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.
For advanced quantum information technology, sources of photon pairs in quantum mechanically factorable states are of great importance for realizing high-fidelity photon-photon quantum gate operations. Here we experimentally demonstrate a technique to produce spectrally factorable photon pairs utilizing multi-order quasi-phase-matching (QPM) conditions in spontaneous parametric downconversion (SPDC). In our scheme, a spatial nonlinearity profile of a nonlinear optical crystal is shaped with current standard poling techniques, and the associated phase-matching function can be approximated to a Gaussian form. By the measurement of a phase-matching function and the second-order autocorrelation function, we demonstrate that telecom-band photon pairs produced by our custom-poled crystal are highly factorable with > 95% single-photon purity. I. INTRODUCTIONQuantum photonics technology plays an important role in various quantum applications such as quantum computing [1, 2], quantum communication [3,4], quantum metrology [5], and bridging solid and atomic quantum systems at a distance [6]. One of the central requirements in the quantum photonics technology is to produce pure single photons. Most multi-photon applications rely on interference of independent single photons [7], and only pure and indistinguishable photons can exhibit perfect interference. Photon-pair generation via spontaneous parametric downconversion (SPDC) has been widely accepted in quantum optics and photonic quantum information experi-
Photon-pair generation via spontaneous parametric downconversion (SPDC) has been widely used for optical quantum information experiments. We demonstrate high-precision spectral measurements of SPDC sources, utilizing frequency-resolved sum-frequency generation, a reverse process of SPDC.
We experimentally demonstrate multi-order quasi-phase-matching conditions for spectral shaping of photon pairs produced by spontaneous parametric downconversion. Incorporated with a group-velocity matching condition, our multi-order quasi-phase-matching scheme generates single photons high spectral purity.
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