Observation of two output light pulses from a partial wavelength converter preserving phase of an input light at a single-photon level

TL;DR

This study demonstrates that a wavelength converter can preserve phase information of single-photon level light pulses, producing two output pulses with high interference visibility regardless of conversion efficiency.

Contribution

It experimentally shows phase preservation in both converted and unconverted light pulses at a single-photon level during wavelength conversion.

Findings

High interference visibility (>0.88) across various conversion efficiencies.

Near-perfect visibilities (0.98 and 0.99) at 50% conversion efficiency.

Preservation of phase information in both output pulses.

Abstract

We experimentally demonstrate that both a converted and an unconverted light pulses after wavelength conversion with various conversion efficiencies preserve phase information of an input light at a single-photon level. In our experiment, we converted temporally-separated two coherent light pulses with average photon numbers of $\sim$ 0.1 at 780 nm to light pulses at 1522 nm by using difference-frequency generation in a periodically-poled lithium niobate waveguide. We observed a single-photon interference between temporally-separated two modes for both the converted and the unconverted light pulses at various values of the conversion efficiency. We observed interference visibilities greater than 0.88 without suppressing the background noises for any value of the conversion efficiency the wavelength converter achieves. At a conversion efficiency of $\sim$ 0.5, the observed visibilities are 0.98 for the unconverted light and 0.99 for the converted light.