We describe a method for reducing the temperature sensitivity for spontaneous parametric down conversion (SPDC) suitable for generating entangled time-frequency quantum photonic states in the telecommunication band at a wavelength of 1550 nm. We present specific design examples using periodically-poled LiNbO3 waveguides, which are commercially available and have a high second-order nonlinear coefficient that improves the overall device efficiency. One method uses two distinct poling regions, each with a poling period selected for phase-matching at different temperatures. The temperature bandwidth is increased from ~5 °C to ~17 °C using this approach, while only reducing the overall efficiency of the process by a factor of 4. The bandwidth is large enough so that controlling the overall temperature of the nonlinear crystal is likely not needed in a laboratory environment or can be achieved using a low size, weight, and power temperature controller design for field-deployed sources. To improve the stability of the design, we propose using a differential heating method where only the difference in the temperature is stabilized. The differential temperature stabilization method requires a smaller, lighter weight heater and lower power than other approaches, thus greatly simplifying the overall design. Additionally, we show that the system is less sensitive to fluctuations in the pump laser’s wavelength for the two-region approach. Finally, we generalize the design to multiple poling regions.
Entangled photon sources (EPS), the major building block for a variety of quantum communication protocols, are commonly developed by utilizing the spontaneous parametric down-conversion (SPDC) in
χ2 nonlinear bulk optical materials. While high nonlinearity and long interaction length have established the superiority of the periodically poled crystals for EPSs, the phase-matching condition of such crystals is very sensitive to the fluctuation of the crystal temperature and the laser wavelength. As a result, deploying such sources outside the laboratory, for example, satellite-based applications, demands a stringent mass and power budget, thus enhancing the mission complexity and cost. We report a bright, stable entangled photon source with a relaxed requirement of crystal temperature and laser wavelength stabilization. Using a periodically poled KTP crystal inside a polarization Sagnac interferometer producing degenerate, type-0 phase-matched entangled photon pairs at 810 nm in an annular ring, we have transformed the SPDC ring into a "perfect" ring with the help of two common optical elements, axicon, and lens. Despite the variation of the SPDC ring size from Gaussian to annular of different diameters due to the change of crystal temperature over
7oC, and laser wavelength over 300 GHz, we observe the size of the "perfect" ring to be constant. The new EPS, having a spectral brightness as high as 22.58
± 0.15 kHz/mW collected using single-mode fiber with a Bell's parameter, S = 2.64
± 0.05, and quantum state fidelity of 0.95
± 0.02, requires a crystal temperature stability of
± 0.8oC, almost five times relaxation as compared to the previous EPS. The generic scheme can be used for non-collinear SPDC photons in all crystals to develop EPS at any wavelength and timescales for resource-constrained applications.
Efficient entangled photon pair sources are the main component for several applications based on quantum imaging. Specifically for ghost imaging, different wavelengths of signal (imaging photons) and idler (interaction with the object) photons are desired. An efficient and narrowband generation of entangled photons exploiting spontaneous parametric down-conversion using periodically poled (pp) nonlinear crystals is therefore a fundamental preliminary requirement to achieve (the process of) ghost imaging. This work presents the design and implementation of a precise and efficient crystalheater as a variable photon pair source and compares the achieved experimental values of the SPDC-wavelengths with theoretical calculations. A periodically poled nonlinear crystal from potassium titanyl phosphate (ppKTP) can generate various non-degenerate wavelengths from a pump radiation of 405 nm by temperature changes and satisfaction of energy conservation and quasi-phase-matching conditions. For this purpose, the crystal is securely housed in a custom-built mechanical mount. A computation and adjustment of various control parameters, as well as a precise determination of the current temperature via two temperature sensors allow the heater to set the target temperature with an accuracy of 0.1 °C±0.015 °C. A method for the theoretical determination of the temperature-dependent shift of the nondegenerate wavelengths, provides a foundation from which experimental verification of achievable wavelengths and intensities can be compared. By experimental verification, the efficiency and functionality of the photon pair source and SPDC-process is verified. These presented investigations and the design of the crystal-heater provide the basis for a precise and effective photon pair source, for subsequent studies in the field of ghost imaging.
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.
The “quantum” system of optical radiation metrology based on spontaneous parametric down-conversion (SPDC) correlated photon sources and coincidence measurement technology has important applications in the field of extremely weak optical radiation detection. To acquire widely tunable entangled photon sources, 532 nm continuous laser was used to pump periodically poled LiNbO3 (PPLN) crystal. And 676 nm~2500 nm correlated photons were got by switching the crystal period and tuning the temperature. Firstly, the parameters such as polarization period, length and temperature control precision of PPLN crystal were made a theoretical study. Thus, the polarization period of PPLN crystals, the bandwidth distribution curve of the signal light in different wavelength, and temperature control precision curve of the different wavelength of signal light are further calculated. The results show that, the 676 nm~2500 nm correlated photon can be got by using 9 PPLN crystals in the temperature range of 50°C~200°C. The bandwidth of the signal light has negative correlation with the crystal length; The temperature control precision of PPLN crystals was from 3.96°C to 7.64°C, and the temperature control precision requires more higher with the increase of signal light wavelength. The research results will lay a foundation for the design of high-throughput wide-spectrum tunable entangled photon sources.
We proposed and demonstrated a simple but deterministic scheme for generating polarization-entangled photon pairs at telecommunication wavelengths with type-II quasi-phase-matched spontaneous parametric down-conversion (QPM-SPDC) having two poling periods. We fabricated a LiNbO3 crystal having two poling periods so as to generate entangled photons at two wavelengths, i.e., 1506 nm and 1594 nm. We characterized the two-photon polarization state with state tomography and confirmed that the state was highly entangled.
As some of the most promising quantum applications such as high resolution quantum optical coherence tomography (QOCT) [1] with dispersion cancellation as well as quantum key distribution require frequency entanglement. Therefore, frequency entanglement light sources are an essential importance for implement of many quantum optical applications. So Far, the most prominent method used for the entangled photon pair generation is spontaneous parametric down-conversion (SPDC) in nonlinear crystals. Recently, with adopting chirp quasi-phase matching (QPM) PPSLT (periodically poled stoichiometric lithium tantalate) has been used as source of entangled photons with large bandwidth [2]. However, improving the overall system efficiency is still remain challenging.