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arXiv:0905.3849 · Physical Review A

Eliminating spectral distinguishability in ultrafast spontaneous parametric down-conversion

Hou Shun Poh, Jiaqing Lim, I. Marcikic, Antı́a Lamas-Linares, Christian Kurtsiefer

Published Oct 14, 2009• 19 citations
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Abstract

Generation of polarization-entangled photon pairs with a precise timing through down-conversion of femtosecond pulses is often faced with a degraded polarization entanglement quality. In a previous experiment, we have shown that this degradation is induced by spectral distinguishability between the two decay paths, in accordance with theoretical predictions. Here, we present an experimental study of the spectral compensation scheme proposed and first implemented by Kim and Grice [J. Mod. Opt. 49, 2309 (2002)]. By measuring the joint spectral properties of the polarization correlations of the photon pairs, we show that the spectral distinguishability between the down-converted components is eliminated. This scheme results in a polarization visibility of $97.9\ifmmode\pm\else\textpm\fi{}0.5%$ without any spectral filtering.

DOI
10.1103/physreva.80.043815

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Generation of polarization-entangled photon pairs with a precise timing through down-conversion of femtosecond pulses is often faced with a degraded polarization entanglement quality. In a previous experiment, we have shown that this degradation is induced by spectral distinguishability between the two decay paths, in accordance with theoretical predictions. Here, we present an experimental study of the spectral compensation scheme proposed and first implemented by Kim and Grice [J. Mod. Opt. 49, 2309 (2002)]. By measuring the joint spectral properties of the polarization correlations of the photon pairs, we show that the spectral distinguishability between the down-converted components is eliminated. This scheme results in a polarization visibility of 97.9\ifmmode±\else\textpm\fi0.597.9\ifmmode\pm\else\textpm\fi{}0.5%97.9\ifmmode±\else\textpm\fi0.5 without any spectral filtering.
Polarization-entangled photon pairs can be efficiently prepared into pure Bell states with a high fidelity via type-II spontaneous parametric down-conversion (SPDC) of narrow-band pump light. However, the use of femtosecond pump pulses to generate multiphoton states with precise timing often requires spectral filtering to maintain a high quality of polarization entanglement. This typically reduces the efficiency of photon pair collection. We experimentally map the polarization correlations of photon pairs from such a source over a range of down-converted wavelengths with a high spectral resolution and find strong polarization correlations everywhere. A spectrally dependent imbalance between contributions from the two possible decay paths of SPDC is identified as the reason for a reduction in entanglement quality observed with femtosecond pump pulses. Our spectral measurements allow us to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.
The quantum state of the photon pair generated from type-II spontaneous parametric down-conversion pumped by a ultrafast laser pulse exhibits strong decoherence in its polarization entanglement, an effect which can be attributed to the clock effect of the pump pulse or, equivalently, to distinguishing spectral information in the two-photon state. Here, we discuss novel spectral engineering techniques to eliminate these detrimental decoherence effects. In addition, spectral engineering provides a means for generating polarization entangled states with novel spectral characteristics: the frequency-correlated state and the frequency-uncorrelated state. Such states may find usefulness in experimental quantum information science and quantum metrology applications.
The quantum state of the photon pair generated from type-II spontaneous parametric down-conversion pumped by a ultrafast laser pulse exhibits strong decoherence in its polarization entanglement, an effect which can be attributed to the clock effect of the pump pulse or, equivalently, to distinguishing spectral information in the two-photon state. Here, we propose novel temporal and spectral engineering techniques to eliminate these detrimental decoherence effects. The temporal engineering of the two-photon wavefunction results in a universal Bell-state synthesizer that is independent of the choice of pump source, crystal parameters, wavelengths of the interacting photons, and the bandwidth of the spectral filter. In the spectral engineering technique, the distinguishing spectral features of the two-photon state are eliminated through modifications to the two-photon source. In addition, spectral engineering also provides a means for the generation of polarization-entangled states with novel spectral characteristics: the frequency-correlated state and the frequency-uncorrelated state.
The quantum state of the photon pair generated from type-II spontaneous parametric downconversion pumped by an ultrafast laser pulse exhibits strong decoherence in its polarization entanglement, an effect which can be attributed to the clock effect of the pump pulse or, equivalently, to distinguishing spectral information in the two-photon state. Here, we propose novel temporal and spectral engineering techniques to eliminate these detrimental decoherence effects. The temporal engineering of the two-photon wavefunction results in a universal Bell-state synthesizer that is independent of the choice of pump source, crystal parameters, wavelengths of the interacting photons and the bandwidth of the spectral filter. In the spectral engineering technique, the distinguishing spectral features of the two-photon state are eliminated through modifications to the two-photon source. In addition, spectral engineering also provides a means for the generation of polarization-entangled states with novel spectral characteristics: the frequency-correlated state and the frequency-uncorrelated state.
We experimentally investigate a double-pass parametric down-conversion scheme for producing pulsed, polarization-entangled photon pairs with high visibility. The amplitudes for creating photon pairs on each pass interfere to compensate for distinguishing characteristics that normally degrade two-photon visibility. The result is a high-flux source of polarization-entangled photon pulses that does not require spectral filtering. We observe quantum interference visibility of over 95% without the use of spectral filters for 200fs200\phantom{\rule{0.3em}{0ex}}\mathrm{fs}200fs pulses, and up to 98.1% with 5nm5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}5nm bandwidth filters.