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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Characterization of coincident-frequency entangled source in quantum synchronization application

Dong Ruifang, Runai Quan, Hou Feiyan, Mengmeng Wang, Zhaoyang Tai, Tao Liu, Shougang Zhang

Published May 1, 2014• 0 citations
Publisher

Abstract

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.

DOI
10.1109/fcs.2014.6859841

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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.
The frequency entangled biphoton source generated via spontaneous parametric down-conversion process (SPDC) has found numerous applications in quantum information processing and relevant fields. We report on an experimental generation of coincident-frequency entanglement from periodically poled potassium titanyl phosphate, pumped by an ultra-short pulsed optical source with duration less than 20 fs. Based on the Hong-Ou-Mandel interferometric coincidence measurement setup, a visibility of about 42% is demonstrated, which indicates degraded frequency indistinguishibility of the down-converted biphotons. Through theoretical investigation, such a degradation can be perfectly explained by the nonnegligible second-order dispersion terms in the Taylor-expanded phase mismatching function for the case of ultra-broadband spectrum of the pulsed pump. The fitting to the experimental results is further used and perfect agreement is achieved. The results imply that the spectral bandwidth of the pump can affect the generated coincident-frequency entanglement.
Tailoring spectral properties of photon pairs is of great importance for optical quantum information and measurement applications. High-resolution spectral measurement is a key technique for engineering spectral properties of photons, making them ideal for various quantum applications. Here we demonstrate spectral measurements and optimization of frequency-entangled photon pairs produced via spontaneous parametric downconversion (SPDC), utilizing frequency-resolved sum-frequency generation (SFG), the reverse process of SPDC. A joint phase-matching spectrum of a nonlinear crystal around 1580 nm is captured with a 40 pm resolution and a > 40 dB signal-to-noise ratio, which is significantly improved compared to traditional frequency-resolved coincidence measurements. Moreover, our scheme is applicable to collinear degenerate sources whose characterization is difficult with previously demonstrated stimulated difference frequency generation (DFG). We also illustrate that the observed phase-matching function is useful for finding an optimal pump spectrum to maximize the spectral indistinguishability of SPDC photons. We expect that our precise spectral characterization technique will be useful tool for characterizing and tailoring SPDC sources for a wide range of optical quantum applications.
We demonstrate a new class of frequency-entangled states generated via spontaneous parametric down-conversion under extended phase-matching conditions. Biphoton entanglement with coincident signal and idler frequencies is observed over a broad bandwidth in periodically poled KTiOPO4. We demonstrate high visibility in Hong-Ou-Mandel interferometric measurements under pulsed pumping without spectral filtering, which indicates excellent frequency indistinguishability between the down-converted photons. The coincident-frequency entanglement source is useful for quantum information processing and quantum measurement applications.
The frequency entangled biphoton source generated via spontaneous parametric down-conversion (SPDC) process has found important applications in the fields of quantum clock synchronization, quantum communication, quantum information processing, etc. As quantum technologies evolve, quantitative characterization of the frequency entanglement becomes necessary and has been implemented by measuring the spectral properties of the biphoton state. However, due to the high dark rate and low quantum efficiency of the InGaAs single-photon detectors, direct measurement of the spectral properties of the biphoton state at optical communication wavelength is hard to implement. In this paper, we report the measurement of the spectral properties of a biphoton state at optical communication wavelength which is generated from periodically poled potassium titanyl phosphate (PPKTP) pumped by an ultra-short pulsed optical source at 787 nm. Based on the coincidence measurement setup together with two infrared spectrometers, the spectra of the signal and idler photons are obtained with their center wavelengths being 1574.4 nm and 1574.9 nm, while their 3-dB bandwidths being 35.3 nm and 37.6 nm respectively. The joint spectrum of the photon pair is observed as well and shows a coincident-frequency entanglement and a joint spectrum bandwidth of 3 nm. According to the ratio of the single-photon spectral bandwidth to the joint spectral bandwidth of the photon pairs, the degree of frequency entanglement is quantified to be 12, denoting a relatively high quality of the entanglement.
Abstract Frequency entangled biphoton sources have played an important role in quantum information. There are two key characteristics which determine the extent of their applications: the spectral indistinguishability and the degree of frequency entanglement. Previously, the spectral indistinguishability of the biphoton state is measured by the Hong–Ou–Mandel (HOM) interference visibility, whereas the degree of frequency entanglement is normally characterized with independent spectral measurements. However, these two quantum features have not been successfully measured with a single experimental setup. In this paper, we deduce the temporal distribution of a short-pulse pumped type-II spontaneous parametric down converted (SPDC) biphoton source and its relevant temporal entanglement parameter R t for operationally quantifying the degree of frequency entanglement. Furthermore, we theoretically and experimentally demonstrate, due to the inherent group delay, the difference between the signal and idler photons at the exit of the type-II nonlinear crystal, resulting in a HOM-shape dip at both sides of the center fringed envelope of the MZ interferometric coincidence diagram. By measuring the HOM-shape dip depth, the separation between the two sideband dips as well as the MZ fringed envelope width, both the spectral indistinguishability and the temporal entanglement parameter can thus be simultaneously quantified. This implementation provides us a unified and convenient method to characterize the frequency entanglement of the short-pulse pumped biphotons.
The generation of entangled photon pairs through spontaneous parametric down conversion (SPDC) is widely used and considered one of the key entanglement generation techniques in quantum optics. The quality of the entanglement produced depends significantly on the physical properties of the nonlinear crystal employed in the SPDC process. In this study, we investigate the influence of the crystal parameters such as phase matching configurations on the degree of entanglement. Our results demonstrate the importance of crystal selection for achieving highly entangled photon sources, which are essential for applications in quantum communication and quantum information.