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New Journal of Physics

Time uncertainty of a photon pair creation in a bulk periodically poled potassium titanyl phosphate pumped by a femtosecond laser

Bao‐Sen Shi, Akihisa Tomita

Published Mar 10, 2006• 9 citations
Publisher

Abstract

Periodically poled crystals, such as periodically poled potassium titanyl phosphate (PPKTP) or lithium niobate, recently became more attractive because of their high efficiency for producing a photon pair in both cw and pulse cases. The time uncertainty of photon pair creation in a crystal is crucial for many important applications. In this paper, we experimentally demonstrate that the time uncertainty of the photon pair creation in a PPKTP crystal is much larger than the coherence time of a femtosecond pump laser by using phase-sensitive two-photon interference using a Michelson interferometer, which means the femtosecond laser cannot be considered as a clock that announces the creation of the photon pairs. The experimental results can be well explained by an effective Franson-type interferometer, if the PPKTP is regarded as a narrow band filter at the same time. Our experiment also shows a way by which the Franson-typetwo-photon interference phenomenon can be observed even if the coherence length of the pump laser is less than the path-length difference between the two interfering beams.

DOI
10.1088/1367-2630/8/3/038

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Periodically poled crystals, such as periodically poled potassium titanyl phosphate (PPKTP) or lithium niobate, recently became more attractive because of their high efficiency for producing a photon pair in both cw and pulse cases. The time uncertainty of photon pair creation in a crystal is crucial for many important applications. In this paper, we experimentally demonstrate that the time uncertainty of the photon pair creation in a PPKTP crystal is much larger than the coherence time of a femtosecond pump laser by using phase-sensitive two-photon interference using a Michelson interferometer, which means the femtosecond laser cannot be considered as a clock that announces the creation of the photon pairs. The experimental results can be well explained by an effective Franson-type interferometer, if the PPKTP is regarded as a narrow band filter at the same time. Our experiment also shows a way by which the Franson-typetwo-photon interference phenomenon can be observed even if the coherence length of the pump laser is less than the path-length difference between the two interfering beams.
It was shown by Riedmatten et al. [J. Mod. Opt. 51 1637 (2004)] that two different four-photon states could be generated according to the relation between the duration time of the pump laser T and the coherence time t of the single photon from spontaneous parametric down-conversion (SPDC). In this paper, using four photons generated by the SPDC in a Periodically Poled Potassium Titanyl Phosphate (PPKTP), we experimentally observe the four-photon interference in a Michelson interferometer. All experimental results can be explained by the product of two independent two-photon interferences. Our experimental condition corresponds to the case of T ≫ t. This experiment clearly confirms the theory given by Riedmatten et al.
We demonstrate efficient generation of collinearly propagating, degenerate pulsed photon pairs based on a bulk periodically poled potassium titanyl phosphate pumped by an ultrashort-pulse laser. Using a single-mode fiber as a spatial mode filter, we detect approximately 3200 coincidence counts per second per milliwatt pump power in a Hanbury–Brown–Twiss-type experiment. The estimated photon-pair production rate is approximately 1.05 MHz/mW pump power. This is very promising for the realization of sources for quantum communication and metrology.
We demonstrate a scheme for femtosecond pulsed spontaneous parametric down-conversion in periodically poled potassium titanyl phosphate crystals. Our scheme is based on a crossed crystal configuration with collinear quasi-phase-matching. The nondegenerate photon pairs are split in a fiber-based wavelength division multiplexer. The source is easier to align than common pulsed sources based on bulk beta barium borate crystals and exhibits high-quality polarization entanglement as well as nonclassical interference capabilities. Hence, we expect this source to be a well-suited candidate for multiphoton state generation, e.g., for linear optical quantum computation and quantum communication networks.
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.
We report on the generation of indistinguishable photon pairs at telecom wavelengths based on a type-II parametric down conversion process in a periodically poled potassium titanyl phosphate (PPKTP) crystal. The phase matching, pump laser characteristics and coupling geometry are optimised to obtain spectrally uncorrelated photons with high coupling efficiencies. Four photons are generated by a counter-propagating pump in the same crystal and anlysed via two photon interference experiments between photons from each pair source as well as joint spectral and g((2)) measurements. We obtain a spectral purity of 0.91 and coupling efficiencies around 90% for all four photons without any filtering. These pure indistinguishable photon sources at telecom wavelengths are perfectly adapted for quantum network demonstrations and other multi-photon protocols.