelengths. Today, squeezed states are most efficiently produced at near-infrared wavelengths. Due to the lack of appropriate nonlinear media it is difficult to produce them with conventional techniques at visible or even ultra-violet wavelengths. In future work we plan to reduce the optical loss of our setup to be able to demonstrate strong squeezing at visible wavelengths.
Fig. 2: Photograph of parts of the experiment. In total, the experiment required five frequency conversion steps. First, a 1064 nm continuous-wave laser beam was frequency doubled. The produced 532 nm beam was used to generate two beams at 1550 nm and 810 nm via optical parametric oscillation. The 1550 nm light was frequency doubled and the generated 775 nm light used to pump a parametric down converter to produce squeezed vacuum states at 1550nm. The final step was the up-conversion as shown in Fig. 1.
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