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Conference on Lasers and Electro-Optics

Novel Narrow-Band Spectral Interference Filter with Very High Transmittance

Jan Bogdanski, Ariel Danan, Scott M. Jobling, Kevin McCusker, Stephan Quint, Alex Z. Smith, Jake Smith, Paul G. Kwiat

Published Jan 1, 2010• 0 citations
Publisher

Abstract

We report a novel scheme to improve the effective transmission of a standard interference filter, and demonstrate 96% passband transmission. Such high efficiency is critical for quantum information applications, e.g., high-efficiency single-photon generation utilizing parametric down-conversion.
DOI
10.1364/qels.2010.qthk5

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We report a novel scheme to improve the effective transmission of a standard interference filter, and demonstrate over 97% passband transmission. Such high efficiency is critical for quantum information applications, e.g. high-efficiency single-photon generation utilizing parametric down-conversion. The scheme can also be modified to function with a tilted filter, thereby allowing tuning of the passband frequency. In addition, the tilted configuration creates an infinite number of consecutive reflections from and transmissions through the filter, further improving the net filter transmission. Because spectral interference filters are a key element in optical quantum information experiments (both on the source and detection side, e.g. to exclude background photons as well as to determine the spectra of the desired photons), our scheme of enhanced interference filter transmission should lead to significant performance improvements in such experiments.
Spectrally pure and indistinguishable single photons are essential for quantum network platforms, where high-visibility interference underpins many quantum information protocols. However, most practical single-photon sources emit spectrally multimode states with reduced purity. Conventional spectral intensity filtering can partially improve purity but cannot select a well-defined temporal mode (TM). Here, we demonstrate coherent temporal filtering of a multimode parametric down-conversion (PDC) source using a quantum pulse gate (QPG) and benchmark its performance against conventional intensity filtering. The generated PDC photons exhibit strong spectral correlations, rendering extraction of pure heralded photons from the pair a challenge. We demonstrate that QPG filtering consistently generates heralded photons with purities above 0.90 regardless of the filter shape. Contrariwise, using spectral intensity filters yields mixed photons. Photon purities are probed with chronocyclic Q-function tomography. Furthermore, we demonstrate the versatility of QPG filtering by extracting structured TMs, including superposition of picosecond time bins. These results establish the QPG as a practical coherent filtering tool for quantum network applications.
Photon pairs produced by parametric down-conversion or four-wave mixing can interfere with each other in multiport interferometers or carry entanglement between distant nodes for use in entanglement swapping. This requires the photons to be spectrally pure to ensure good interference and have high heralding efficiency to know accurately the number of photons involved and to maintain high rates as the number of photons grows. Spectral filtering is often used to remove noise and define spectral properties. For heralded single photons high purity and heralding efficiency are possible by filtering the heralding arm, but when both photons in typical pair sources are filtered, we show that the heralding efficiency of one or both of the photons is strongly reduced even by ideal spectral filters with 100% transmission in the passband: any improvement in reduced-state spectral purity from filtering comes at the cost of lowered heralding efficiency. We consider the fidelity to a pure, lossless single photon, symmetrize it to include both photons of the pair, and show this quantity is intrinsically limited for sources with spectral correlation. We then provide a framework for this effect for benchmarking common photon-pair sources and present an experiment where we vary the photon filter bandwidths and measure the increase in purity and corresponding reduction in heralding efficiency.