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.
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.
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.
We report the generation and characterization of a frequency anti-correlated entangled biphoton source at the wavelength of 1560nm via a continuous-wave laser pumped spontaneous parametric down-conversion process. The spectra of the signal and idler photons are measured to have their center wavelength being 1559.7nm and 1559.8nm while their 3-dB bandwidths being 3.2nm and 3.15nm respectively. The joint spectrum of the photon pair is observed to be frequency anti-correlated and have a spectral bandwidth of 0.5nm. 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 6.4. By investigating the relative overlap between the achieved two-photon spectral intensity function and its transpose, the spectral indisinguishablity of the photon pairs is expected to be 90%. Based on a HOM interferometric coincidence measurement setup, a visibility of 89.8% is demonstrated, which shows a good agreement with the expectation.
The frequency entangled photon pairs generated by spontaneous parametric down-conversion (SPDC) possess wide applications in quantum optics and relevant fields.To facilitate the practical quantum information technologies,particularly in optical fiber links,a frequency entangled source at telecommunication wavelength with features of compactness,portability,high efficiency and miniaturization is highly desired.In this paper,we report the experimental generation of a miniaturized frequency entangled source in telecommunication band from a 10 mm-long type-Ⅱ periodically poled lithium niobate (PPLN) waveguide pumped by a 780 nm distributed Bragg reflector (DBR) laser diode.The frequency entangled photon pairs generated by SPDC possess wide applications in quantum optics and relevant fields.When the DBR laser diode is driven by a current of 170 mA at a temperature controlled to 20℃,the output power is measured to be 70.4 mW with a central wavelength of 780.585 nm.Under this pump,the orthogonally-polarized photon pairs are generated and output from the PPLN waveguide.After filtering out the remaining pump by three high-performance long-pass filters mounted on an adjustable U-type fiber bench,the photon-pair generation rate,spectral and temporal properties of the generated frequency entangled source are measured.The results show that the generation rate of the photon pairs,after being corrected for the detection efficiencies of the single photon detectors and the optical losses,is achieved to be 1.86×107 s-1 at a pump power of 44.9 mW (coupled into the waveguide).Optimizing the working temperature of the waveguide and fixing it at 46.5℃,the frequency degeneracy of the SPDC generated photon pairs is realized.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 of 1561.43 nm and 1561.45 nm,and their 3-dB bandwidths of 3.62 nm and 3.60 nm respectively.The joint spectrum of the photon pair is observed,showing a joint spectrum bandwidth of 3.18 nm.The degree of frequency entanglement is quantified to be 1.13 according to the bandwidth ratio between the single photon spectrum and the joint spectrum.Furthermore,based on the Hong-Ou-Mandel (HOM) interferometric coincidence measurement setup,a visibility of about 96.1% is observed,which indicates the very good frequency indistinguishibility of the down-converted biphotons.The measured 3-dB width of the HOM dip is 1.47 ps and shows good agreement with the measured single-photon spectral bandwidth.The experimental results lay a solid foundation for developing portable,miniaturized frequency entangled sources at telecommunication band for the further applications in quantum information areas,such as quantum time synchronization.
We report on experimental measurements of coincidence and single particle spectral shapes of biphoton signals when frequency entangled states are generated by SPDC crystals pumped by short pulses.It is shown evidence for biphoton coincidence spectrum narrower and single-particle one wider than the pump spectrum, with a large contrast between coincidence and singles distributions.The investigation of biphoton correlations has been performed by fixing one monochromator at the maximal transmission wavelength on down-converted signal gate and scanning the one placed in the idler branch in order to observe the spectral distribution of single counts and coincidences.The degree of entanglement is evaluated by the ratio of the FWHM of single particle and coincidence distributions: the greater the ratio, the greater is the entanglement.The degree of entanglement in frequency variables can be very high: it increases linearly with the length of the crystal, and decreases with the growing pump pulse.
We demonstrate an efficient generation of frequency anti-correlated entangled photon pairs at telecom wavelength. The fundamental laser is a continuous-wave high-power fiber laser at 1560 nm, through an extracavity frequency doubling system, a 780-nm pump with a power as high as 742 mW is realized. After single-passing through a periodically poled KTiOPO4 (PPKTP) crystal, degenerate down-converted photon pairs are generated. With an overall detection efficiency of 14.8 %, the count rates of the single photons and coincidence of the photon pairs are measured to be 370 kHz and 22 kHz, respectively. The spectra of the signal and idler photons are centered at 1560.23 and 1560.04 nm, while their 3-dB bandwidths being 3.22 nm both. The joint spectrum of the photon pair is observed to be frequency anti-correlated and have a spectral bandwidth of 0.52 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 6.19. Based on a Hong–Ou–Mandel interferometric coincidence measurement, a frequency indistinguishability of 95 % is demonstrated. The good agreements with the theoretical estimations show that the inherent extra intensity noise in fiber lasers has little influence on frequency entanglement of the generated photon pairs.
Time-frequency domain is a promising platform for optical quantum technologies due to large Hilbert space available for quantum information encoding. Moreover, photons appear as perfect candidates for interface between different quantum systems as they can be transmitted over large distances in a low-loss manner. Obviously mostly desired would be such link at telecommunication wavelengths, because it can be integrated with classical communication schemes. We report on a tunable telecom-wavelength photon pair source based on bulk periodically poled potassium titanyl phosphate (PPKTP) pumped by femtosecond laser pulses. The pairs are produced via type-II spontaneous parametric down conversion (SPDC). The spectra of photons, which lies in telecommunication range, in our source can be affected via both changing the spectrum of pumping laser and changing the phase matching by using crystals of different lengths. By appropriate choice of these parameters either the pair of photons occupy single-mode in frequency and is in a separable state or is multimode and entangled [1]. Here we report on experimental active modification the photon pairs’ spectral properties. We employ fast electro-optic temporal phase modulation to induce a deterministic change of photons’ joint spectral intensity (JSI). Central wavelength of one photon is shifted by up to 0.2 nm. The measurement of the spectrum of single photons is based upon frequency to time mapping, implemented by large group delay dispersion (GDD) in chirped fiber Bragg grating (CFBG) [2]. An unprecedented sub-10 pm resolution of correlated single-photon spectral measurements has been achieved. Further work will be done to modify not only the position, but also the shape of JSI by using techniques based on the time-lens principle [3]. References: [1] P. G. Evans, R. S. Bennink, W. P. Grice, T. S. Humble, Phys. Rev. Lett. 105, 253601 (2010) [2] A. O. C. Davis, P. M. Saulnier, M. Karpinski, B. J. Smith, arXiv:1610.03040 (2016) [3] M. Karpiński, M. Jachura, L. J. Wright, B. J. Smith, Nature Photonics 11, 53–57 (2017)