Quantum Up-Conversion

Quantum up-conversion is a versatile tool in today’s quantum optics experiments. It has the potential to overcome frequency differences between disparate subsystems within a quantum information network. Today, the generation and fibre-based transmission of nonclassical states of light are most efficient at near infra-red wavelengths, for instance at the telecommunication wavelength of 1550 nm. Future quantum memories based on trapped atoms, ions or atomic ensembles will potentially use shorter wavelengths, up to the visible spectrum. Furthermore, the up-conversion into the visible regime enables the efficient detection of infrared single photons with commercially available, low-noise and easy-to-handle silicon avalanche photodetectors.

Quantum Up-Conversion of Squeezed Vacuum States of Light

In our recent work [publication in Physical Review Letters or at 2physics.com (free)] 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’. 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.



Schematic of quantum up-conversion (QUC) of a squeezed vacuum field from 1550 nm to 532 nm. The nondegenerate sum-frequency generator is pumped with a coherent light field at 810 nm being the shortest intense wavelength in this setup.



Schematic of the experimental setup. The left half shows the (conventional) generation of squeezed vacuum states of light at 1550 nm and the generation of intense radiation at 810 nm based on cavity enhanced parametric down-conversion (PDC) and above-threshold nondegenerated optical parametric oscillation (NOPO), respectively. Both fields are superimposed and mode matched into the sum-frequency generator (SFG) for quantum up-conversion (QUC). Here photon pairs at 1550 nm are converted unconditionally to photon pairs at 532 nm with a measured conversion efficiency of 75%. SHG: second-harmonic generation; DBS: dichroic beam splitter for separating different wavelengths; PBS: polarizing beam splitter; BS: balanced beam splitter.


Experimental proof of up-converted vacuum squeezing at 532 nm via balanced homodyne detection. A nonclassical noise reduction of 1.5 dB below the vacuum noise and an antisqueezing of 2.4 dB was measured.





Quantum Up-Conversion of Single Photons

Some quantum communication protocols require non-Gaussian states to work such as photon number states. In this project we successfully implemented the up-conversion of heralded single photons. In a spontaneous parametric down-conversion process correlated photon pairs at 810 nm and 1550 nm are generated, where the former are used as triggers and the latter are up-converted to 532 nm [See arXiv-Version of the publication].

Schematic of the experimental setup. Two doubly resonant optical parametric oscillators are pumped above (OPO) and below (SPDC) threshold with a continuous wave 532 nm pump field, producing bright fields and twin photons at 810 nm and 1550 nm. The 1550 nm photons are up-converted to 532 nm in the quantum up-converter (QUC) which is pumped with a strong pump field at 810 nm and analysed in a Hanbury Brown and Twiss setup with Si-APDs (APD-A and -B). The 810 nm photons heralding the existence of a 532 nm photon are detected at APD-T after transmitting the filter cavity (FC).


Histogram of the two-fold coincidence detections at APD-T and APD-A (red), and APD-T and APD-B (yellow) with theoretical curves. Clearly, three-fold coincidences (grey dots) do not contribute significantly to the statistics, indicating that single photons have been observed.




This work was supported by the Deutsche Forschungsgemeinschaft (DFG), by the Centre for Quantum Engineering and Space- Time Research (QUEST), and by the International Max Planck Research School for GravitationalWave Astronomy (IMPRS-GW).



Contact: Christoph Baune

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