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Optics Express

Inherent resolution limit on nonlocal wavelength-to-time mapping with entangled photon pairs

Ye Yang, Xiao Xiang, Feiyan Hou, Runai Quan, Baihong Li, Wei Li, Ninghua Zhu, Tao Liu, Shougang Zhang, Ruifang Dong, Ming Li

Published Feb 17, 2020• 19 citations
Publisher

Abstract

Nonlocal wavelength-to-time mapping between frequency-entangled photon pairs generated with the process of spontaneous parametric down-conversion is theoretically analyzed and experimentally demonstrated. The spectral filtering pattern experienced by one photon in the photon pair will be non-locally mapped into the time domain when the other photon propagates inside a dispersion-compensation fiber with large group velocity dispersion. Our work, for the first time, points out that the spectral bandwidth of the pump laser will become the dominated factor preventing the improvement of the spectral resolution when the involved group velocity dispersion is large enough, which provides an excellent tool for characterizing the resolution of a nonlocal wavelength-to-time mapping for further quantum information applications.

DOI
10.1364/oe.387146

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Nonlocal wavelength-to-time mapping between frequency-entangled photon pairs generated with the process of spontaneous parametric down-conversion is theoretically analyzed and experimentally demonstrated. The spectral filtering pattern experienced by one photon in the photon pair will be non-locally mapped into the time domain when the other photon propagates inside a dispersion-compensation fiber with large group velocity dispersion. Our work, for the first time, points out that the spectral bandwidth of the pump laser will become the dominated factor preventing the improvement of the spectral resolution when the involved group velocity dispersion is large enough, which provides an excellent tool for characterizing the resolution of a nonlocal wavelength-to-time mapping for further quantum information applications.
The dispersive wavelength to time mapping with the entangled photon source is an effective way of measuring the spectral information of the entangled photon pairs. This approach avoids the usage of spectral filtering equipment like the monochromator, which reduces the measuring time and the system’s complexity. The wavelength-to-time mapping method can be divided into local mapping and nonlocal mapping depending on whether the measurement utilizes the frequency correlation of the entangled photon pairs. For local mapping, the spectral information of signal photons is directly mapped to the time domain through dispersion without utilizing the frequency correlation between the photon pair. For nonlocal mapping, the signal photons with spectral information are directly detected. And the corresponding idler photons are dispersed. With the help of the frequency correlation between the photon pair, the spectral information on signal photons can be recorded in count measurement. In this letter, the two types of mapping results are theoretically and experimentally compared. The theoretical result indicates that the two types of mapping results are the same when the pump light of the entangled source is ideal monochromatic with infinite linewidth. However, when using a real pump light with finite linewidth, the theoretical and experimental result of the two types of mapping is different. The difference in the result indicates the potential influence of the mapping method, which can further help to select a more suitable mapping method for different measuring conditions.
Recently the engineering of the entanglement for photon pairs generated during the spontaneous parametric down conversion process (SPDC) can be achieved via manipulation of pump wavelength behind a \c{hi}(2)-based type II SPDC process [1]. Such effect is used in this paper for demonstration of non-classical dispersion cancellation phenomenon in both local and nonlocal detections, theoretically. The following results are analytically achieved: I) For local detection, if narrow pump laser (highly entangled photons) are used, the dispersive broadening cancelation is directly depends on the degree of entanglement. The higher entanglement degree, the more compensation occurs. The results indicate that by increasing the FWHM of the pump the impact of the entanglement degree is decreased in such a way that for a pump with FWHM=4 nm the entanglement has no effect on the broadening. Therefore, the dispersive broadening is only depends on the temporal walk-off between generated photon pairs and the pump. II) For nonlocal detection, it is also shown that entanglement cancels the dispersive broadening if and only if each of the generated paired photons propagate through dispersive material with identical length with opposite group velocity sign.
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.
Benefiting from the unique quantum feature of nonlocal dispersion cancellation (NDC), the strong temporal correlation of frequency-entangled photon pair source can be maintained from the unavoidable dispersive propagation. It has thus played a major role in many fiber-based quantum information applications. However, the limit of NDC due to finite frequency entanglement has not been quantified. In this study, we provide a full theoretical analysis of the NDC characteristics for the photon pairs with finite frequency entanglement. Experimental examinations were conducted by using two spontaneous parametric down-conversion photon pair sources with frequency correlation and anticorrelation properties. The excellent agreement demonstrates the fundamental limit on the minimum temporal correlation width by the nonzero two-photon spectral correlation width of the paired photons, which introduces an inevitable broadening by interaction with the dispersion in the signal path. This study provides an easily accessible tool for assessing and optimizing the NDC in various quantum information applications.
We analyze the process of photon-pair generation via spontaneous parametric down-conversion in a quadratic nonlinear asymmetric waveguide coupler. The two waveguides have different geometry, such that light coupling only occurs within a narrow bandwidth of one of the generated (signal) photon modes, while the other (idler) photon together with the pump stay localized in one (driven) arm of the coupler. We demonstrate that such a setup represents a powerful and flexible tool for engineering spectral properties of generated photon pairs. Mode hybridization and dispersion of coupling can be utilized for shifting the balance between group velocities of interacting pump, signal, and idler fields, subsequently leading to a significant increase of spectral factorizability (purity) of photons. We also show that for interaction lengths shorter than one beat length, generated pairs with signal photon being localized in the auxiliary (not pumped) arm of the coupler appear to be spectrally localized in both signal and idler components. The bandwidth of such intrinsic filtering of generated photons can be controlled by several geometrical parameters.
The frequency entangled photon pairs generated via spontaneous parametric down-conversion process (SPDC) have been found the main resources in quantum clock synchronization protocols. The spectral indistinguishibility of the photon pairs is a key factor for the achievement of quantum clock synchronization. In this paper, we characterize both theoretically and experimentally the factors degrading the spectral indistinguishibility of the generated coincident-frequency entangled photon pairs under extended phase matching conditions. The results show that due to the nonnegligible second-order dispersion contribution to the phase mismatching, the broadening of the pulse pump bandwidth and the lengthening of the nonlinear crystal all lead to the degradation.