We extend our earlier results of a bright source of non‐degenerate polarization entangled photon‐pairs by making a thorough theoretical and experimental investigation of its single‐mode emission and state‐decoherence. Using two perpendicular bulk crystals of periodically poled KTP, the source directly creates the polarization‐entangled state (|HH〉 + |VV〉)/√ with signal and idler wavelengths of 810 nm and 1550 nm, respectively. In order to optimize the coupling into single‐mode fiber we have developed a numerical algorithm which determines the emission mode and the fiber coupling efficiency for a given crystal length and focussing of the pump. For a 4.5 mm long crystal the optimal focussing of the pump is to a beam waist radius of 14 μm. With these focussing conditions the computer calculations give the lowest M2 value for the emission mode, M2 ≈ 3, and the best possible fiber coupling efficiency, γ ≈ 75%. Using the theoretical results to optimize our experimental setup we obtained photon count rates of 1.4 MHz singles and 12 kHz coincidences, and visibilities H, V: 99.6% and 45°: 94.2%.
We report results regarding a source of polarization entangled photon pairs created by the process of spontaneous parametric downconversion in two orthogonally oriented, periodically poled, bulk
KTiOPO4 crystals. The source emits light colinearly at the nondegenerate wavelengths of 810 and
1550nm, and is optimized for a single-mode optical fiber collection and long-distance quantum communication. The configuration favors long crystals, which promote a high photon-pair production rate at a narrow bandwidth, together with a high pair probability in fibers. The quality of entanglement is limited by chromatic dispersion, which we analyze by determining the output state. We find that such a decoherence effect is strongly material dependent, providing for long crystals an upper bound on the visibility of the coincidence fringes of 41% for
KTiOPO4, and zero for
LiNbO3. The best obtained raw visibility, when canceling decoherence with an extra piece of crystal, was $91\ifmmode\pm\else\textpm\fi{}0.2%$, including background counts. We confirm by a violation of the CHSH-inequality ($S=2.679\ifmmode\pm\else\textpm\fi{}0.004$ at $55\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1∕2}$ standard deviations) and by complete quantum state tomography that the fibers carry high-quality entangled pairs at a maximum rate of $55\ifmmode\times\else\texttimes\fi{}{10}^{3}\phantom{\rule{0.3em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{THz}}^{\ensuremath{-}1}\phantom{\rule{0.2em}{0ex}}{\mathrm{mW}}^{\ensuremath{-}1}$.
We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasi-phase matched bulk crystals for the use in quantum communication sources. We show that, by careful design, one can attain well defined modes close to the fundamental mode of optical fibers and obtain high coupling efficiencies also for bulk crystals, these being more easily aligned than crystal waveguides. We distinguish between singles coupling, ${\ensuremath{\gamma}}_{\mathrm{s}}$ and ${\ensuremath{\gamma}}_{\mathrm{i}}$, conditional coincidence, ${\ensuremath{\mu}}_{\mathrm{i}\ensuremath{\mid}\mathrm{s}}$, and pair coupling, ${\ensuremath{\gamma}}_{\mathrm{c}}$, and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal pumped by a
532nm laser creating photon pairs at
810nm and
1550nm. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation
L∕zR to be $\ensuremath{\approx}1$ to 2 for the pump mode and $\ensuremath{\approx}2$ to 3 for the fiber-modes, where
L is the crystal length and
zR is the Rayleigh-range of the mode-profile. These results are independent on
L. By showing that the single-mode bandwidth decreases $\ensuremath{\propto}1∕L$, we can therefore design the source to produce and couple narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means both less chromatic dispersion for long propagation distances in fibers, and that telecom Bragg gratings can be utilized to compensate for broadened photon packets---a vital problem for time-multiplexed qubits. Longer crystals also yield an increase in fiber photon flux $\ensuremath{\propto}\sqrt{L}$, and so, assuming correct focusing, we can only see advantages using long crystals.
This study presents an ultrabright polarization‐entangled photon source that is optimally coupled into single‐mode fibers (SMFs). This study theoretically and experimentally examines the characteristics of spontaneous parametric down‐conversion (SPDC) photons, including their spectrum, bandwidth, emission angle, and intensity, as functions of crystal length, temperature and beam waist condition. Notably, this study measures the collinear spatial modes of photon‐pairs and collection optics under various beam waist conditions and analyzes them using a collinear Gaussian approximation model. By employing a simple mode‐matching optical setup, it optimizes the SMF coupling and heralding efficiencies of the photon‐pairs. Consequently, it achieves a spectral brightness exceeding 2.0 MHz mW −1 nm −1 from a fiber‐coupled entangled photon source, utilizing a 30 mm ppKTP crystal inside a polarization Sagnac interferometer. This represents the highest spectral brightness of SPDC photons generated using a continuous‐wave (CW) laser pumped bulk crystal to date. Polarization entanglement is verified by a quantum state tomography and a polarization‐correlation measurement. The fidelity of the entangled state is measured to be 97.8% and the Bell‐Clauser‐Horne‐Shimony‐Holt value S = 2.782 ± 0.04. The results obtained here provide practical insights for designing high‐performance SPDC sources for satellite‐based communication and long‐distance optical links with extremely high‐photon loss.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.
We demonstrate a broadband entangled photon source based on a single-pass, type-0 phase-matched, non-collinear spontaneous parametric down-conversion (SPDC) process. By spatially multiplexing the intrinsically random transverse momentum states of biphotons emitted in an annular spatial distribution from a single-frequency-pumped, 2-mm-long periodically poled KTP crystal, we generate polarization-entangled pair photons with a spectral bandwidth of ∼130 nm centered at 810 nm. The negligible group-velocity mismatch of the co-polarized near-degenerate photons in single-pass and low birefringence of standard optical components preserves indistinguishability across the broadband emission without phase compensation, enabling high-quality polarization entanglement over the entire emission bandwidth. The source achieves a brightness of 0.57 Mpairs/s/mW and a two-photon coherence length of 6.97 μm, while exhibiting an interference visibility exceeding 95%, a Bell–CHSH parameter of 2.526 ± 0.005 that remains stable over 12 h of continuous operation and violates the Bell inequality by more than 100 standard deviations, and a state fidelity of 96.3%. The generic scheme providing the broadest bandwidth entangled biphotons at 810 nm, to our knowledge, can readily be used for other wavelengths as required for multiparty quantum networks and quantum sensing applications.