Joint spectrum mapping of polarization entanglement in spontaneous parametric down-conversion

Abstract

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 multi-photon 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 to predict the polarization correlations for arbitrary filter profiles when the frequency degree of freedom of the photon pairs is ignored.

Shaping the Waveform of Entangled Photons

“…In this letter, we demonstrate experimentally this mapping and report experiments that demonstrate the feasibility of using noncollinear SPDC as a tool to control the type of frequency correlations using as tunable parameter the size of the pump beam waist and the angle of emission of the downconverted photons. In the past, measurements of the joint spectrum have been reported [5,19]. However, to the best of our knowledge this is the first time that manipulations of the joint spectrum have been demonstrated experimentally.…”

“…Quantum light has been proved to be useful in many quantum information applications and the most appropriate form of the joint spectrum depends on the specific realization under consideration. For example, uncorrelated pairs of photons can be used as a source of heralded single photons with a high degree of quantum purity [1,2]; the tolerance against the effects of mode mismatch in linear optical circuits can be enhanced by using photons with appropriately tailored waveform shape [3]; the use of frequency-correlated or anticorrelated photons allows erasing the distinguishing information coming from the spectra when considering polarization entanglement [4,5]; some protocols for quantum enhanced clock synchronization and positioning measurements rely on the use of frequency anticorrelated [6] or correlated photons [7]. Moreover, the entanglement in the frequency domain offers by itself a new physical resource where to explore quantum physics in a high-dimensional Hilbert space [8].…”

Focusing on factorability: space–time coupling in the generation of pure heralded single photons

“…The predicted heralded purity from the amplitude distribution of a source using this set of parameters is 0.953 for positive spatial chirp but 0.839 if the spatial chirp is negative, as estimated using the numerical model outlined above. To test this prediction, two grating spectrometers were used to map the joint spectral intensity of the photon pairs [23,24]. Such a measurement determines the degree of spectral correlation, although it provides no information about the phase of the joint spectral amplitude and therefore the degree of temporal correlation remains unknown.…”

Toward a downconversion source of positively spectrally correlated and decorrelated telecom photon pairs

“…Spectral filtering [1][2][3][4][5][6] makes it possible to minimize correlations, but positive correlations can be achieved only by careful source design. In a typical scenario, the SPDC process naturally leads to spectral anticorrelations [7][8][9][10][11]. Positively spectrally correlated photons at 1500 nm were suggested in theory [12] but were never shown in experiment.…”