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Photonics for Quantum (PfQ)

Design and implementation of a tunable crystal-heater for spectral variation of entangled non-degenerate photon pairs generated by spontaneous parametric down-conversion

Robin Horn, Richard Fütterer, Maik Rosenberger, Gunther Notni

Published Jun 5, 2023• 0 citations
Publisher

Abstract

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.

DOI
10.1117/12.2670656

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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 “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 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.
The Periodically Poled Potassium Titanyl Phosphate (PPKTP) is a type of non linear crystal that serves as a source of production of entangled pairs of photons. The produced pair of photons, known as the Signal and Idler photons, possess unique characteristics within their entanglement that allow the production of new technologies such as quantum cryptography and quantum teleportation. The creation of the Signal and Idler photons occur through a process named Spontaneous Parametric Down Conversion (SPDC), which occurs when the pump laser beam penetrates a non linear medium. SPDC follows conservation of momentum and energy, thus imposing phase matching conditions on the generated photon pairs. These conditions result in specific space and frequency correlations of the photons. The aim of this project is to characterize the entangled photon pairs in their spatial profile and frequency spectrum, known as the Joint Spectrum.
A periodically poled lithium niobate (PPLN) crystal has been used as an efficient source of non-collinearly generated polarization-entangled photon pairs at 810 and 1550 nm. The PPLN crystal was endowed with a specially designed poling pattern and the entangled photons were generated via the nonlinear optical process of spontaneous parametric down conversion (SPDC). A novel design based on overlapping two concurrent type-I quasi-phase-matching structures in a single PPLN crystals produced correlated pairs of alternatively polarized photons in largely separated spectral regions. The phase of the resulting two-photon state is directly linked to parameters of the nonlinear grating. Continuous tunability of the generated Bell state, from Phi(+) to Phi(-), has been demonstrated by translating a slightly wedged crystal perpendicular to the pump beam.
Spontaneous parametric down-conversion (SPDC) is a reliable and robust source of photons for quantum information applications. For applications that involve operations such as entanglement swapping or single-photon heralding, two-photon states are required to be factorable (uncorrelated) in their spectral and spatial degrees of freedom. We report the design and experimental characterization of an SPDC source that has been optimized for high spectral and spatial purity. The source is pumped by the 776 nm output of a mode-locked Ti:Sapphire laser and consists of a periodically-poled Potassium Titanyl Phosphate (PPKTP) crystal phase-matched for collinear type-II SPDC. The dispersive properties of PPKTP at these wavelengths is such that it is possible to minimize the spectral entanglement by matching the widths of the pump to the spectral phase-matching function. The spatial entanglement is minimized through careful control of the pump focus, yielding nearly single-mode emission. An advantage of this approach is that the emission rate into the collection modes is very high, resulting in a very bright SPDC source. We also report a scheme that employs the output of collinear sources such as these to produce polarization-entangled photon pairs. The scheme, which requires only simple polarization elements, can be scaled to N-photon GHZ states.
We demonstrate a polarization-entangled photon-pair source obtained via a type-II non-collinear quasi-phase-matched spontaneous parametric down-conversion process with a 10-mm periodically poled KTiOPO4 crystal, which is as stable and wavelength-tunable as the well-known Sagnac configuration scheme. A brightness of 4.2 kHz/mW is detected and a concurrence of 0.975 is estimated using quantum state tomography. Without loss of entanglement and brightness, the photon-pair wavelengths are tunable through control of the crystal temperature. This improvement is achieved using the non-collinear configuration and a stable interferometric distinguishability compensator.