Description
Nonclassical photon sources of high brightness are key components of quantum communication technologies. We here demonstrate the generation of narrowband, nonclassical photon pairs up to the ultimate limit of achievable generated spectral brightness at which successive photon pairs start to overlap in time.
Our biphoton source employs spontaneous four-wave mixing in an optically-dense ensemble of cold Rubidium atoms within a hollow-core fiber. To avoid collisions of the cold atoms with the fiber wall, thereby reducing the coherence time, the atoms are guided by an optical dipole trap. By using cold atoms we achieve an orders of magnitude lower biphoton bandwidth as compared to fibers filled with warm gases. As an additional benefit of the hollow-core fiber, all light fields involved in the FWM process are guided in the same optical mode. Thus, we obtain intrinsically optimal mode-matching as well as strong light-atom coupling at orders of magnitude lower pump powers compared to free space experiments. This results in a generated spectral brightness per pump power of up to $2\times10^9 \ \mathrm{pairs}/(\mathrm{s}\cdot\mathrm{MHz}\cdot\mathrm{mW})$, which is a 10-fold increase over the previous record. Additionally, our results are achieved at a 100-fold reduced pump power and exhibit a 10-fold lower bandwidth of $2\pi \times 6.5\ \mathrm{MHz}$, which is directly compatible with atomic quantum memories. We verify the nonclassical character of the generated photons with a 97-fold violation of the Cauchy-Schwarz inequality. We further demonstrate that our source can be used as a heralded single-photon source with a heralded auto-correlation as low as $0.08$. Moreover, we show that our biphoton source can be tuned to the ultimate achievable limit of generated spectral brightness, at which successive photon pairs start to overlap in time. In this regime, the photon correlations clearly start to deviate from the expected behavior for low brightness sources.
As the photon pairs at wavelengths of $780/795 \ \mathrm{nm}$ exhibit orthogonal polarizations and are emitted into a single spatial mode, they can be efficiently separated and interfaced to photonic networks as well as atomic quantum memories. Our source combines the advantages of atomic ensembles and waveguide environments, thus representing a step towards interfacing ensemble-based building blocks with photonic quantum networks.
| Presenter name | Alexander Bruns |
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| How will you attend ICAP-27? | I am planning on in-person attendance |
