Quantum Up-Conversion of Squeezed Vacuum States from 1550 to 532 nm

Squeezed vacuum states constitute a particularly useful resource in quantum information as well as in quantum metrology. The frequency conversion of these states is important to provide the bridge between different wavelengths within a sequence of downstream applications and also to provide a way for squeezed-state generation at so-far inaccessible wavelengths. Here we demonstrate the external quantum up-conversion of carrier-light-free squeezed vacuum states for the first time. Our result proves that nondegenerate sum-frequency generation preserves the coherences that are present between photon pairs and higher-order photon pairs of the squeezed input state.

Figure 1

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. In contrast to a direct squeezing generation at 532 nm via parametric down-conversion (PDC), our scheme does not require an intense pump field at half the squeezing wavelength (here 266 nm). Since absorption and vulnerability for photorefractive damage of typical nonlinear media increases towards shorter wavelengths our approach opens the possibility for so far inaccessible short wavelengths.

Figure 2

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 quantum efficiency of 75%. The coherences between the photon pairs are conserved since balanced homodyne detection proved a squeezed shot noise when overlapping the up-converted squeezed vacuum state with a bright local oscillator at 532 nm. SHG: second-harmonic generation; DBS: dichroic beam splitter for separating different wavelengths; PBS: polarizing beam splitter; BS: balanced beam splitter.

Figure 3

Experimental proof of up-converted vacuum squeezing at 532 nm. For this measurement the balanced homodyne detector (BHD) output was bandpass filtered around a Fourier frequency of $f=8\mathrm{MHz}$. The noise power of the corresponding zero-point fluctuation (vacuum noise) was taken with a blocked signal input port of the BHD and then used to normalize all three traces. Corrections to the data such as dark noise subtraction were not applied. A nonclassical noise reduction of 1.5 dB below the vacuum noise and an antisqueezing of 2.4 dB were measured. These values correspond to an initial 4 dB highly pure squeezed state at 1550 nm, a quantum up-conversion efficiency of 75% and a total detection efficiency of 71% . These values were independently verified.